The term K-complex was originally used to describe the characteristic large potential waves following tone stimulation during sleep. These EEG waves lasted about a second and consisted of a “swing down” negativity of several hundred microvolts followed by a “swing up” positivity (Loomis et al., 1938
). Isolated versions of these waves were also observed during lighter stages of sleep which did not appear to be elicited by any source outside the brain. These characteristic waves, both spontaneous and induced, were deemed “K-complexes” (Loomis et al., 1938
). Although auditory stimulation was most effective in producing them, K-complexes were noted in response to a variety of different stimuli, all of which produced the same “diffuse, non-specific, delayed electrical response” in the human brain (Davis et al., 1939
). Later works, like the Roth paper, used several different types of stimuli but did not show direct comparisons, and with limited channel montages it is unlikely that any differences would have been easily observed. Colrain et al. (1999)
and others gave the idea that K-complexes were the same, regardless of stimulation type more definitive support by examining the scalp topography of both auditory evoked and respiratory-occlusion evoked K-complexes. In this study, the authors evoked K-complexes either by means of auditory stimulation or through respiratory occlusion and recorded the responses with an extended EEG montage (29 channels). Respiratory occlusion, it should be noted, is a form of somatosensory stimulation. They then compared the average evoked response topography at each electrode between the two conditions and found no differences. The results of this study confirmed the similarity of the scalp topographies at the negative peak of the K-complex. In an excellent and thorough review of the K-complex literature by Colrain several years later (Colrain, 2005
), which summarizes several of the studies also discussed in this chapter, the author emphasizes the notion that the K-complex is a modality-independent, sleep specific response to stimulation, noting that the evoked K-complex does not involve sensory-relay thalamocortical pathways.
The degree to which the cortex receives information about peripheral stimulation during sleep has long been controversial. Most studies in humans have relied on evoked potential studies. These studies have consistently suggested that while the early components, typically thought to reflect brainstem or nerve conduction are preserved during sleep, later components are usually disrupted (for a review see Bastuji and Garcia-Larrea, 1999
). The lack of brain responsiveness during sleep is usually attributed to a thalamic gating (Steriade et al., 1990
). While there have been few stimulation studies in naturally sleeping animals (for reviews see Velluti, 1997
; Hennevin et al., 2007
), one recent report suggests that in naturally sleeping monkeys there is preservation of responses from both primary and secondary auditory cortical area to acoustic stimulation (Issa and Wang, 2008
). Other studies have shown that the phase of the cortical slow oscillation can be an important factor in determining the degree to which sensory information reaches the cortex (Massimini et al., 2003
; Rosanova and Timofeev, 2005
Neuroimaging techniques have also recently been used to address the issue of the degree of cortical response to sensory stimulation during sleep. A landmark study in this regard was published in Neuron by Portas et al. (2000)
. The auditory cortex activation observed during the presentation of the subject's name or a beep was remarkably conserved between waking and NREM sleep. In a study using fMRI and combined EEG-PET imaging, however, visual stimulation was shown to decrease activity in the occipital cortex and another study showed significant decreases in auditory cortex with tone stimulation using EEG-fMRI during NREM sleep compared to wakefulness (Born et al., 2002
; Czisch et al., 2002
). In contrast, an even more recent study by Czisch used EEG-fMRI and showed auditory cortical activation in response to tones, but only when the tones produced a K-complex (Czisch et al., 2009
). Interestingly during the tone evoked K-complexes activation was also found in middle frontal gyri and cingulate areas. Consistent with these results, data presented elsewhere in this journal (Maquet this volume
) also show enhanced auditory cortical activation during slow wave sleep in the presence of an evoked K-complex using tone-triggered fMRI.
