27 Place Pitch Discrimination and Speech Recognition in Cochlear Implant Users Johan J Hanekom Department of Electrical and Electronic Engineering University of Pretoria Robert V Shannon Head: Department of Auditory Implant and Perception House Ear Institute Los Angeles, California, United States of America ABSTRACT The considerable variability in speech perception performance among cochlear implant patients makes it difficult to com- pare the effectiveness of different speech processing strategies. One result is that optimal individualized processor param- eter setting is not always achieved. This paper investigates the relationship between place pitch discrimination ability and speech perception to establish whether pitch ranking could be used as an aid in better patient-specific fitting of processors. Three subjects participated in this study. Place pitch discrimination ability was measured and this information was used to design new channel to electrode allocations for each subject. Several allocations were evaluated with speech tests with consonant, vowel and sentence material. It is shown that there is correlation between the perceptual pitch distance between electrodes and speech perception performance. The results indicate that pitch ranking ability might be used both as an indicator of the speech perception potential of an implant user and in the choice of better electrode configurations. OPSOMMING Die beduidende verskille in spraakherkenningsvermoe van kogleere-inplantpasiente bemoeilik die vergelyking van die effektiwiteit van verskillende spraakverwerkingsstrategiee. 'n Gevolg is dat die individuele instelhng van spraakverwerkerparameters 'vir pasiente nie altyd optimaal gedoen word nie. Hierdie artikel ondersoek die verband tussen plek-toonhoogtediskriminasie en spraakherkenning om te bepaal of toonhoogte-rangskikking nuttig is as hulpmiddel vir beter gebruiker-spesifieke passing van spraakverwerkers. Drie proefpersone het aan hierdie ondersoek deelgeneem. Plek- toonhoogtediskriminasie is gemeet en die inligting Meruit is gebruik vir die ontwerp van nuwe afbeeldings van elektrodes op kanale. Verskeie afbeeldings is evalueer met spraaktoetse met vokaal-, konsonant- en sinsmateriaal. Daar word aangetoon dat daar korrelasie bestaan tussen toonhoogtediskriminasie en spraakherkenningsvermoe. Die resultate wys dat toonhoogte- rangskikkingsvermoe gebruik kan word as beide 'n indikator vir die spraakherkenningspotensiaal van 'n kogleere- inplantgebruiker en vir die beter keuse van elektrodekonfigurasies. KEY WORDS: cochlear implants, multi-electrode stimulation, pitch discrimination, speech recognition, neural selectiv- ity, perceptual distance. INTRODUCTION Two parameters, which influence the speech perception abilities of cochlear implant users, are the quality of spec- tral information and the quality of temporal information received by their electrically activated auditory systems. This paper focuses on the importance of spectral informa- tion. Multiple-electrode stimulation is preferred in coch- lear implants, because it is generally accepted that the tonotopic organization found along the length of the coch- lea in the healthy auditory system is retained to some degree for electrical hearing. Many research studies, in- cluding earlier work by Eddington (1980) and a recent study by Nelson, Van Tasell, Schroder & Soli (1995), have shown that electrodes stimulating the more basal areas of the cochlea result in higher perceived pitches (or sharper tonal quality) and stimulation closer to the apex results in lower perceived pitches. It would be natural to assume that tonotopic organiza- tion in electrical hearing is retained by multiple electrodes selectively stimulating discrete neural populations. How- ever, the assumption that discrete neural populations can be activated is not always true. When electrodes are closely spaced, considerable overlap occurs in the neural populations excited by the stimulation current, a problem which was addressed by Tbwnshend & White (1987). This Die Suid-Afrikaanse Tydskrif vir Kommunikasieafwy kings, Vol. 43, 1996 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) 28 is the result of spread of electrical current in the biologi- cal medium of the cochlea. The implication is that if two electrodes stimulate the same neural population or overlapping neural populations, sound sensations elicited by the two stimuli might be con- fused or might even be indistinguishable. This reduces the number of independent channels of information that can be conveyed to the cochlear implant user's auditory sys- tem. It is important to realize that the number of inde- pendent channels of information is not equal to the number of electrodes. The question which presents itself then, is how much overlap in stimulation of auditory neurons does occur in the cochlea, and how important is this in the patient's ability to understand speech? Although it is generally as- sumed to be true, is it really beneficial to have excitation of distinct neural populations? In other words, on the one hand it is assumed that multi-electrode implants perform well because they utilize the tonotopical organization of the cochlea, but on the other hand current spread inside the cochlea might defeat this purpose by having different electrodes stimulating identical neural populations. If many electrodes stimulate the same neural population, why do some patients perform so well on open set speech recognition tasks? A secondary question which follows is whether using simpler implants with a smaller number of electrodes (which in turn could potentially be cheaper and more reliable) would not perform as well as implants with many electrodes. The questions formulated above and related questions have been investigated by a number of researchers. The effect of the number of electrodes on speech perception performance has been addressed in Lawson, Wilson, Zerbi & Finley (1996). They found that by increasing the number of electrodes, speech perception performance is improved, but for as few as four to seven electrodes, speech perform- ance levels are close to what can be achieved by using ten or twenty electrodes. Busby, Whitford, Blarney, Richardson & Clark (1994) studied patients' abilities to rank electrode pitch as a func- tion of stimulation mode of the Nucleus1 implant device. (Clark, Tong & Patrick (1990) give detailed descriptions of this device). As will be explained in the text to follow, the Nucleus device can utilize different stimulation modes, which produce differences in the amount of current spread from the electrodes. Busby et al. (1994) found that the ability to rank pitch for stimulation on a specific electrode (in other words, place pitch) was related to the mode of stimulation used (and thus the amount of current spread). Nelson et al. (1995) studied the relationship between pitch ranking ability (or electrode ranking ability) and consonant perception in ten subjects using the Nucleus cochlear implant. They found correlation between the con- sonant perception and pitch discrimination, but they found little correlation between recognition of consonants based on recognition of place cues and place pitch perception. However, they concluded that this might be related to the speech processing strategy not taking full advantage of the user's ability to do place pitch ranking. This paper addresses some of the questions mentioned