The sensitivity of listeners to interaural time differences (ITDs) in pulse-train stimuli decreases with increasing rate beyond a few hundred pulses per second (pps). Hafter and Dye [J. Acoust. Soc. Am. 73: 644-651] showed that the decreasing sensitivity is related to the reduced effectiveness of the ongoing signal and called the effect “binaural adaptation.” We hypothesized that binaurally-coherent temporally-random (“jittered”) pulse trains would improve ITD sensitivity at high pulse rates by causing a recovery from binaural adaptation. Perceptual ITD sensitivity was measured in six normal-hearing listeners in a left-right discrimination task. For rates higher than 400 pps, we found that ITD sensitivity was much better for jittered pulse trains than for periodic pulse trains.
To investigate the neural basis for this effect, we recorded from single units in the inferior colliculus (IC) of bilaterally-implanted, anesthetized cats. Responses to pulse trains were measured as a function of pulse rate, jitter, and ITD. For pulse rates above 300 pps, ongoing neural firing rates were greater when the pulse trains were jittered. Action potentials tended to occur at sparse preferred times across repeated presentations of a jittered pulse train. Neurons stimulated with jittered high-rate pulse trains showed ITD tuning comparable to that produced by low-rate pulse trains. Thus, jitter appears to improve neural ITD sensitivity by restoring sustained firing in IC neurons.
In addition, the response to jittered pulse trains was observed in a physiologically-based model of auditory nerve. We found that the random nature of the pulse trains caused increased firing at specific times across repeated presentations, which is consistent with the firing pattern observed in the IC.
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