Lifetime of Human Visual Sensory Memory: Properties and Neural Substrate

The classic partial-report procedure was modified to optimize the condition to measure the transient decay of visual sensory memory (VSM, also known as iconic memory). A model was developed to isolate the VSM and visual working memory (VWM) underlying the partial-report performance. The decay of VSM in each subject was well characterized by a single exponential function, thus a lifetime could be defined for VSM decay in individual subjects. It was found that intensive practice with partialreport task prolonged VSM lifetime. This practice effect shows an unexpected adaptive property of VSM and reveals VSM lifetime as a specific dimension for perceptual learning. Of the stimulus parameters, a change of the mean luminance of the stimuli from that of the background shortened the VSM lifetime. Such a "luminance effect" is consistent with the temporal properties of the spatial frequency channels in the visual pathway, most likely revealing the differences in the time course of the decay of the memory traces in these channels. To identify the neural substrate of VSM, the lifetime for the decay of the neural activation trace in the human primary visual cortex (area V1) were deduced from the visually evoked potential (VEP) recordings for each subject. There was a precise match between the V1 lifetime and the VSM lifetime for each subject. The match even held when both psychophysical and physiological lifetimes were prolonged by practice, or shortened by enhancing the mean luminance of the stimuli above the background. This precise match indicates that the cortical location for VSM is V1.