Neural sensitivity to interaural time differences: Beyond the Jeffress model

Interaural time differences (ITDs) are a major cue for localizing the azimu thal position of sounds. The dominant models for processing ITDs are based on the Jeffress model and predict neurons that fire maximally at a common I TD across their responsive frequency range. Such neurons are indeed found i n the binaural pathways and are referred to as "peak-type." However, other neurons discharge minimally at a common ITD (trough-type), and others do no t display a common ITD at the maxima or minima (intermediate-type). From re cordings of neurons in the auditory cortex of the unanesthetized rabbit to low-frequency tones and envelopes of high-frequency sounds, we show that th e different response types combine to form a continuous axis of best ITD. T his axis extends to ITDs well beyond that allowed by the head width. In Jef fress-type models, sensitivity to large ITDs would require neural delay lin es with large differences in path lengths between the two ears. Our results suggest instead that sensitivity to large ITDs is created with short delay lines, using neurons that display intermediate- and trough-type responses. We demonstrate that a neuron's best ITD can be predicted from (1) its char acteristic delay, a rough measure of the delay line, (2) its characteristic phase, which defines the response type, and (3) its best frequency for ITD sensitivity. The intermediate- and trough-type neurons that have large bes t ITDs are predicted to be most active when sounds at the two ears are deco rrelated and may transmit information about auditory space other than sound localization.