No obligatory trade-off between the use of space and time for working memory

Space-time interference: The asymmetry we get out is the asymmetry we put in

Temporal judgments are more affected by space than vice versa. This asymmetry has often been interpreted as primacy of spatial representations over temporal ones. This interpretation is in line with conceptual metaphor theory that humans conceptualize time by spatial metaphors, but is inconsistent with the assumption of a common neuronal magnitude system. Here we review the accumulating evidence for a genuinely symmetric interference between time and space and discuss potential explanations as to why asymmetric interference can arise, both with respect to the interaction between spatial size and temporal duration, and the interaction between traveled distance and travel time. Contrary to the view of hierarchical representations of time and space, our review suggests that asymmetric interference can be explained on the basis of working memory processes and the aspect of speed inherent in dynamic stimuli. We conclude that the asymmetry we often get out (space affects time more than vice versa) is a consequence of the asymmetry we put in (by using biased paradigms and stimuli facilitating spatial processing).

Attentional shifts bias microsaccade direction but do not cause new microsaccades

Brain circuitry that controls where we look also contributes to attentional selection of visual contents outside current fixation, or content within the spatial layout of working memory. A behavioural manifestation of this contribution comes from modulations in microsaccade direction that accompany spatial attention shifts. Here, we address whether such modulations come about because attention shifts trigger new microsaccades or whether, instead, spatial attention only biases the direction of ongoing 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 robust spatial microsaccade modulations and compared microsaccade rates following colour retrocues that were carefully matched for sensory input, but differed in whether they invited an attention shift or not. If attention shifts 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, despite robust modulations in microsaccade direction. This implies that shifting spatial attention biases the direction of ongoing microsaccades without changing the probability of microsaccade occurrence. These findings help to explain why microsaccades and visual-spatial shifts of attention are often correlated but not obligatorily linked.

Microsaccades Track Location-Based Object Rehearsal in Visual Working Memory

Besides controlling eye movements, the brain's oculomotor system has been implicated in the control of covert spatial attention and the rehearsal of spatial information in working memory. We investigated whether the oculomotor system also contributes to rehearsing visual objects in working memory when object location is never asked about. To address this, we tracked the incidental use of locations for mnemonic rehearsal via directional biases in microsaccades while participants maintained two visual objects (colored oriented gratings) in working memory. By varying the stimulus configuration (horizontal, diagonal, and vertical) at encoding, we could quantify whether microsaccades were more aligned with the configurational axis of the memory contents, as opposed to the orthogonal axis. Experiment 1 revealed that microsaccades continued to be biased along the axis of the memory content several seconds into the working memory delay. In Experiment 2, we confirmed that this directional microsaccade bias was specific to memory demands, ruling out lingering effects from passive and attentive encoding of the same visual objects in the same configurations. Thus, by studying microsaccade directions, we uncover oculomotor-driven rehearsal of visual objects in working memory through their associated locations.