Implicit memory

One challenge of neurology is to explain consciousness. Clinically, we avoid the issue by equating level of consciousness with arousal, knowing that consciousness has aspects that are much harder to think about and still harder to test. Seeck et al.¹ probe the mechanisms of consciousness by asking whether there is a cortical neurophysiologic correlate of conscious recollection. Their work illustrates progress in memory research that reveals a dichotomy between explicit memory, knowledge that we know we possess; and implicit memory, knowledge without awareness.

In earlier studies, they recorded visual evoked responses (VERs) to flashed pictures of human faces.² Visual evoked responses in the hippocampus and temporal cortex differed when evoked by a familiar face versus that of a stranger. Now they have analyzed where face recognition occurs and found that the localization depends on whether the subject realizes that the face has been seen before. Using frontal and temporal depth electrocorticography, in patients undergoing diagnostic evaluation for intractable epilepsy, they recorded VERs to pictures of unfamiliar human faces. The pictures were presented as a series of sequential pairs of flashed stimuli. The subject was asked if either face in the most recent pair was a repeat from the previous pair. The VERs evoked by the flashed face stimuli were sorted by whether they were evoked by a repeat face or a new one. The authors found that temporal and frontal cortical sites, but not hippocampal sites, show differences in VERs to the repeated faces. Remarkably, the VERs differed even though the subjects failed to consciously recognize which faces were repeats. Thus, the VERs indicated if a face had been …