Depth perception and cortical physiology in normal and innate microstrabismic cats

Abstract

AbstractEvidence is presented that innate microstrabismus and abnormal cortical visual receptive-field properties can occur also in cats without any apparent involvement of the Siamese or albino genetic abnormalities in their visual system. A possible cause for microstrabismus in these cats may be sought in an abnormally large horizontal distance between blind spot and area centralis indicated by a temporal displacement of the most central receptive fields on both retinae.Depth perception was found to be impaired in cats with innate microstrabismus. Behavioral measurements using a Y-maze revealed in four such cats that the performance in recognizing the nearer of two random-dot patterns did not improve when they were allowed to use both eyes instead of only one. The ability of microstrabismic cats to perceive depth under binocular viewing conditions only corresponded to the monocular performance of five normal cats.Electrophysiological recordings were performed in the visual cortex (areas 17 and 18) of four awake cats, two normal, and two innate microstrabismic animals. Ocular dominance and orientation tuning of single neurons in area 17 and 18 were analyzed quantitatively.The percentage of neurons in area 17 and 18 which could be activated through either eye was significantly reduced to 49.7% in the microstrabismic animals when compared to the normal cats (74.8%). “True binocular cells,” which can only be activated by simultaneous stimulation of both eyes, were significantly less frequent (1.6%) in microstrabismic cats than in normal animals (10.4%). However, subthreshold binocular interactions were identical in both groups of animals. In the strabismic animals, long-term binocular stimulation of monocular neurons did not give a clear indication of alternating use of one or the other eye.The range of stimulus orientations leading to discharge rates above 50% of the maximal response, i.e. the half-width of the orientation tuning curves, was the same in the two groups of cats. However, orientation sensitivity, i.e. the alternation in discharge rate per degree change in stimulus orientation, was higher in cortical cells of normal cats than in those of microstrabismic cats.In normal and microstrabismic cats, no clear sign of an “oblique effect,” i.e. the preference of cortical neurons for vertical and horizontal orientations compared to oblique orientations, could be found neither in the incidence of cells with horizontal or vertical preferred orientation nor in the sharpness of orientation tuning and sensitivity of these neurons.In summary, the receptive-field properties reported here for awake innate microstrabismic cats are similar to those reported in the literature for anesthetized cats with varying degrees of albinism and for cats with artificial symmetrical strabismus surgically induced by sectioning the equivalent extraocular muscles in both eyes. Our innate microstrabismic cats may provide, however, an animal model for investigating the etiology of one form of naturally occurring strabismus.