| Briefly-presented visual stimuli were used to reveal two new effects that are quickly reduced by longer presentations of the visual test probe. One is the attentional repulsion effect, the effect of visual attention on the perception of position. Attention was focused at a specific location either by a briefly flashed cue or by a voluntary effort. In both cases, briefly-presented probes appeared displaced away from the focus of attention. The results showed that the effect of cue-induced attention was transient whereas the effect of voluntary attention was long lasting. Control experiments ruled out hypotheses based on classic figural aftereffects and apparent motion. The second effect is the shape contrast effect, the interaction of visual images at the level of shape representation. A briefly-presented line interacted with a subsequently flashed circle--the circle always appeared elongated, perpendicular to the line. The distortion effect depended little on the relative positions of the line and the circle, indicating that the two stimuli interacted at a processing level where shapes are coded independently of the retinal parameters. The results also suggest that for the two shapes to interact, they must be perceived as separate, but related events. It is speculated that both the attentional repulsion and the shape contrast effects result from a general mechanism which operates to improve the spatial and figural selectivity of the visual system. Possible mechanisms are discussed based on the suppression, recruitment, and shrinking of receptive fields for position and shape coding. |
