Loss of brain synchrony may explain working memory limits, says study

Everyday experience makes it obvious -- sometimes frustratingly so -- that our working memory capacity is limited and we can only keep so many things consciously in mind at once. The results of a new study, which is published in the journal Cerebral Cortex, may explain why: The authors suggest that the 'coupling', or synchrony, of brain waves among three key regions breaks down in specific ways when visual working memory load becomes too much to handle. This loss of synchrony means the regions can no longer communicate with each other to sustain working memory.

Maximum working memory capacity -- for instance the total number of images a person can hold in working memory at the same time -- varies between individuals but averages about seven. This new study tries to understand what causes the memory to have this intrinsic limit.

The study's lead author, Dr Dimitris Pinotsis, a lecturer at the Department of Psychology at City, University of London, and a research affiliate at the Department of Brain and Cognitive Sciences at MIT, said: "At peak memory capacity, the brain signals that maintain memories and guide actions based on these memories, reach their maximum. Above this peak, the same signals break down."

As researchers have previously correlated working memory capacity with intelligence, understanding what causes working memory to have an intrinsic limit is important because it could also help explain the limited nature of conscious thought and how it might break down in diseases.

"Because certain psychiatric diseases can lower capacity, the findings could explain more about how such diseases interfere with thinking," said Professor Earl Miller, the study's senior author and the Picower Professor of Neuroscience at MIT's Picower Institute for Learning and Memory. The study's other author is Dr Timothy Buschman, assistant professor at the Princeton University Neuroscience Institute.

To investigate working memory limits, the researchers carried out a detailed statistical analysis of data when animal subjects played a simple game. They had to spot the difference when they were shown a set of squares on a screen and then, after a brief blank screen, a nearly identical set in which one square had changed colour. The number of squares involved, hence the working memory load of each round, varied so that sometimes the task exceeded the animals' capacity.

As the animals played, the researchers measured the frequency and timing of brain waves produced by ensembles of neurons in three regions presumed to have an important -- though as yet unknown -- relationship in producing visual working memory: the prefrontal cortex (PFC), the frontal eye fields (FEF), and the lateral intraparietal area (LIP).

Using sophisticated mathematical techniques, they found that the regions essentially work as a committee, without much hierarchy, to keep working memory going. They also found changes as working memory approached and then exceeded capacity. In particular, the researchers found that above capacity the PFC's coupling to the FEF and LIP at low frequency stopped.

As previous studies have suggested that the PFC's role might be to employ low-frequency waves to provide the feedback the keeps the working memory system in sync, the researchers suggest that when that signal breaks down, the whole enterprise may as well. This observation may also explain why memory capacity has a finite limit.

Professor Miller said: "We knew that stimulus load degrades stimulus processing in various brain areas, but we hadn't seen any distinct change that correlated with reaching capacity, but we did see this with feedback coupling. It drops off when the subjects exceeded their capacity. The PFC stops providing feedback coupling to the FEF and LIP."

The findings could also help optimise heads-up displays in cars and to develop diagnostic tests for diseases like schizophrenia and dementia, among other applications.

"Understanding brain signals at peak load can help us understand the origins of cognitive impairments. This could lead to new therapeutic approaches for people in need, like schizophrenics," said Dr Pinotsis.

The US National Institute of Mental Health and the MIT's Picower Institute Innovation Fund supported this study.