Complex visual processing in older adults

Abstract

Declines in performance on complex visual tasks have been shown to predict declines in functional independence and mobility in older adults, but the specific perceptual processing mechanisms that decline with age and lead to functional declines are not well understood. Two measures of perception shown to predict functional outcomes (e.g. safe driving) in older adults are measures of the Functional Field of View (FFOV) and of global coherent motion. Age-related change in capacity for the deployment of visual attention and sensitivity of the sub-cortical magnocellular (M) pathway were hypothesised to explain the FFOV and motion processing in older adults. The thesis therefore had four aims: 1) to investigate differences in the capacity of older and younger adults to use bottom-up and top-down attention in the performance of a FFOV task; 2) to investigate the association between FFOV performance and the sensitivity of the M pathway, particularly in older adults; 3) to investigate differences in the capacity of older and younger adults to use top-down attention to facilitate the perception of coherent motion; 4) to investigate the association between motion coherence thresholds and the sensitivity of the M pathway, particularly in older adults. Study 1 (Chapter 4) aimed to explore how manipulating attentional demands impacted the error rates for a FFOV task in older adults, and how the effect of age on the FFOV varied under different attentional conditions. A specially designed FFOV task with differing levels of bottom-up and top-down attentional demands was conducted using a sample of 42 younger (M = 27.38 years, SD = 5.41 years, 21 women) and 42 older (M = 72.11 years, SD = 5.92 years, 23 women) adults. Older adults had higher error rates in all conditions and the effect of age did not vary across conditions indicating similar capacity to attend to and ignore salient peripheral stimuli in older and younger adults. The performance cost of dividing attention between central and peripheral vision in the older group indicated older adults narrowed the focus of attention for an attentionally demanding central task, at the expense of accuracy on the peripheral task. The results indicate that the need to compensate for reduced sensory processing by manipulating the focus of attention contributes to reduced FFOV efficiency in older adults. Study 2 (Chapter 5) investigated the degree to which FFOV performance (from Study 1) could be explained by the sensitivity of the M pathway. FFOV error rates and contrast sensitivity thresholds were measured for 44 younger (M = 27.18 years, SD = 5.40 years, 22 women) and 44 older (M = 72.18 years, SD = 5.82 years, 23 women) adults. Contrast sensitivity was measured using the steady and pulsed pedestal task (Pokorny & Smith, 1997). Older adults performed more poorly on all measures. In older adults, M pathway contrast sensitivity explained a significant proportion of variance in FFOV error rates. These findings support the hypothesis that rapid bottom-up visual attention is reliant on M pathway input, at least in older adults. Study 3 (Chapter 6) investigated differences in coherent motion thresholds between older and younger men and women, in the presence and absence of a top-down attentional cue. In addition, the extent to which motion coherence thresholds could be explained by sensitivity of the M pathway was explored. Motion coherence thresholds were measured for 40 younger (M = 26.05 years of age, SD = 5.47 years, 20 men, 20 women) and 35 older (M = 72.11 years of age, SD = 5.92 years, 17 men, 18 women) adults using a dot motion task in which half the dots were white and half were black. In conditions with no attention cue, motion coherence thresholds were higher in older than younger adults and higher in women than in men. Addition of a cue alerting participants to which dots (black or white) contained the coherent motion signals, improved coherence thresholds for younger women and older men, but not for younger men and older women. The cue helped younger women form a coherent percept of motion but was redundant for younger men. The absence of improvement in older women may reflect resource limitations. Perception of coherent motion is more demanding for women and may require increased top-down attention, which will impact older women because older adults have limited top-down attentional resources. Differences in motion coherence thresholds within the older group, and between younger and older adults, were associated with increased contrast discrimination thresholds in the M and P pathways suggesting that with the tasks used in the current study, reduced coherent motion perception in older adults, is not simply attributable to a reduction in sensory sensitivity specific to the M pathway. The results indicate that the FFOV and motion coherence tasks assess different perceptual processes that change with age and are impacted in different ways by changes in visual attention and the sensitivity of the M pathway. These findings are important given the increasing proportion of older adults in modern society who rely on functional vision for mobility and independence.