the cerebral cortex of mammals , including humans , displays oscillatory spindle activity ( 1015  hz ) across the lifespan . in adults , sleep spindles , a specific type of spindle activity , occur exclusively during non - rem sleep [ 14 ] and have been implicated in memory retention and skill learning [ 57 ] . in humans ,
sleep spindles first appear 49 weeks postterm and become more prominent and frequent over the next several months .
there exists another type of spindle activity  called spindle bursts  that is phenomenologically similar to sleep spindles in that they share a similar frequency range , duration , and spindle - shaped waveform .
for example , spindle bursts predominate early in development : they are readily observed in premature human infants   and are thought to disappear soon after birth . in rats ,
spindle bursts have been recorded in sensorimotor and visual cortex soon after birth through at least the end of the second postnatal week [ 8 , 10 , 11 ] .
in addition , unlike sleep spindles , spindle bursts are not exclusive to non - rem sleep but rather can occur across the sleep - wake cycle . finally , spindle bursts are most closely associated with self - generated or evoked activity in the sensory periphery ; for example , manually stimulating the limb of a rat or human , during sleep or wakefulness , elicits spindle bursts in sensorimotor cortex [ 8 , 9 , 11 , 12 ] . across all sensory modalities ,
however , self - generated activity in the sensory periphery is the predominant trigger of spindle bursts during early development . in the visual system , for example
, retinal ganglion cells fire spontaneously before eye opening , producing waves of retinal activity that provide downstream afferent input to visual brain areas [ 10 , 13 , 14 ] . in the auditory system , cochlear spiral ganglion cells fire bursts of activity before the onset of hearing that provide downstream afferent input to the auditory system ( although spindle bursts have not yet been recorded in auditory cortex , it is likely that they occur ) [ 1517 ] . by providing substantial sensory experience during early development , retinal waves and cochlear bursts shape and
refine functional links between the sensory periphery and developing brain networks [ 14 , 18 , 19 ] . in the developing sensorimotor system
, spontaneous activity takes the form of myoclonic twitching , which is characterized by jerky movements of the forelimbs and hindlimbs , tail , whiskers , and eyes during rem sleep [ 8 , 9 , 11 , 20 , 21 ] . in neonatal rats ,
sensory feedback from twitching limbs triggers activity throughout the nervous system and has been hypothesized to contribute to the development of sensorimotor circuits [ 8 , 11 , 20 , 22 , 23 ] .
moreover , just as retinal waves trigger spindle bursts in visual cortex , sensory feedback from twitching limbs ( i.e. , reafference ) triggers spindle bursts in somatosensory [ 8 , 2426 ] and motor [ 11 , 12 ] cortex .
twitches , however , differ from retinal waves in two fundamental ways : first , although retinal waves are transient features of development , twitches persist into adulthood [ 28 , 29 ] and second , although retinal waves seem to occur independently of behavioral state , twitches are an exclusive feature of rem sleep . to reiterate , twitches trigger spindle bursts in sensorimotor cortex [ 8 , 11 , 12 , 24 , 25 ] , occur exclusively during rem sleep , originate prenatally [ 9 , 31 ] , and persist into adulthood [ 28 , 29 ] . curiously , however , spindle bursts have not been reported during rem sleep in adults .
one possible explanation for this absence is that twitches in adults are somehow prevented from triggering spindle bursts in sensorimotor cortex . given how reliably twitches activate the cerebral cortex in early development , this possibility requires that a mechanism emerges at some point between infancy and adulthood that gates twitch - triggered reafference .
for example , there is evidence in adult cats of spinally mediated sensory gating that is stronger during rem sleep than during wakefulness .
however , even during rem sleep , this gating mechanism only partially blocks sensory feedback .
therefore , it remains possible , if not likely , that twitches continue to trigger spindle bursts in the adult sensorimotor cortex .

