Measuring correlations and interactions among four simultaneously recorded brain regions during learning.

Brain function depends on coordinated interactions in spatially distributed neuronal populations. Thanks to recent technological advances, it is now possible to monitor the activity of large groups of neurons. Although significant progress has been made in analyzing neuronal interactions across large samples of simultaneously recorded cells, most of the available approaches do not allow direct visualization of multidimensional correlations in the time domain. This study describes a novel analysis technique, termed four-dimensional spike-triggered joint histogram (4-d STJH) that permits the study of co-modulations of unit activity across four simultaneously recorded brain regions while preserving the time domain. To illustrate how this technique works, we recorded simultaneously from basolateral amygdala (BLA) and medial prefrontal (mPFC), as well as perirhinal and entorhinal neurons in animals learning an appetitive trace-conditioning task. Using the 4d-STJH, we show that coincident activity in the BLA and mPFC modulates the interactions between perirhinal and entorhinal neurons in a manner that cannot be explained by a linear combination of the individual BLA and mPFC-related modulations. We conclude with a discussion of the strengths and limitations of 4-d STJH and offer recommendations regarding optimal conditions for its use.