The integrative role of the intralaminar system of the thalamus in visual orientation and perception in the cat.

Electrical stimulation with repetitive pulses (6/sec) was applied to the internal medullary lamina and produced tonic contraversive turning movements of eyes and head which were associated with recruiting responses in the visual cortex. Optical gratings of a spatial frequency of 0.5 c/deg which alternated with diffuse illumination of equal intensity were then presented to the animal during conjugated adversive eye movements induced by intralaminar stimulation with continuous sine-wave alternating currents of 5000 c.p.s. The head of the animal was fixed except for rotation in the horizontal plane. Prior to intralaminar stimulation it was turned in the direction opposite to the induced eye movements so that the animal always viewed the central area of the screen. The potential recorded in the visual cortex in response to the gratings consisted of an initial positive-negative potential of the order of 20 muV followed by later deflections of irregular shape. The size of this initial potential was measured during induced gazing and in the absence of intralaminar stimulation. A significantly greater potential in response to the gratings was found during gazing. Diminishing the visual acuity of the animal resulted in a marked decrease of the initial potential and in this condition very little increase was obtained during gazing. No enhancement of the potential was seen when stimulation was applied to other thalamic structures that yielded no visual orientation. It is concluded that perception of contrast is improved during visual orientation and that the effect is due to a concomitant activation of unspecific intralaminar afferents to the visual cortex.