Disentangling perceptual from non-perceptual expectation biases in short-term memory

There is debate about how many items can be represented in visual consciousness at once. Evidence against the view that visual consciousness consists of many detailed items has come from a study using the partial-report paradigm. De Gardelle et al. (2009) showed that rotated letters shown in non-cued (i.e., unattended) parts of a briefly presented letter array are sometimes illusorily perceived as upright. Presumably, the expectation that letters are generally seen upright modulated a rotated letter's visual representation towards an upright one. The present study elaborates on this finding by comparing reports of rotated letters to two kinds of controls: letters shown upright, and letters that were not shown in the preceding stimulus. Results showed that participants were able to discriminate non-cued letters from not-shown letters, providing evidence that such letters remained available for some time after stimulus offset. This was found for letters that were shown upright but also for letters shown inverted. Upright letters were reported as upright more often than not-shown letters were, showing that information about letter orientation was preserved. Inverted letters were also reported as upright but, importantly, not more so than not-shown letters were. This replicates the earlier finding that inverted letters can be reported as upright but shows that such errors might be attributable to non-perceptual stages of processing. Post-perceptual biases influencing reports of visual representations might present a new ground to distinguish between phenomenally conscious contents and reports thereof.