Top — Down Control of Visual Perception: Attention in Natural Vision

Abstract

IntroductionPre-emptive perception can be used to describe a situation in which a motor command influences perception. An example might be a command for a saccadic eye movement, in which a corollary discharge cancels the changed sensory input produced by the movement of the eyes, contributing to stability of visual perception (Wurtz and Sommer 2004). Here, I consider a somewhat different situation in which prior expectations influence visual perception. These prior expectations might consist of top^down attentional influences on object or spatial processing, or they might consist of prior knowledge of the world which can influence what is seen, as in, for example, the perception of a Necker cube as three-dimensional. A particular focus of this paper is on how attention operates in natural, cluttered, visual scenes, where its influence is much more restricted than when two objects are presented on a blank background. The neurophysiological approach to how attention operates in natural vision leads to hypotheses about the binding problem in perception; about how object representations are interfaced to action systems; and about change blindness. It is shown that even cognitive influences produced by a word descriptor can by top^down processing`pre-empt' affective perception in the olfactory, taste, flavour, somatosensory, and emotional systems, by a top^down modulation of representations in the orbitofrontal cortex. These top^down effects do not pre-empt perception in the sense that they override it. Indeed, it is important that bottom^up inputs dominate the processing to avoid hallucinatory activity. Instead, top^down processes can bias visual perception, in ways for which we now have clear computational-neuroscience models at the level of integrate-and-fire neurons, which show that the top^down effects are most evident when the bottom^up input is weak or ambiguous. The processes described here provide a way for understanding not only top^down effects in attention, but also top^down effects that are Abstract. Top^down perceptual influences can bias (or pre-empt) perception. In natural scenes, the receptive fields of neurons in the inferior temporal visual cortex (IT) shrink to become close to the size of objects. This facilitates the read-out of information from the ventral visual system, because the information is primarily about the object at the fovea. Top^down attentional influences are much less evident in natural scenes than when objects are shown against blank backgrounds, though are still present. It is suggested that the reduced receptive-field size in natural scenes, and the effects of top^down attention contribute to change blindness. The receptive fields of IT neurons in complex scenes, though including the fovea, are frequently asymmetric around the fovea, and it is proposed that this is the solution the IT uses to represent multiple objects and their relative spatial positions in a scene. Networks that implement probabilistic decision-making are described, and it is suggested that, when in perceptua...

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