Hippocampal Theta Waves Synchronize With Paw Movements During Rat String Pulling

Neuroscientists recording hippocampal local field potentials in rats performing a skilled string-pulling task discovered a striking dissociation between fine-motor paw movements and whole-body locomotion. While hippocampal theta oscillations are widely recognized for increasing in frequency and power as rodents run faster across physical or virtual spaces, this canonical speed-tracking relationship completely vanished during the forelimb-dominated string-pulling task. Rather than accelerating alongside vigorous paw motion, hippocampal theta oscillations exhibited a lower fundamental frequency and lacked the robust harmonic structure observed during circular track running. Furthermore, instead of tracking gross movement speed, hippocampal theta power and inter-trial phase locking peaked selectively during the slow, downward push phase of the reach-pull cycle. These experimental findings, detailed in the journal iScience, challenge conventional models of hippocampal function by demonstrating that hippocampal rhythms do not merely encode spatial translation or body velocity, but actively synchronize with discrete sensorimotor micro-actions, effort exertion, and goal-directed limb mechanics.

For decades, empirical investigations have documented that the frequency and amplitude of theta waves correlate positively with running velocity, acceleration, and the spatial displacement of an animal through its environment. Nevertheless, neurobiologists lacked detailed knowledge regarding how hippocampal field potentials operate during continuous, rhythmic fine-motor tasks that require bimanual coordination without translational whole-body locomotion across a physical floor.

To examine whether hippocampal theta dynamics scale with non-locomotor limb velocity, the research team implemented a highly structured string-pulling paradigm paired with in vivo electrophysiological recording. In this experimental setup, five adult male rats were trained prior to surgery to pull at least two meters of string per pulling bout across ten or more bouts within a thirty-minute training session. Bouts were quantitatively detected using a rotary encoder that logged physical string displacement, defining an active bout as continuous string movement lasting at least one second and concluding when string movement ceased for a full second. Animals mastered this behavioral criterion in fewer than eight days before undergoing surgical implantation of recording tetrodes.

Following behavioral pre-training, researchers recorded hippocampal local field potentials while rats alternated between blocks of string pulling and running on a circular track. Each recording session yielded extensive behavioral datasets, with rats executing at least forty-two string-pulling bouts per session, averaging approximately five and a half seconds per bout and cumulating in roughly ninety meters of string pulled per session. Pulling speed gradually decreased across twenty minutes in twelve out of sixteen sessions, likely reflecting progressive physiological fatigue or appetitive satiation. Power spectral density analyses of paw trajectories and rotary encoder string records revealed that string speed oscillated rhythmically at approximately five hertz, whereas paw movements cycled at roughly two and a half hertz. Notably, mean movement speeds between circular track running and string pulling showed no significant statistical difference across animals.

Despite comparable average movement velocities between behaviors, local field potential spectral analyses revealed profound physiological divergences in the hippocampal rhythms. Robust theta band oscillations occurred during both behaviors, yet theta frequency was significantly lower across all five rats during string pulling compared to track running. Remarkably, theta frequency recorded during the fastest string-pulling episodes (approximately sixty centimeters per second, yielding theta near 7.2 hertz) remained lower than theta frequency observed during the slowest track running (ten centimeters per second, yielding theta near 8.2 hertz). Furthermore, the prominent theta harmonic—an oscillation occurring at twice the fundamental frequency, reflecting waveform asymmetry driven by coordinated firing among hippocampal interneurons and pyramidal cells—was significantly attenuated during string pulling. To verify that this harmonic diminution did not simply stem from lower fundamental frequencies, the investigators matched spectral profiles at an identical fundamental frequency of 7.8 hertz, confirming that harmonic power remained significantly diminished during string manipulation.

When the investigators examined the relationship between instantaneous movement speed and hippocampal oscillatory dynamics, the conventional positive correlations completely broke down during the string-pulling task. During circular track running, theta frequency scaled linearly with running speed and acceleration, replicating well-established findings from decades of spatial navigation literature. During string pulling, however, this positive relationship was entirely absent when evaluating instantaneous string velocity and acceleration. When the research team smoothed string speed using a one-second moving average—encompassing approximately four consecutive paw-pulling cycles to reflect slower operational timescales comparable to whole-body center-of-mass translation—they uncovered a statistically significant negative correlation between string velocity and theta frequency. In parallel, analysis of theta power revealed no significant linear slope or correlation with string speed or string acceleration in either behavioral condition. These observations establish that forelimb movement speed within an abstract string space does not drive hippocampal frequency acceleration in the manner characteristic of locomotor navigation.

