INTERAURAL DELAY SENSITIVITY AND THE CLASSIFICATION OF LOW BEST-FREQUENCY BINAURAL RESPONSES IN THE INFERIOR COLLICULUS OF THE GUINEA-PIG

Monaural and binaural response properties of single units in the infer ior colliculus (IC) of the guinea pig were investigated. Neurones were classified according to the effect of monaural stimulation of either ear alone and the effect of binaural stimulation. The majority (309/33 4) of IC units were excited (E) by stimulation of the contralateral ea r, of which 41% (127/309) were also excited by monaural ipsilateral st imulation (EE), and the remainder (182/309) were unresponsive to monau ral ipsilateral stimulation (EO). For units with best frequencies (BF) up to 3 kHz, similar proportions of EE and EO units were observed. Ab ove 3 kHz, however, significantly more EO than EE units were observed. Units were also classified as either facilitated (F), suppressed (S), or unaffected (O) by binaural stimulation. More EO than EE units were suppressed or unaffected by binaural stimulation, and more EE than EO units were facilitated. There were more EO/S units above 1.5 kHz than below. Binaural beats were used to examine the interaural delay sensi tivity of low-BF (BF < 1.5 kHz) units. The distributions of preferred interaural phases and, by extension, interaural delays, resembled thos e seen in other species, and those obtained using static interaural de lays in the IC of the guinea pig. Units with best phase (BP) angles cl oser to zero generally showed binaural facilitation, whilst those with larger BPs generally showed binaural suppression. The classification of units based upon binaural stimulation with BF tones was consistent with their interaural-delay sensitivity. Characteristic delays (CD) we re examined for 96 low-BF units. A clear relationship between BF and C D was observed. CDs of units with very low BFs (< 200 Hz) were long an d positive, becoming progressively shorter as BF increased until, for units with BFs between 400 and 800 Hz, the majority of CDs were negati ve. Above 800 Hz, both positive and negative CDs were observed. A rela tionship between CD and characteristic phase (CP) was also observed, w ith CPs increasing in value as CDs became more negative. These results demonstrate that binaural processing in the guinea pig at low frequen cies is similar to that reported in all other species studied. However , the dependence of CD on BF would suggest that the delay line system that sets up the interaural-delay sensitivity in the lower brainstem v aries across frequency as well as within each frequency band.