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Michael J Wright, Louise Alston; Limitations of visual memory in spatial frequency discrimination. Journal of Vision 2003;3(9):26. doi: https://doi.org/10.1167/3.9.26.
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© ARVO (1962-2015); The Authors (2016-present)
Spatial frequency (SF) discrimination of single stimuli showed little loss of precision when measured across an interstimulus interval (ISI). However the use of multiple target arrays gave large set size effects. Stimuli consisted of two 100 or 150 msec arrays of 1 to 8 Gabor patches, randomised in SF, with a 2 sec ISI. This was an “all change” task, in which every Gabor patch changed its SF across the ISI, but only one patch (the target) was cued, and required a SF discrimination response. The task was designed to elicit visual short term memory (VSTM) of multiple individual SFs. Unlike many other two-frame discrimination tasks using the method of constant stimuli, it was not soluble on the basis of single-frame information, nor by averaging across stimuli. Analysis of psychometric functions showed that observers reacted to delayed cues by distributing attention across targets (sharing model) rather than “betting” on a single target (mixture model). Cueing before the onset of the first frame was optimal, in the sense that pre-cueing a single target gave the same thresholds as presenting that target alone. There was a modest (0.1–0.2 log unit) rise in thresholds for cues placed immediately after the first frame, requiring the spatial distribution of attention across multiple targets. Within the 2sec ISI itself, there was little further increase in thresholds with cue delay, suggesting a relatively durable representation of multiple stimuli, once registered. Noise masks > 50msec after the first frame were ineffective, confirming the memory during the ISI is not iconic, but Gabor masks placed in the ISI disrupted performance,in agreement with reported “memory masking” effects. The greatest rise in thresholds (0.4–0.8 log unit) occurred for cues placed after the second frame. The results are interpreted in terms of distributed attention and interference effects within VSTM.
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