The Cortical Correlate of Pattern Vision

Abstract

The cortical correlate of pattern vision is defined as the physiol. (electrical) brain processes associated with the psychological fact of seeing a pattern. To minimize difficulties in recording these correlates, an object differing not too greatly in luminosity from the background, and moving slowly (velocity of 6-12[degree] of visual angle/sec.) across the visual field is used. These visual objects consist of white or gray strips of cardboard moving on a dark background or moving bars of light projected on a screen. Records are obtained from 13 human subjects seated in a shielded cage. Silver-silver chloride electrodes are used, one usually being placed slightly above the occipital protuberance (where the foveal region of the visual cortex is located), the other at the vertex. Amplification and recording are accomplished with a General Motors breaker-type d-c amplifier and a General Electric photoelectric recorder. Reproductions of 11 representative records are presented in the article. In all cases, the application of the visual stimulus is associated with the development of a potential between the electrodes, with the occipital (active) electrode being positive to the vertex electrode, and the response increases as the object approaches the visual fixation point. With appropriate instrumentation the alpha waves are demonstrated superimposed on the above deflections, but the alpha waves are somewhat distorted by the visual response. By placing the 2 electrodes in the occipital region, one on the left, the other on the right of the median plane, a diphasic response is obtained in which, when the object moves from right to left and the left electrode is "active," a positive deflection during the 1st part of the record is followed by a negative during the 2d. These and other results are in agreement with the concept that the retinal cells on the left side of the central meridian are connected with cortical tissue in the left hemisphere, and vice versa.