Cells responsive to free-field auditory stimuli in guinea-pig superior colliculus: distribution and response properties.

We have investigated the responses of superior colliculus neurones in the anaesthetized guinea-pig to free-field auditory stimulation. The auditory cells were located throughout the deeper laminae and also in the lower part of the stratum opticum. Auditory cells were not found in the rostral pole of the superior colliculus. The auditory responses consisted of a few spikes at stimulus onset with a latency from stimulus arrival at the ear of 7-27 ms. Frequency response areas were measured for forty-five neurones; many of these areas were broad or multipeaked although some were well defined and 'V' shaped. White noise was a more effective stimulus than tones. The majority of cells in our sample responded best to sounds from a restricted horizontal location. Two major response types were found: (1) neurones responding to the same localized area of space despite changes in sound level and (2) neurones responding only to a localized area of space near threshold, but to an extensive area for louder sounds. As the site of the recording electrode was moved from the rostral to the caudal part of the superior colliculus, the location of the auditory receptive fields shifted from the anterior to the posterior field of the animal, thus indicating the presence of a map of auditory space. The visual projection to the guinea-pig superior colliculus was determined and found to be similar to that in other vertebrates. Comparison of visual and auditory space maps in the guinea-pig superior colliculus reveals that receptive fields are coincident over a wide range, but severe discrepancies were evident between the visual and auditory receptive field positions represented at single locations in rostral and caudal colliculus.