above. Specifically, we investigate the question of the re- One of the most widely used cochlear implant devices is the Nucleus, manufactured by Cochlear Pty Limited in Australia and by their United States subsidiary, Cochlear Corporation. Johan J Hanekom en Robert V Shannon lationship between speech perception performance and the stimulation of overlapping neural populations in the co- chlea. The approach used is to find a measure of the amount of overlap among stimulated neural populations. This is accomplished by determining the amount of elec- trode confusion with a pitch discrimination experiment. An additional question, which we also address, is Whether it is possible to improve speech discrimination with the correct design of electrode configuration. EXPERIMENTAL APPROACH One way to quantify the amount of overlap among neu- ral populations stimulated by different electrodes, is to measure the amount of confusion between electrodes. We measure the pitch discrimination between electrodes as a measure of the amount of electrode confusion, which is then also a measure of the amount of overlap in the neu- ral populations stimulated. This is in turn a measure of the amount of current spread resulting from electrical stimulation. The procedure we use is to compile a place pitch ranking matrix (or electrode discrimination matrix) which is transformed to perceptual distance values, as explained below. Based on the pitch discrimination infor- mation, we then design various maps and evaluate the speech perception performance for these. We focus primarily on the relationship between pitch discrimination data and speech perception performance for various different maps in a single subject, while re- peating only some of the tests in other subjects. In this respect our approach differs from that used by Nelson et al. (1995). They compared speech perception abilities in ten subjects using their everyday maps2, and related this to the subjects' place pitch ranking abilities. However, they did not study the effect of using different maps in the same patient. The rationale for this would be to investigate to which dimensions of place pitch discrimination speech perception is related. The term dimensions refers to the fact that place pitch discrimination ability might be re- lated to various physical electrode parameters, for exam- ple electrode spatial separation and current spread from the electrodes, but also to perceptual distance between electrodes. Our study evaluates nine different maps in the same subject, to establish whether speech recognition perfor- mance is related to the perceptual distance between elec- trodes or to other dimensions of pitch discrimination ability. It will become clear in the description of the properties of the pitch discrimination abilities of each of the subjects why more maps were evaluated in one specific subject. The three subjects who participated included a good user, The term map used in this context, usually refers to the patient-specific settings that are made to the Nucleus processor. A map is a table of values with the threshold and uncomfortable loudness stimulation current values for each electrode. The map also holds information on the specific frequency allocation table used. The frequency allocation table is a table specifying the filter cutoff frequencies used in the twenty channels of the Nucleus speech processor. Different frequency allocation tables are available. The software used in this study to program the Nucleus device allows the user an extra option, namely to allocate multiple filter channels to the same electrode. When we refer to map in this article, we are actually referring to this filter-to-electrode allocation. The other parameters we used for the maps were from the subjects' everyday maps and remained unchanged throughout the experiments, unless noted otherwise. Everyday map refers to the regular maps that the subjects used daily. The South African Journal of Communication Disorders, Vol. 43, 1996 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) Place Pitch Discrimination and Speech Recognition fair user and a relatively poor user. We relate our find- ines to this observation. perceptual distance between stimuli is quantified by d', a asure often used in psychophysical studies to express per- m ® a l distances in forced choice experiments. This meas- w a s u s e d in the context of pitch discrimination studies b^Townshend e t a i (1987) and more recently by Nelson et al (1995)· Smaller values of d' indicate more confusion between stimuli. Negative values of d' in the pitch dis- crimination experiment indicate pitch reversal. For no confusion between stimuli, the largest value of d' (3.29) is achieved. The d's can be calculated from signal detection theory as described in Green & Swets (1966). Hacker & Ratcliff (1979) tabulated the values of d' for a two alter- native forced choice experiment such as is described here. We used a vowel test, a consonant test and a sentence test in the evaluation of speech perception ability with each map. In previous studies (for example Nelson et al., 1995) consonant perception was used to assess speech perception ability. We included a vowel test, because vow- els are recognized primarily by their formant structure (Dubno & Dorman, 1987) and as such their recognition should be dependent on the ability to activate discrete neural populations selectively. Sentence material was in- cluded in the speech perception tests to evaluate open set speech understanding for the various different maps. The number of electrodes used in the maps in our ex- periments was six or seven throughout. The reason for this choice was that it was found that seven-electrode maps gave speech perception performance levels that allowed some play for the speech perception scores to improve or deteriorate. A study by Lawson et al. (1996) showed that speech recognition demonstrated a rapid decline when the number of electrodes was lowered from seven to four to two and to one, and that with a seven-electrode map it is possible to get speech discrimination close to what can be achieved by ten or twenty-electrode maps. An advantage of using a reduced number of electrodes is that the stimu- lation rate of the Nucleus processor increases. Higher stimulation rates have proved to result in better speech recognition performance (Wilson et al., 1991). A corollary of this study is to establish a procedure which could optimize an electrode map when only a small number of electrodes are used. Fewer electrodes than the twenty available in the Nucleus might be used for several reasons. Other implants that use fewer electrodes are in Cochlear Implant Users 29 available. It is also possible that only a small number of stimulation sites are available as a result of poor neural sur- vival, or that a reduced number of electrodes are available because of electrode damage. Another possible application for fewer electrodes would be a future lower cost device. The rest of this paper is discussed in two sections. In the first section, the pitch discrimination experiment is described and in the second section the results are used in the design of maps which are evaluated with speech per- ception experiments. PITCH DISCRIMINATION EXPERIMENT METHODOLOGY Subjects Three users of the Nucleus cochlear implant partici- pated in this study. All of them were users of the Nucleus Spectra speech