Recognizing that gross string velocity might obscure microscale paw mechanics, the researchers performed fine-grained kinematic analyses on the paw contralateral to the hippocampal implant. Left paw speed and acceleration varied dramatically across the reach-pull cycle, with velocity peaking during the lift and advance phases and dropping to its lowest values during the pull and push phases. Instantaneous theta measurements revealed that theta frequency and theta power were significantly modulated across these discrete movement categories, reaching distinct peaks specifically during the downward push phase. Continuous circular coordinate mapping of paw trajectories confirmed that theta power peaked systematically during the transitional junction between the pull and push phases.

Beyond amplitude and frequency modulation, the researchers quantified trial-to-trial oscillatory consistency by calculating inter-trial phase coherence across consecutive reach-pull cycles. Theta oscillations demonstrated statistically significant phase locking that peaked tightly around the transition from the pull to the push phase. The researchers validated this finding using Rayleigh z statistics and confirmed its specificity through temporal-shuffle controls, in which shifting local field potentials by a randomized interval abolished phase-locking effects across animals. Because the downward push represents the mechanically demanding phase that directly advances the string to deliver reward, the authors noted that theta phase alignment mirrors previous observations during instrumental operant actions, such as lever presses or button presses. This phase resetting may serve as a temporal gating mechanism, synchronizing hippocampal networks with proprioceptive and somatosensory feedback during the specific sensorimotor actions that directly mediate goal attainment.

At the macroscale level, visual inspection of hippocampal field potentials revealed a sudden, discrete surge in theta frequency at the conclusion of each pulling bout. Wavelet spectrograms aligned to behavioral bout boundaries showed that while theta frequency remained steady during bout initiation, it jumped rapidly by more than one hertz precisely when the bout ended. Behavioral tracking demonstrated that this terminal frequency jump coincided with a sharp increase in nose velocity as rats pivoted their heads and bodies to orient toward a food reward located thirty centimeters behind the string apparatus. Because forward translational locomotion did not commence until at least half a second after the turn, the researchers attributed this transient theta acceleration to the abrupt increase in vestibular stimulation generated by the rapid turning motion. This contrast underscores how head angular acceleration and vestibular input strongly invigorate hippocampal rhythms, whereas isolated forelimb manipulation operates under distinctly attenuated ascending vestibular drive.

The investigators highlighted several methodological limitations that contextualize their conclusions and outline essential directions for subsequent research. First, recordings were conducted using fixed hand-made wire tetrodes rather than high-density silicon probes, preventing current-source density analyses across hippocampal laminar depths and precluding thorough evaluations of theta waveform asymmetry. Second, pharmacological manipulations using cholinergic antagonists such as atropine were not administered, leaving open whether the recorded signals represented atropine-insensitive type I theta or atropine-sensitive type II immobility theta. Third, the study examined exclusively male rats, necessitating future research to evaluate whether sex-dependent differences in oscillatory dynamics or cortico-hippocampal coherence exist in female rodents. Finally, the authors noted that the hippocampus receives bilateral somatosensory inputs through the lateral entorhinal cortex and extensive commissural pathways, meaning that bilateral limb coordination during bimanual pulling likely engages inter-hemispheric sensorimotor integration. Overall, the study demonstrates that hippocampal theta oscillations operate across multiple distinct scales, dynamically decoupling from gross kinematic velocity while synchronizing with fine-motor mechanics and effortful goal-directed manipulation.

Subject of Research: Hippocampal theta oscillation dynamics during rat string pulling and track running

Article Title: Hippocampal theta aligns to paw movements during a forelimb-dominated sensorimotor behavior

Article References: Holguin, G., Jorgensen, K., Tapia, A., Jordan, G. A., Vishwanath, A., Barnes, C. A., & Cowen, S. L. (2026). Hippocampal theta aligns to paw movements during a forelimb-dominated sensorimotor behavior. iScience, 29(12), Article 118167. https://doi.org/10.1016/j.isci.2026.118167

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.118167

Keywords: hippocampal theta, local field potentials, string pulling, sensorimotor integration, phase locking, inter-trial phase coherence, running speed, paw kinematics, rat behavior, CA1, DeepLabCut, oscillatory power, theta harmonic, track running, proprioception, rotary encoder, power spectral density, tetrode recording