Each side of the brain has its own memory

Each brain hemisphere has its own discrete visual working memory store. At least that's what a new study with two rhesus monkeys suggests (PNAS: tinyurl.com/62wz4jx). The monkeys had to look at an array of coloured squares, there was an 800 to 1000ms delay, then a second array appeared, the same as the first except one of the squares had changed colour. The monkeys' task was to look at the square that had changed colour. The more squares there were in the arrays, the more difficult the task became.

Whilst the monkeys performed the task, Timothy Buschman at MIT and his colleagues used electrodes to record from individual neurons in the parietal and frontal cortices of the monkeys' brains.

The monkeys' performance suggested a memory capacity of 3.88 squares (human capacity is typically around 4 squares). Beyond that number their performance accuracy dropped off abruptly. Most intriguing, adding extra squares to one side of the screen didn't affect performance for the other side, suggesting that each hemisphere has its own independent capacity. This independence was also confirmed at a neuronal level.

Whilst memory capacity was divided separately between the hemispheres, another finding was that memory capacity within each hemisphere was divided in a graded fashion - each extra item diminished the memory for all items, rather than memory slots being filled in an all-or-nothing fashion.

By comparing frontal and posterior brain activity for when there were fewer versus more squares, the researchers further showed that memory capacity was limited by a 'bottom up' failure to encode more squares, rather than by a 'top down' failure, in the prefrontal cortex, to store already encoded information.

"Our results suggest that visual capacity limits result from competition for encoding within independent, but limited pools of neural information that can each be divided among multiple objects," the researchers wrote.

In a university press release Buschman said the new findings have practical applications: "For example, heads-up displays [of the kind used by drivers or pilots] show a lot of data. Our results suggest that you want to put that information evenly on both sides of the visual field to maximize the amount of information that gets into the brain."