processor. This processor implements the SPEAK speech processing strategy, which is described in Skinner et al. (1994). Table 1 contains detailed informa- tion on the three subjects. Electrode parameters All three subjects used the Nucleus 22 electrode array (described in Clark et al., 1990, p. 114), implanted into the scala tympani. The electrode bands in this array are separated by 0.75 mm. We refer to the two electrodes of a stimulation pair as the stimulation electrode and the re- turn electrode. Stimulation was always with current-bal- anced biphasic pulsatile waveforms, with the positive pulse preceding the negative pulse. The stimulation electrode was the electrode on which a positive first biphasic pulse could be measured if the other electrode was used as ref- erence. The return electrode was always the more apical of a stimulation pair. The electrode numbering conven- tion used in this paper is as follows: an electrode pair is referred to by the stimulation electrode number, and with the stimulation mode known (explained below), the return electrode is implicitly known. Electrode 1 was the most basal electrode and electrode 20 was the most apical elec- trode used as stimulation electrode. The Nucleus speech processor allows different stimu- lation modes. In BP mode3, the stimulation electrode and TABLE 1. Subject information for the three subjects who participated in this study. Insertion depth refers to the number of electrode bands inside the cochlea. The first twenty-two electrodes are active. Subject Age Gender Age of onset of profound hearing loss Time of implant use Processor type Insertion depth Cause of deafness EWB 55 Male 45 6 years Spectra 27 trauma JEM 39 Male 35 4 years Spectra 26 trauma REK 54 Male 47 6 months Spectra 22 progressive hearing loss 3 The abbreviation BP, for bipolar, is used throughout this paper. This is the standard abbreviation used in texts by Cochlear Pty Limited to indicate the Nucleus device's stimulation modes. Die Suid-Afrikaanse Tydskrif vir Kommunikasieafwykings, Vol. 43, 1996 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) 30 the return electrode are adjoining. In B P + 1 mode one elec- trode separates the stimulation electrode and the return electrode: In B P + 2 mode two electrodes separate the Ξ Γ ™ t r ° d e a n d ; e t U r n e l e C t r ° d e - Simulation modeUs up to BP+3 were used in this study. The symbol ΔΕ is used for electrode spatial separation. Stimulus parameters mode of stimulation. For subject JEM w T ^^ ^ e d a BP+1 everyday map, we used stimulation in BP, B P ^ and Ms i Z Z 1 " a ? d l t l ° n 1° B P + 1 m o d e · T h e reasons for this will become clear in the discussions to follow AiJ^stimuli were current-balanced biphasic pulses posi Ζ P Γ / 1 1 " 8 ' ; S t l m u l a t l 0 n r a * e was 1000 puisespersec- The l e n e t h ^ ) f U e d u r a t i o n 200 microsecond . was 500 ms flnHSmg P r e S e n t a t i o n ° n a « ο electrode was 500 ms and no ramping was used. Stimuli were pre ut below Ι ο : ' Z t T T l e V e l ° f S t l m u l a t - above 50%, but below 80% of the dynamic range of the subject. Acorn c o r d e d T h T , T n n e r a t e d ^ a p p r ° p r i a t e stimuli and " S cor L t f ^ / r " 0 " ! ? ' T h e S t i m u l i W e r e — d e d in me correct format to enable presentation directly to the internal receiver of the Nucleus device. The coded stTmuli were p r e s e n t e d directly to the internal receiver οο Γ ο the subjects (the subjects' processors were not used) ^ a a custom m t e r f a c e (described in Shannon, Adams Fe^rel Pal umbo & Grandgenett, 1990). ' Psychophysical procedure eact of t h 7 h d l S T m m a t l 0 n m a t r i x was measured for u s S f o r all th B P + 1 S t l m u l a t i o n - o d e was u i p i n> κ i S U b j e C t S ' a n d i n a d d i t i ° n we also meas- ured pitch discrimination matrices for BP, B P + 2 and Β ρ Λ mode for subject JEM. + 3 The pitch discrimination matrix was compiled by us- stirmilTof 500 P S y c h ° P h ^ P-cedure. Consecutive stimuli of 500 ms, separated by a brief quiet interval of The S = e n W 0 n , t W ° ° f ^ S ^ C t ' s e l e S e piched r t T " t 0 J U d g e W h i c h s t i m u l u * was higher S a m Tbl « W 3 S C ° n t r 0 l l e d ^ a computer p r o - S Z r ί ! Γ h a d t 0 i n d i c a t e h i s c h o i c e by depressing th^firqt w o buttons, with one button c o r r e s p o n d to the first S o U n d and the other button to the second sound d e l ^ t S o m t T ° f l t l m U l 1 W a S P-sented X r a delay of 200 ms after the patient had made his choice Ζ Γ : Ζ Τ 0 η ΐ Ο Τ rePetiti°n °f t h e P a i r o f < p n u 7 t P a i r C O n s i s t e d of stimuli on two differ- ent electrodes. The stimuli were balanced for loudness to minimize confusions between loudness and pitch ThTs was done b y l o u d b a l a n d n g o c e Pteh.̂ Ihiswas Z Z f P l t C t d l S C n m i n a t l 0 n experiments. A reference midd e ofTh: C i T ' , 1 l S U a l l y e l e C t r ° d e 1 0 ' w h i c h » - the middle of the electrode array. The subject was asked to potCatheeastir 0ttabl! " ΐ " ^ l 6 V e 1 ' a n d ^ tS Pdur! pose tne stimulation levels were varî H , tween 50% and 80% of the d y Z ^ T S ^ ot thereference stimulus being presented, followed by a w " r peaTed3 a ? " 1 1 6 e l e C t r ° d e S - ^ ^mJns^ X e s e S f t T y 3 8 n e C e S S a ^ i n to find level setting for the second stimulus which had the same Johan J Hanekom en Robert V Shan apparent loudness as the first stimulus, l b be more a, ' rate in pinpointing the stimulation level which was " in loudness to the reference, subjects were also a s k ^ 1 find a level that was just noticeably softer than the r r 1 0 ence and also to find a level just not.eab y loude/th^' the reference^ In this way three datapoints Z e l t hshed for each electrode's simulation level and we J ' able to make a good estimation of the stimulus m r ! ' 6 necessary to have al, the electrode s t i m u ^ ^ * The electrodes used for the stimulation pairs duri the pitch discrimination experiment were compleM8 randomized for each run. One run consisted οί Γ ρ ^ tation of all possible combinations of electrodes fn l "' p W r , o . e S " \ o r m o d e · this amounted to twentv sion between electrodes, a considerable n T m b e r of c l pansons were needed. Twenty runs were Completed Tn B P + 1 mode for each of the subjects, which gave a t o S of forty comparisons of each electrode with evfry other elel to ϊ ί ΐ r e a C t i ° n ' i n d i c a t i n g which stimulus was judged mmssszt electrode of a stimulation pair was iudwH f„ ι· , pitched than the « o r e a p i c a U w h r X T u K tte e M order based o „ the tonotopic organization rftie S f a ) a p , « l eieetade to be higher pitched C X l ™ ' ^ re S d ° i f a T " " 1 ; Ρ * Γ · W M ™ d e S o n d very aistinct pitch difference, resulting in a l a m A' without the pitch difference being very W e a large d , probably an indication of w h e t h e / o v e l p p f g or ^ neural populations are stimulated n u m h ^ f P i d t y W a S p r e s e n t > electrodes with lower numbers (located more basally) would be expected t 7 Z consistently judged higher in /u expected to be higher m i m w ! 7 f ! P t c h t h a n electrodes with companson i u n T ^ ^ ^ e l e c t r o d e s ) in a paired tfonsT™ , ' l f c o m P l e t e l y separate neural popula- tions (non-overlapping) were stimulated by each electrode i e T t r T C l ° S e e l e C t r ° d e S p a d n ^ ' n ° c o ^ u s t n b e t e S electrodes was expected. On the other hanH 7 s M f A S S K W f t i f f l a f e S . s a n ^ c a t , o n of the amount of current The South Africa Journal of Communication Disorders, Vol. