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Stimulus parameters
All stimuli were charge-balanced, 200 lls/phase biphasic pulses, with anodic phase first, and were presented at a stimulation rate of 1000 pulses per second. Stimuli were presented at a comfortable level of stimulation. The stimuli were loudness balanced across electrodes before the start of the experiment, using a bracketing loudness balance procedure. First, thresholds and upper loudness levels were obtained in each stimulation mode. Then the subjects were asked to choose a comfortable level of stimulation on electrode 10. All subjects chose comfort levels somewhere between 50% and 85% of their dynamic ranges in the various stimulation modes.
All other electrodes were then loudness balanced to this electrode by instructing the subject to adjust the loudness of an adjustable stimulus to be just louder than, then just softer than and finally equal to the reference stimulus. Loudness was adjusted by adjusting pulse amplitude.
This was repeated as many times as was necessary to obtain consistent decisions about the relative loudnesses. Loudness balancing was repeated for all conditions (each level of stimulation in each stimulation mode).
Gaps were presented between two 200 ms stimuli. These two stimuli were presented on the same electrodes in the baseline condition and on different electrodes otherwise. Gap thresholds were measured as a function of the separation of the two electrodes. In a single run, the first electrode position was held constant, and gap thresholds were measured for different positions of the second electrode. The experiment was performed in BP, BP+1, BP+2, BP+3 and a pseudo monopolar mode as described above. A computer program generated the appropriate stimuli and recorded the subject responses. The stimuli were encoded in the correct format to enable presentation directly to the internal receiver of the Nucleus device (without using the subjects’ processors), via a custom interface (Shannon et aI., 1990).
Psychophysical procedure
Gap thresholds were collected using an adaptive, two-interval, forced-choice procedure. The gap was initially 100 ms and two consecutive correct decisions led to a decrease in gap size, and one error increased gap size. This procedure estimates the gap size required for 70.7% correct responses (Levitt, 1971). Initially the increase or decrease was by a factor of two, but after four reversals this factor was 1.3. Data collection was for twelve reversals and the mean of the last eight reversals was used to estimate the gap threshold. Gap detection thresholds were obtained in BP+ 1 mode for all three subjects using all the evennumbered electrodes as standard (the first stimulus). For each standard, the gap thresholds were measured as a function of probe electrode Gust even numbered or both even and odd numbered) separation from the standard. Three repetitions were made for each measurement, which resulted in six measurements of gap threshold for each combination of stimulation electrodes when using both orderings of electrodes. (That is, when electrode i was used as standard, three measurements were obtained for probe electrode j, and when j was the standard, another three measurements were obtained with i as probe). Also, gap detection thresholds were obtained in BP, BP+2, BP+3 and AR modes for all three subjects using electrodes 6, 10 and 14 as standard. Again, gap thresholds were measured on even numbered electrodes as a function of probe electrode separation from the standard. In this task two to six measures were taken at each probe electrode location.
Gap threshold as a function of electrode separation
Figure 2.1 compares gap threshold data from Formby et al. (1996) (in normal hearing listeners)to gap threshold data from cochlear implant patients. Formby et al. (1996) measured gap thresholds as a function of marker frequency separation and the study described in this chapter measured gap thresholds as function of electrode separation in cochlear implant patients. The electrode axis in figure 2.1 is scaled to match the approximate location of the linearly spaced electrodes to the cochlear frequency-position function of Greenwood (1990). There is good agreement between the two sets of data in the shape of the gap threshold curves and in the absolute values of gap thresholds for implant subjects N3 and N7. Gap thresholds for implant listener N4 were consistently lower than those from Formby et al. at every comparison point.
Gap thresholds were measured as a function of electrode separation for ten standard electrodes (all the even numbered electrodes) for each of the three subjects (figures 2.2 to 2.4). The lowest gap thresholds were always achieved when the two stimuli that bound the gap were presented on the same electrode. The minimum values of gap threshold were near 1 ms for most electrodes for N4 and 3 to 4 ms for the other two subjects. This is consistent with the range of gap thresholds reported by Shannon (1989).
