Behavioral and Quantitative Anatomical Studies of the Primary Visual Cortex in the Aging Rhesus Monkey PDF Download

Author: Eustathia Lela Giannaris
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Languages : en
Pages : 236

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Book Description
Abstract: Mild declines in visual function with age are observed in both humans and monkeys, independent of ocular pathology, suggesting an involvement of structures along the central visual pathway. Electrophysiological studies of cells in the primary visual cortex of the aging rhesus monkey have demonstrated age-related decreases in orientation and direction selectivity of cortical neurons (Schmolesky et al., 2000). These physiological impairments improve with the administration of drugs that enhance the inhibitory neurotransmitter GABA (Leventhal et al., 2003). In order to quantify behavioral declines in visual function with age in the rhesus monkey, a novel behavioral metric was derived from scores on subsets of the delayed recognition span task (DRST) that differ only in the visual salience of the stimuli. This metric demonstrated an age-related decline in visual function across the lifespan (R=-0.288, P=0.001 one-tailed, N=110). While many factors might account for this decline, a loss of neurons in primary visual cortex could be responsible by disrupting the balance between principal neurons and the GABAergic interneurons. To investigate this, in a sample of 26 behaviorally tested male and female rhesus monkeys ranging from 7.4 to 31 years of age, design-based stereology was used to quantify total numbers of neurons, glia, and parvalbumin-positive GABAergic interneurons across the entire primary visual cortex, as well as within three laminar zones: supragranular (layers II-IVB), granular (layer IVC), and infragranular (layers V-VI). Results demonstrated preservation of total neuron numbers (R=-0.018 P=0.931 two-tailed, N=26) as well as the parvalbumin-positive GABAergic interneurons (R=0.171, P=0.624 two-tailed, N=26). In contrast to the stability of neuron numbers, there was a significant increase in the total number of glia in the infragranular layers of the cortex (R=0.459, P=0.018 two-tailed, N=26). Interestingly, an increase in glial density correlated with decreased visual function as assessed by the DRST behavioral metric (R=0.446, p=0.028 two-tailed, N=24). This observation likely reflects sublethal pathology to myelinated axons, a ubiquitous feature of normal aging, as they enter and exit through the infragranular layer of primary visual cortex. Further studies of this area are needed to fully understand the implications of glial increases.