Given these recent results, we decided to revisit the notion that evoked K-complexes are entirely modality independent. Therefore, we source modeled high-density EEG recording (256 channel HydroCel Geodesic Sensor Net Electrical Geodesic Inc.) to directly compare the cortical sources of K-complexes evoked with three different kinds of stimulation: auditory (50ms tones, 2000 Hz pure tones delivered through earphones or speakers); somatosensory (0.3 ms constant current squarewave pulses delivered to the median nerve of the dominant hand at intensities around motor but below pain threshold); and visual (stroboscopic flashes, 10 (sec duration, at or below the maximum flash intensity of 20 lumen-sec/ft2
placed approximately three feet away from the subject's face). In order to compare different modalities of K-complexes in the most direct manner possible, we employed a within subject, within night comparison of the responses. Two minute blocks of the same stimulus modality were delivered with a 5 second within block interstimulus interval (ISI) during sleep stages N2 and N3. In order to maximize the number of stimulation blocks within the night we chose to use the minimum ISI which is still expected to be longer than the K-complex refractory period (Colrain, 2005
). Stimulation blocks were separated by at least one minute. The type of stimulation was pseudorandomly distributed throughout the night. Intensity of stimulation was manually modulated between blocks so as to maximize the chance to produce a K-complex without arousing the subject from sleep. Of the seven subjects (males age 22-36, 2 left-handed) who participated in the experiment, one subject was consistently aroused by the visual stimulation and another showed substantial high-amplitude, low frequency artifact which precluded identification of K-complexes. Therefore, data from 5 subjects were included in the final analysis. Approximately 1000 total stimuli were delivered to each subject (mean = 1032, range 595-1379) and different modality stimulations were evenly distributed throughout the night. EEG was sampled at 1000 Hz, re-referenced to the average of the mastoids (originally referenced to the vertex) and band-passed filtered (0.1 Hz 1st
order highpass, 0.5-40 Hz). Visually identified bad channels were replaced using spline interpolation. Responses were identified as K-complexes if there was a negative peak of at least 50 μV in the butterfly plot of all 256 channels between 200 – 1100 ms after the stimulus and if the negative peak amplitude was larger than the activity in the 500 ms prior to the stimulation. Success for evoking K-complexes was similar to what has been previously reported (mean +/− range: auditory 32% +/−21-47%; somatosensory 17% +/−14-21; visual 43% +/−33-55%). Surprisingly, visual stimulation was more successful than auditory or somatosensory stimulation in evoking K-complexes for 4 out of the 5 subjects.
show EEG butterfly plot traces (all 256 channels overlaid) of the grand average across subjects for slow waves evoked by each stimulation modality. The largest component of the average evoked response for the three different stimulations was a negative peak occurring between 400-700 ms. In the evoked potential literature, this peak is called the N550. Since the N550 peak is only evident in average responses during sleep that include K-complexes, it is commonly utilized as a means to investigate the characteristic differences between K-complexes across different stimulation conditions (Bastein et al., 2002
). Our analyses initially focused on this peak, which is the EEG reflection of the cellular downstate evoked by stimulation. However, we also wanted to take advantage of the temporal resolution of EEG to further explore the time course of any topographical differences. Given the latency variability of the responses across conditions and subjects even after averaging, we segmented the evoked potential response for each subject and condition based on the evoked components observed during NREM sleep (Bastien et al., 2002
). This segmentation applied to the grand averages is shown in and included the P200 (0-300 ms auditory, 0-200 ms somatosensory, and 0-260 ms visual in the grand averages), the N350 (300-445 ms auditory, 200-315ms somatosensory, 260-420 ms visual), N550 (445-775 ms auditory, 315-545 ms somatosensory, 420-650 ms visual), and the P900 components (900-1535 ms auditory, 1030-1560 ms somatosensory, 850-1580 ms visual).
