COINCIDENCE DETECTION BY BINAURAL NEURONS IN THE CHICK BRAIN-STEM

1. Neurons in nucleus laminaris (NL) of birds are the first to receive binaural information and are presumed to play a role in encoding inte raural time differences (ITDs). We studied extracellular single-unit r esponses of NL neurons in slices of the auditory brain stem of the chi ck. The afferents to NL were activated by electrical stimulation of nu cleus magnocellularis (NM) or the auditory nerve. Changes in response were measured as the delay between trains of bilateral stimuli (the si mulated interaural time difference or S-ITD, n = 26) was varied and as the interstimulus interval and stimulus amplitude were varied (n = 61 ). 2. The probability of an action potential and the action-potential latency varied as a function of interstimulus interval. Most NL neuron s showed a greater response probability and a shorter response latency to an interstimulus interval between 2.5 and 3.5 ms. The interstimulu s interval that produced the minimum response latency was slightly lon ger than the interval that produced the maximum response probability. In contrast, NM neurons (n = 4) showed no preferred rate, instead, the probability of firing increased as the interstimulus interval increas ed. 3. Responses to bilateral stimulation showed that NL neurons can a ct as coincidence detectors. NL neurons responded most reliably when a ctivated simultaneously by their two inputs and, at favorable S-ITDs, two subthreshold inputs combined to produce an action potential. 4. NL neurons also exhibited inhibition during bilateral stimulation. At un favorable S-ITDs a subthreshold input combined with a suprathreshold i nput produced fewer action potentials than evoked by the suprathreshol d input alone. 5. The latency of the bilateral response varied as a fu nction of S-ITD. At S-ITDs near coincidence the latency of the bilater al response was shorter than the latency of either of the unilateral r esponses. Away from coincidence, the latency of the bilateral response was largely determined by the latency of the stronger unilateral resp onse. When the unilateral responses were of similar strength, the earl ier stimulus determined the latency of the bilateral response. 6. The range of S-ITDs producing a maximal response varied as a function of s timulus strength but was never less than approximately 300 mus. This i s greater than the maximum possible ITD of sound calculated for the ch ick's head size. From these data we hypothesize that, in the chick, si ngle units cannot uniquely encode ITDs, but rather ITDs may be coded b y the proportion of maximally firing cells along an isofrequency band in NL.