Receptive Fields of Neurons in the Owl's Auditory Brainstem Change Dynamically

Binaural neurons in the barn owl's auditory brainstem have spatial receptive fields. It is shown here that both the frequency tuning of these neurons and their tuning to interaural time difference (ITD), the prime cue for azimuthal sound localization, improves with time after stimulus onset, a process I shall term 'dynamic sharpening'. Thus, the receptive fields of these neurons also have a temporal dimension. Data were collected in four hierachically ordered nuclei concerned with the computation of ITD: the nucleus ventralis lemnisci lateralis, pars anterior (VLVa), and three subnuclei in the inferior colliculus. Dynamic sharpening in the frequency tuning curves is evident from a dynamic reduction of tuning width. When stimulated with a tone, the response of all neurons varies in a cyclic manner with ITD. The ITDs at the response peaks differ by one period of the stimulus tone. The responses with noise stimulation are similar to the responses with tonal stimulation in all but the hierarchically highest nucleus, the external nucleus of the inferior colliculus. In this nucleus are found neurons that, if stimulated with noise, respond maximally only to one ITD while the responses at the other peaks are suppressed. This sidepeak suppression is a dynamic process. Dynamic sharpening of ITD tuning is also evident from a dynamic reduction of the tuning width around each of the response peaks. The proportion of neurons showing dynamic sharpening of ITD tuning is the same in all collicular subnuclei, but is lower in VLVa. Therefore, a major component of dynamic sharpening of ITD tuning occurs at the first station of the inferior colliculus. Lateral inhibition is one of the mechanisms underlying dynamic sharpening. Part of the inhibition may be mediated by GABA (Fujita and Konishi, 1988). The function of dynamic sharpening of ITD tuning may be to increase the fine representation of auditory space in single neurons.