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) lace tc Discrimination and Speech Recognitionin Cochlear Implant Users 31 RESULTS t i m u l u s - r e s p o n s e matrices for the three subjects T h e S in fi srures 1 to 4 after they have been converted a r e shown i " " 6 1 0.36 0.62 2 -0.07 3 1 0.36 0.62 2 -0.07 1 0.36 0.62 2 -0.07 0.18 0.36 0,51 .0.36 0.25 0.62 0.00 -0.62 -0.18 0.43 0.18 0.36 0,51 .0.36 0.25 0.62 0.00 -0.62 -0.18 0.43 0.07 •0.18 0.18 0.36 0,51 .0.36 0.25 0.62 0.00 -0.62 -0.18 0.43 0.07 •0.18 0.00 0.07 1.53 1.66 .0.25 0.62 1 53 0.00 -0.62 -0.18 0.43 0.07 •0.18 0.00 0.07 1.53 1.66 .0.25 0.62 1 53 0.00 -0.62 -0.18 0.43 0.62 1.00 0.51 0.07 1.53 1.66 1.81 2.09 7 33 1 53 1.53 1.35 1.14 0.25 0.07 1.53 1.66 1.81 2.09 7 33 2.33 2.33 2.33 1.24 1.66 0.36 1 81 3.29 3.29 1.81 1.66 2.09 1.66 1.24 0.91 2.90 3.29 3.29 2.90 2.90 3.29 2.33 1.81 1.53 0.51 3?9 2.90 3.29 3.29 3.29 3.29 2.90 2.90 2.09 3.29 1.81 1.66 1.14 190 2.90 3.29 3.29 2.33 2.90 3.29 3.29 1.81 1.66 1.81 1.24 0.91 3.29 3.29 3->9 3 29 3.29 2.90 3.29 3.29 2.90 2.90 2.33 1.66 2.33 1.66 1.14 3.29 3.29 3->9 3.29 3.29 3.29 3.29 3.29 2.90 2.90 3.29 2.90 2.09 1.53 1.35 1.14 3 79 3 29 3.29 3.29 3.29 3.29 3.29 2.90 3.29 2.90 1.53 2.33 2.33 1.81 1.81 0.18 3.29 1.29 3.29 3.29 3.29 2.90 2.90 3.29 2.90 2.09 2.90 3.29 1.66 1.53 1.66 1.00 0.91 3.29 3.29 3.29 1.29 3.29 3.29 3.29 2.33 3.29 2.33 3.29 2.90 3.29 3.29 2.09 2.90 1.66 1.24 1.14 0.62 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 2.90 2.90 2.33 2.33 2.09 2.09 2.09 1.14 0.78 0.70 0.36 FIGURE 1. Pitch discrimination matrix for subject JEM for BP+1 mode of stimulation. Positive values of d' in the table indicate that the electrode num- bered at the top was judged to have a higher pitch than the electrode numbered at the left, d' values of 3.29 indicate 100% consistency in pitch judgement and values higher than 1.5 indicate an 85% consist- ency. JEM had a large area of good pitch discrimi- nation. 10 11 12 13 14 15 16 17 0.18 1.19 0.74 0.00 0.18 -0.74 0.18 0.74 -0.55 0.54 0.54 0.18 -0.74 0.18 0.18 > 1.19 0.18 0.18 0.74 0.95 0.36 [ 1.81 1.47 1.47 2.33 1.81 1.47 0.95 1 2.33 3.29 1.81 2.33 3.29 1.81 2.33 1.47 1 3.29 2.33 2.33 3.29 3.29 2.33 2.33 2.33 0.74 3.29 3.29 3.29 3.29 3.29 2.33 3.29 1.81 1.81 0.95 2.33 3.29 3.29 3.29 3.29 3.29 3.29 3.29 2.33 3.29 1.81 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 1.19 1.47 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 1.47 0.18 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 2.33 2.33 1.81 1.47 0.18 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 1.81 0.74 0.36 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 1.19 1.19 0.18 0.36 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 3.29 2.33 1.47 3.29 2.33 0.95 0.95 0.54 0.18 FIGURE 2. Pitch discrimination matrix for subject JEM for BP+3 stimulation mode. Note that the number of available electrodes was only 18 instead of 20. That is because the return electrode, for elec- trode 18 as stimulation electrode in BP+3 mode, is electrode band 22, which is the most basal electrode band. Also, note that the pitch discrimination pat- tern differs from that in figure 1. Some electrodes which did not exhibit good discrimination in BP+1 mode, had good discrimination in BP+3 mode. Com- pare the d' between electrodes 11 and 12 in these stimulation modes. The inverse was also true for some electrodes. ' to d' values. Two aspects are immediately evident from these matrices. First, the distribution of areas where elec- trode discrimination was better, varied considerably for the three subjects. This might have been dependent on electrode placement, with areas of good electrode discrimi- nation being where the electrodes were situated closer to the (surviving) nerves. This result underlines the fact that there are two important but uncontrollable factors in per- ception of sound in cochlear implants: (1) placement of elec- trode and (2) nerve survival. If a d' of 1.5 was (arbitrarily) taken as a criterion for largely independent neural populations (this corresponds 13 14 15 16 17 18 19 0.36 0.36 0.36 1.47 0.95 0.54 0.74 0.74 -0.36 0.00 1.19 1.19 1.81 0.18 0.18 0.95 1.47 1.81 0.74 0.36 0.18 1.19 0.95 0.74 0.00 0.55 0.74 0.18 0.54 0.18 0.36 0.00 1.47 0.95 1.19 0.74 1.47 1.19 1.47 0.54 0.00 0.74 0.00 0.54 1.47 1.81 2.33 2.33 1.19 1.47 0.95 0.18 0.54 1.47 0.54 2.33 1.81 1.19 1.81 0.74 0.54 1.19 1.19 1.19 0.74 0.36 3.29 3.29 1.81 1.19 1.47 1.19 0.95 0.74 3.29 0.74 0.18 0.18 2.33 3.29 2.33 3.29 0.54 1.19 0.74 0.54 1.47 0.18 0.18 0.54 0.00 2.33 3.29 1.81 1.81 0.95 1.19 1.19 1.47 1.81 0.54 0.36 0.54 0.36 •0.36 3.29 2.33 1.81 1.81 1.81 0.54 0.18 1.81 1.47 0.74 0.36 0.36 0.00 0.18 0.00 3.29 0.95 1.81 2.33 0.36 0.36 0.54 0.74 1.47 1.19 0.36 0.18 0.36 0.18 •0.36 0.36 1.47 0.00 0.36 0.36 •0.36 •0.18 •0-18 0.00 0.74 •0.55 0.95 1.47 -0.74 •0.18 -0.74 •1.19 0.95 0.18 •0.74 «.36 0.55 •0.18 •1.47 •1.47 •1.47 0.95 •2.33 •1.81 1.47 •1.19 1.81 •1.47 -1.81 0.00 0.55 1.19 0.55 0.36 •0.18 •0.18 1.19 0.74 •0.36 0.55 0.95 •1.19 •1.47 1.19 1.81 1.19 •1.47 •1.81 •0.36 1.47 FIGURE 3. Pitch discrimination matrix for subject REK. The values in the table are the calculated d' values. Positive values of d' indicate that the elec- trode numbered at the top was judged higher in pitch than the electrode numbered at the left more than half of the time. For normal tonotopic organi- zation, the d' values are expected to be positive, as electrode 1 is the most basal electrode and electrode 20 the most apical. REK had a region of good pitch discrimination toward the left side of the matrix. ι 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 -0.07 -0.70 0.00 0.14 -0.14 0.00 0.54 -0.07 0.62 0.54 -0.21 0.07 -0.21 0.21 0.14 -0.43 -0.14 -0.07 -0.07 -0.43 0.36 0.29 -0.07 -0.21 -0.21 -0.21 -0.21 -0.14 0.14 -0.29 0.62 0.21 -0.07 0.00 0.07 -0.14 0.87 -0.14 0.36 •0.07 0.43 0.62 0.07 0.29 0.36 0.54 0.21 0.87 0.07 0.14 0.00 0.21 0.43 0.43 -0.14 0.78 0.29 0.70 0.14 -0.14 0.70 0.14 0.21 0.29 0.36 -0.07 0.36 0.00 0.54 0.07 0.21 0.54 0.36 0.54 0.54 -0.14 -0.36 0.00 0.29 -0.21 0.21 -0.21 0.00 0.07 0.21 0.21 0.14 -0.43 0.00 -0.62 -0.29 0.43 -0.07 0.07 0.14 -0.36 0.14 0.07 0.00 0.14 0.07 -0.78 -0.43 0.14 0.21 0.54 -0.21 0.00 -0.36 0.00 -0.43 -0.14 0.07 -0.70 -0.70 -0.36 -0.87 -0.95 -0.36 0.07 0.95 0.54 1.14 0.62 0.70 0.70 0.87 0.87 0.54 0.21 0.70 0.07 0.29 1.05 0.78 1.05 1.05 0.78 0.87 1.24 0.87 0.87 0.62 1.35 0.95 0.70 0.70 0.70 0.29 0.78 1.14 1.24 0.43 1.66 0.78 1.14 1.14 1.14 1.35 0.95 1.99 1.35 0.70 1.05 0.95 0.78 1.24 1.05 1.81 1.05 -0.07 1.14 1.47 1.05 1.05 1.05 1.66 1.35 1.47 0.95 1.24 1.24 0.87 1.05 1.66 1.24 1.47 1.14 1.14 0.78 FIGURE 4. Pitch discrimination matrix for subject EWB. This subject exhibited poor pitch discrimina- tion throughout the electrode array, although he did a little bit better toward the apical side. Die Suid-Afrikaanse Tydskrif vir Kommunikasieafwy kings, Vol. 43, 1996 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) 32 to 85% consistency in pitch ranking), then subject EWB had almost no discrimination between any of his electrodes, although he did a little better toward the apical side. Sub- ject REK had a region of good pitch discrimination, but primarily for large electrode separations. Subject JEM had very good electrode