 figure 1 presents local field potentials ( lfps ) in infants rats recorded from the hindlimb region of sensorimotor cortex in relation to twitches detected in the hindlimb electromyogram ( emg ) ( data from ) . at postnatal days ( p ) 4 and 810 ,
when background lfp activity is low , hindlimb twitches trigger easily discernible cortical spindle bursts . indeed , because of the low background activity at these ages , visual inspection of the lfp alone
 that is , without guidance provided by the hindlimb emg  allows one to confidently state when a spindle burst occurred .
also , the lfp alone is sufficient to predict with high confidence when hindlimb twitches occurred . in contrast , by p12 , lfp background activity has increased such that spindle bursts are now much harder to detect . in this case , the lfp alone is not sufficient to predict with high confidence when hindlimb twitches occurred . given this dramatic increase in background activity between p810 and p12 , we expect spindle bursts to disappear further into the background over the ensuing days and weeks . but
the precise relationship between twitches and spindle bursts is revealed using a twitch - triggered averaging method . in the case of sleeping infant rats at p4 , p810 , and p12
, averaging lfp power in relation to hindlimb twitches reveals increased activity in the hindlimb region of sensorimotor cortex within ~150  ms of a twitch ( figure 2(a ) ) .
time - frequency spectrograms reveal that these peaks in twitch - related power occur at spindle burst frequency ( figure 2(b ) ) .
note that , at p4 , the spectrogram displays a discrete  hotspot  immediately after hindlimb twitches .
by contrast , at p12 , there is increased background activity in the spindle frequency range ; nonetheless , a concentrated twitch - related  hotspot  in spindle frequency is still apparent .
these results suggest that , with increasing age , spindle bursts are harder to detect because they are obscured by increases in background lfp activity .
event - triggered averaging , such as that illustrated in figure 2 , is routinely used to reveal hidden or obscured cortical events and oscillations .
for example , cognitive neuroscientists use event - related potentials ( erps ) to reveal cortical activity associated with sensory , motor , or cognitive process ( figure 3 ; ) . because eeg electrodes reflect ongoing activity from thousands of neurons , the resulting signals are necessarily noisy . as a result , an individual event
( e.g. , a flash of light ) may not be evident in the raw eeg signal .
however , by averaging over many event - triggered trials , random noise that is not associated with neural processing of the stimulus cancels out , leaving behind a distinct and stereotyped cortical pattern  the erp  that reflects underlying neural processes .
we propose that twitch - related spindle bursts have not yet been detected in the activated eeg of human adults during rem sleep because event - triggered averaging would be necessary to observe them .
undoubtedly , there are many published studies  in human and nonhuman adult animals  in which cortical activity recorded across behavioral states can be related to peripheral motor activity , including twitching ( e.g. , see ) .
critically , to our knowledge , no study in adults has specifically used twitches for event - triggered averaging of cortical activity in somatotopically related areas of sensorimotor cortex .
species - typical and behaviorally relevant body parts  the bill of a platypus , the star appendage of a star - nosed mole , and the digits of a raccoon  have magnified representations in sensorimotor cortex .
the degree of magnification reflects the innervation of these peripheral morphological features as well as their use .
if twitches are functionally important for the development and maintenance of sensorimotor circuits [ 8 , 23 ] , then we expect the quantity and patterning of twitching to reflect an appendage 's innervation density , biomechanics , and behavioral importance across species [ 23 , 36 ] . like other primates ,
humans are a highly visual species that execute hundreds of thousands of saccadic eye movements ( 3 per second ) during the day .
interestingly , much like the skeletal muscles that control the limbs , the extraocular skeletal muscles that move the eyes twitch during rem sleep .
in fact , the resulting  rapid eye movements  give rem sleep its name .
moreover , much like the limbs of the body , proprioceptive reafference from eye movements in rats and humans is relayed to sensorimotor cortex [ 39 , 40 ] . since twitches of all skeletal muscles studied thus far trigger spindle bursts [ 8 , 11 , 21 ]
because rapid eye movements are a prominent feature of rem sleep in human adults , the sensorimotor system that controls eye movement could be an ideal model for exploring the contributions of spindle bursts to the calibration , maintenance , and repair of neural networks . whereas humans , which are diurnal , rely heavily on the visual system for spatial navigation , rats , which are nocturnal , rely heavily on the whiskers .
the whiskers of rats and other rodents are controlled by an elaborate set of extrinsic and intrinsic striated muscles [ 41 , 42 ] .
importantly , in infant rats , these muscles twitch during rem sleep and reafference from twitches activates the whisker thalamus and barrel cortex .
