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.
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