Threshold interaural time differences and the centroid model of sound localization

Abstract

The centroid display model of sound lateralization hypothesizes a two-dimensional array of brain-stem cells with wide ranges of best frequencies ($f-c$) and best interaural time delays (ITD, $\ tau$). The cells are distributed according to a cell population density function $p(f-c,\tau)$, and images are lateralized according to the centroid of an excitation pattern on this array, the rate-ITD function, weighted by $p(f-c,\tau)$. The ability of the centroid display to predict the frequency dependence of human ITD thresholds was tested by calculations using model cells for the medial superior olive, as the origin of the rate-ITD function. The cells had synaptic inputs, membrane channel dynamics, and time constants established by physiological measurements. Cells were driven with realistic frequency-dependent synchrony. The centroid model could successfully account for the high-frequency human data, but greatly underestimated thresholds for lower frequencies. Similar calculations with a rate-difference model was successful in the lower frequency range but failed at high frequencies. Therefore, we propose a hybrid display model consisting of a centroid code at high frequencies and a rate code at low. © 2013 Acoustical Society of America.