Neural Shape-Up: Brain anticipates object perception

Many neuroscientists suspect that in order to see, a person first sorts through edges, contours, and other basic visual features using a brain area called the primary visual cortex. Then so-called higher visual areas of the brain assemble these features into perceptions of shapes and objects.

However, according to a new investigation, at least one of these higher areas shapes what we see from the get-go, and it does it by actually suppressing activity in the primary visual cortex.

This give and take of higher and lower parts of the visual system allows the brain to use predictions about the world, generated from experience and presumably stored in neural structures, to perceive objects under often ambiguous circumstances, propose neuroscientist Scott O. Murray of the University of California, Davis and his coworkers.

“Higher visual areas anticipate the structure of incoming visual information, and if the input matches what’s expected, neural activity declines in the primary visual cortex,” Murray says.

This is surprising, he adds, because feedback from one brain region to another is typically thought to enhance, rather than inhibit, neural activity. The new findings will appear in the Proceedings of the National Academy of Sciences.

Murray’s group used functional magnetic resonance imaging to measure blood-flow changes in the lateral occipital complex (LOC), a higher visual area involved in object recognition, and in the primary visual cortex. Jumps and dips in blood flow provide indirect markers of neural activity.

In three experiments, a total of 18 adults viewed lines and dots that were perceived as either meaningless jumbles or geometric shapes. Jumbles elicited substantially more activity in the primary visual cortex than in the LOC. In contrast, coherent shapes–especially three-dimensional ones–yielded much stronger LOC responses accompanied by diminished blood flow in the primary visual cortex.

In the first experiment, volunteers saw haphazard lines and drawings of two-dimensional and three-dimensional shapes.

In the second experiment, participants saw three computer displays: a set of stationary, randomly arrayed dots; randomly arrayed dots rotating on an axis around drawings of cubes and other shapes to create three-dimensional perceptions; and dots moving at random.

The third experimental stimulus consisted of a moving diamond on a computer screen, with the diamond’s four corners invisible to different degrees. The diamond’s four visible segments then moved from side to side on the screen. Volunteers sometimes perceived segments as moving haphazardly and at other times, as moving together as a diamond. All experimental stimuli can be seen at http://www.demos.kersten.org/.

The new findings suggest that the primary visual cortex sifts through basic visual elements until the LOC recognizes an object and terminates most low-level activity, remarks neuroscientist Michael S. Beauchamp of the National Institute of Mental Health in Bethesda, Md. Still, he says, other researchers need to confirm the results and see whether a similar pattern of electrical responses occurs in visual areas.

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