Perceptual consequences of temporal disparities in the visual field: The case of the line motion illusion

Background and Goal. When a line is preceded by a cue, it appears to be drawn from the end where the cue appeared. This is known as the illusory line motion (Hikosaka, Satoru & Shimojo, 1993). In this study, we characterized the changes in perception of the illusion over the visual field to explore whether these changes correspond to temporal performance field asymmetries -information is processed faster along the horizontal than vertical meridian, and at the lower than upper vertical meridian (Carrasco, McElree & Giordano, VSS 2002). Method. Twenty observers were presented with a 100 ms line either preceded (illusion) or not preceded (control) by the cue. The line appeared on the horizontal and vertical meridians at 4 or 9 eccentricity. In the 'focused attention' condition, the cue appeared only at the location adjacent to that at which the line would appear. In the 'distributed attention' condition 4 cues appeared adjacent to all 4 possible locations. Observers responded in a 3AFC task with either of 2 directions of motion (E vs. W, or N vs. S) or no motion. Results and Conclusion. (a) With focused attention, the illusion was more pronounced along the horizontal than vertical meridian, and least pronounced at the North location. This pattern of results was consistent with the temporal performance field asymmetries aforementioned. (b) With distributed attention, the illusion decreased at all locations to the point that its magnitude became comparable at all locations. These results are consistent with the fact that attention speeds information accrual (Carrasco & McElree, 2000) and that attention eliminates temporal asymmetries in the visual field (Carrasco, Giordano & McElree, VSS 2003). Taken together, these results clearly illustrate the perceptual consequences of disparities in the speed of processing at different locations.