The deep layers of the superior colliculus (SC), which participate in the orientation of gaze to sensory stimuli, receive visual and non-visual inputs that are spatiotopically organized. In anesthetized animals these sensory maps are in rough spatial register with each other and are thought to superimpose upon a """"""""motor map"""""""". This arrangement is held to account for how tectal activity evoked by a visual or non-visual stimulus specifies the gaze shift needed to fixate the stimulus. However, for behaving animals, whose eyes and head move independently, a system of tectal visual and auditory maps that are fixed relative to the retina and head seem awkward; a deviation of the eyes will cause visual or auditory stimulation from the same point in space to activate different regions of the tectal motor map and hence specify gaze shifts unappropiately. Using alert cats, we will test the """"""""dynamic remapping"""""""" hypothesis that auditory and visual inputs to the deep SC are modulated to compensate for eye position, keeping the sensory maps in register so that they conform to motor maps specifying eye or head movements. Dynamic remapping is observed in monkeys: auditory receptive fields (RFs) move as the eyes move (Jay and Sparks, 1984). This conforms to the above hypothesis since a particular locus in primate SC represents a given """"""""retinocentric"""""""" motor error (i.e. a desired change in eye position). Harris et al, (1980) found no such remapping of auditory RFs in cat SC. But recent evidence suggests that tectal motor organization in cat differs from that in monkeys. Rostral SC in cats has a retinocentric motor map like primates, but caudal SC appears to have a craniocentric motor map, each tectal locus representing a desired change in head position. The dynamic remapping hypothesis predicts that: a) in rostral SC, auditory RFs will depend on eye position and visual RFs will be retinally fixed: b) in caudal SC, auditory RFs will be craniocentrically fixed but the retinal locus of visual RFs will depend on eye position. We will record from visual, auditory and bimodal units in both zones, using computer-controlled stimulation and electromagnetic eye-position monitoring to map RFs as a function of eye position. We will 1) test the generality of the dynamic remapping principle of tectal sensorimotor integration by determining if it occurs in cats (as in primates) and in both visual and auditory senses; 2) to determine whether visual inputs to caudal SC are transformed into a head-centered reference system, and hence whether sensory organization confirms the existence of retinocentric and craniocentric motor maps in different regions of cat SC; and 3) resolve the Sparks-Harris discrepancy.
| Hartline, P H; Vimal, R L; King, A J et al. (1995) Effects of eye position on auditory localization and neural representation of space in superior colliculus of cats. Exp Brain Res 104:402-8 |
