The traditional view is that perceptual information is processed in early sensory regions of the brain, such as primary visual cortex, before being passed on to memory systems subserved by “high-order” brain areas, such as the prefrontal and inferotemporal cortices. Aside from being an oversimplification, a further problem with this account is that the the high-order brain regions don’t have the precision to handle visual detail, thus leading to the conundrum of how exactly we store visual detail in short-term memory.
Past research has shown that activity in primary visual cortex often decays rather rapidly after a visual stimulus disappears, thus seeming to preclude these regions from serving a memory function. However, a new study by psychologists Stephenie Harrison and Frank Tong suggests that early visual processing areas do in fact hold visual information in storage – a finding that suggests we need to have re-think about the functional roles we ascribe to early visual cortex.
Harrison and Tong scanned the brains of six volunteers while they watched two “gratings” (i.e. scrolling bars; see image above) presented in succession on a computer screen, each with a different orientation. A cue then indicated which grating should be remembered, and finally a third test grating appeared after a delay and the participants had to say whether the test grating matched the earlier grating they were cued to remember.
Remarkably, the researchers were able to decode the orientation of the to-be-remembered grating using activity patterns they’d recorded from the participants’ early visual cortices during the retention period, between the disappearance of the first two gratings and the appearance of the test grating. And they were able to do this with astonishing accuracy (80 per cent and upwards, where chance performance would be 50 per cent). This remained true even though activity levels in these regions dropped dramatically for half the participants – as has been found in past research.
“This indicates that maintaining an orientation in working memory is associated with widespread changes in orientation selective activity throughout the early visual system, including V1 [primary visual cortex], the first stage of orientation processing,” the researchers said.
In a second experiment, the researchers again recorded activity patterns in the participants’ early visual cortices when they were exposed to gratings, but this time the gratings were irrelevant to the task at hand. The patterns of activity in early visual cortex triggered by the irrelevant gratings matched closely the patterns found when the gratings were held in memory in the first experiment. In other words, storing an orientation in working memory seems to recruit many of the same orientation-selective neuronal subpopulations as when the same grating orientation is merely observed but not attended to.
The researchers said: “We find that early visual areas are not only important for processing information about the immediate sensory environment, but can also maintain information in the absence of direct input to support higher-order cognitive functions.”
Stephenie A. Harrison, Frank Tong (2009). Decoding reveals the contents of visual working memory in early visual areas. Nature DOI: 10.1038/nature07832