Posts Tagged ‘cross-frequency coupling’

Phase and Frequency of Activity Associated with Long Term Memory

September 15, 2019

This post is based on an important book by Scott D. Slotnick titled “Cognitive Neuroscience of Memory.” Remember to consult the website
to see the anatomical information referred to in this post.

Frequency refers to the rate of change in magnitude over time. Frequencies can be low, changing slowly over time, or high, changing rapidly over time. Brain activity time courses can be considered from a frequency perspective, with lower frequencies corresponding to slower changes in signal over time, and higher frequencies corresponding to the faster changes in signal over time. Certain frequencies of brain activity have been associated with memory and have been linked to particular brain regions. Specifically, memory has been associated with brain activity that oscillates in the theta frequency band (4 to 8 Hertz), the alpha frequency band (8 to 12 Hertz) and the gamma frequency band (greater than 30 Hertz). In the fields of visual perception and visual attention, gamma activity is known to reflect binding of features that are processed in different cortical regions (such as shape and color). Gamma activity is a mechanism that underlies the perception of unified objects. Theta activity (4 to 8 Hertz) reflects the interaction the hippocampus and cortical regions have during long-term memory, and alpha activity reflects cortical inhibition.

In addition to modulation of activity within theta, alpha, and gamma frequency bands during memory, there is evidence that brain regions with different frequencies of modulation can be in phase with each other. This is called cross-frequency coupling and indicates two brain regions interact. In a long-term memory electroencephalography (EEG) study, participants viewed picture of objects during the study phase and then during the test phase were presented with old and new pictures of objects and made “remember’ “new” judgments. Subsequently remembered items as compared to subsequently forgotten items were associated with an increase in beta activity in right frontal regions, a decrease in alpha activity in anterior and posterior regions, and an increase in gamma activity in parietal and occipital regions (from 300 to 1300 milliseconds after stimulus onset). Moreover, there was greater cross-frequency coupling for subsequently remembered than subsequently forgotten items between frontal theta activity and parietal-occipital gamma activity. The identical pattern of results for theta activity and gamma activity was observed with the same experimental protocol during memory retrieval. Based on the known role of gamma activity in visual perception and attention, it can be assumed that the increase in parietal-occipital gamma activity in these studies reflected an increase in visual object processing associated with remembered items, and frontal theta activity may have modulated the gamma activity. Of special importance, the cross-frequency coupling evidence suggests that frontal regions and parietal-occipital regions interacted during long-term memory encoding and retrieval.

To summarize succinctly, theta activity reflects the interaction between the hippocampus and cortical regions during long-term memory, alpha activity reflects cortical inhibition, and gamma activity reflects process of features in different cortical regions that are combined to create a unified memory.