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Abstract

Visual space is represented in the human cortex in a highly ordered manner. Each position in the visual field corresponds to a well-defined region in the cortical area VI, and similarly at several subsequent stages of the visual stream. An experiment to determine how far out in the peripheral visual field the orientation of a small line segment can be detected is presented. The fixation target was centered in the stimulus window, and a test bar was presented along the horizontal meridian to the left or to the right of that spot. In the right hemifield of the right eye, the basic pattern of orientation detection performance is overruled by an additional peculiarity appearing at eccentricities between 15° and 18°. In this range, the performance is reduced to zero values for all stimulus contrasts. This behavior was to be expected from the fact that the blind spot is located at a particular position in the visual field, and therefore the subjects should not be able to detect the stimuli at all in that region. The fact can be used to control the reliability of the subjects' fixation by placing one test bar position in the blind spot. It is found that performance for orientation detection of a bar of a given size decreases with decreasing stimulus contrast.