Attention and microsaccades: do attention shifts trigger new microsaccades or only bias ongoing microsaccades?
Brain circuitry that controls where we look also contributes to attentional focusing of visual contents outside of current fixation or contents held within the spatial layout of working memory. A behavioural manifestation of the contribution of oculomotor brain circuitry to selective attention comes from modulations in microsaccade direction that accompany attention shifts. Here, we address whether such modulations come about because attention itself triggers new microsaccades or whether, instead, shifts in attention only bias the direction of ongoing microsaccades - i.e., naturally occurring microsaccades that would have been made whether or not attention was also shifted. We utilised an internal-selective-attention task that has recently been shown to yield clear spatial microsaccade modulations and compared microsaccade rates following colour retrocues that were matched for sensory input, but differed in whether they invited an attention shift or not. If shifts in attention trigger new microsaccades then we would expect more microsaccades following attention-directing cues than following neutral cues. In contrast, we found no evidence for an increase in overall microsaccade rate following attention-directing cues, despite observing robust modulations in microsaccade direction. This implies that shifting attention biases the direction of ongoing microsaccades without changing the probability that a microsaccade will occur. These findings provide relevant context for complementary and future work delineating the links between attention, microsaccades, and upstream oculomotor brain circuitry, such as by helping to explain why microsaccades and attention shifts are often correlated but not obligatorily linked.