Memory and incidental learning for visual frozen noise sequences

Five experiments explored short-term memory and incidental learning for random visual spatio-temporal sequences. In each experiment, human observers saw samples of 8 Hz temporally-modulated 1D or 2D contrast noise sequences whose members were either uncorrelated across an entire 1-s long stimulus sequence, or comprised two frozen noise sequences that repeated identically between a stimulus' first and second 500 ms halves ("Repeated" noise). Presented with randomly intermixed stimuli of both types, observers judged whether each sequence repeated or not. Additionally, a particular exemplar of Repeated noise (a frozen or "Fixed Repeated" noise) was interspersed multiple times within a block of trials. As previously shown with auditory frozen noise stimuli (Agus, Thorpe, & Pressnitzer, 2010) recognition performance (d') increased with successive presentations of a Fixed Repeated stimulus, and exceeded performance with regular Repeated noise. However, unlike the case with auditory stimuli, learning of random visual stimuli was slow and gradual, rather than fast and abrupt. Reverse correlation revealed that contrasts occupying particular temporal positions within a sequence had disproportionately heavy weight in observers' judgments. A subsequent experiment suggested that this result arose from observers' uncertainty about the temporal mid-point of the noise sequences. Additionally, discrimination performance fell dramatically when a sequence of contrast values was repeated, but in reverse ("mirror image") order. This poor performance with temporal mirror images is strikingly different from vision's exquisite sensitivity to spatial mirror images.