Christopher P. Taylor

Aging reduces center-surround antagonism in visual motion processing.

Discriminating the direction of motion of a low-contrast pattern becomes easier with increasing stimulus area. However, increasing the size of a high-contrast pattern makes it more difficult for observers to discriminate motion. This surprising result, termed spatial suppression, is thought to be mediated by a form of center-surround suppression found throughout the visual pathway. Here, we examine the counterintuitive hypothesis that aging alters such center-surround interactions in ways that improve performance in some tasks. We found that older observers required briefer stimulus durations than did younger observers to extract information about stimulus direction in conditions using large, high-contrast patterns. We suggest that this age-related improvement in motion discrimination may be linked to reduced GABAergic functioning in the senescent brain, which reduces center-surround suppression in motion-selective neurons.

The effect of aging on the orientational selectivity of the human visual system.

Leventhal et al. (Science, 2003, 300(5620), 812-815) reported that orientation selectivity of V1 neurons was significantly reduced in older macaque monkeys, which suggests that mechanisms that encode orientation in humans may become more broadly tuned in old age. We examined this hypothesis in two experiments that used sine-wave masking and notched-noise masking to estimate the bandwidth of orientation-selective mechanisms in younger (age approximately 23 years) and older (age approximately 68 years) human adults. In both experiments, the orientation selectivity of masking was essentially identical in younger and older subjects.

The effect of aging on the spatial frequency selectivity of the human visual system

Changes in the physiological properties of senescent V1 neurons suggest that the mechanisms encoding spatial frequency in primate cortex may become more broadly tuned in old age (Zhang et al., European Journal of Neuroscience, 2008, 28, 201-207). We examined this possibility in two psychophysical experiments that used masking to estimate the bandwidth of spatial frequency-selective mechanisms in younger (age approximately 22years) and older (age approximately 65years) human adults. Contrary to predictions from physiological studies, in both experiments, the spatial frequency selectivity of masking was essentially identical in younger and older subjects.

Spatial frequency summation in visual noise

Kersten [Vision Res. 27, 1029 (1987)] reported that absolute efficiency for the detection of static, one-dimensional bandpass noise was high and approximately constant for stimulus bandwidths ranging from 1 to 6 octaves. This result implies that human observers integrated information efficiently across a wide range of spatial frequency. One interpretation of this result--and similar results obtained with auditory stimuli [J. Acoust. Soc. Am. 32, 121 (1960)]--is that human observers, like ideal observers, can detect stimuli using an internal filter that has an adjustable bandwidth. The current experiments replicate Kerstens findings, extend them to the case where observers are uncertain about stimulus bandwidth, and use the classification image technique to estimate the filter used to detect noise stimuli that differ in bandwidth. Our results suggest that observers do not adjust channel bandwidth to match the stimulus and that detection thresholds are consistent with the predictions of a multiple-channel model.