Visual discrimination in noise: behavioural correlates of age-related cortical decline.
Neurophysiological evidence has consistently shown that, compared to younger animals, single neurons in aged mammalian visual cortex exhibit reduced selectivity to stimulus direction and orientation. It has been suggested that this may be due, in part, to increased internal noise in the aged visual system. This study measured contrast thresholds for judging the motion direction (left vs. right) and spatial orientation (vertical vs. horizontal) of sinusoidal gratings presented alone and with additive noise in young (20-29 years) and older (65-79 years) adults. Compared to their young counterparts, older adults demonstrated reduced sensitivity to direction and orientation when no noise was present. However, when gratings were presented in conjunction with additive noise, older adults required a greater increase in external noise to elicit a corresponding reduction in sensitivity. Subsequent analysis assessing equivalent noise and sampling efficiency attributed performance differences to an increase in equivalent noise with age.