Dealing with illumination in visual scenes: effects of ageing and Alzheimer's disease

Abstract

Various visual functions decline in ageing and even more so in patients with Alzheimer's disease (AD). Here we investigated whether the complex visual processes involved in ignoring illumination-related variability (specifically, cast shadows) in visual scenes may also be compromised. Participants searched for a discrepant target among items which appeared as posts with shadows cast by light-from-above when upright, but as angled objects when inverted. As in earlier reports, young participants gave slower responses with upright than inverted displays when the shadow-like part was dark but not white (control condition). This is consistent with visual processing mechanisms making shadows difficult to perceive, presumably to assist object recognition under varied illumination. Contrary to predictions, this interaction of "shadow" colour with item orientation was maintained in healthy older and AD groups. Thus, the processing mechanisms which assist complex light-independent object identification appear to be robust to the effects of both ageing and AD. Importantly, this means that the complexity of a function does not necessarily determine its vulnerability to age- or AD-related decline.We also report slower responses to dark than light "shadows" of either orientation in both ageing and AD, in keeping with increasing light scatter in the ageing eye. Rather curiously, AD patients showed further slowed responses to "shadows" of either colour at the bottom than the top of items as if they applied shadow-specific rules to non-shadow conditions. This suggests that in AD, shadow-processing mechanisms, while preserved, might be applied in a less selective way.

Figure 1. Example stimulus displays for a) the search task with dark shadows, upright; b) the search task with white non-shadows, inverted; c) the detection task.

Each trial began with presentation of a central fixation cross, for a fixed 1 s (a/b) or a variable 1–2 s (c). Displays were then presented until the participant keypress indicating target identification (a/b) or item detection (c). The subsequent display of letters allowed the target location to be reported.

Figure 2. Performance data for each sample group in the search task, showing interaction of shadow type with item orientation.

Upper panel shows errors, middle panel shows raw median RTs and lower panel shows RTs normalised according to single-stimulus RTs. Dark bars/circles represent dark shadows and light bars/triangles, white non-shadows. Error bars represent ±1 SEM.