Preferred Hemifields Learn Less
Subjects were tested on orientation and colour pop-out tasks. Orientation pop-out stimuli were 3 × 3, 5 × 5, or 7 × 7 arrays of vertical light (white) bars (on a black background) with a target element of diagonal (45°) orientation (present on half of the trials). Colour pop-out stimuli consisted of 60° oriented bars with blue distractor and yellow target elements (on a gray background). Inter-element distance was 40 min arc. The arrays were presented eccentrically, in the right or left hemifield, so that the distance of their centres from the fixation cross was 2.5 – 6.5 deg. First we tested performance in the right and left hemifields, with interleaved trials. Most subjects (7/9) had a right-hemifield preference for the orientation and/or colour task. The remaining 2 had a left-hemifield preference. For the six subjects tested on both tasks, all but one had more laterality for colour than for orientation. Thus, for the 15 conditions tested (9 orientation + 6 colour), most (8/15) showed better performance in the right hemifield; the minority (2/15) were better on the left; and a third were about equal in both hemifields. Subjects were recalled for a second session with the same tasks, to test improvement due to first session training. Almost all showed significantly greater improvement for the non-preferred hemifield. This effect was so strong that often preference was switched. Surprisingly, preferred-hemifield performance actually declined for many subjects. Thus, the effect seems related to competition, and perhaps an automatic attention-directing mechanism.