Abstract
We measured the stimulus duration required to detect target lines that differed in orientation from distractor lines. Tilted targets amongst vertical distractors required shorter durations than vice-versa. Surrounding the stimulus with a square frame tilted by the same amount as the tilted lines reduced or reversed this asymmetry; a vertical frame had no effect. Treisman and Gormican (1988 Psychological Review 95 15 ^ 48) found similar results using reaction times. Li (2002 Trends
in Cognitive Sciences 6 9 ^ 16) proposed that V1 mechanisms determine salience in visual search. According to this proposal, the advantage for tilted targets could arise from weaker iso-orientation suppression of obliquely tuned V1 cells, since fewer cells encode oblique orientations. The frame effect can be explained by proposing that the sides of the frame inhibit responses to lines parallel to the frame. This predicts no effect of a frame constructed from elements with orientation perpendicular to the side of the frame. This prediction was supported by some subjects, but not others. When alternate frame elements were black and white (on a grey background), so that a large V1 receptive field aligned with the side of the frame would show no response, the frame effect disappeared for some subjects.
