McIntosh et al 1998 ARVO abstract

NEURAL SYSTEMS OF VISUAL MEMORY IN
YOUNG AND OLD OBSERVERS

A.R. McIntosh, A.B. Sekuler, C. Penpeci, M.N. Rajah,
P.J. Bennett, C.L. Grady, R. Sekuler*

Department of Psychology, University of Toronto
and *Volen Center, Brandeis University

Purpose.
Memory for spatial frequency is robust and long lasting. Because sinewave gratings have no semantic context, measures of spatial frequency memory provide an excellent assay of purely visual memory. The goal of the current study was to determine what neural systems underlie such visual memory and also how age affects those systems.
Methods.
10 young (20-30 yrs) and 9 old (60-80 yrs) observers underwent PET scans while performing a spatial frequency discrimination task for stimuli presented in the near periphery of the visual field. Measures of performance and of regional cerebral blood flow (rCBF) were taken while two sinewave gratings were presented either simultaneously (side by side) or successively, with interstimulus intervals (ISIs) of 500 and 4000 ms. Partial Least Squares (PLS) analysis was used to determine the pattern of activity across the whole brain that best related to performance on the spatial frequency memory task.
Results.
Behavioral results for young and old observers did not differ. For both, average discrimination thresholds were about 7% for the simultaneous and 500-ISI conditions, and thresholds increased to about 12% for the 4000-ISI condition.

Brain imaging results differed dramatically for young and old observers, despite the two groups similar behavioral performance.
For young observers, higher activity in the hippocampal and left pre-frontal regions corresponded to lower thresholds, whereas higher activity in the supramarginal, cuneus and right posterior pre-frontal areas corresponded to higher threholds.
For old observers, higher activity in the bilateral occipitotemporal cortices and in the posterior thalamus corresponded to lower thresholds, whereas higher activity in the right anterior pre-frontal cortex corresponded to higher thresholds.
Conclusions.
Age does not affect performance on this purely visual memory task. However, the neural systems underlying performance change dramatically with age. The neural system related to purely visual memory in young subjects overlaps systems implicated in conventional explicit memory tasks using semantically rich stimuli. The pattern of overall brain activity related to visual memory in the old was orthogonal to that of the young: the neural systems supporting visual memory reorganize with age.

Supported by Alzheimer's Association Grant PRG-95-209, and NSERC OGP0105494 & OGP0042133.