"A physiologically based model of ITD discrimination"
Abstract
Interaural time differences (ITD) are cues to the location of low-frequency sounds and are represented in the inferior colliculus (IC) by cells that respond maximally at a particular best delay (BD). In the conventional view of ITD coding, characteristic frequency (CF) and BD are distributed independently within the ITD processing mechanism to form a place map. Recent evidence from the guinea pig indicates that BD actually varies with CF, such that the best phase (BP) distribution is nearly independent of CF, suggesting a population rate code for ITD. This finding was confirmed in the IC of anesthetized cats, generalizing the observation across species and head sizes.
Rate-versus-ITD curves were fit with a cross-correlation model of ITD sensitivity and the parameters were used to constrain a population rate model of ITD coding. The model was tested by simulating the ability of human listeners to discriminate ITDs. While previous studies have demonstrated that pooling of information across neurons is not necessary to produce fine ITD acuity on the midline, this study suggests that pooling is required to account for the deterioration of acuity away from the midline, and that ITD discrimination is likely performed at level higher than the IC. The results further suggest that ITD acuity is best accounted for by a system in which ITD sensitivity is the result of internal phase shifts rather than internal conduction delays.