Acuity of Visual Perception of Direction in the Dark for Various Positions of the Eye in the Orbit

Abstract

Viewing monocularly in a dark room, Ss reported the location of a 4′ 100-msec. flash relative to the location of a fixation target extinguished 3 sec. earlier. In one experiment the flashes were randomly preselected from a horizontal array centered on the fixation target and S reported the horizontal displacement of the flash (left, right, same); fixation was either in primary position or in secondary position 34½° to the right or to the left of primary position. In a second experiment the flashes were randomly preselected from a vertical array centered on the fixation target and S reported the vertical displacement of the flash (above, below, same); fixation was either in primary position or in a secondary position 34¼° above or 23¼° below the fixation target. JNDs were about 20′ of arc in primary position and increased considerably in all secondary positions of fixation. In the first experiment PSEs shifted to the right as fixation position was shifted to the left; in the second experiment PSEs shifted upward as fixation position shifted downward.