For the last five decades, the medial temporal lobes have been generally understood to facilitate enduring representation of certain kinds of information. In particular, knowledge about the relations among items and concepts appears to rely on that region of the brain. Recent results suggest that those same structures also play a subtle role in the apprehension of ongoing events, enhancing the integration of information over very brief intervals. This capacity is referred to as "on-line processing", and several lines of experimentation are proposed to investigate and characterize deficits in on-line processing that accompany MTL damage.
Damage to the medial temporal lobes (MTL) results in profound memory impairments and has been shown to interfere with the ability to maintain complex stimuli across short delays. Evidence for impairments in the maintenance of simple stimuli remains equivocal as different results indicate a range of performance from normal to deficient. A visual search task performed in both simultaneous and short-delay conditions was used to explore the possibility of subtle deficits in maintenance of simple visual stimuli. Stimuli were basic shapes varying by condition across six visual dimensions, and participants searched a small array for a match to a trial-specific sample. Eye-movement data collected during search revealed that both normal and MTL-lesion participants fixated array items for durations corresponding to the similarity of the item to the sample, but that this effect was attenuated in the MTL-lesion patients in the short-delay condition.
Medial temporal lobe structures may play a role in the maintenance of complex visual information across short intervals. This phenomenon was investigated by testing neurological patients and healthy comparisons with a novel visual search task employing complex stimuli that varied in their similarity to the target. Behaviorally, patients with medial temporal lobe damage performed less well than comparisons, and simultaneous recording of eye movements revealed further differences. All participants exhibited a relationship between the similarity of fixated array lures to the sample item and the duration of those fixations. However, patients searched the array differently: fixating the sample item more frequently; making shorter fixation paths from the sample item into the search array; and during those fixation paths, patients' later fixations were less likely to exhibit the effect of item similarity on duration. This evidence suggests that the representations available after medial temporal lobe damage degrade during search perhaps due to interference. By implication, the same structures would insulate representations in healthy brains, indicating a role for this region in on-line processing.
Patients with damage to the structures of the medial temporal lobes (MTL) exhibit gross impairments in the ability to create enduring new memories, but are generally thought to possess an intact capacity for the brief maintenance and manipulation of information. Five tests employing degraded visual stimuli and requiring just those capacities were administered to groups of lesion patients and healthy comparison participants. Results show that damage to the MTL does not yield a clean dissociation of brief and enduring representations, and that patients with such lesions are impaired on tasks requiring little or no maintenance. MTL structures appear to contribute rapidly and crucially to the processing of complex visual stimuli in addition to their known role in preserving information across long intervals.
Results described in preceding chapters reinforce and extend the emerging view that structures of the medial temporal lobe contribute to the on-line processing of visual stimuli. These experiments uniquely demonstrate that the MTL is necessary for normal apprehension of even single stimuli when degraded, and for normal discrimination of even very simple representations after a brief delay. Changes were observed in a maintained representation over time in the face of interference, but only in the presence of MTL lesions. Explanation of these findings requires a theory of substantial scope that can explain why information cannot be easily integrated over time. One candidate is an extension of relational memory theory, which previously has addressed primarily enduring representations. However, an inability to bind together even simultaneously available information might manifest in the fashion reported here. Theoretical contributions of other authors are also considered, but the available data do not permit definitive adjudication between the alternatives. Even so, my results and those of others clearly indicate that MTL structures are active participants in the representation of visual stimuli.
|Advisor:||Cohen, Neal J.|
|School:||University of Illinois at Urbana-Champaign|
|School Location:||United States -- Illinois|
|Source:||DAI-B 71/01, Dissertation Abstracts International|
|Subjects:||Psychobiology, Experimental psychology|
|Keywords:||Amnesia, Eye tracking, Hippocampus, MTL, Medial temporal lobe, On-line, On-line processing|
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