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Research over the last three decades has yielded a wealth of information about the neural mechanisms underlying vision. Dozens of cortical visual areas have been characterized (Fig. 1), and the visual information encoded by neurons has been shown to differ greatly between areas (1). (Figure 1 omitted) Whereas neurons in the primary visual area V1 (striate cortex) respond well to edges or bars of light, those at later stages of processing represent increasingly complex aspects of the retinal image (2). Neurons in later stages of the visual cortex can be extremely selective, responding only to specific, complex forms or patterns of motion (3). Thus, vision is supported by levels of cortical processing that collectively cover a range of stimulus attributes, from simple to complex. A widely held view is that the primary reason for these multiple levels is to generate this range of sensory representations.
Creating representations of the retinal image is, however, just one component of vision. Vision is an active process that selects a limited part of the visual image for concentrated attention. Although unselected portions of the image are not lost to perception, at any moment we can give full attention only to a severely limited amount of visual information (4). Once this subset of signals has been selected, it must then be interpreted. Thus, the events leading to visual awareness include a substantial editing process that de-emphasizes irrelevant information and adds interpretations and inferences about the meaning of the targeted information.
Studies of macaque monkeys have shown that this editing of visual signals begins in relatively early stages of processing in the cerebral cortex. What the observer is trying to see and what that observer knows about the visual scene have considerable impact on what is represented in the visual cortex. These studies show that most areas in the visual cortex, even those at relatively early stages of processing do not give equal weight to all parts of the retinal image. Instead, they preferentially represent those elements to which the observer is paying attention. As more studies have examined the influence of attention on cortical representations, it has come to be seen as an increasingly important factor in determining patterns of activity in the visual cortex.
These findings are changing the...