discrimination throughout his elec- trode array when the electrode spatial separation was two or more. His best region was near to the middle of his elec- trode array. More details concerning the analysis of the properties of the subjects' pitch discrimination matrices are discussed when the maps used in the speech perception experiment are described below. SPEECH PERCEPTION EXPERIMENT METHODOLOGY Objective The speech perception experiment was used to investi- gate whether there was correlation between subjects' abili- ty to discriminate among electrodes based on place pitch (as reflected by the information in the pitch discrimina- tion matrices), and speech perception performance. Two parameters (which could be deduced from the pitch dis- crimination matrices) were identified as being important in speech perception: the perceptual distance between pitch sensations elicited by electrodes as reflected by the d' val- ues and ΔΕ, the electrode spatial separation. The maps used where chosen to reflect the effect on speech percep- tion of varying these two parameters. Speech materials We used vowel, consonant and sentence material to evaluate speech perception performance. All the speech material was available on laser disk. The vowel test used a set of eight vowels in a /hVdI context uttered by a male talker. Each vowel syllable was repeated three times in randomized order. The consonant test used sixteen consonants in a /aCa/ context. The con- sonant syllables were repeated five times in randomized order during a single consonant test. The sentence test consisted of a set of thirty-six sen- tences uttered by a female speaker. No repetitions of the same sentences were used and a new set of sentences was used for the evaluation of every map. None of the subjects had been tested with the specific sentence material be- fore. Procedure The software used for the creation of the subject maps allowed the programmer to allocate the output of any of the Nucleus processor's filters to any electrode. Multiple filter outputs could be allocated to the same electrodes. This enabled us to use maps utilizing a reduced set of elec- trodes, while still presenting all the spectral information from the twenty filters to the electrodes. The operation of the Nucleus implant is such that reducing the number of electrodes increased the stimulation rate on the electrodes that were used (Shannon et al., 1990). Subjects were tested with their regular BP+1 maps and frequency allocation tables which they had been using daily Johan J Hanekom en Robert V Shannon for several months. The only changes to these maps were that the regular filter-to-electrode allocations were re- placed by maps with a reduced number of electrodes of which multiple filter channels were allocated to each elec- trode. The specific choices of electrodes which were used in the various different maps are explained below. The subjects wore these experimental maps for a period of two full days before evaluation with the speech material. Tests were conducted in a sound-isolated booth. Speech stimuli were presented at 60 dB SPL. Speech stimuli were played from a laser disc player through high quality au- dio loudspeakers. A computer program controlled the pres- entation of the speech material to the subjects. The sub- jects responded by indicating their choice on a computer keyboard. The computer program recorded the subject re- action for the vowel and consonant tests, and compiled stimulus-response matrices for these. The computer pro- gram also controlled the laser disc player for presentation of the sentences. The subject had to repeat as many words as he could understand from the sentence material, which was then recorded by the experimenter. Map parameters Details of the maps that were used for each patient are explained below. As explained earlier, all maps were seven- electrode maps, except two of JEM's maps, which were six-electrode BP+3 maps. Maps were chosen to give either large or small cumulative values of d', and maps with simi- lar cumulative d' were evaluated using different electrode spatial separations. Both orderly and disorderly electrode spatial separations were used. Table 2 summarizes the maps used. The motivation for the choice of each of the maps is given in the descriptions below. We evaluated more maps for JEM than for the other two subjects. This was primarily dictated by the fact that subject JEM's pitch discrimination matrix provided more degrees of freedom in the choice of various maps. This statement will become clear in the explanation of the pro- cedure used to choose electrodes to be used in the maps. This procedure was very simple and was as follows: A list of all possible combinations forming seven-electrode maps was compiled (for example electrodes 1, 2, 3, 4, 5 6 7 or electrodes 1, 3, 5, 7, 9, 11, 13 and so on): Then, 'for each subject, the cumulative d' was calculated for all these electrode sets by simply adding the corresponding six d' values. For example for subject REK, for the electrode set consisting of electrodes 1, 2, 3, 4, 5, 6 and 7, the six d' values to be added were 0.36 (d' between electrode 1 and 2), 0.36 (the d' between electrode 2 and 3), -0.36, 0, 0.18 and 0.18. The cumulative d' was thus calculated as 0.36. It was assumed that electrodes were distinguishable if d' was larger than 1.5. From subject REK's pitch discrimi- nation matrix in figure 2, it can be seen that only a small region in the lower left corner of this matrix produced d' values in the region of 1.5 or larger, and then for an elec- trode spacing of four or larger. For smaller a criterion level of d'>l, a larger area in the d' matrix was identified. Be- tween electrodes 9 and 10 a d' of 1.47 was produced but for all other electrodes the electrode spatial separation had to be at least three to find a d'larger than 1. It can also be seen that electrodes could not be discriminated at the api- cal end of the electrode array. In this area all electrodes were confused and d' values were generally very low This means that to find a seven-electrode set with a large value The South African Journal of Communication Disorders, Vol. 43, 1996 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) lace Pitch Discrimination and Speech Recognitionin Cochlear Implant Users 33 of cumulative d', the electrodes had to be chosen more to- ward the basal side of the array. Even then we would not be able to choose an electrode set with all d' values greater than 1. For example, if we started with electrode 1, the next electrode had to be electrode 5 for d' greater than 1. Then, the third electrode had to be electrode 11 to have a d' greater than 1 between electrode 5 and the third elec- trode to be used. From electrode 11 toward the apical side of the electrode array, all electrodes were confused. Comparing this to JEM's pitch discrimination matrix, it becomes clear that for JEM there was much more flex- ibility in possible choices of electrodes for good cumula- tive d'. A d' of greater than one was found for all elec- trodes if electrode spatial separation was eight, and for most electrodes if the spatial separation was only two or larger, excluding electrodes to the more basal side of the array. This gave us many different possibilities of choice of seven-electrode maps with good cumulative d'. Confu- sion between electrodes occurred primarily on the basal electrodes. There seemed to be total confusion