Distribution in the visual field of the costs of voluntarily allocated attention and of the inhibitory after-effects of covert orienting.

By using a simple reaction time (RT) paradigm we have investigated the spatial distribution of the benefits and costs of voluntarily directed attention and of the inhibitory after-effects of covert orienting. In the first experiment subjects deliberately allocated attention to each one of five stimulus positions disposed along the horizontal meridian, while at the same time fixing their eyes on the central position. The separation in visual angle between the central position and the two nearest positions, one on the left and the other on the right, was 10 degrees; that between the central position and the two most eccentric positions was 30 degrees. By comparing RT to brief flashes of light presented at each position during directed attention with RT to identical flashes at the same position during diffuse attention (i.e. in a condition in which subjects paid equal attention to all five positions), it was possible to determine that benefits, that is RT decreases relative to the diffuse-attention condition, were strictly limited to the attended position. Costs, i.e. RT increases relative to the diffuse-attention condition, showed a more diffuse and complex spatial pattern. When attention was directed to one of the noncentral positions, costs were apparent at the two contralateral positions and at the central position, but not at the ipsilateral position. When attention was directed to the central position, costs occurred at all other positions. This suggests a special role for the vertical meridian in delimiting the area of costs when one covertly orients towards the opposite right or left visual half field. Work of others and our preliminary evidence indicate that the area of costs is similarly limited by the horizontal meridian when one orients toward the opposite upper or lower visual field. In the second experiment we studied the inhibitory after-effect of covert orienting. Orienting to a light stimulus without moving the eyes to it may induce a short-lived facilitation of the speed of response to a second stimulus presented at the same position, but this facilitation is followed by a profound and prolonged RT retardation. By using a two-flashes paradigm we observed this RT retardation not only when the two stimuli appeared at the same position, but also when they occurred at different locations in the same altitudinal or lateral visual hemifield. There were no inhibitory after-effects when the two stimuli appeared on opposite sides of the vertical or horizontal meridian.(ABSTRACT TRUNCATED AT 400 WORDS)