Attention to local form information can prevent access to semantic information.

Humphreys and Boucart (1997) have shown that when processing of local form is required for response, surrounding global information is automatically processed to a semantic level. The generality of this effect was investigated in two experiments in which the perceptual load of the relevant local form information was manipulated, as was uncertainty about perceptual load. Participants attended to a single line segment inside a picture of an animal or a vehicle and then decided which of two pictures in the target/distractor display that followed contained a line segment that matched the first in orientation. The perceptual load of the relevant local form information was either low (the matching line segment was one of the solitary line segments presented in the centre ofthe target and distractor pictures), or high (the matching line segment was part of one of the solitary letters presented in the centre of the target and distractor pictures). The semantic relations among the pictures were manipulated, and when perceptual load remained constant across trials, semantic relations affected performance in the low-load but not in the high-load condition. When perceptual load varied from trial to trial, there was no evidence of any semantic processing on either low- or high-load trials. The results suggest that in form-matching tasks, attentional priority can be given to local information, thus preventing access to semantic information derived from global shape. They further suggest that the distribution of attention is not dictated entirely by the actual perceptual demands of the relevant stimulus, indicating an important role for top-down processing.