Cochlear and neural delays for coincidence detection in owls

The auditory system uses delay lines and coincidence detec-tion to measure the interaural time difference (ITD). Both axons and the cochlea could provide such delays. The stereausis theory assumes that differences in wave propagation time along the basilar membrane can provide the necessary delays, if the coincidence detectors receive input from fibers innervating dif-ferent loci on the left and right basilar membranes. If this hypothesis were true, the left and right inputs to coincidence detectors should differ in their frequency tuning. The owl’s nucleus laminaris contains coincidence detector neurons that receive input from the left and right cochlear nuclei. Monaural frequency-tuning curves of nucleus laminaris neurons showed small interaural differences. In addition, their preferred ITDs were not correlated with the interaural frequency mismatches. Instead, the preferred ITD of the neuron agrees with that pre-dicted from the distribution of axonal delays. Thus, there is no need to invoke mechanisms other than neural delays to explain the detection of ITDs by the barn owl’s laminaris neurons. Key words: owl; nucleus laminaris; frequency tuning; coinci-dence detection; delay lines; sound localization; stereausis The auditory systems of birds and mammals use coincidence detector neurons and delay lines to measure interaural time differences (ITDs). Both the coincidence detectors and axonal delay lines are known in these animals, although the evidence for axonal delay lines in mammals is anatomical (Carr and Konishi, 1990; Yin and Chan, 1990; Smith et al., 1992). The use of axons as delay lines resembles the model put forth by Jeffress (1948) (Fig. 1A). In the avian auditory system, axons from the cochlear nucleus magnocellularis (NM) and neurons of the nucleus lami-naris (NL) are thought to constitute the delay lines and coinci