Visual search and the aging brain: discerning the effects of age-related brain volume shrinkage on alertness, feature binding, and attentional control

Abstract

Objective: Decline in visuospatial abilities with advancing age has been attributed to a demise of bottom-up and top-down functions involving sensory processing, selective attention, and executive control. These functions may be differentially affected by age-related volume shrinkage of subcortical and cortical nodes subserving the dorsal and ventral processing streams and the corpus callosum mediating interhemispheric information exchange.

Method: Fifty-five healthy adults (25-84 years) underwent structural MRI and performed a visual search task to test perceptual and attentional demands by combining feature-conjunction searches with "gestalt" grouping and attentional cueing paradigms.

Results: Poorer conjunction, but not feature, search performance was related to older age and volume shrinkage of nodes in the dorsolateral processing stream. When displays allowed perceptual grouping through distractor homogeneity, poorer conjunction-search performance correlated with smaller ventrolateral prefrontal cortical and callosal volumes. An alerting cue attenuated age effects on conjunction search, and the alertness benefit was associated with thalamic, callosal, and temporal cortex volumes.

Conclusion: Our results indicate that older adults can capitalize on early parallel stages of visual information processing, whereas age-related limitations arise at later serial processing stages requiring self-guided selective attention and executive control. These limitations are explained in part by age-related brain volume shrinkage and can be mitigated by external cues.

Figure 1

Design of the visual search task with four conditions: feature search and organized, disorganized and cued conjunction search. For cued conjunction search for display purposes, cue and search display are shown here superimposed, whereas in the actual task the cue appeared 150 ms prior (and not simultaneously) to the search display. Specific effects were visual load calculated as the difference between high load (8 set size) and low load (4 set size) [visual load = 8 – 4 items], cue validity calculated as the difference between invalid and valid trials [validity = invalid –valid], with benefits for valid cueing calculated as the difference between neutral and valid trials [benefit = neutral – valid], cueing costs as the difference between invalid and neutral trials [cost = invalid – neutral], and alertness calculated for low and high visual-load conditions as the difference between uncued and neutrally-cued trials [alertness = no cue – neutral cue].

Figure 2

Visual-load, validity and alertness effects: Z-transformed mean reaction times for A) feature search and organized, disorganized, and cued conjunction searches; and for B) valid, neutral, invalid and uncued conditions. Data were collapsed over target and non-target trials.

Figure 3

Correlation between age and processing speed (A) and visual search-specific effects: (B) visual load, (C) cue validity, and (D) alertness. Removal of outliers did not affect statistical significances.

Figure 4

Correlations between regional brain volumes (ROIs) and specific conjunction-search effects: Visual load for organized and disorganized displays, cue validity, and alertness for high load disorganized conjunction search conditions. Removal of outliers did not affect statistical significances.