of electrode pitch for all electrodes in subject EWB's case, except for a few electrodes near the basal end of the array, l b find a d' of larger than 1, electrode spatial separation had to be at least 14 in the more apical half of the electrode array, and no d's of 2 or larger were found. This gave very little flex- SUBJECT: JEM Map no Electrodes used Stim mode d' C ΔΕ a D No of sites Vowel test Cons test Sent test 1 3,4,9,13,15,17,19 BP+1 10.3 2.7 2.2 5 92 88 95 2 6,7,8,9,10,11,12 BP+1 2.54 1 1 1 54 76 20 3 2,4,6,9,11,17,2 BP+1 4.98 3 2 2 88 88 88 4 4,5,10,11,16,17,19 BP+1 7.46 2.5 4 3 79 83 85 5 3,4,9,13,15,17 BP+3 6.53 2.8 3 3 83 89 92 6 2,4,6,9,11,17 BP+3 7.27 3 2.3 4 79 85 82 7 12,13,14,15,16,17,18 BP+1 5.42 1 1 1 62 81 66 8 2,5,8,11,14,17,20 (BP,BP,BP2,BP2,BP2, BP2,BP1) j mixed 13+ 3 1 5 92 90 97 9 2,4,7,10,13,16,Jl9 BP+1 6.64 2.8 1.1 3 88 85 98 SUBJECT: REK Map no Electrodes used i 1 Stim mode d' C ΔΕ a D No of sites Vowel test Cons test Sent test 1 1,3,6,10,12,14,116 . BP+1 4.37 2.5 1.8 2 46 56 38 2,-, 1,2,5,12,13,14,19 BP+1 0.60 3 5 1 58 65 74 3 2,4,6,8,10,12,14 BP+1 2.00 2 1 1 50 55 38 4 2,5,8,11,14,17,20 BP+1 -0.87 3 1 1 46 88 94 5 1,4,7,10,13,16,19 BP+1 0.41 3 1 1 67 74 87 SUBJECT: EWB Map no Electrodes used Stim mode d' C ΔΕ a D No of sites Vowel test Cons test Sent test 1 3,5,8,16,17,18,20 BP+1 3.72 2.8 3.25 1 38 49 30 2 1,4,7,9,12,14,16 BP+1 -0.55 2.5 1.5 1 62 53 33 3 1,4,7,10,13,16119 BP+1 0.86 3 1 1 67 49 57 TABLE 2. Results for the speech perception experiment. The number/) is a simple measure for the disorder in electrode spacing as calculated by equation 1. ΔΕλ is the average spacing between electrodes. d'c is the cumulative d'. No of sites refer to the number of discrete stimulation sites when d ' > 1.5 is used as criterion. Cons and Sent are abbreviations of Consonant and Sentence, respectively. The table lists the stimulation modes used for each electrode for the mixed stimulation mode map for JEM (map 8). The abbreviations BP1 and BP2 are used for BP+1 and BP+2 stimulation modes, respectively. Die Suid-Afrikaanse Tydskrif vir Kommunikasieafwy kings, Vol. 43, 1996 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) 34 Johan J Hanekom en Robert V Shannon ibility in choice of electrodes if a seven-electrode map was to be constructed, because although a range of different cumulative values for d' could be found, the electrode pitch sensations could not be discriminated for many of the pos- sible electrode pairs. The hypothesis we wanted to test was whether electrodes that were clearly distinguishable were better choices to achieve good speech perception per- formance. At least we wanted to be able to compare good and poor electrode discrimination. Consequently, only three different maps were tested for subject EWB. Two choices of cumulative d' were con- trasted: a map with EWB's best possible cumulative d' (3.72), and a map with a very poor cumulative d' of -0.55 were used. Map 2 (with the poorer cumulative d') had the electrodes spaced more orderly than map 1, with ΔΕ at least 2. ΔΕ was 2 toward the apex and 3 toward the base. Map 1 had a very disorderly electrode spacing with a big gap between electrodes 8 and 16 and some electrodes hav- ing spatial separation of only 1. The third map had an intermediate value of cumulative d' and all ΔΕ were 3 throughout. Five maps were tested for subject REK. Map 1 utilized both a good cumulative d' and a reasonably good electrode spacing. Although the electrode spacing was irregular, ΔΕ was equal to or larger than 2. The cumulative d' of 4.37 was close to the best cumulative d' of 5.12 achievable for REK. The second map had a small cumulative d' of only 0.6, but with the interesting property that the d' values between every second electrode were relatively large. This map possibly stimulated only four distinct sites in the co- chlea. Map 3 had a cumulative d' of 2, which was near 50 % of the maximum achievable d', with a very orderly elec- trode spacing of 2. The electrodes used were situated in the middle of the electrode array. Maps 4 and 5 also used very orderly electrode spacings, but this time the spatial separations used were 3. The cumulative d's were the small value of-0.87 and 0.41 respectively. Nine maps were tested for subject JEM. These maps tested speech perception performance for a spectrum of cumulative d's, from very low to the highest achievable values for this subject. Some of these maps had very or- derly electrode spacing, and some of them had very irregu- lar electrode spacing. Six of the maps used BP+1 mode (which was the stimulation mode used in JEM's everyday map). Two of the maps used BP+3 mode and one used dif- ferent stimulation modes on different electrodes. Map 1 was chosen for maximum d' (10.34) in BP+1 mode. The electrode spacing was irregular with the largest electrode spatial separation of 5 between electrodes 4 and 9. Map 2 was chosen for low d' (2.54) and had an electrode spacing of 1. Map 3 had an irregular spacing with a large separa- tion gap between electrodes 11 and 17. The map was cho- sen for its cumulative d' of 4.98, in the middle of the d' value of maps 1 and 2. Map 4 was chosen to be similar to map 2'of subject REK. Although this map had a large cu- mulative d' of 7.46, electrodes were grouped into pairs, with the spacing between the pairs being only 1, and these electrodes were essentially stimulating the same areas within the cochlea. Map 5 en map 6 used exactly the same electrodes as maps 1 and 2 respectively, but BP+3 stimu- lation mode was used. These maps were chosen to test whether the wider spread of stimulation current in BP+3 mode would influence the speech results negatively. Thus, if d' was a good indication of current spread, then larger current spread would result in smaller d' values and poorer speech perception performance. Map 7, a BP+1 map, tested another ΔΕ = 1 map, this time closer to the base, where JEM had better electrode discrimination. Map 8 was an attempt to create a map with the maximum achievable d', with the best possible electrode spacing. Different stimu- lation modes were used on different electrodes. The stimu- lation modes were chosen to give the best achievable d's. To obtain the d's for the other stimulation modes, pitch discrimination matrices were compiled for these. The cu- mulative d' for this multi-stimulation mode map was larger than 13. The exact value is not known, because the d's between electrodes 5 and 8, and also between electrodes 17 and 20, were unknown because the electrodes in these pairs used different stimulation modes for each electrode. Map 9 was a ΔΕ = 3 map with an intermediate value of d'. RESULTS Although it was clear that electrode spatial separation was a parameter in determining speech perception per- formance, we had to find a way to quantify this param- eter. Various simple measures of how well the electrodes were spaced, were used. We calculated a value for the av- erage ΔΕ, ΔΕβ, by simply adding all the inter-electrode separations and dividing by the number of inter-electrode separations (six for a seven-electrode map). We also calculated a number, D, for the amount of elec- trode disorder caused by using irregular electrode spac- ing. The rationale for using this measure of disorder was that when we used these seven-electrode maps, we always added the same filter outputs, but for each choice of map these filter outputs were relayed to a different electrode. In