Theta Frequency Reset in Memory Scanning

In a 1998 article in the Journal of Neuroscience, authors Jensen & Lisman describe how the "7 plus or minus two" capacity limit of short-term memory might arise from a specific type of oscillatory neural network in prefrontal cortex. Specifically, as I have mentioned before, items in working memory would become serially active during each successive gamma cycle, and by the end of a theta cycle, this process would repeat. The capacity of STM, then, is dependent on the interaction of theta and gamma oscillations.

For the skeptics in the audience, remember that recent evidence has shown that this kind of multiplexing "sequence compression" is known to occur in hippocampus, and so this form of neural computation is not as outlandish as it might otherwise seem.

In order to explain behavioral data from the Sternberg memory task (which I discussed earlier this week as being sensitive to variations in brainwave oscillations), a few complications to this basic account are necessary. First, memory scanning must be initiated at the trough of a theta cycle. Second, the theta period must increase with increasing memory load (so that more gamma cycles can fit within each theta cycle). In other words, dominant theta frequencies should decrease with increasing memory load, from a maximum of 10Hz to a minimum of 6Hz.

Alternatively, the phase of theta oscillations can be reset rather than requiring theta frequencies to adapt with memory load. This means that whatever pattern generator creates the driving theta frequency would essentially change the phase of the theta rhythm at the start of a memory scanning process. This model requires that dominant theta frequencies fall within the range of 4 to 7 Hz.

A more recent article by Jensen & Tesche, from the European Journal of Neuroscience, suggests that this latter model is more likely to be correct, given that theta frequencies actually remain constant during the Sternberg task, and the amplitude increases with load. Other recent evidence also supports the idea that theta-clocked oscillations reset on the presentation of probe stimuli.

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