Distribution of frequency sensitivity in the superior colliculus of the guinea pig.

A field potential could be recorded in the deep layers of the superior colliculus (SCd) in the guinea pig following presentation of pure tone bursts at frequencies between 1 and 22 kHz, bursts of white noise, and clicks presented in the free field. The potentials evoked by these stimuli at different intensities had two negative and one positive component associated with the onset of the stimulus. The amplitude of the negative components of the potential varied according to the location of the recording electrode along the dorso-ventral and rostro-caudal axes of the SC and with the frequency of pure tone stimuli. The largest amplitude potentials were recorded in the deep grey layer of the SCd midway along the rostro-caudal axis of the nucleus. The distribution of frequency sensitivity along the rostro-caudal axis of the SCd was determined by recording the amplitude of the negative components of the field potential evoked by 10 ms pure tone bursts at a constant intensity. The distribution of frequency sensitivity across the nucleus was not cochleotopic. The caudal pole of the nucleus had largest responses to frequencies above 15 kHz, with a peak sensitivity around 20 kHz. In contrast, the rostral pole of the nucleus was most sensitive to frequencies around 10 kHz and the sensitivity to frequencies around 20 kHz was relatively low. At a point midway along the rostro-caudal axis of the SCd, the nucleus was sensitive to a broad range of frequencies from 5 to 25 kHz. The patterns of frequency sensitivity recorded in the rostral, middle and caudal SCd are qualitatively similar to the frequency transfer characteristics of the auditory periphery for sounds located in the anterior, orthogonal and posterior regions respectively of contralateral space. The correspondence between these two sets of data suggests that the pattern of frequency sensitivity along the SCd may provide a mechanism by which the nervous system can encode the spectral cues which are generated at the auditory periphery.