Serial Oscillations and the Frequency Following Response

Several previous posts cover the role of specific frequencies of neural oscillations, in everything from anticipation to face processing. I have also mentioned a neural network model of short-term memory in which multiplexed gamma and theta oscillations give rise to memory capacity limits. A fascinating paper by Burle and Bonnet in Cognitive Brain Research delves into the implications of this "serial oscillation" account by using the Sternberg task in conjunction with the frequency following response.

In the Sternberg task, subjects are presented with a list of items and are later presented with sample items, which they must judge as belonging or not belonging to the previous list. The characteristic result is that reaction time linearly increases with the length of the studied list - in fact, it increases around 40 ms per item. This linear slope has been seen as the "memory scanning time" for a serial processor - but interestingly the RTs are the same for both positive and negative trials, suggesting that the memory search process is necessarily exhaustive.

A recent neural network model by Lisman et al provides an interesting perspective on this data. According to their model, short term memory capacity may arise as an interaction between theta and gamma oscillations, such that each item stored in short-term memory is "refreshed" at a rate of 40 Hz (gamma) once every 100-200 ms (theta). This model can account for many aspects of the RT distributions seen in the Sternberg task, and importantly provides a much-needed connection between prefrontal neural activity and behavioral results.

Burle and Bonnet realized that if these oscillations really are responsible for memory capacity limits, we should be able to manipulate the pace of the oscillations and see an effect on behavior. Consequently, they hypothesized that by playing a repetitive stimulus with a frequency close to that of the neural oscillations, they should be able to shift the neural oscillations slightly in the direction of the external stimulus (this phenomenon is known more generally as the "frequency following response," and is at the heart of the excellent bwgen software).

Therefore, they conducted an experiment in which subjects completed the Sternberg task while auditory 'clicks' were played at frequencies that would be close to 40 Hz (in fact, they used half that frequency, 20 Hz, because of the difficulty for subjects to temporally resolve 40 clicks per second). The results showed that these "click trains" slowed reaction time between 21-21.5 Hz, but quickened reaction time at 22 Hz. This pattern is precisely what one would expect from a system in which some slower click trains slow the neural oscillations important for memory scanning, while slightly faster click trains increase these memory scanning processes.

These results suggest that the pacemaker frequency hypothetically involved in memory scanning has a harmonic somewhere between 21.5 and 22 Hz, and also provides support for the Lisman et al serial oscillatory model of short term memory.

Related Posts:
Entangled Oscillations
Lost Keys: Memory Search Failure
Sequential Order in Precise Phase Timing