Increased levels of the neurotransmitter GABA have in the past been shown to produce greater neural plasticity. Here researchers link higher GABA levels in one part of the brain with a greater capacity of working memory. This preliminary finding is a very long way from being the basis for some form of therapy for age-related loss of memory function or enhancement technology to improve memory at all ages, but that is where investigations of the biochemistry of memory will eventually lead.
Working memory is the brain function that lets you carry on a phone conversation while adding three numbers in your head and remembering that you need to steer the car onto the freeway exit in about two minutes - all this time not forgetting who you're talking to. Working memory serves as a buffer where information, derived from the senses or retrieved from long-term memory, can be temporarily placed so the conscious brain can process it. It's tied to assessments of cognitive capacity such as IQ, and to real-world outcomes such as academic performance. As most people eventually find out, working memory declines with age.
A new study teases apart three key components of working memory and shows that one component, but not the other two, is tied to the amount of a chemical called GABA in a brain area known as the dorsolateral prefrontal cortex, or DLPFC. This component, referred to as load, is a measure of the number of separate bits of information a person's working memory can store at the same time. A second component, maintenance, denotes how long information can be stored in working memory before it's lost. A third, distraction resistance, gauges how well an individual's working memory holds onto information in the face of interfering stimuli. The DLPFC, a broad swath of neural tissue on the forebrain surface, has been shown in animal studies and in observations of brain-damaged patients to be integral to high-level executive functions in the brain, such as planning, prioritizing and avoiding distractions. It has likewise been strongly implicated in working memory. The DLPFC orchestrates activity in numerous distant centers throughout the brain, including the visual cortex, which is located near the brain's surface but in the hindbrain.
In the study, 23 healthy participants ages 19-32 were subjected to batteries of tests of working memory. The researchers reasoned that different components of working memory would involve different neurotransmitter inputs. They devised working-memory tests that separated the measurement of load, maintenance and distraction resistance. Participants repeated several related tasks. In the simplest, they were shown a drawing of a face and then, after a two-second delay, shown a second face and asked whether it was the same as or different from the first one. Variations of this task - initially presenting two faces instead of just one; lengthening the intervening delay; or displaying a different, irrelevant face between the initial and final displays - tested load, maintenance and distraction resistance, respectively. The investigators compared individuals' error rates on the simple version of the task with outcomes on tasks taxing one or another working-memory component more heavily. The smaller the deterioration in performance on a test of a particular working-memory component, the greater the individual's capacity regarding that component was judged to be.
Using an advanced imaging method, the scientists measured GABA levels in the DLPFC and, for comparison, in the visual cortex. GABA, secreted by nerve cells, is an inhibitory neurotransmitter: Its uptake by other nerve cells inhibits their firing. The researchers also measured levels of an excitatory neurotransmitter, glutamate. By far the two most abundant neurotransmitters in the brain, GABA and glutamate are considered to be that organ's stop and go signals. Individuals with higher levels of GABA in their DLPFC performed better on tests of their load capacity - the ability to juggle more bits of information - the researchers found. In contrast, no significant association emerged linking GABA levels in the DLPFC to maintenance or to distraction resistance, or tying participants' load capacity to GABA levels in the visual cortex. Nor did imaging reveal any connection between performance on tests of load capacity and levels of glutamate in the DLPFC.