Orientation Feature Binding in Pigeons
Type of Degreethesis
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Manipulating conditions under which humans search for a target is a standard approach for studying perceptual errors. An important early finding was that visual search for a combination of features (e.g., color, orientation, shape, and size) was much slower than a search for any given single featurej (e.g., just color). This slow-down during “conjunction” searches is a major component of the Feature Integration Theory of Attention. The theory posits that visual scene processing is composed of two separable stages: a pre-attentive segregation phase and an attention-mediated integration phase. The second stage is slower as it requires focused attention, thus ensuring that searches requiring integration of features will take longer than single-feature searches that can be conducted in the absence of focused attention. If attention can be interrupted before features are properly integrated, or if insufficient time for binding is allowed, errors in binding may occur. First termed “illusory conjunctions,” the existence of binding errors in human visual perception has been extensively studied, but the conditions that give rise to them are still debated. To date, no evidence for binding errors has been observered for non-human animals. Past research has demonstrated the slow down for conjunction targets (as compared to single-feature targets) in pigeons. The current study is a first attempt to determine systematically if pigeon target search can be affected by binding errors. Through several phases, pigeons learned to search for a plus sign (+) in a two-item target search using vertical or horizontal bars as distractors. After acquiring the task, the stimulus viewing time was titrated until accuracy was within the 70-80% range. To demonstrate the presence of binding errors, false alarm rates were compared between trials in which a binding error would not be possible (feature trials; trials with only horizontal or only vertical bars) to trials in which a binding error could have occured (conjunction trials; one horizontal and one vertical bar). The results of the current study failed to show unequivocal evidence of binding errors in pigeon visual perception. Instead, under the reduced viewing time condition, most birds decre ased responding to the “target present” response, lowering both target-present accuracy and false alarm rates. This shift in response profile renders any conclusions tentative, but despite the decreased frequency of false alarms, some birds committed more errors on conjunction trials than on feature trials. Future research should consider the role of reinforcement history in solving the target search as well as the possibility of a species difference in the processing of visual scenes between pigeons and humans.