Modulation of Visual Processing by Attention

Leslie G. Ungerleider, PH.D
Laboratory of Brain and Cognition, NIMH
Bethesda, Maryland 20892

Abstract

A typical scene contains many different objects that compete for neural representation due to the limited processing capacity of the visual system. Evidence from functional magnetic resonance imaging (fMRI) studies in humans indicates that the competition among multiple objects for representation in visual cortex can be biased by both bottom-up sensory-driven mechanisms and by top-down influences. For example, stimulus salience, such as a stimulus of high contrast, provides a bottom-up bias favoring neurons that represent the salient stimulus at the expense of less salient stimuli. Top-down influences on visual cortex, as in the case of selective attention, can also bias the competition and even override sensory-driven inputs. We have found that biasing signals due to selective attention can affect neural processing in several ways, which include: 1) the enhancement of neural responses to attended stimuli; 2) the filtering of unwanted information by counteracting the suppression induced by nearby distracters; and 3) the biasing of signals in favor of an attended location by increases of baseline activity in the absence of visual stimulation. Thus, attentional modulation of activity in visual cortex can occur not only in the presence, but also in the absence, of visual stimulation. Although the competition among objects for representation is ultimately resolved within visual cortex, the source of top-down biasing signals derives from a network of areas in frontal and parietal cortex. Our results indicate that attention-related activity in these areas does not reflect attentional modulation of visually evoked responses, but rather the attentional operations themselves. Ultimately, the object that wins the competition for representation in visual cortex will gain further access to memory systems for mnemonic encoding and to motor systems for guiding behavior.

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