Similarity of scalp and source topographies of K-complex responses
In agreement with previous results, there was a clear similarity in the scalp topography of the N550 peak across stimulations (). The relative scalp topography of the largest evoked negative peak for all three modalities had a hot spot of negativity centered near the approximate 10-10 location of AFz. However, the scalp topographies were not completely redundant, with the most obvious difference being that the negative peak hot spot extends to include Cz in the visual condition. To examine the cortical sources of the evoked K-complexes, we source modeled each time point of the average evoked response for each modality and each subject using a 4-shell forward model consisting of 2447 cortical voxels and the standardized low resolution electromagnetic tomography inverse solution (sLORETA, Geosource 2.0) with Tikhanov regularization (10−1
). The source modeling technique used here was similar to what was previously utilized to examine spontaneous slow waves (Murphy et al., 2009
) but removed the necessity of using baseline data to normalize the results because the sLORETA constraint accounts for measurement and biological noise directly within the algorithm itself (Pascual-Marquis, 2002
). To compare relative current source topographies across subjects, current sources for each time point were z-score transformed before averaging across time periods. Flat maps of the z-transformed cortical sources corresponding to the N550 peak time period are displayed in . In terms of current source topography, the flat maps reveal that the current sources for the negative peak of the K-complex, like the scalp topographies, are relatively consistent across modalities, although a slight increase in current is present in occipital areas for K-complexes evoked by visual stimulation. Across stimulations, the cortical areas where the evoked K-complexes were most pronounced in terms of relative current across stimulation conditions were bilaterally in the anterior cingulate, middle frontal, inferior frontal, orbital, and rectal gyri. The largest current coldspot (minimal current) was the inferior parietal lobule. The topographic consistency of the N550 peak across stimulation modalities and the fact that primary sensory areas are not the areas with the largest amount of current generally confirms the notion that peripheral stimuli likely utilize a common mechanism for producing cortical slow waves that does not directly involve the primary sensory pathways (Roth et al., 1956
; Bastien et al., 2002
; Colrain 2005
Still, is there no role for the primary sensory pathways in peripherally evoked slow waves? In order to answer this question, we chose to directly compare the cortical sources with a more fine-grained analysis to explore whether the evoked K-complex slow wave responses were entirely sensory pathway independent. To do so, we used a nonparametric Quade test to compare cortical sources across stimulation conditions. The Quade test is an extension of the Wilcoxon signed ranks test for cases of multiple related conditions and is especially powerful when the number of conditions is less than five (Conover, 1999
). Each of the 2447 cortical source voxels for each subject was ranked according to the amount of relative (z-transformed) current for the three stimulation conditions. Essentially, the higher the current, the higher the rank. The Quade test establishes the statistical significance based on the consistency of these rankings across subjects. For statistically significant voxels, Quade rankings can be examined directly to determine the stimulation condition with the largest relative current compared to the other two (Garcia et al., 2010
). The rankings are also submitted to post hoc procedures to determine the significant differences for all possible comparisons.
The direct comparison of current sources at the negative peak of the evoked K-complex slow wave revealed that there are in fact cortical areas which show differences in relative current between stimulation conditions (). Interestingly, not only are some cortical regions significantly different across the stimulation conditions, but the statistically different regions exhibit a pattern of relatively increased currents that suggests an obvious relationship with the type of stimulation. For instance, there is a left hemisphere area that includes Brodmann areas 17, 18 and 19, comprising primary and secondary visual cortex, which is significantly different across stimulation modalities (). Based on Quade rankings, the negative peak has the largest currents in this area when a K-complex is evoked by visual stimulation, compared to the other stimulations ). Post hoc analysis of the Quade rankings confirm that visual stimulation in these areas is significantly larger than one or both of the other stimulation modalities (). Other areas in the occipital gyrus were also different including portions of the posterior cingulate and precuneus and showed larger relative current peaks during the negative portion of the scalp EEG K-complex evoked by visual stimulation. Similarly, a large cortical area that included parts of the primary somatosensory and motor cortices and extended anteriorly to include the supplementary motor areas (Brodmann areas 6 and 8) bilaterally was also statistically different across stimulations and showed the largest relative currents when the K-complex was evoked by somatosensory stimulation. In contrast, relatively few areas showed differences where the highest relative current values during the negative peak of K-complexes were evoked by auditory tone stimulation. .
