The effect of mixed visual contrast schedules on detection times for both free and horizontally structured visual search.

Abstract

An experiment was carried out in which Ss' detection times were compared for free and systematic visual search for a small target. The systematic search pattern consisted of a horizontal zig-zag between pairs of lines drawn on the visual field. Two different levels of target contrast were employed. Groups of four Ss were presented with one of three mixed stimulus schedules containing different percentages of high and low contrast targets; each S being tested on both free and systematic visual search for that schedule. The composition of the schedules was as follows: schedule (1), 75 per cent low and 20 per cent high contrast; schedule (2), 50 per cent low and 50 per cent high; schedule (3), 25 per cent low and 75 per cent high. Ss were not informed about either the level of target contrast before individual trials, or the schedule to which they had been allocated. The main results were as follows: systematic search using the structured field reduced detection time for low contrast targets in schedules (1)...