Medial temporal lobe damage impairs representation of simple stimuli

Medial temporal lobe (mtl) damage in humans is typically thought to produce a circumscribed impairment in the acquisition of new enduring memories,but recent reports have documented deficits even in short-term maintenance. we examined possible maintenance deficits in a population of mtl amnesics,with the goal of characterizing their impairments as either representational drift or outright loss of representation over time. patients and healthy comparisons performed a visual search task in which the similarity of various lures to a target was varied parametrically. stimuli were simple shapes varying along one of several visual dimensions. the task was performed in two conditions,one presenting a sample target simultaneously with the search array and the other imposing a delay between sample and array. eye-movement data collected during search revealed that the duration of fixations to items varied with lure-target similarity for all participants,i.e.,fixations were longer for items more similar to the target. in the simultaneous condition,patients and comparisons exhibited an equivalent effect of similarity on fixation durations. however,imposing a delay modulated the effect differently for the two groups: in comparisons,fixation duration to similar items was exaggerated; in patients,the original effect was diminished. these findings indicate that mtl lesions subtly impair short-term maintenance of even simple stimuli,with performance reflecting not the complete loss of the maintained representation but rather a degradation or progressive drift of the representation over time.