Two kinds of bias in visual comparison illustrate the role of location and holistic/analytic processing differences

A number of studies have shown that two stimuli appearing successively at the same spatial location are more likely to be perceived as the same, even though location is irrelevant to the task. This bias to respond “same” when stimuli are at the same location is termed spatial congruency bias. The experiments reported here demonstrate that the spatial congruency bias extends to letter strings: Participants tend to respond “same” when comparing two strings appearing successively at the same location. This bias may arise because successive stimuli at the same location are more likely to be perceived as a single object. Bias is also affected by the nature of the comparison task. We show that if letters must be compared individually (analytical comparison), there is a bias to respond “different,” but if letter strings are compared as unified wholes (holistic comparison), there is no bias or a bias to respond “same.” This analytical bias is apparently separate from the spatial congruency bias. It appears whether the task requires localization of differences between strings, or counting the number of differences, or ignoring differences in some parts of the stimuli while attending to others. All of these analytical comparison tasks require that letters be selected individually, and the analytical bias may reflect difficulty in preventing interference from neighboring letters in this selection process. Each type of bias reflects a different aspect of visual processing, and both can be measured to probe how processing changes across different tasks.