Washington: Researchers at the Sainsbury Wellcome Centre at UCL examined the reciprocal relationships between two brain areas that encode visual working memory in mice.
The findings of the research were published in the journal 'Nature'.
The researchers discovered a co-dependency on instantaneous timeframes in the communication between these two working memory sites, the parietal cortex and the premotor cortex.
"There are many different types of working memory and over the past 40 years, scientists have been trying to work out how these are represented in the brain. Sensory working memory, in particular, has been challenging to study, as during standard laboratory tasks many other processes are happening simultaneously, such as timing, motor preparation, and reward expectation," said Dr Ivan Voitov, Research Fellow in the Mrsic-Flogel lab and first author on the paper. To overcome this challenge, the SWC researchers compared a working memory-dependent task with a simpler working memory-independent task. In the working memory task, mice were given a sensory stimulus followed by a delay and then had to match the next stimulus to the one they saw prior to the delay.
This meant that during the delay the mice needed a representation in their working memory of the first stimulus to succeed in the task and receive a reward. In contrast, in the working memory-independent task, the decision the mice made on the secondary stimulus was unrelated to the first stimulus.
By contrasting these two tasks, the researchers were able to observe the part of the neural activity that was dependent on working memory as opposed to the natural activity that was just related to the task environment.
They found that most neural activities were unrelated to working memory, and instead working memory representations were embedded within 'high-dimensional' modes of activity, meaning that only small fluctuations around the mean firing of individual cells were together carrying the working memory information.
To understand how these representations are maintained in the brain, the neuroscientists used a technique called optogenetics to selectively silence parts of the brain during the delay period and observed the disruption to what the mice were remembering.
Interestingly, they found that silencing working memory representations in either one of the parietal or premotor cortical areas led to similar deficits in the mice's ability to remember the previous stimulus, implying that these representations were instantaneously co-dependent on each other during the delay.
To test this hypothesis, the researchers disrupted one area while recording the activity that was being communicated back to it by the other area. When they disrupted the parietal cortex, the activity that was being communicated by the premotor cortex to the parietal cortex was largely unchanged in terms of average activity.
However, the representation of working memory activity specifically was disrupted. This was also true in the reverse experiment when they disrupted the premotor cortex and looked at the parietal cortex and also observed working memory-specific disruption of cortical-cortical communication. "By recording from and manipulating long-range circuits in the cerebral cortex, we uncovered that working memory resides within co-dependent activity patterns in cortical areas that are interconnected, thereby maintaining working memory through instantaneous reciprocal communication," said Professor Tom Mrsic-Flogel, Director of the Sainsbury Wellcome Centre and co-author on the paper. —ANI
