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| Figure 1 |
Local field potentials (LFP), and other physiological recordings that
emphasize patterns of organized activity occurring across relatively large
neural populations, frequently reveal large-scale oscillatory brain events
with a range of peak frequencies and foci (Niedermeyer, 1993; see also
Chrobak & Buzsaki, 1996; Steriade, Contreras, Amzica, & Timofeev, 1996; Wehr
& Laurent, 1996). Mu, typically defined as a 7-10 Hz modulation of activity
centered over somatosensory cortex of awake but relaxed animals, is one of
the most easily observed brain rhythms, variously thought to occur in
50-100% of normal human or non-human animals tested (Arroyo et al., 1993;
Tiihonen, Kajola, & Hari, 1989). This rhythm typically arises during quiet
immobility, and vanishes immediately following stimulus presentation or
before movement. Such observations have engendered the theory that μ may
reflect some sort of "idling" mode in the brain. The fact that human
subjects' performance on certain cognitive tasks is degraded during periods
of prominent μ activity is consistent with this idea. Other data, however,
has led researchers to suggest instead that μ represents "readiness" to
process information: bouts of rhythmic brain activity frequently precede
exploration, and somatosensory stimulation provided during these μ bouts are
coded differently than that provided at other times. While these two
theories seem contradictory, they may not in fact be mutually exclusive, as
μ may represent the shifting of the organism into a mode that is optimized
for some sorts of tasks and not for others (Fanselow and Nicolelis, 1999).
There are at least three reasons to expect that μ activity might be observed
in gustatory (insular) cortex (GC). First, the fact that GC is adjacent to
and connected to primary somatosensory cortex, and that GC contains many
neurons that respond to somatosensory stimulation of the oral region, makes
it reasonable to suppose that somatosensensory rhythms might appear among
taste responses. Second, the fact that oral behaviors such as licking and
chewing tend themselves to be modulated at frequencies in the range of μ
leaves open the possibility that these behaviors might drive (or be driven
by) gustatory motor rhythms originating in GC. Finally, the suggestion that
olfactory processing may be intrinsically oscillatory leads naturally to the
possibility that gustatory processing is similarly organized. It is
reasonable, therefore, to examine GC LFPs for the presence of oscillations
as rats perform an ingestive task.
We have trained restrained, water-restricted rats to perform a simple
variant of a timing/delayed reinforcement task. Small doses of water were
supplied when rats pressed a lever on a long (30 sec) fixed interval
schedule. The rats developed lever-pressing styles consistent with other
such tasks, entering a response "up-state" toward the end of single trial
intervals. At the full-session level, they progressed through a series of 3
"behavioral states" that were easily identifiable on the basis of
lever-pressing regimes; one of these states appeared to reflect
disengagement from (boredom or frustration with) the task. Transitions
between states were relatively sudden and simple to pinpoint. Concurrent
neural recordings from GC, meanwhile, revealed large-amplitude, broadly
coherent spike-and-wave-type events in the μ frequency range (see Figure).
These events could be terminated by stimulus delivery, and were endogenously
quenched when the rats prepared to lever-press; thus they are consistent
with earlier descriptions of μ. They did not appear to be related to
gustatory processing itself -- periods of intense rhythmic mastication were
seldom associated with μ episodes. In fact, such episodes appeared to be
associated with disengagement from task performance, emerging solely during
periods in which lever pressing was unrelated to time since last
reinforcement, and may even predict such disengagement.
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Bibliography:
- Arroyo, S., Lesser, R. P., Gordon, B., Uematsu, S., Jackson, D., & Webber, R. (1993). Functional significance of the mu rhythm of human cortex: An electrophysiologic study with subdural electrodes. Electroencephalography and Clinical Neurophysiology, 87: 76-87.
- Chrobak, J. J., & Buzsáki, G. (1996). High-frequency oscillations in the output networks of the hippocampal-entorhinal axis of the freely behaving rat. Journal of Neuroscience, 16: 3056-3066.
- Fanselow, E. E., & Nicolelis, M. A. (1999). Behavioral modulation of tactile responses in the rat somatosensory system. J Neurosci, 19(17): 7603-7616.
- Niedermeyer, E. (1993). The normal EEG of the waking adult. In E. Niedermeyer & F. Lopes da Silva (Eds.), Electroencephalography: basic principles, clinical applications, and related fields. (pp. 131-152). Baltimore, MD: Williams & Wilkins.
- Steriade, M., Contreras, D., Amzica, F., & Timofeev, I. (1996). Synchronization of fast (30-40 Hz) spontaneous oscillations in intrathalamic and thalamocortical networks. Journal of Neuroscience, 16(8): 2788-2808.
- Tiihonen, J., Kajola, M., & Hari, R. (1989). Magnetic mu rhythm in man. Neuroscience, 32(3): 793-800.
- Wehr, M., & Laurent, G. (1996). Odour encoding by temporal sequences of firing in oscillating neural assemblies (w/commentary). Nature, 384: 162-166.
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