Effect of stimulus contrast on performance and eye movements in visual search

According to the visual span control hypothesis, eye movements are controll ed in relation to the size of visual span. In reading, the decrease of cont rast reduces visual span, saccade sizes, and reading speed. The purpose of the present study is to determine how stimulus contrast affects the speed o f two-dimensional visual search and how changes in eye movements and visual span could explain changes in performance. The task of the observer was to search for, and identify, an uppercase letter from a rectangular array of characters in which the other items were numerals. Threshold search time, i .e. the duration of stimulus presentation required for search that is succe ssful with a given probability, was determined by using a multiple-alternat ive staircase method. Eye movements were recorded simultaneously by using a video eye tracker. Four different set sizes (the sizes of stimulus array) (3 x 3-10 x 10), and five different contrasts (0.0186-0.412) were used. At all set sizes, threshold search time decreased with increasing contrast. Al so the average number of fixations per search decreased with increasing con trast. At the smallest set size (3 x 3), only one fixation was needed excep t at the lowest contrast. Average fixation duration decreased and saccade a mplitudes increased slightly with increasing contrast. The reduction of the number of fixations with increasing contrast suggests that visual span, i. e. the area from which information can be collected at one fixation, increa ses with increasing contrast. The reduction of the number of fixations toge ther with reduced fixation duration result in reduced search times when con trast increases. (C) 2001 Elsevier Science Ltd. All rights reserved.