Shifting attention in space is a fundamental biological function. Without attention, it would be impossible to scan the environment and to detect prey, predators, or conspecifics. However, attention does not only allow detecting a stimulus of interest. It can be deployed across regions of space that are devoid of any stimulus, or within boundaries defined by an object. While experiments with human participants suggest that different mechanisms may underlie space-based and object-centered attention it has been impossible so far to determine whether both also rely on distinct brain regions.
A neuropsychological study conducted at the University Hospital Geneva now sheds light on this question by proposing a neuroanatomical model of the interactions between space-based and object-centered processing. Previous research has shown that the right cerebral hemisphere has a dominant role for spatial attention. Patients with damage to the right frontoparietal cortex show severe deficits in deploying attention to the left and appear magnetically attracted by salient stimuli shown in their right visual field.
The new study tested patients with right hemisphere lesions on four tasks that differentially recruit space-based and object-centered processing. The first two tasks required participants to find and cancel out tiny targets dispersed among distractors on a sheet of paper. Such visual search tasks are known to rely on space-based exploration. In the third task, patients had to bisect lines of different lengths. Finally, they were asked to read words scattered on a sheet of paper. Since lines and words are processed as visual entities the latter two tasks are thought to recruit object-centered attention.
The researchers determined to what extent the lesions of each individual patient overlapped with one of four brain regions that are known to be of particular importance for visual attention: the superior and inferior parietal lobe, the middle frontal cortex, and the insula. These lesion data were then entered in a statistical model as possible predictors of performance in the behavioral tasks.
The results were clear-cut: while performance on cancellation tasks was predicted by an underlying ‘space-based’ factor, bisection and reading were exclusively related to a distinct ‘object-centered’ factor. The crucial question, however, was whether these two factors were causally linked to damage in one of the four brain regions. The results showed that space-based attention only correlated with damage to the superior parietal cortex. In contrast, the involvement of the inferior parietal cortex was a selective predictor of object-centered performance. Neither the middle frontal cortex nor the insula contributed significantly to space-based or object-centered attention.
These findings thus support the distinction between space-based and object-centered processing, both in terms of cognitive mechanisms as well as the underlying brain anatomy. They provide a framework for understanding how space-based and object-centered attention interact and how they are implemented in the neural architecture of the human brain.
These findings are described in the article entitled A neuroanatomical model of space-based and object-centered processing in spatial neglect, published in the journal Brain Structure and Function. This work was led by Radek Ptak from the University of Geneva.