engleski

Time-Intensity Trading: Ongoing Temporal Coding of Broadband Noise in the Auditory Nerve as a Function of SPL

Human azimuthal sound localization relies on two cues: interaural level differences (ILDs) and interaural time differences (ITDs). Processing of these cues is thought to occur in separate brainstem circuits, but psychophysical experiments have shown that under some conditions ILDs and ITDs can be traded, which could reflect interaction between the cues at a low anatomical level. The latency hypothesis (Jeffress 1948) postulates that a decrease in latency between stimulus and neural response with increasing stimulus intensity accounts for this interaction. Previous physiological studies examined and to some extent confirmed the hypothesis by studying onset responses of binaural neurons, mostly using transient stimuli. However, human time-intensity trading has been observed to occur at low frequencies and to both transient and ongoing stimuli. Also, the timing of onset and ongoing responses depends on sound pressure level (SPL) in different ways. We tested the basic tenet of the latency hypothesis by studying the effect of changes in SPL on the ongoing timing of the auditory nerve (AN) responses of cats. We compared spike times in response to broadband noise, obtained from single AN fibers at multiple SPLs, using a crosscorrelation analysis. We found that changes in SPL cause small but systematic shifts in the ongoing timing in the AN, resulting in longer delays between stimulus onset and neural response at lower SPLs. The size of the shifts depended on characteristic frequency (CF). Also, for fibers with CFs between 1 and 5 kHz, paradoxical shifts were seen, with high intensities causing delayed spike times. These results show that a limited and rather complex time-intensity trade exists at the most peripheral neural level. Comparison of autocorrelograms with revcors suggests that these effects reflect changes in cochlear filtering with SPL.

auditory neuroscience

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