Reaching for your morning coffee may feel like the simplest task of your day, but a new study led by York University Professor Doug Crawford reveals it relies on a critical brain mechanism that matches vision to action.
Left: Doug Crawford
"What we’ve found is the key place in the brain where you go from the visual map to the motor map," said Crawford, professor of psychology and York’s Canada Research Chair in Visual-Motor Neuroscience at York’s Centre for Vision Research. "Normally it’s hard to draw a firm line between these two stages. We made this line clear by measuring brain activity while people looked through optical prism lenses that reverse vision left-for-right. Our results are useful for stroke rehabilitation and might also help us to understand why some dyslexic people have trouble distinguishing left from right."
The study, published online in the journal Cerebral Cortex on Wednesday, Jan. 10, is a collaborative effort between Crawford and other scientists working at York and the University of Western Ontario*. It answers a question that has vexed scientists and surgeons for years: Where in the brain does visual perception stop and control of movement begin?
Doctors understand that damage to the visual cortex – located at the back of the head – produces fairly basic deficits in the visual map. And they know that damage to the primary motor cortex – located near the top of the head – produces problems with controlling muscles. However, it’s much harder to understand damage to the intermediate area of the brain called the parietal cortex. Damage to the parietal cortex results in strange deficits: a stroke patient with right parietal damage may, for example, completely ignore left visual space or have problems reaching to the left.
"For years, neurologists, neurosurgeons and neuropsychologists have wondered if this is a problem with vision or movement," said Crawford.
To test this, Crawford’s team used a brain imaging technology called Functional Magnetic Resonance Imaging (fMRI) to locate a parietal area that only "lights up" when subjects point or reach toward visual targets. Then they trained subjects to point while looking through reversing prisms. Under normal circumstances, the left parietal reach region is activated when a person points to a target on the right. The question posed by the research team was what would happen in the brain after participants had adapted to the reversing prism?
The study found that after prism adaptation, the left parietal cortex was no longer activated for rightward pointing – it was now activated for pointing to the left. In other words, even though the region is just activated for reaching, it also cares about where you see the object (which was reversed with the prism). This shows that the parietal cortex does not code vision or movement per se, said Crawford, but rather something in between like the visual goal for a movement.
Crawford is now using the study results in collaboration with Toronto-based doctors to better understand the symptoms of stroke patients with damage to the parietal cortex and to rehabilitate these patients.
*The study, "Human parietal ‘reach region’ primarily encodes intrinsic visual direction, not extrinsic movement direction, in a visual-motor dissociation task," was a collaborative effort by principal investigator Crawford, University of Mexico Professor Juan Fernandez-Ruiz (while visiting York), York Professor Joseph De Souza, and University of Western Ontario researchers Herbert Goltz and Tutis Vilis.