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. 2010 May 18;4:35.
doi: 10.3389/fnhum.2010.00035. eCollection 2010.

Medial Temporal Lobe Damage Impairs Representation of Simple Stimuli

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Free PMC article

Medial Temporal Lobe Damage Impairs Representation of Simple Stimuli

David E Warren et al. Front Hum Neurosci. .
Free PMC article

Abstract

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.

Keywords: MTL; STM; WM; amnesia; decay; delay; hippocampus; memory.

Figures

Figure 1
Figure 1
Sample stimuli. Clockwise from top left: linetilt; luminance; orientation; shape; spatial frequency; proportion.
Figure 2
Figure 2
Trial sequence for simultaneous presentation condition. From left to right: fixation; sample presentation (2s); sample persists and search array appears (6s); response screen.
Figure 3
Figure 3
Trial sequence for delayed condition. From left to right: fixation; sample presentation (2s); sample masked (∼4.5s); fixation (∼1.5s); search array presentation (6s); response screen.
Figure 4
Figure 4
Explanation of the difference measure. Six values along the continuum (here orientation) always comprised the search array. Eleven different values were used for sample items: six values that matched one array item; and five values falling between the values of array items. Our “difference” predictor was based on the number of potential sample items between the value of a given trial's sample item and the value of a fixated array item.
Figure 5
Figure 5
Overall effect of perceptual difference on fixation duration. Overall means and model-fitted curve.
Figure 6
Figure 6
Fixation duration, overall means and model-fitted curves: (A, top) simultaneous condition; (B, bottom) delayed condition. In (A), 95% HPDI curves reflect the main effects of perceptual difference and MTL damage, while in (B), the HPDI curves reflect the interactions of those terms with delay.

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