Journal of Neurophysiology, Vol 38, Issue 2 219-230, Copyright © 1975 by APS
Visual receptive fields and their images in superior colliculus of the cat
J. T. McIlwain
1. The receptive fields of collicular neurons in the cat, recorded in a
single microelectrode penetration, were not centered on a point in visual
space, but nested eccentrically with the smaller fields displaced toward
the area centralis. The eccentric nesting was not eliminated by correcting
the fields for the tangent screen distortion or by making penetrations
normal to the collicular surface in coronal and parasagittal planes. These
findings do not support the idea that collicular cells form topographically
organized columns oriented normal to the collicular surface. 2. When the
receptive fields were plotted in the visual coordinate system of the
collicular map, the nesting became much more concentric, suggesting that
the eccentric nesting of the receptive fields in visual space was largely a
product of the retinotectal coordinate transformation. 3. The profile of a
collicular receptive field, plotted in the collicular visual coordinate
system is called the receptive-field image. Receptive-field images tended
to have oval shapes with the long axis oriented mediolaterally. Clusters of
receptive-field images, plotted for single penetrations, appeared similar
wherever they occurred in the collicular map, suggesting that a common
pattern of neural convergence determines the geometry of the
receptive-field images in all parts of the colliculus. 4. The neural
substrate of the receptive-field images was examined by tracing the
theoretical patterns of neural activity which a point stimulus would
produce in the retinotectal system. This analysis suggested that the shape
and dimensions of the receptive-field images, and consequently the
receptive fields, might be accounted for in large part by the geometry of
collicular dendritic fields, the dimensions of the visual receptive fields
of afferent fibers, and the retinotectal coordinate transformation. 5.
Because it adjusts for the retinotectal distortion of visual space, the
receptive-field image may be used to outline the distribution of collicular
cells excited by a point stimulus. This makes it possible to show that a
point stimulus activates large-field cells in the superficial gray layer
over an area of about 2.5 by 1.5 mm in the central parts of the colliculus.
It is suggested that such cells may organize the directional signals
required by the oculomotor system for visual orienting behavior.