Gap thresholds increased considerably as electrode separation increased. In general, gap thresholds increased by almost a factor of 10 as the two electrodes were separated. The absolute values and ranges of gap thresholds varied considerably among the subjects, particularly when electrodes were widely separated. Subject N4 had gap thresholds of between 10 and 20 ms for widely separated electrodes, while subject N7 had maximum gap thresholds of 20-70 ms, and N3 had maximum gap thresholds of 100-200 ms. For most electrodes towards the basal end of the array, the spatial selectivity of the gap threshold curves was sharpest for N4, while N7 had broader selectivity, and N3 had broad « spatial tuning » that covered most of the length of the electrode array. For simplicity, the gap threshold curves will be referred to as « tuning curves ». Many of the gap detection tuning curves have two portions: a sharply tuned « tip » region in the vicinity of the standard electrode, and a shallow, bowl-shaped portion for electrodes distant from the standard. These two sections may reflect two different mechanisms relating electrode similarity to gap detection.
Figures 2.2 to 2.4 show a general tendency for the slopes of the bowl-shaped portion to become steeper on the apical side of the gap threshold tuning curves (i.e. towards electrode 20) and shallower on the basal side as the standard moved from base to apex. For electrodes near the base, asymmetry was towards the apex (slopes were shallower on the apical side). This is consistent with measurements of electrode interaction in the same three subjects, using forward masking (Chatterjee and Shannon, 1998). The shallower slopes towards the base (for apical electrodes) suggest larger current flow towards the basal region, but the shallower slopes towards the apex (for basal electrodes) suggest larger current flow towards the apex. Previous measures of electrode interaction using forward masking (Lim et aI., 1989) suggested larger current flow towards the basal region for stimulated electrodes at all cochlear locations, but the data presented here does not confirm this observation.
Chapter 1: INTRODUCTION
1 ISSUES IN COCHLEAR IMPLANT RESEARCH
2 MODEL BASED RESEARCH
3 OBJECTIVES OF THIS THESIS
4 HYPOTHESES OF THIS THESIS
5 THESIS OUTLINE
Chapter 2: GAP DETECTION AS A MEASURE OF ELECTRODE INTERACTION IN COCHLEAR IMPLANTS
1 INTRODUCTION
2 METHODS
3 RESULTS
4 DISCUSSION
5 CONCLUSIONS
Chapter 3: MODELS OF GAP DETECTION IN ACOUSTIC HEARING
1 INTRODUCTION
2 A MODEL FOR GAP DETECTION IN ACOUSTIC HEARING
3 RESULTS
4 DISCUSSION
5 CONCLUSIONS
Chapter 4: MODELS OF GAP DETECTION IN ELECTRIC HEARING
1 INTRODUCTION
2 A MODEL FOR GAP DETECTION IN ELECTRIC HEARING
3 RESULTS
4 DISCUSSION
5 CONCLUSION
Chapter 5: A SPATIAL MODEL OF FREQUENCY DISCRIMINATION IN ACOUSTIC HEARING
1 INTRODUCTION
2 POPULATION CODING
3 POINT PROCESS DESCRIPTION OF SPIKE TRAINS
4 MODELLING AND SIMULATION
5 METHODS
6 RESULTS
7 DISCUSSION
8 CONCLUSIONS
Chapter 6: A MODEL OF FREQUENCY DISCRIMINATION WITH OPTIMAL PROCESSING OF AUDITORY NERVE SPIKE INTERVALS
1 INTRODUCTION
2 METHODS
3 RESULTS
4 DISCUSSION
5 CONCLUSIONS
Chapter 7: WHAT DO COCHLEAR IMPLANTS TEACH US ABOUT THE ENCODING OF FREQUENCY IN THE AUDITORY SYSTEM?
1 INTRODUCTION
2 METHOD
3 RESULTS
4 DISCUSSION
5 CONCLUSIONS
Chapter 8: CONCLUSION
1 DISCUSSION OF HYPOTHESES
2 RESEARCH CONTRIBUTION
3 IMPLICATIONS FOR COCHLEAR IMPLANTS
4 FUTURE RESEARCH DIRECTIONS
REFEREN CES
