For example, it has been known for some time that when a stroke damages the right side of the parietal cortex (the upper posterior part of our brain) the result is often "neglect," a bizarre syndrome in which the patient seems unaware of the left side of space.

This unawareness appears to be caused by an attention deficit; neglect patients can see the left side of space but fail to notice it; they "forget" or "ignore" objects or people on their left side. This has led scientists to speculate that the right side of the parietal cortex mediates attentional processing of the left visual field.

A case study in today's issue of Neuron challenges this classic view by reporting that patients with lesions in this area of the brain also show more subtle attention deficits in their “good” right visual field.

In their study, Lorella Battelli at Harvard University and colleagues assessed the visual perception of several patients with a battery of tests. While most of these tests were consistent with the traditional view and showed visual impairment in the left visual field only, the researchers found one exception.

When two dots are flashed sequentially at slightly different locations, a motion illusion occurs: we think the dot has moved from the first location to the second.

Surprisingly, Battelli's patients were unable to perceive this illusion -- they saw independently flashing dots instead of moving dots -- not only when it was presented in their left visual field but also when it was presented in their right one.

Thus, the right parietal cortex would appear to be particularly important for this perceptual illusion. The effortless nature of this illusion appears to recruit a new attentional mechanism that involves both halves of the brain.

Further studies will determine if patients with left parietal lesions suffer similar deficits, and will seek to understand the nature of this attentional deficit in detail.

[Contact: Lorella Battelli]

20-Dec-2001