But the latest imaging research from his team shows that, in monkeys, this is not the case. “We asked: Can visually driven activity in the region of the primary visual cortex that corresponds to the retinal injury recover to pre-lesional levels in the months following the lesion??said Smirnakis. “The answer is, in that time interval the primary visual cortex did not achieve anything like normal responsivity.?/p>
To arrive at this conclusion, Smirnakis and his group first photocoagulated the retinas of four monkeys with a laser, creating small blind spots on the same sides of the field of vision. The retina sends signals that the brain interprets as light, color, or objects. Each section of the retina corresponds to a specific location in the primary visual cortex. Without any visual signal to interpret, the cortical area corresponding to each monkey's blind spot fell silent, generating no activity.
The team measured the size and shape of each of these cortical quiet spots. They placed the lightly anesthetized monkeys into a fMRI machine, which measures blood flow, and hence, brain activity. With the monkey's eyes held open, the team focused various grid and circle patterns on the animal's retina, centered on the fovea—a small depression in the retina where vision is most acute—and covering the blind spot. They made baseline measurements of the cortical quiet zone two to three hours after the laser surgery and compared them to new readings taken every few weeks for up to seven and a half months.
“If the visual cortex of the monkeys did reorganize, it would happen as they were behaving normally in their cages in between scans,?said Smirnakis. “And then, when we brought them back to the scanner, the region of their cortex corresponding to the blind spot would have shrunk.?Instead, though, the silent region remained the same size each time. The neurons surrounding it did not reach out to fill it in.
Source:Howard Hughes Medical Institute