some map we connected two neighbouring filters to elec- trodes that were spaced far apart. Thus, we were actually introducing spectral distortion, and the number D calcu- lated for electrode disorder gave some indication of how much this distortion was. A third measure of the quality of our choice of electrode spacing was to find a measure for the number of discrete stimulation sites activated. This could be found from the d's under the assumption that the perceptual distance between two electrodes was an indication of how much overlap there was in neural excitation. We assumed that a d' of 1.5 reflected reasonably little overlap, because this value of d' translated to an 85% consistency in pitch judge- ments. | Table 2 summarizes the results for all the maps tested for the three subjects. The electrodes used in each map, the cumulative d's and the three measures reflecting the electrode spatial separations are included in the table. The results are given as percentage correct scores for tlie vowel, consonant and sentence tests. Several interesting obser- vations can be made from these results when we investi- gate the relative contribution of the two parameters (d' and electrode spacing) to speech perception performance. Simple linear regression analysis was used to relate various map parameters to the results of the speech per- ception tests. Regression lines and correlation coefficients are given in the figures. Significance o f Correlation was tested by the t-test and a 5% level of significance was used. Speech perception as a function of cumulative d' As explained earlier, perceptual pitch distance as meas- ured by d' was used as measure of the amount of overlap The South African Journal of Communication Disorders, Vol. 43, 1996 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) Place Pitch Discrimination and Speech Recognitionin Cochlear Implant Users 35 in neural population stimulated by electrodes. For a spe- cific seven-electrode map, cumulative d' is also simply a measure of a subject's ability to pitch rank this set of elec- trodes. Figures 5 to 7 relate speech perception results to this measure for each of the subjects. The three figures for each subject give results for vowels, consonants and sentences, respectively. As was also demonstrated by Nelson et al. (1995), sig- nificant correlation between pitch ranking ability and speech perception performance was evident. There was, however, considerable variation in how pitch ranking abil- ity was correlated with speech perception. The regression lines and correlation coefficients for linear regression are given in the figures. In general, for subject JEM, for whom the most data were available, higher cumulative values of d' were corre- lated to better performance on the three speech percep- tion measures. There was a correlation of around 70% for all three speech perception measures to cumulative d'. There was a significant increase in vowel perception with larger values of d', the total range being nearly 40%. Con- sonant perception increased over a smaller range of 14%. Far too few data points were available for subject EWB to be conclusive. No correlation between vowel perception and cumulative d' was evident (r=0.03), but there was cor- relation with consonant perception (r=0.95) and sentence perception (r=0.68). The vowel test did not have significant correlation to the cumulative d ' for subject REK. The weakest vowel performance was achieved for the smallest and the larg- est values of d'. This subject showed an inverse relation- ship between cumulative d' and consonant scores. The best consonant recognition scores were achieved for small val- ues of d'; increasing the cumulative d' decreased conso- nant perception. The correlation coefficient of 0.84 is sig- nificant. The range of consonant performance scores was υ CD « Β CD eg e 1.5. Based on this criterion, both REK and EWB had no or very little discrimination be- tween different electrodes in the maps used. Figure 11 shows the speech perception performance as a function of the number of electrode sites for JEM. All the measures of speech performance showed significant correlation (r=0.7 or larger) with the number of discrete stimulation sites. Speech perception as a function of position of elec- trodes Most of the maps for subject JEM were spread over the entire range of electrodes, except his map 2 and map 7. Map 2 was in the middle of the electrode array, and map 7 was on the apical end of the electrode array. Vowel and consonant scores improved slightly with the electrodes in the more apical position, but the sentence score improved dramatically from 20% to 66%. No similar data were meas- ured for the other two subjects. Speech perception as a function of the stimulation mode Subject JEM was the only subject for whom speech per- ception tests were conducted as a function of stimulation mode. Maps 1 and 3 in BP+1 mode were repeated in BP+3 mode. The goal was to evaluate the effect of larger cur- rent distribution on speech perception. Current spread is larger in BP+3 mode than in BP+1 mode (Busby et al., 1994), therefore it was assumed that neural selectivity decreased and it was expected that speech perception per- formance would decrease. The BP+3 maps utilized the same electrodes as the cor- responding BP+1 maps, except that the.most basal elec- trode iri each map was omitted. This decreased the number of discrete stimulation sites for map 5 (a BP+3 map) in comparison to map 1 (a BP+1 map), but, contrary to ex- pectation, the number of discrete stimulation sites in- creased for map 6, the BP+3 counterpart for map 3 (a BP+1 map). For both maps significant (10%) decreases in vowel perception scores were observed. This might be explained by the BP+3 maps using only six instead of seven elec- trodes. An alternative explanation might be the fact that vowels are primarily recognized by their formant struc- ture, and for the BP+3 mode the formant structure was less pronounced. However, there is no significant decrease in performance for the consonant and sentence tests. DISCUSSION THE RELATIONSHIP BETWEEN SPEECH PERCEP- TION AND PITCH DISCRIMINATION ABILITY OF SUBJECTS The experiments clearly identified both physical elec- trode spacing and perceptual electrode distance (as re- flected by the cumulative d') as parameters determining speech perception performance. (Note that pitch ranking ability as measured by cumulative d' is dependent on the specific set of electrodes used in a map.) These two pa- rameters are related: the physical electrode spacing influ- ences the amount of current distribution from electrodes. This in turn determines the neural selectivity that can be achieved. The actual number of discrete neural sites that are activated is also a function of other factors, including electrode distance from neurons, neural survival (Zimmer- mann, Burgess & Nadol, 1995) and the conductive prop- erties of the biological medium. Thus, electrode spacing is Vowels r =0.79 Consonants r = 0.70 Sentences r = 0.72 Number of discrete stimulation sites FIGURE 11. Speech perception as a function of the number of discrete stimulation sites for subject JEM. Linear regression lines are fitted through the data and correlation coefficients are given above each panel. The number of discrete sites were calculated for each map by taking a d ' value of larger than 1.5 as criterion for two electrodes stimulating discrete