Posterior parietal cortex and the filtering of distractors

Abstract

Neural systems for visual processing can focus attention on behaviorally relevant objects, filtering out competing distractors. Neurophysiological studies in animals and brain imaging studies in humans suggest that such filtering depends on top-down inputs to extrastriate visual areas, originating in structures important for attentional control. To test whether the posterior parietal cortex may be a necessary source of signals that filter distractors, we measured the ability of a patient with bilateral parietal lesions to discriminate the features of a target surrounded by distractors of variable contrast. In the presence of distractors, the patient was impaired at discriminating both grating orientation and faces, and the magnitude of the impairment increased with distractor salience. These attentional deficits are remarkably similar to those caused by damage to monkey extrastriate regions V4 and͞or TEO, which are thought to be recipients of top-down attentional feedback. In contrast to the effects of V4 and TEO lesions, however, the parietal lesions impaired performance even with widely spaced targets and distractors, a finding consistent with the projections of parietal cortex to visual processing areas covering a wide range of receptive field sizes and eccentricities.A typical visual scene contains many different objects, not all of which can be fully processed at any given moment. Attentional mechanisms are therefore needed to focus visual processing on the most behaviorally relevant stimuli and to filter out competing distractors. A possible mechanism for this resolution of competition between objects has been described in ventral stream visual areas of monkeys, where neurophysiological studies have found that in the absence of attention, multiple stimuli in the receptive field (RF) of a cell will compete for the response of the cell. However, when attention is directed to a target stimulus in the RF, responses are biased in favor of the target, and the influence of distracting stimuli in the RF is filtered out (1-11). Likewise, brain imaging studies in humans show a similar biasing of competition in favor of attended stimuli (12)(13)(14)(15)(16)(17)(18)(19)(20), and human subjects often experience little awareness for distractor stimuli outside the focus of attention (21-23).Lesion data also support the idea that ventral stream areas are sites where top-down inputs bias the competition in favor of attended targets compared with unattended distractors. Both monkeys with lesions of areas V4 and͞or TEO and humans with lesions of area V4 are impaired at visual discrimination tasks when target stimuli are presented in the presence of salient visual distractors (24)(25)(26). Consistent with the neurophysiological data, the attentional filtering impairments after these extrastriate lesions are limited to configurations in which both the targets and distractors are located close to one another, within an area equal to the average size of V4 and͞or TEO RFs. When normal filtering mechanisms are compromised, the pre...

Language regions of brain are operative in color perception

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Impaired attentional selection following lesions to human pulvinar: Evidence for homology between human and monkey

“…are attributable to involvement of dorsal parts of pulvinar, which is known in primates to have neurons with bilateral receptive fields (15) and reciprocal connections with posterior parietal cortex (16). Indeed, a patient with bilateral parietal lobe damage has also been shown to perform poorly on the same filtering task used here (42).…”

Prioritizing new over old: An fMRI study of the preview search task

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