Functional organization in the superior colliculus of the golden hamster

The superior colliculus of the golden hamster was investigated by means of multi-unit and single unit recording. The retinotopic map, which probably embraces a projection from the entire retina of the contralateral eye, is organized as in other vertebrates, with the central field represented in the anterior colliculus, the upper field medially. Magnification factor is fairly uniform and is about 0.02 mm/deg. There is a small binocular segment (where almost half of all neurones have input from the ipsilateral eye) in the anterior colliculus, representing the area of field around the area centralis and the anterior pole of the field. In the more superficial layers, units have small (about 10 deg diameter) receptive fields, which can be classified as symmetrical, responding to slow movement (80%), very fast movement detectors (6%), directional movement detectors (13%) and axial movement detectors (1%). In the deeper layers, below the stratum opticum, receptive field size increases dramatically and many cells habituate rapidly, making them sensitive only to new events. Receptive fields can be classified as movement detectors (89%), directional movement detectors (10%) and axial movement detectors (2%). All directional receptive fields, at least in the upper visual field, have an upward component in their directional preferences. About 42% of deeper layer cells have somatic sensory input, responding to light touch on the fur or whiskers of the contralateral half of the body. Some 5% of cells respond to complex sounds on the contralateral side of the animal. Many of these somatic and auditory cells also have visual receptive fields and, throughout the colliculus, there is general correspondence between the maps of visual space, auditory space and the body surface. This correlation may be important in the regulation of orienting behaviour towards novel peripheral stimuli.