Perception: ECVP 2001 abstracts

ECVP 2001 abstract

Microsaccades reflect covert attention shifts
Z M Hafed, J Clark (Center for Intelligent Machines, McGill University, 3480 University Street, Montréal, Québec H3A 2A7, Canada; e-mail:

We present evidence that microsaccades reflect covert attention shifts. Subjects fixated on a central point (FP) and shifted attention to one of four peripheral points (PP) (with eccentricities between 4 and 8 deg) using a cue halfway between the FP and PP. After a random number of cue jumps, the PP that was last cued showed a brief colour flash. Subjects were instructed to maintain fixation and discriminate the colour of the flash (yellow/green). Microsaccades (< 1 deg) were observed in roughly a quarter of the trials. These often occurred in pairs of opposing movements (couplets or square-wave jerks). The leading edges of most couplets occurred approximately 250 ms after cue onset and were predominantly in the cue direction. We hypothesise that couplets arise from an attention shift to the cued PP followed by a shift back to the FP. Unpaired microsaccades were generally in the direction of the cue for short latencies; otherwise, they were in the opposite direction. This suggests that unpaired microsaccades are leading/lagging edges of couplets for which the other edges are successfully suppressed. We propose that attention shifts always result in the generation of saccades unless suppressed by fixation activity. Microsaccades result when this suppression is incomplete.

© 2001 Pion Ltd