What came out of visual memory: Inferences from decay of difference-thresholds.

Means and standard deviations of psychophysical measurements are first and second statistical moments that reflect neural mechanisms underlying the measured sensation: for example, standard deviations of hue matches are difference-thresholds that reflect the color pathways' spectral response. Might memory psychophysics yield similar insight into neural mechanisms underlying recognition? Five experiments measured memory means and difference-thresholds: Means of monochromatic hue matches made from memory hardly changed with memory time. Hue memory remained consistent even though the variability of those matches increased exponentially as memory time increased to 14 s. This indicates the loss of accuracy in the memory resulted from a random decay of the stored information. Nevertheless, the hue memory difference-thresholds as a function of wavelength continued to resemble sensory difference-threshold functions as memory time increased. Similar results obtained by dichoptic matches indicated this resemblance was not a retinal effect. Line and grating orientation memory difference-thresholds also retained their similarity to sensory difference-threshold functions as memory time increased. The similarity was not some artifact of measuring difference-thresholds by matching because such results were also obtained using a forced-choice method. Similarity of memory discrimination functions to sensory functions is quantitative evidence that, despite decay, activity retrieved from memory resembles the sensory responses used when two stimuli are compared simultaneously. Feasibility of this conclusion was tested by modeling the memory-matching procedure. It produced difference-thresholds resembling those obtained with simultaneous matches when the stored signal had decayed to 10%. Together these results provide "inside" evidence that cortical activity resembling sensory responses underlie recognition.