Foveopetal are easier to detect than foveofugal motions: the effect of attention

BARRAZA, JOSÉ F.; MARTÍN, ANDRÉS

Sarasota (Miami - EEUU)

Congreso; Vision Science Society 7th Annual meeting; 2007

Studies about flash-lag effect revealed that the perceptual latency is shorter for foveopetal than for foveofugal motions. The same tendency was found in subsequent experiments showing that the reaction time for detecting a stimulus is shorter when it moves towards the fovea than when it moves away. We wondered whether this anisotropy holds for motion detection in the periphery and whether attention is involved in this phenomenon. We performed an experiment in which subjects had to indicate by pointing with the mouse which of four simultaneously displayed patches contained a coherent motion (test). Stimuli were random dot patterns. In the test, dots moved coherently in one of four possible directions (Northwest, Northeast, Southeast, and Southwest). In the dummies they underwent Brownian motion. The independent variable of the experiment was the proportion of test dots moving incoherently (noise). The patches positions were top-left, top-right, bottom-right, and bottom-left respect to the fixation point. Both motion direction and position were randomized along trials. We determine for each condition the proportion of correct answers as a function of the test noise (4AFC) and computed the noise threshold for a performance of 82%. Three subjects performed the experiment. Motions were grouped to compute the thresholds in three categories: foveopetal vs foveofugal, clockwise vs counterclockwise, and tilted to the left vs to the right. For example, a Northeast motion in the top-left position is clockwise, but the same motion in the bottom-left position is foveopetal. Results show that thresholds for foveopetal motions are higher (less sensitive to noise) than for foveofugal motions. This anisotropy does not appear in the other classifications. Interestingly, when subjects are told about what motion is going to appear in each trial (foveopetal or foveofugal) the anisotropy disappears, which would suggest that some mechanism of selective attention is favoring foveopetal motions.