Binaural response characteristics in isofrequency sheets of the gerbil inferior colliculus.

The spatial distribution of neurons with different binaural response properties was studied in two isofrequency areas of the inferior colliculus (IC) of the gerbil (Meriones unguiculatus). Single units were recorded in dorsomedial-to-ventrolaterally oriented electrode penetrations at different rostrocaudal positions, within a low (1-2 kHz; N = 108) and a high (23-27 kHz; N = 90) 'iso-frequency' sheet. Response types were not equally represented within the two layers. Within low-frequency regions, 41% of the units were excited following sound stimulation of either ear (EE), 21% received an excitatory input from one and an inhibitory input from the other ear (EI), and 12% were monaural (E0), while within high-frequency regions the distribution was 20% EE, 47% EI and 18% E0. However, the spatial arrangement of EE, EI and E0, within the dorsomedial-to-ventrolateral extent of the isofrequency sheets, was on the whole comparable within the two regions: EE-units were accumulated dorsomedially and EI-units ventrolaterally in the respective isofrequency sheets. E0-units were distributed equally within the low-frequency sheet but occurred more frequently in the middle part of the high-frequency lamina. About equal proportions of the units (low frequencies 9%; high frequencies 7%) had complex binaural response characteristics and were characterized as E/IE. In the rostrocaudal dimension of the nucleus, orderly arrangement of different unit types was less obvious. The results show that there is a tendency for grouping of different binaural unit types within the inferior colliculus. However, these unit types are not strictly segregated. Furthermore, it was shown that, despite the differences found in low- and high-frequency sites, there is a common scheme of 'binaural representation' across frequencies within the IC. Provided that EE-, EI-, and monaural response characteristics originate from separate sources, the data point to a convergence of binaural brainstem afferents within the IC.