Abstract

We investigated the effects of added noise on the visibility of dot pattern stimuli as a function of the visual field position using the pattern discrimination perimeter.¹ The stimuli were 20 × 20-dot patches of nonrandom dots embedded in a field of dynamic random dots. The dots subtended 15 min of arc, and the black/white dot ratio was 1/1 for both stimulus and surround. The stimuli were located on the four principal diagonals at an eccentricity of 12.7°. The stimulus pattern was either an upward-scrolling spatially random pattern presented for eight surround frames or a static checkerboard presented for only one surround frame. The surround frame rate was 15 Hz for the scrolling stimulus and 1.875 Hz for the checkerboard; thus stimulus duration was 0.533 s for both patterns. Coherence thresholds were measured with a staircase procedure by randomizing known percentages of stimulus dots per surround frame. Checkerboard coherence thresholds were higher for the superior than the inferior field, in agreement with asymmetries typically reported for luminance increment thresholds. Scrolling coherence thresholds, however, were lower for the superior than the inferior field, suggesting that motion detection mechanisms are more sensitive for the superior field, whereas spatial pattern detection mechanisms are more sensitive for the inferior field. A control experiment confirmed this finding for downward, leftward, and rightward scrolling stimuli, but thresholds were surprisingly lower for upward scrolling than for the other three directions, regardless of position.

© 1988 Optical Society of America