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The Journal of Neuroscience, June 14, 2006, 26(24):6518-6522; doi:10.1523/JNEUROSCI.0190-06.2006

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Behavioral/Systems/Cognitive
Exploratory Whisking by Rat Is Not Phase Locked to the Hippocampal Theta Rhythm

Rune W. Berg,¹ Diane Whitmer,^2,3 and David Kleinfeld^1,3,4

¹Department of Physics, ²Division of Biological Sciences, ³Graduate Program in Computational Neurobiology, and ⁴Center for Theoretical Biological Physics, University of California, San Diego, La Jolla, California 92093

Correspondence should be addressed to Dr. Rune W. Berg, Department of Medical Physiology, The PANUM Institute, Room 12.5.5, Blegdamsvej 3, DK-2200 Copenhagen, Denmark. Email: rune{at}mfi.ku.dk

The rat has a strong 6–9 Hz rhythm of electrical activity in the hippocampus, known as the theta rhythm. Exploratory whisking, i.e., the rhythmic movement of the rat’s vibrissas to acquire tactile information, occurs within the same frequency range as the theta rhythm and provides a model system to examine the relationship between theta rhythm and active sensory movements. In particular, it has been postulated that these two rhythms are phase locked as a means to synchronize sensory and hippocampal processing. We tested this hypothesis in rats trained to whisk in air. Theta activity was measured via field electrodes in the hippocampus, and whisking was measured via the mystacial electromyogram. We calculated the spectral coherence between these two signals as a means to quantify the extent of phase locking. First, we found that the fraction of epochs with high coherence is not significantly greater than that expected by chance (seven of eight animals and as a population average). Second, we found that the trial-averaged coherence is low (coherence, < 0.1) and, as an average across all animals, statistically insignificant. We further asked whether the strength of the theta rhythm correlated with that of whisking, independent of the lack of cycle-by-cycle coherence. We observe that the correlation is weak and insignificant (six of eight animals and as a population average). We conclude that there is no relationship between the whisking and theta rhythms, at least when animals whisk in air.

Key words: EMG; electromyogram; hippocampal function; vibrissa; whisker; hippocampus; motor; rat; movement; motion; motor activity; theta rhythm; oscillator; whisking

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Received Jan. 16, 2006; revised April 7, 2006; accepted May 5, 2006.

Correspondence should be addressed to Dr. Rune W. Berg, Department of Medical Physiology, The PANUM Institute, Room 12.5.5, Blegdamsvej 3, DK-2200 Copenhagen, Denmark. Email: rune{at}mfi.ku.dk

This article has been cited by other articles:

				D. N. Hill, R. Bermejo, H. P. Zeigler, and D. Kleinfeld Biomechanics of the Vibrissa Motor Plant in Rat: Rhythmic Whisking Consists of Triphasic Neuromuscular Activity J. Neurosci., March 26, 2008; 28(13): 3438 - 3455. [Abstract] [Full Text] [PDF]

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