Concurrent maintenance of visual imagery and short-term memory provides evidence for their distinct representations

Recent research indicates there is overlap in the neural resources used during imagery and visual short-term memory. But do visual short-term memory and visual imagery operate on similar representations during recall? Here we investigated this question by asking participants to perform a delayed match to sample task for the contrast of visual gratings as cues. In the "Imagery" condition, participants were asked to form an accurate mental image of the visual cue, and at the end of the trial, perform a matching task on the mental image contrast. In the "Memory" condition, participants were not required to perform visual imagery but merely instructed to perform the delayed contrast-matching task. In Experiment 1, participants were told at the beginning of each block whether to engage in memory or imagery. The results showed that for the relevant contrast feature, matching judgments were more accurately in the "Memory" than in the "Imagery" condition. Thus imagery did not maintain an accurate representation of the encoded image, even when the visual features could still be maintained in visual short-term memory. In Experiment 2, participants were required to engage in memory and imagery simultaneously, and were told which to base their judgment on after each trial. The key finding was that the superior accuracy for memory over imagery remained, indicating that the contents of VSTM and imagery are based on distinct representations.