The impact of simulated hemianopia on visual search for faces, words, and cars.

Tests of visual search can index the effects of perceptual load and compare the processing efficiency for different object types, particularly when one examines the set-size effect, the increase in search time for each additional stimulus in an array. Previous studies have shown that the set-size effect is increased by manoeuvres that impede object processing, and in patients with object processing impairments. In this study, we examine how the low-level visual impairment of hemianopia affects visual search for complex objects, using a virtual paradigm. Forty-two healthy subjects performed visual search for faces, words, or cars with full-viewing as well as gaze-contingent simulations of complete left or right hemianopia. Simulated hemianopia lowered accuracy and discriminative power and increased response times and set-size effects, similarly for faces, words and cars. A comparison of set-size effects between target absent and target present trials did not show a difference between full-view and simulated hemianopic conditions, and a model of decision-making suggested that simulated hemianopia reduced the rate of accumulation of perceptual data, but did not change decision thresholds. We conclude that simulated hemianopia reduces the efficiency of visual search for complex objects, and that such impairment should be considered when interpreting results from high-level object processing deficits.

Visual search for complex objects: Set-size effects for faces, words and cars.

To compare visual processing for different object types, we developed visual search tests that generated accuracy and response time parameters, including an object set-size effect that indexes perceptual processing load. Our goal was to compare visual search for two expert object types, faces and visual words, as well as a less expert type, cars. We first asked if faces and words showed greater inversion effects in search. Second, we determined whether search with upright stimuli correlated with other perceptual indices. Last we assessed for correlations between tests within a single orientation, and between orientations for a single object type. Object set-size effects were smaller for faces and words than cars. All accuracy and temporal measures showed an inversion effect for faces and words, but not cars. Face-search accuracy measures correlated with accuracy on the Cambridge Face Memory Test and word-search temporal measures correlated with single-word reading times, but car search did not correlate with semantic car knowledge. There were cross-orientation correlations for all object types, as well as cross-object correlations in the inverted orientation, while in the upright orientation face search did not correlate with word or car search. We conclude that object search shows effects of expertise. Compared to cars, words and faces showed smaller object set-size effects, greater inversion effects, and their search results correlated with other indices of perceptual expertise. The correlation analyses provide preliminary evidence supporting contributions from common processes in the case of inverted stimuli, object-specific processes that operate in both orientations, and distinct processing for upright faces.