Micro-timing of iconic memory readout

The human visual system is immensely efficient at extracting and briefly retaining visual information. The classic Sperling task has suggested the existence of a high-capacity memory store—iconic memory—that decays rapidly following stimulus offset. But when exactly does perception end and when does iconic memory begin? We measured the availability of visual information at the temporal transition between perception and iconic memory. In a partial report task, we parafoveally presented radial arrays of 4, 8, 12, and 16 consonant letters, coupled with exogenous cues shown before, simultaneously with, or after the stimulus. Contrary to the expectation of ultra-high iconic capacity, we observed a significantly reduced availability of information when the probe was presented immediately at stimulus offset compared to during stimulus presentation. At first sight, this may suggest that iconic memory, even at its highest capacity, is a significantly degraded representation of the stimulus. Instead, we propose that this decrease in available information is partially due to a cue-readout delay, the time it takes to process the spatial cue and initiate the read-out from the stimulus representation. To estimate the length of the cue-readout delay, we modeled the availability of visual information in two stages: the exogenous stage when sensory information is available externally from the stimulus; and the endogenous stage when information is read out of a rapidly decaying iconic memory, approximated by a three-parameter exponential function. Our model indicated very short cue-readout delays on the order of 25 ms. Our findings suggest that the estimated time constants of iconic memory decay need to be adjusted for these cue-readout delays. We routinely assume that memory begins only after the offset of a stimulus, but our data suggest that experimental probing of memory contents may be required already during stimulus exposure, a stage that would usually be considered “perception”.