Detection of Direction of Contrast

Abstract

Abstract Because D.B., when he acknowledges having some sort of impression or feeling when a stimulus is abruptly presented, fails to ascribe any sense of ‘brightness’ to the experience — in fact, on one occasion he said, ‘it is as though I imagine that I am moving my finger in front of my eyes in the dark’ — it seemed possible that he would be unable to respond to the contrast of a stimulus on the ground. That is, he might treat a black spot on a grey ground as equivalent to a white spot on the same ground. We put this to the test in February 1975. Two grey cards were prepared, 24º × 17º in size, with a small black or white square, 3.6º × 3.6º, positioned in the centre. The cards were clipped to the Aimark perimeter at an eccentricity of 45° on the horizontal meridian. They were covered by a masking card, removed for a period of two seconds during each trial. The overhead fluorescent lamps were on, giving a light level on the card of 0.70 log ft lamberts. The level of the black square was 0 log ft lamberts, and of the white square, 1.1 log ft lamberts.