Visual Search - Relationship With The Parietal Cortex

Relationship With The Parietal Cortex

Visual search can proceed efficiently or inefficiently. During efficient search, performance is unaffected by the number of distractor items. The reaction time functions are flat, and the search s is assumed to be a parallel search. In contrast, during inefficient search, the reaction time to identify the target increases linearly with the number of distractor items present. The posterior parietal cortex is involved during inefficient visual search. Patients with parietal lesions are impaired during inefficient but not efficient search directed to the opposite side of space to the lesion. Brain-imaging studies have also shown the activation of the posterior parietal cortex during visual spatial orienting.

All visual search tasks, as determined by Nobre et al., (2003), activated an extensive network of cortical regions in the parietal, frontal, and occipital cortex and the cerebellum. Multiple regions within the posterior parietal cortex were activated bilaterally, including the superior and inferior parietal lobules, and the intraparietal sulcus. Activation of multiple regions within the posterior parietal cortex further suggests multiple functional contributions by different parietal areas.

Studies using only covert visual search conditions found enhanced activation in multiple posterior parietal areas and in frontal areas during inefficient relative to efficient visual search. Therefore, the participation of posterior parietal and frontal brain areas in visual search are not constrained to their involvement in eye movements. Whereas, studies using overt visual search conditions have shown that areas in the superior parietal cortex and intraparietal sulcus are more active during inefficient visual search.

Findings by Nobre et al., (2003) confirmed that the posterior parietal cortex is indeed involved with visual search and that activation of the parietal lobe is mainly sensitive to the degree of efficiency in visual search but less sensitive to the binding of features. Search efficiency has a more substantial effect on participating brain regions in visual search than feature binding. Search efficiency exerts enhanced activity bilaterally in the superior parietal lobule, intraparietal sulcus, and in the right angular gyrus, in inefficient relative to efficient search conditions. Enhanced activity is also found in the frontal, occipital, and cerebellar regions. Frontal activations include the right dorsolater prefrontal cortex and bilateral ventrolateral premotor/prefrontal cortex. Feature binding exerts only sparse effects on brain activations. Search for conjunction targets compared to search for feature targets activates small clusters in the superior parietal lobule, which overlap with the activation from search efficiency. No brain region is selectively activated only by conditions requiring the binding of features.

The engagement of attention on a target is controlled by the pulvinar nucleus in the thalamus which blocks input from unattended stimuli. The superior colliculus is involved in the movement of attention from one location to another and the disengagement of attention is governed by the parietal lobe so that another stimulus can be processed.