Oscillations of the Granular Retrosplenial Cortex

Abstract

The retrosplenial cortex is essential for spatial memory and navigation. We sought to learn about how the retrosplenial cortex encodes information through oscillations. Our experiment results reveal an oscillation pattern we call “splines”, resembling the similarly-named interlocking teeth on mechanical gears. Splines are 110-160 Hz, precisely coupled to the peaks of local theta rhythms, and observed during both REM sleep and active awake behaviors. We found that splines are distinct from gamma rhythms. While gamma rhythms are in-phase across the two retrosplenial hemispheres, splines are anti-phase across the hemispheres. By sorting theta cycles by either spline or gamma power, we show that retrosplenial splines and gamma oscillations occur independently of each other within any given theta cycle. Splines are also distinct from sharp wave ripples and alternate with sharp wave ripples across REM and NREM sleep, respectively. At higher running speeds, splines become more powerful, more strongly phase-amplitude coupled to theta, and have greater cross-hemispheric coherence. The retrosplenial cortex’s ability to rapidly switch between splines and gamma as distinct modes of rapid interhemispheric communication may allow this region to more effectively integrate information using two mechanistically distinct rhythms.