spindle bursts are readily detected in the infant rat 's barrel cortex , occurring immediately after twitching ; the developmental onset of whisker - related cortical activity could contribute to the refinement of the somatotopic map in barrel cortex .
whisker twitching has also been observed in adult rats   but has not been systematically investigated .
therefore , similar to rapid eye movements in humans , we expect whisker twitching and associated spindle bursts to reflect the outsized role of this sensory modality in rats . extrapolating to other species , highly specialized sensorimotor appendages should exhibit relatively high rates of twitching .
the bill contains a high density of electro- and mechanoreceptors and , consequently , a large proportion of the platypus ' cerebral cortex is devoted to processing sensory input from the bill [ 46 , 47 ] .
as we would expect , platypuses exhibit vigorous twitches of the bill ( as well as the eyes and limbs ) during rem sleep , even as adults [ 48 , 49 ] .
the star - nosed mole uses a specialized set of 22 facial appendages that it uses like a  tactile fovea  to forage for food .
similar to the bill of the platypus , the star appendages contain a high density of somatosensory receptors and , consequently , a large proportion of this animal 's cerebral cortex is devoted to each appendage .
although there is no information on the sleeping habits of the star - nosed mole , we predict that it will be found to exhibit high rates of twitching in the star appendages during rem sleep , as well as twitch - associated spindle bursts in sensorimotor cortex .
it has been argued that local changes in cortical slow - wave activity can be triggered by learning processes that differentially involve specific brain regions .
similarly , after the development of a new waking skill or in response to changes in the sensory periphery ( e.g. , due to injury ) , perhaps twitches contribute to the process of learning , adaptation , or recovery of function .
for example , in one study , human adults wore goggles during the day that decreased their visual field to 5 degrees .
when these subjects were observed over several days during sleep , the duration of rem sleep was unaffected , but the frequency and amplitude of rapid eye movements increased significantly . because the changes in rapid eye movements were a transient response to a waking perturbation of visual experience , these findings suggest to us that twitching  in this case in the form of rapid eye movements  contributes to the process of visual adaptation .
this experimental approach  that is , the manipulation of waking motor experience in human and non - human subjects  could provide the opportunity to explore the contributions of twitching and twitch - related spindle bursts to learning and adaptation in health and disease .
until recently , twitches were widely perceived as functionless byproducts of dreaming , providing little reason to search for their neural consequences . only with the discovery of twitch - related neural activity in infant rats
, it has become apparent that twitches could play a functional role in sensorimotor development , akin to spontaneous activity in other sensory systems .
it is the persistence of twitching into adulthood , in humans and other mammals , that raises the intriguing possibility that twitching has much more to reveal to us about its functional contributions to neural plasticity within the sensorimotor system .
although the idea that twitch - related spindle bursts persist into adulthood is speculative , it can easily be tested using new  or even perhaps existing  recordings . if evidence of twitch - related spindle bursts is found , there will be a strong basis for expanding our understanding of the functions of spindle activity during non - rem sleep  about which we currently know a lot  to include rem sleep as well .
 sleep spindles are brief cortical oscillations at 1015  hz that occur predominantly during non - rem ( quiet ) sleep in adult mammals and are thought to contribute to learning and memory . 
 spindle bursts are phenomenologically similar to sleep spindles , but they occur predominantly in early infancy and are triggered by peripheral sensory activity ( e.g. , by retinal waves ) ; accordingly , spindle bursts are thought to organize neural networks in the developing brain and establish functional links with the sensory periphery . whereas the spontaneous retinal waves that trigger spindle bursts in visual cortex are a transient feature of early development , the myoclonic twitches that drive spindle bursts in sensorimotor cortex persist into adulthood . 
 moreover , twitches  and their associated spindle bursts  occur exclusively during rem ( active ) sleep . curiously , despite the persistence of twitching into adulthood , twitch - related spindle bursts have not been reported in adult sensorimotor cortex . 
 this raises the question of whether such spindle burst activity does not occur in adulthood or , alternatively , occurs but has yet to be discovered . 
 if twitch - related spindle bursts do occur in adults , they could contribute to the calibration , maintenance , and repair of sensorimotor systems . 
1. Introduction
2. Detecting Twitch-Related Spindle Bursts
3. Exploring the Functionality of Twitching and Associated Spindle Bursts across the Lifespan in Diverse Species
4. Conclusions