Pointing at an object may not seem complicated, but even such a simple act requires an intricate network of brain activity. Scientists traditionally thought this network included a one-way “information highway” between the brain’s visual system and its motor and sensory systems, but research at Washington University School of Medicine in St. Louis now challenges this long-held theory.
The study presents surprising evidence that the brain’s visual system is not only responsible for seeing, or perceiving, objects outside the body, but also is involved when individuals sense and manipulate their own bodies.
Such insight may help scientists understand puzzling disorders like anosognosia, which is characterized by unusual perceptual experiences. For example, individuals with this disorder may not recognize their arms as part of their own bodies.
“Vision apparently is far more complicated and integrated than we suspected,” says Maurizio Corbetta, M.D., associate professor of neurology, of radiology and of anatomy and neurobiology. “Areas thought to be exclusively involved in perceiving the world around us apparently are involved in integrating visual, spatial and sensory-motor signals to help each of us develop an internal representation of our body and its position in space.”
Corbetta, who also is head of stroke and brain injury rehabilitation at the School of Medicine and the Rehabilitation Institute of St. Louis, led the study, which was published in the May 2004 issue of the journal Nature Neuroscience. The first author is Serguei Astafiev, Ph.D., research associate in radiology. Gordon Shulman, Ph.D., staff scientist in neurology, also is an author.
When a person points at an object, the brain has to see the object, determine where the person and the object are in space, command the arm to move in the correct direction and coordinate muscles to execute that directive.
According to the traditional theory, the visual system, located in the back of the brain, perceives an object in space and sends that visual information to the brain areas that control movement, located toward the front of the brain. Those areas then command the body to move and adjust those actions based on sensory feedback.
Corbetta and his colleagues were trying to determine which parts of the brain are involved in planning an action in space when they discovered unexpected activity in a peripheral part of the visual system known as the extrastriate body area (EBA). This region recently had been implicated in the perception of other people’s bodies, but there was no evidence that it also was involved in an individual’s perception of self.
They decided to investigate further. Using functional magnetic resonance imaging (fMRI), the researchers took brain scans of volunteers doing one of three tasks. Each task started out the same: Participants stared at a fixation point in the center of a diamond shape on a computer screen. Then either the left or the right half of the diamond briefly flashed, followed by an asterisk on the same side of the screen.
Instructions on what to do when the asterisk appeared were different for the three tasks. In the first condition, participants were asked to notice the asterisk when it appeared, but to continue fixating on the center of the screen; in the second task, participants were asked to look at the asterisk when it appeared, using the flash of light beforehand to prepare to move their eyes; in the third task, participants pointed to the asterisk, again using the brief flash to get ready to make their response.
The team found that the EBA was more active during the pointing task than when participants either looked at or noticed the asterisk, suggesting the EBA plays a role in planning and executing the pointing motion.
“We were very surprised by these findings,” Corbetta says. “Visual areas are not supposed to be involved in actions. We always thought information travels from the back of the brain, where vision is processed, to the front of the brain, where actions originate. Moreover, it is quite intriguing that an area involved in coding other people’s bodies also responds when you move your own body. Some researchers have speculated that the EBA is where a distinction between others and ourselves is beginning to be coded in the brain.”
To make sure this phenomenon did not result from the fact that participants could see their hands moving during the pointing task, a second group of volunteers performed two of the original three tasks — either pointing to the asterisk or simply noticing it. This time, though, participants were situated in the scanner so that they could not see their hands move.
Even without visual feedback, the EBA was significantly more active during the pointing task than during the attention task, and the effect did not depend on whether volunteers used their right or left hand. When participants were asked to point to the asterisk with their feet instead of their hands, the EBA also became more active than during the attention task, though not as much as in the hand-pointing task.
The EBA also was more active when participants pointed at the asterisk than when they prepared themselves to point at it, demonstrating that EBA activity was modulated specifically by action, not by mental imagery.
“Showing that EBA activity is present even when people don’t see their hands or their feet is the key part of this study, illustrating that this is not a perception effect but rather is the result of a motor or sensory signal going back to the visual system,” Corbetta explains.
Activity induced by moving a body part or the eyes also was found in two regions, the calcarine sulcus and lingual gyrus, which are considered part of the “primary” and “secondary” visual areas located even deeper in the visual system. The fact that areas at the heart of the visual system are activated during tasks involving movements of the arms or eyes was surprising.
“Our paper demonstrates for the first time that visual areas are influenced by the execution of motor actions, even in the absence of visual feedback,” Astafiev says. “That should help us understand how the brain performs goal-directed behavior.”
Astafiev SV, Stanley CM, Shulman GL, Corbetta M. Extrastriate body area in human occipital cortex responds to the performance of motor actions. Nature Neuroscience, vol. 7(5), May 2004.
Funding from the National Institutes of Health supported this research.
The full-time and volunteer faculty of Washington University School of Medicine are the physicians and surgeons of Barnes-Jewish and St. Louis Children’s hospitals. The School of Medicine is one of the leading medical research, teaching and patient care institutions in the nation, currently ranked second in the nation by U.S. News & World Report. Through its affiliations with Barnes-Jewish and St. Louis Children’s hospitals, the School of Medicine is linked to BJC HealthCare.