RESPONSES OF NEURONS IN THE AUDITORY PATHWAY OF THE BARN OWL TO PARTIALLY CORRELATED BINAURAL SIGNALS

1. Extracellular single-unit recording in anesthetized barn owls was u sed to study neuronal response to dichotic stimuli of variable binaura l correlation (BC). Recordings were made in the output fibers of nucle us laminaris (NL), the anterior division of the ventral lateral lemnis cal nucleus (VLVa), the core of the central nucleus of the inferior co lliculus (ICcC), the lateral shell of the central nucleus of the infer ior colliculus (ICcLS), and the external nucleus of the inferior colli culus (ICx). 2. The response of all neurons sensitive to interaural ti me difference (ITD) varied with BC. The relationship between BC and im pulse number fits a linear, a parabolic, or a ramp model. A Linear or parabolic model fits most neurons in low-level nuclei. Higher order ne urons in ICx did not respond to noise bursts with strong negative bina ural correlation, creating a ramp-lice response to BC. 3. A neuron's a bility to detect ITD varied as a function of BC. Conversely, a neuron' s response to BC changed with ITD. Neurons in NL, VLVa, and ICcC show almost periodic ITD response curves. In these neurons peaks and trough s of ITD response curves diminished as BC decreased, creating a flat I TD response when BC = O. When BC was set to -1, the most favorable ITD became the least favorable one and vice versa. The ITD response curve of ICx neurons usually has a single dominant peak. The response of th ose neurons to a negatively correlated noise pair (BC = -1) showed two ITD peaks, flanking the position of the primary peak. 4. The paraboli c BC response of NL neurons fits the prediction of the cross-correlati on model, assuming half-wave rectification of the sound by the cochlea . Linear response is not predicted by the model. However, the paraboli c and the linear neurons probably do not belong to two distinct groups as the difference between them is not statistically significant. Thus , the cross-correlation model provides a good description of the binau ral response not only in NL but also in VLVa and ICcC. 5. Almost all r amp neurons occurred in either ICx or ICcLS where neurons are more bro adly tuned to frequency than those in the lower nuclei. The synthesis of this response type requires, however, not only the convergence of d ifferent frequency channels but also inhibition between different ITD channels. We modeled the ramp response as a three-step process. First, different spectral channels converge to create broad frequency tuning . The response to variation in BC will be linear (or parabolic) becaus e it is a sum of Linear (parabolic) responses. Second, the activity in some adjacent ITD channels is subtracted by lateral inhibition. Final ly, the result is rectified using a high threshold to avoid negative a ctivity.