Stimuli, procedures, and analyses were similar to those in experiment 1. In the behavioural task, psychometric functions were measured for comparisons between the “4–6” pulse train and that of isochronous pulse trains having IPIs of 2, 3, 4, 5, 6, 7, and 8 ms. The duration of all pulse trains was 100 ms, so that the 4–6 train consisted of 20 pulses. ECAPs were obtained for the first ten pulses of each stimulus used in the behavioural experiment for each subject.

The results of the behavioural experiment were analysed to determine the IPI of an isochronous pulse train judged equal in pitch to the 4–6 pulse train. These values were converted to pitch shifts in percent, relative to the 5-ms mean IPI in the 4–6 stimulus. They are shown in the third column in bold type in Table 40.1. This ­average difference of 12.4 % corresponds to an IPI of 5.7 ms, which is in good agreement with previous results (Carlyon et al. 2002, 2008).

ECAPs to the 2nd through 9th pulse of the 4–6 pulse train are shown for four subjects by the dashed lines in Fig. 40.1. The summary modulation depth values for these ECAPs are shown by the third column in faint type in Table 40.1.

Figure 40.1 and Table 40.1 show that there can be a substantial pitch shift associated with the 4–6 stimulus and that the physically modulated pulse train (from experiment 1) can produce a larger ECAP modulation but a much smaller pitch shift. It therefore seems that modulation in the AN response – at least as measured by the ECAP – is not entirely responsible for the fact that the pitch of a 4–6 pulse train is lower than the 200 Hz that would be expected from the mean IPI in that stimulus. A possible caveat is that behavioural responses in experiment 2 were obtained for 100-ms stimuli, whereas ECAPs were obtained only to the first 50 ms of the modulated 200-pps and unmodulated 4–6 pulse trains. However, for this to explain the differing relationship between pitch shift and ECAP modulation observed in the two experiments, the ECAP modulation depth would need to decrease over time for the modulated stimulus and for this effect to be substantially larger than for the 4–6 stimulus. No such trend was observed over the first ten pulses. In addition, additional data (not shown) from listeners S6 and S7 showed similar modulation depth for the first and second ten pulses of modulated pulse trains.