  Performance limitations in visual short-term memory (VSTM) tasks have
traditionally been explained in terms of resource or capacity limitations. It
has been claimed, for example, that VSTM possesses a limited amount of
cognitive or neural "resources" that can be used to remember a visual display.
In this paper, we highlight the potential importance of a previously neglected
factor that might contribute significantly to performance limitations in VSTM
tasks: namely, a mismatch between the prior expectations and/or the internal
noise properties of the visual system based primarily on its adaptation to the
statistics of the natural environment and the statistics of the visual stimuli
used in most VSTM experiments. We call this 'model mismatch'. Surprisingly, we
show that model mismatch alone, without assuming a general resource or capacity
limitation, can, in principle, account for some of the main qualitative
characteristics of performance limitations observed in VSTM tasks, including:
(i) monotonic decline in memory precision with increasing set size; (ii)
variability in memory precision across items and trials; and (iii) different
set-size dependencies for initial encoding rate and asymptotic precision when
the duration of image presentation is varied. We also investigate the
consequences of using experimental stimuli that more closely match the prior
expectations and/or internal noise properties of the visual system. The results
reveal qualitatively very different patterns of behavior for such stimuli,
suggesting that researchers should be cautious about generalizing the results
of experiments using ecologically unrealistic stimulus statistics to
ecologically more realistic stimuli.
