The effect of aging on visual orientation and spatial frequency

Abstract

<p> Although nearly one third of the Canadian population is projected to be over the age of 65 by the year 2030, we know relatively little about how aging affects brain function generally, let alone how aging affects visual perception. The current dissertation was conducted as part of a research programme designed to better characterize how aging affects visual perception. </p> <p> Older persons exhibit a variety of deficits for perception of complex visual forms. The perception of these complex forms-including everyday forms such as faces and objects-is subserved by low-level channels that are selective, or tuned, for the orientation and spatial frequency of luminance-defined contours in the visual scene. The bandwidth of these channels is inversely related to the amount of information that they can pass on to higher visual processes; narrowly-tuned channels are better. Single-cell physiological investigations of primates suggest that visual cortex neurons thought to subserve these channels exhibit broader tuning in senescence. If these channels become broadly-tuned in older aging, this could explain age-related deficits for complex form perception. In Chapters 2 and 3 of the current thesis, I measured the tuning of these channels in otherwise healthy, older humans using psychophysical masking techniques. In Chapters 4 and 5, I measured the average tuning of the neurons thought to underlie these channels in older human adults, physiologically, using electroencephalography (EEG). Despite the aforementioned reports of functional decline in senescent neurons, psychophysical and physiological orientation and spatial frequency tuning did not differ between younger and older adults. One explanation for this discrepancy is that there is a methodological issue in the single-cell primate literature wherein anesthetics interact with senescence to produce seemingly broader neural tuning. Another explanation is that older humans do have otherwise detuned neurons and channels, but are able to tune their neurons and channels by the action of consciousness, attention, or age-related compensatory brain reorganization. </p>