N550 peak shows modality specific differences in cortical sources that include primary cortical areas
The intriguing modality specificity of the cortical currents during the largest EEG component of the evoked slow wave prompted us to explore the time course of these current differences in K-complexes induced by peripheral stimulation Therefore, we next examined the other components of the evoked response, the P200, N350 and P900 time periods. The results of the Quade test for these two time periods are shown in . Like the K-complex negative peak, the P200 period includes significantly different cortical sources that include primary visual and somatosensory areas (). In addition, the Rank and Post Hoc plots combine to show that stimulation causes a comparative increase in relative currents specific to the stimulation modality. That is, visual stimulation produced higher relative currents in visual cortex while somatosensory stimulation was responsible for higher relative current in somatosensory cortices. Interestingly, the significant differences seemed to involve more circumscribed areas within the primary cortices than during the N550 peak, especially for sources ranking highest in the somatosensory condition. It should also be noted that there were cortical voxels that were significantly different outside of the primary cortical areas. For instance, parts of the inferior temporal cortex and fusiform gyri were significantly different showing increased current during somatosensory stimulation, although these areas are not generally thought to be involved in somatosensory information processing. The P200 time period comparison included some sizeable cortical areas where the significant differences across stimulation modality were being driven by auditory stimulation, including areas in the anterior cingulate, middle frontal, inferior frontal, and rectal gyri, . These areas were among the highest in current for K-complexes in general, during both the negative peak and the P200 period (data not shown), regardless of stimulation. The finding that cortical areas specific for auditory stimulation are common to all K-complexes but are more strongly activated by tones, likely reflects the fact that auditory stimulation is more effective in consistently evoking stereotypical slow waves. The fact that researchers often include a criterion for K-complex identification that includes a frontocentral maximum may also contribute to the reason why auditory stimulation is found to be more reliable.(Colrain, 2005
). Whether primary auditory cortex involvement is truly absent, is shorter in time than other stimulation modalites and thus more easily averaged over, or is an area that is particularly difficult to image with source modeling hd-EEG is unknown.
The initial positive peak (P200), but not N350 or P900, shows modality specific differences in primary cortical areas
The modality specific differences during the P200 and N550 time periods may be a residual effect of the stimulation that fades over time. Thus, the further away in time from the stimulation, the less apparent the effect. However, the time period between the P200 and N550 K-complex peaks, which is labeled N350 in and included an obvious negative component around this time in both the auditory and visual evoked responses, did not show significant differences in either somatosensory or visual primary cortical areas. The N350 component has often been attributed to vertex sharp waves (Colrain et al 2000). The lack of modality specific differences in primary cortical areas for the N350 peak, as there is in P200 and N550 time period suggests that the mechanisms underlying the production of vertex waves are indeed different in some distinct way from those involved in the K-complex, or at least do not show any reflection of primary sensory pathway involvement. In the P900 comparisons, there are relatively few cortical areas that are significantly different across conditions, and there is no apparent relationship with the primary sensory areas for either the visual and somatosensory conditions, as there was during the P200 and N550 component time periods.
These results confirm that during sleep, different types of stimuli evoke a stereotypic, global, K-complex slow wave. However, the spatial and temporal resolution of source modeling hd-EEG allowed us to observe aspects of the evoked K-complex response that were not stereotypic. For somatosensory and visual stimulation, the P200 and the N550 components showed differential involvement of primary cortical areas consistent with the activation of primary sensory pathways. One possibility is that the global K-complex is produced by an initial activation of the primary cortical area that expands throughout the cortex via corticocortical connections or indirectly through other structures. An alternative explanation is that the evoked K-complex response reflects the parallel activation of both specific and nonspecific ascending sensory pathways to the cortex (Brodal, 1981
). Specific pathways relay detailed sensory information about to primary cortical areas, while the nonspecific pathways often involve collateral innervations of the reticular formation, and are capable of diffusely activating the entire cortex (Brodal, 1981
; Jones, 2003
). During sleep, the main function of the nonspecific pathways may be to provide an early warning signal of danger and potentially even wake the organism (Halasz, 2004
). Thus, the stereotypical K-complex response common to all modalities of stimulation may be the result of nonspecific pathway involvement (Colrain 2005
) and the modality specific differential activations apparent in the P200 and N550 components may result from parallel processing via the classical sensory pathways. This interpretation is consistent with the fact that auditory stimulation produces the most stereotypic K-complexes. In microosmatic animals, like humans, the auditory system is the most useful for detecting potential dangers in the environment during sleep and is therefore likely to be the most efficient in activating the nonspecific sensory pathways to produce the most stereotypical K-complex slow waves. While further investigations are needed to characterize the neural circuitry underlying these events, the hd-EEG data presented here suggest a previously unrecognized involvement of primary cortical areas at specific time periods during peripherally evoked slow waves.