When a monkey has to remember something, it holds that thought in its mind's eye, a new study suggests. A low-level part of the visual system just one step removed from the retina buzzes while a monkey maintains a visual memory--a power once ascribed only to more sophisticated parts of the brain.
Earlier memory research showed that neurons in higher order brain regions--such as the frontal lobes--fire madly when monkeys (or people) remember something briefly. In contrast, the primary visual cortex was once thought to simply sort incoming lights and shadows before passing basic information on to higher brain centers for interpretation.
But the new research suggests that primary visual cortex is more than a mere conduit. Its neurons can continue to fire even when they don't see light and shadow, apparently because they're retaining a visual memory. Hans Supèr and colleagues at the University of Amsterdam in the Netherlands taught monkeys to watch a computer screen filled with flickering black and white pixels. A small, rectangular patch of pixels somewhere in the monkey's peripheral vision would occasionally jerk to one side and then quickly vanish into the background flicker. Then, after as long as 2 seconds, the monkey moved its eyes to where the patch had been and earned a treat.
The team monitored primary visual cortex neurons that were tuned to the spot where a patch sometimes flashed. The neurons continued firing even after the patch disappeared, presumably when the monkey was keeping the location in mind. If the firing dwindled, the monkeys often made a mistake--apparently because they forget where the patch had been, the researchers report in the 6 July issue of Science.
The team suggests that primary visual cortex neurons are communicating with other areas of the monkey's brain that are responsible for understanding the task and formulating a plan to respond. The neurons contribute by holding onto the exact location that has to be remembered. As vision scientist Jeffrey Schall of Vanderbilt University in Nashville, Tennessee, puts it, primary visual cortex "is a lot smarter than it used to be."