Visual Working Memory for Motion Sequences

Abstract

The cognitive mechanisms of visual working memory for sequentially presented objects are not fully understood. We investigated the precision of memory for motion direction of sequentially presented colored objects. Observers viewed random dot motion stimuli displayed sequentially at fixation. Within a sequence, each stimulus was displayed in a different color. At the end of each sequence, participants were asked to adjust a colored probe's direction to match the direction of motion of the stimulus with the same color. We quantified precision as the reciprocal of the standard deviation of error in response direction. The results show a decrease in precision for motion directions presented within longer sequences. There was a significant effect of serial position of the target within each sequence; the last item was remembered best. Importantly, this recency effect was influenced by the number of preceding items; precision of memory for the last item was lower when presented in longer sequences. We applied a probabilistic model of performance to integrate possible sources of error in memory. The model takes into account possible changes in variability in memory for target direction, the probability of responding to a non-target direction and the probability of responding at random. The fall in precision for the last item presented in longer sequences was purely explained by an increase in variability in memory for target direction in these sequences. A similar decrease in precision within longer sequences was observed in other serial positions of the target. This was explained by an increase in probability of responding to a non-target motion direction. These results are compatible with a dynamic resource model of memory whereby a limited resource is shared between items presented in a sequence. The amount of memory allocated to each item is affected by the preceding items and the items that follow it.