Rapid switching of in-phase and anti-phase interhemispheric rhythms within a single theta cycle: a mechanistic explanation
Abstract
The granular retrosplenial cortex (RSG) supports memory consolidation and spatial navigation. In vivo recordings of local field potential (LFP) from RSG reveal coexisting gamma (30-80 Hz) and spline (100-160 Hz) oscillations, coupled to distinct phases of the background theta (∼8 Hz) rhythm. Remarkably, bilateral recordings reveal that gamma and splines exhibit divergent interhemispheric phase relationships: gamma is in-phase across hemispheres, while splines are anti-phase. Here, using numerical bifurcation analysis of next-generation neural mass models, we show that RSG oscillations within a single hemisphere can be understood as alternation of PING (pyramidal-interneuron gamma) and ING (interneuron gamma) mechanisms in an excitatory-inhibitory network. Theta-frequency external drive to the inhibitory population switches the network between these two rhythms. Next, we build canonical interhemispheric circuits, composed of two such excitatory-inhibitory modules, delay-coupled across hemispheres. These circuits switch between in-phase gamma and anti-phase splines within a single theta cycle, reproducing the cross-spectral statistics of experimental RSG recordings. Finally, we present analytical results for threshold-linear neural mass models, showing cross-hemispheric phase patterning is governed by the ratio of cross-hemispheric delay to local inhibitory delay; we obtain explicit ranges of this ratio for which RSG-like patterning could be expected. Our work establishes that the unique oscillatory profile of RSG can be explained parsimoniously by minimal interhemispheric circuit configurations and provides a framework for experimental interrogation of interhemispheric rhythms.
Significance statement: The granular retrosplenial cortex (RSG) contributes to our sense of spatial orientation. Recordings from RSG reveal nested rhythmic activity, with two fast rhythms, gamma and splines, embedded within a slower theta rhythm. Intriguingly, gamma rhythms are synchronized across hemispheres (in-phase), while splines are antisynchronized (anti-phase): spline peaks in one hemisphere correspond with troughs in the other. Here, we present and analyze mathematical models which capture how the observed oscillation patterns can arise out of simple neural circuit configurations. These models make testable predictions about what network architectures could produce the oscillations, and how the oscillations are controlled by external inputs to RSG, supporting future experiments which aim to better understand left/right brain communication in health and disease.
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BibTeX
@misc{kailasa2026rapid,
title = {{Rapid switching of in-phase and anti-phase interhemispheric rhythms within a single theta cycle: a mechanistic explanation}},
author = {Sameer Kailasa and Izabela Jedrasiak-Cape and Daniele Avitabile and Victoria Booth and Omar J. Ahmed},
howpublished = {bioRxiv},
year = {2026},
month = oct,
doi = {10.64898/2026.10.23.761406},
url = {https://www.biorxiv.org/content/10.64898/2026.10.23.761406v1},
note = {CC BY-NC-ND},
}
RIS
TY - UNPB
TI - Rapid switching of in-phase and anti-phase interhemispheric rhythms within a single theta cycle: a mechanistic explanation
AU - Sameer Kailasa
AU - Izabela Jedrasiak-Cape
AU - Daniele Avitabile
AU - Victoria Booth
AU - Omar J. Ahmed
PY - 2026
DA - 2026/10/23
PB - bioRxiv
DO - 10.64898/2026.10.23.761406
UR - https://www.biorxiv.org/content/10.64898/2026.10.23.761406v1
AB - The granular retrosplenial cortex (RSG) supports memory consolidation and spatial navigation. In vivo recordings of local field potential (LFP) from RSG reveal coexisting gamma (30-80 Hz) and spline (100-160 Hz) oscillations, coupled to distinct phases of the background theta (∼8 Hz) rhythm. Remarkably, bilateral recordings reveal that gamma and splines exhibit divergent interhemispheric phase relationships: gamma is in-phase across hemispheres, while splines are anti-phase. Here, using numerical bifurcation analysis of next-generation neural mass models, we show that RSG oscillations within a single hemisphere can be understood as alternation of PING (pyramidal-interneuron gamma) and ING (interneuron gamma) mechanisms in an excitatory-inhibitory network. Theta-frequency external drive to the inhibitory population switches the network between these two rhythms. Next, we build canonical interhemispheric circuits, composed of two such excitatory-inhibitory modules, delay-coupled across hemispheres. These circuits switch between in-phase gamma and anti-phase splines within a single theta cycle, reproducing the cross-spectral statistics of experimental RSG recordings. Finally, we present analytical results for threshold-linear neural mass models, showing cross-hemispheric phase patterning is governed by the ratio of cross-hemispheric delay to local inhibitory delay; we obtain explicit ranges of this ratio for which RSG-like patterning could be expected. Our work establishes that the unique oscillatory profile of RSG can be explained parsimoniously by minimal interhemispheric circuit configurations and provides a framework for experimental interrogation of interhemispheric rhythms. Significance statement: The granular retrosplenial cortex (RSG) contributes to our sense of spatial orientation. Recordings from RSG reveal nested rhythmic activity, with two fast rhythms, gamma and splines, embedded within a slower theta rhythm. Intriguingly, gamma rhythms are synchronized across hemispheres (in-phase), while splines are antisynchronized (anti-phase): spline peaks in one hemisphere correspond with troughs in the other. Here, we present and analyze mathematical models which capture how the observed oscillation patterns can arise out of simple neural circuit configurations. These models make testable predictions about what network architectures could produce the oscillations, and how the oscillations are controlled by external inputs to RSG, supporting future experiments which aim to better understand left/right brain communication in health and disease.
SN - 2692-8205
ER -