neural populations. The South African Journal of Communication Disorders, Vol. 43, 1996 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) Place Pitch Discrimination and Speech Recognitionin Cochlear Implant Users 39 nhysical parameter determining electrical current 3 read. On the other hand, perceptual distance is a psy- chophysical parameter, meaning that this is a derived ° rameter, dependent on neural selectivity as well as ex- Perimenta'l and subject-dependent variables. Thus, al- though there is correlation between current distribution and neural selectivity, the relationship is subject-depend- ent This is demonstrated by comparing the neural selec- tivity for BP+3 and BP+1 modes for JEM, as reflected by the d' matrices in figures 1 and 2. The results also indicate that better overall pitch dis- crimination ability (measured over the set of all twenty electrodes) is related to better speech perception ability. This is demonstrated by comparing overall pitch discrimi- nation ability to speech perception abilities. One measure of overall place pitch discrimination ability of a subject is the maximum cumulative d ' achievable for a specific number of electrodes. In this study, JEM achieved a maxi- mum seven-electrode cumulative d' of 10.34 in BP+1 mode. This can be compared to a maximum achievable cumula- tive d' of just more than 5 for REK and 3.72 for EWB. Relating this to speech perception abilities as reflected by this study, JEM was the best implant user in the group and EWB derived the least benefit from his implant. The relationship between the amount of benefit that the user derives from the implant and the pitch discrimi- nation ability can also be quantified with two other meas- ures of overall pitch discrimination ability. These are (1) the area of the d' matrix with large d' values (say d'>1.5) and (2) the minimum ΔΕ required for electrode discrimi- nation. If d'>1.5 is used as the criterion for electrode discriminability, it is seen that JEM generally required a ΔΕ of 2, while REK required a ΔΕ of 4 or more and EWB required large ΔΕ values to have d'>1.5. SPEECH PERCEPTION AND SPECTRAL DISTOR- TION Disorder in the choice of electrode spacing did not seem to be an important parameter determining speech percep- tion performance. However, the results obtained by Shan- non et al. (1995) suggested that spectral distortion might be an important factor determining cochlear implant us- ers' speech perception abilities. An explanation for this ap- parent anomaly might be that different current distribu- tions from each electrode, overlap in neural excitation and neural survival patterns might already introduce so much spectral distortion that the effect of poor electrode alloca- tion is simply swamped. ' SPEECH PERCEPTION AND THE NUMBER OF DIS- CRETE NEURAL SITES Although the number of discrete neural excitation sites in the cochlea (therefore, the number of discrete spectral locations) is correlated to speech perception performance, other information in the speech signal may also be uti- lized to understand electrical speech. This was evident from results achieved by EWB and REK. Although being poorer users than JEM, both demonstrated significant speech perception even with the excitation of so few dis- crete neural sites. These subjects might rely on temporal information for speech recognition. This paper emphasised the importance of cochlear place information (spectral in- formation), but other studies have demonstrated the im- portance of temporal information in speech recognition (see, for example, Shannon, Zeng, Kama'th, Wygonski & Ekelid (1990) or Dorman et al. (1990)). USING PITCH RANKING DATA TO DETERMINE BETTER SUBJECT-SPECIFIC MAPS It is possible to make better choices of electrodes to be used in a reduced electrode map by basing the electrode choices on pitch discrimination data. The choice of a smaller number of electrodes leads to improved stimula- tion rates in the Nucleus processor (Shannon et al., 1990). It has been shown that higher stimulation rates lead to better speech perception performance (Wilson et al., 1991). Also, the careful choice of electrodes can be used to choose the set of electrodes with the best neural selectiv- ity. The best neural selectivity is achieved by paying at- tention to two parameters: the physical electrode spacing, which determines current spread, and perceptual distance as measured with the d's, which is related to neural selec- tivity. Furthermore, current distribution can be control- led by choice of stimulation mode. Different stimulation modes result in different pitch ranking ability, as demon- strated in subject JEM and also by Busby et al. (1994). Stimulation modes with larger current spread do not nec- essarily lead to decreased neural selectivity, as was dem- onstrated by map 6 of subject JEM, where more discrete neural channels were achieved than in the BP+1 equiva- lent of this map. It might be possible to achieve better neural selectivity by. using multi-stimulation mode maps. By varying both electrode spacing and stimulation mode, current distribution patterns might be obtained which give larger perceptual distances between electrodes. This was accomplished in map 8 for subject JEM. This technique for individualized fitting is more useful when subjects have good pitch discrimination abilities. For subjects in which pitch discrimination ability is not very good in a specific stimulation mode, other stimulation modes might be used. For subjects with poor pitch dis- crimination ability it might be advantageous to use fewer electrodes in order to increase the stimulation rate. The rationale is that if discrete excitation sites are few, place pitch abilities will be poor regardless of the number of elec- trodes used, and place pitch resolution may be substituted with better temporal resolution by increasing the stimu- lation rate. CONCLUSIONS (1) The ability to discriminate electrodes based on place pitch, varies considerably among subjects. Also, if it is assumed that perceptual distance between electrodes is related to current spread in the cochlea, the pat- terns of current spread vary considerably among sub- jects. (2) The variability in performance among subjects makes it difficult to compare the effectiveness of different speech processing strategies. Also, there is a need for alternative procedures for better individualized fitting of processors. Our results indicate that pitch ranking ability might be used both to assess implant user po- tential and to choose better electrode configurations. (3) Two parameters that can be related to subjects' ability to rank pitch according to place of stimulation influ- ence speech perception performance: electrode spatial ie Suid-Afrikaanse Tydskrif vir Kommunikasieafwykings, Vol. 43, 1996 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) 40 J o h a n J H a n e k o m e n R o b e r t V Shannon separation and perceptual distance between electrodes. (4) It is possible to make better choices of electrodes to be used in a reduced electrode m a p b y basing the elec- trode choices on pitch discrimination data. (5) It might be possible to achieve better neural selectiv- ity b y using multi-stimulation mode maps. A C K N O W L E D G E M E N T S This research was done in the Department of Auditory Implant and Perception at the House Ear Institute in Los Angeles, U S A . 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