BINAURAL RESPONSE CHARACTERISTICS IN ISOFREQUENCY SHEETS OF THE GERBIL INFERIOR COLLICULUS

The spatial distribution of neurons with different binaural response p roperties was studied in two isofrequency areas of the inferior collic ulus (IC) of the gerbil (Meriones unguiculatus). Single units were rec orded in dorsomedial-to-ventrolaterally oriented electrode penetration s at different rostrocaudal positions, within a low (1-2 kHz; N=108) a nd a high (23-27 kHz; N=90) 'iso-frequency' sheet. Response types were not equally represented within the two layers. Within low-frequency r egions, 41% of the units were excited following sound stimulation of e ither ear (EE), 21% received an excitatory input from one and an inhib itory input from the other ear (EI), and 12% were monaural (EO), while within high-frequency regions the distribution was 20% EE, 47% EI and 18% EO. However, the spatial arrangement of EE, EI and EO, within the dorsomedial-to-ventrolateral extent of the isofrequency sheets, was o n the whole comparable within the two regions: EE-units were accumulat ed dorsomedially and EI-units ventrolaterally in the respective isofre quency sheets. EO-units were distributed equally within the low-freque ncy sheet but occurred more frequently in the middle part of the high- frequency lamina. About equal proportions of the units (low frequencie s 9%; high frequencies 7%) had complex binaural response characteristi cs and were characterized as E/IE. In the rostrocaudal dimension of th e nucleus, orderly arrangement of different unit types was less obviou s. The results show that there is a tendency for grouping of different binaural unit types within the inferior colliculus. However, these un it 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 wit hin the IC. Provided that EE-, EI-, and monaural response characterist ics originate from separate sources, the data point to a convergence o f binaural brainstem afferents within the IC.