Abstract
Visual stimuli can be classified so rapidly that their analysis may be based on a single sweep of feedforward processing through the visuomotor system. Behavioral criteria for feedforward processing can be evaluated in response priming tasks where speeded pointing or keypress responses are performed toward target stimuli which are preceded by prime stimuli. We apply this method to several classes of complex stimuli. (1) When participants classify natural images into animals or non-animals, the time course of their pointing responses indicates that prime and target signals remain strictly sequential throughout all processing stages, meeting stringent behavioral criteria for feedforward processing (rapid-chase criteria). (2) Such priming effects are boosted by selective visual attention for positions, shapes, and colors, in a way consistent with bottom-up enhancement of visuomotor processing, even when primes cannot be consciously identified. (3) Speeded processing of phobic images is observed in participants specifically fearful of spiders or snakes, suggesting enhancement of feedforward processing by long-term perceptual learning. (4) When the perceived brightness of primes in complex displays is altered by means of illumination or transparency illusions, priming effects in speeded keypress responses can systematically contradict subjective brightness judgments, such that one prime appears brighter than the other but activates motor responses as if it was darker. We propose that response priming captures the output of the first feedforward pass of visual signals through the visuomotor system, and that this output lacks some characteristic features of more elaborate, recurrent processing. This way, visuomotor measures may become dissociated from several aspects of conscious vision. We argue that “fast” visuomotor measures predominantly driven by feedforward processing should supplement “slow” psychophysical measures predominantly based on visual awareness.
Highlights
Complex stimuli, binding, and the fast feedforward sweep Much has been learned in the last decades about the flow of information within a hierarchy of visual areas
We propose that response priming captures the output of the first feedforward pass of visual signals through the visuomotor system, and that this output lacks some characteristic features of more elaborate, recurrent processing
Different binding tasks are probably solved by different mechanisms: while contour assignment and visual search for conjunction targets might require extensive recurrent processing and selective attention (Roelfsema, 2006), basic object classification tasks might be based on feedforward processes converging on high-level object classifiers (Thorpe et al, 1996)
Summary
Complex stimuli, binding, and the fast feedforward sweep Much has been learned in the last decades about the flow of information within a hierarchy of visual areas. Classical evidence that the visual system faces a binding problem comes from visual search experiments, which have shown that it is difficult to find conjunction targets (e.g., a green X) in a clutter of non-targets sharing the defining target features (e.g., red X’s and green O’s), and that single features may be miscombined to form illusory conjunctions (Treisman and Gelade, 1980; Treisman, 1996; Wolfe and Cave, 1999) This problem especially pertains to the encoding of multi-feature objects, such as complex artificial stimuli or natural images. These findings highlight the need to understand the time course of processing in feedforward as opposed to recurrent networks, recognizing that any area might serve different functions at different points in time
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