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| Image: MRI scans revealed short term "working" memory in action within the premotor cortex, as volunteers participated in an experiment to test data selection from working memory. Credit: Badre lab/Brown University |
For example, imagine you're on the phone with a client at the office, and the receptionist buzzes you to say a group has arrived for a tour of the factory. Meanwhile, you have to send an email to a colleague to say you won't be able to join her for lunch. While you're wrapping up the call, the three demands upon your attention - your client, tour, lunch with a colleague - are held within your working memory; upon ending your call, you'll choose one and act upon it.
At this point, you're recruiting neural circuits running from a specific region withing your emotion-generating limbic system (a part of the striatum known as the caudate) and your brain's executive control center, the prefrontal cortex (in a specific region called the dorsal anterior premotor cortex).
Because of its color, the striatum takes its name from the Latin word for "striped", and the caudate means "tail shaped", because of its shape, which resembles a shrimp. The striatum uses dopamine as a sort of biochemical valve to regulate neural signaling. Dopamine generates positive sensations, a reward that reinforces learning.
For example, when you see a delicious piece of cake, or your best friend, a surge of dopamine allows for freer signal flow throughout your brain and nervous system, resulting in feelings of elation or energized excitement. These positive sensations are central to learning and behavior selection - if an experience feels good, you want to repeat it. The same reward channel is at work in nearly all animal life - even such comparatively primitive organisms as fruit flies.
The caudate comprises a major part of the striatum, and is involved in voluntary actions, memory, learning, sleep, and social interactions. Studies indicate it also appears to be central in OCD - obsessive compulsive
disorder, in which the sufferer attempts to banish repetitive, unwanted thought processes (obsessions) through ritualistic behaviors over which she has difficulty with self-control (compulsions). It is thought the caudaute and another key behavior-related region called the ACC serve as "error checking" circuits, alerting one to a "mistake" in one's thoughts or behavior.
"Dorsal" means "toward the spine", while anterior means "toward the forepart". The premotor area helps plan and coordinate motor activity - muscle contractions for movement. What this means is that choosing from options within working memory recruits some of the same neural circuits used in planning movement.
Brown University Drs. Badre, Chatham, and Frank, conducted a lab experiment upon 22 adults, using magnetic resonance imaging to monitor brain activity while the participants engaged in a working memory exercise. The team measured how quickly the volunteers were able to select data from within their working memory, a process the team dubbed "output gating".
According to Dr. Badre, our limited working memory allows us to hold onto small bits of particularly useful data while engaged in other tasks, at which point our brain engages in output gating or choosing upon which data we will act. Coupled with input gating - choosing what to hold in working memory - output gating enables us to continue along a given course of action, with the capacity to hold options for what to do afterwards.
Both input and output gating occur in different regions, but the experiment in question was concerned specifically with output gating - the choice of behavior. During the experiment, the subjects watched a character sequence comprised of either alphabetic letters or symbols called wingdings. At either the start or the end of the task, they were then given a number to indicate which character in the sequence and which group - either alphabetic or symbolic - was relevant. The group of wingdings was assigned the number 1, while the group of letters was assigned the number 2.
When the relevancy number came before the sequence, the subjects would input gate (select to remember) only letters, and then output gate the correct letter from the sequence held in their working memory. When the relevancy number came after the sequence, the subjects would need to "input gate" (select to remember) everything, meaning the real cognitive operations consisted of output gating, and they needed to hold more characters in their working memory.
Because of the difference in mental power required, the experiment required subjects to respond to an additional situation where "3" meant they had to remember everything they saw, whether the hint came before or after the sequence. In this way, brain activity could be monitored specifically during the input gating sequence and the output gating sequence.
The subjects were able to finish the tasks at a variety of speeds, which was understood in the experiment as the amount of cognitive work the task required. The slowest responses came when subjects had to respond to the relevancy number after the sequence and select a single specific symbol from memory (for example, after seeing a 1, choosing a wingding from the sequence).
The fastest responses came when subjects were given the relevancy number before the sequence and chose a specific symbol based upon the cue (for example, after seeing a 2, choosing a letter that would follow in the sequence).
MRI scans showed the caudate and dorsal anterior premotor cortex were significantly related to participant reaction times. Says Chatham, the striatum is central to gating the data, much like a traffic lights signal along a route, while the cortex interprets the context.
Source: Study reveals workings of working memory, Press Release, February 19, 2014, David Orenstein, Brown University
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