Integrative systems 779
Daytime sleepiness is associated with altered resting thalamocortical connectivity William D.S. Killgore, John R. Vanuk, Sara A. Knight, Sarah M. Markowski, Derek Pisner, Bradley Shane, Andrew Fridman and Anna Alkozei Thalamocortical connectivity is believed to underlie basic alertness, motor, sensory information processing, and attention processes. This connectivity appears to be disrupted by total sleep deprivation, but it is not known whether it is affected by normal variations in general daytime sleepiness in nonsleep deprived persons. Healthy adult participants completed the Epworth Sleepiness Scale and underwent resting-state functional MRI. Functional connectivity between the thalamus and other regions of the cortex was examined and correlated with Epworth Sleepiness Scale scores. Greater sleepiness was associated with inverse (i.e. lower or more negative) connectivity between the bilateral thalamus and cortical regions involved in somatosensory and motor functions, potentially reflecting the disengagement of sensory
Introduction Acute total sleep deprivation (TSD) and prolonged partial sleep restriction are associated with a host of neurocognitive and emotional deficits [1,2], but the underlying neurobiological causes for these deficits are still being explored. After a night of TSD, the brain shows dramatic declines in metabolic activity within key systems involved in information transfer and attention, including the thalamus and higher order brain regions such as the prefrontal and parietal cortex [3]. These same systems are also adversely affected by TSD when measured by functional MRI (fMRI) during working memory tasks, which show reduced task-related activation in primary sensory and attention-related regions [4]. More recently, it has been shown that TSD also disrupts low-level intrinsic temporal correlations (i.e. functional connectivity) between key regions involved in alertness and cognition, including the default mode network and its anticorrelated networks [5–7]. Moreover, recent evidence suggests that TSD might impair cognition by disrupting functional connectivity between the thalamus (a key structure involved in alertness and sensory information transfer) and higher cortical regions [8,9]. Recently, Tomasi and colleagues also posited that TSD leads to compensatory hyperactivation of the thalamus to sustain alertness during prolonged wakefulness. However, such directed thalamic activation may come at a cost; namely it could lead to reduced functional and cognitive capacity within other attention demanding areas such as the parietal and prefrontal cortex [9]. Thus, TSD 0959-4965 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
and motor processing from the stream of consciousness. NeuroReport 26:779–784 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. NeuroReport 2015, 26:779–784 Keywords: functional connectivity, functional MRI, medial prefrontal cortex, motor cortex, neuroimaging, parietal cortex, sensory cortex, sleepiness Social, Cognitive and Affective Neuroscience Lab, University of Arizona College of Medicine, Tucson, Arizona, USA Correspondence to William D.S. Killgore, PhD, Social, Cognitive, and Affective Neuroscience Lab, Department of Psychiatry, University of Arizona College of Medicine, 1501 N Campbell Ave, Tucson, AZ 85724, USA Tel: + 1 520 621 0605; fax: + 1 520 626 6050; e-mail: [email protected] Received 3 June 2015 accepted 15 June 2015
leads to reduced or inverse patterns of thalamocortical connectivity. At present, it remains unknown whether functional connectivity disturbances seen in TSD might extend to less extreme sleep loss situations routinely encountered outside of the laboratory such as variations in general daytime sleepiness among nonsleep deprived individuals. Building upon the functional connectivity findings observed during TSD [8,9], we hypothesized that individuals reporting greater general daytime sleepiness would tend to show reduced thalamocortical functional connectivity during a brief resting-state fMRI scan relative to individuals with minimal daytime sleepiness.
Methods Participants
Sixty (30 male; 30 female) right-handed, healthy adults ranging in age from 18 to 45 years (M = 30.5, SD = 8.1) were recruited from the New England area via posted flyers and Internet advertisements. Participants were initially screened over the telephone by a trained research technician to exclude for any history of severe medical, neurological, or psychiatric conditions, head injury, alcohol or drug treatment, or current use of psychoactive drugs or medications. Participants had obtained an average of 14.9 years of education (SD = 2.2), and described themselves as normal sleepers, averaging 7.3 h (SD = 0.83) on weeknights and 7.7 h (SD = 1.2) on weekends according to self-report. Typical caffeine use DOI: 10.1097/WNR.0000000000000418
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780 NeuroReport 2015, Vol 26 No 13
was reported to be generally low to moderate (M = 138.9 mg/day, SD = 151.4), and caffeine intake on the day of the scan was close to their typical levels (M = 81.9 mg, SD = 103.7). Each participant provided written informed consent and received nominal financial compensation. Other data from a subset of this sample have been reported elsewhere [10–12], but the analyses reported herein linking sleepiness and thalamic connectivity are novel and have not been previously published. The McLean Hospital Institutional Review Board and the US Army Human Research Protections Office approved this research. Materials and procedure
Participants underwent a morning assessment session that involved completing several questionnaires and cognitive tasks. As part of this process, participants completed the Epworth Sleepiness Scale (ESS), a wellvalidated eight-item instrument that measures the severity of general daytime sleepiness on a scale from 0 to 24 [13]. In the early afternoon, participants underwent resting-state functional MRI (rsfMRI) for 6 min, with their eyes open. Magnetic resonance imaging parameters
Data acquisition occurred on a 3.0 T Siemens (Erlangen, Germany) Tim Trio scanner with a 12-channel head coil. First, T1-weighted 3D MPRAGE structural images were collected (TR/TE/flip angle = 2.1 s/2.25 ms/12°) over 128 sagittal slices (256 × 256 matrix) with a voxel size of 1.0 × 1.33 × 1.0 mm. Then, rsfMRI data were collected using a T2*-weighted blood oxygen level dependent echoplanar imaging sequence (TR/TE/flip angle = 2.0 s/30 ms/90°) over 34 transverse interleaved slices, with 180 images per slice (3.5 mm thickness, no skip; 22.4 cm field of view; 64 × 64 acquisition matrix). Image processing
SPM12 (Wellcome Department of Cognitive Neurology, London, UK) was used for preprocessing and statistical analyses. Preprocessing involved standard routines for motion correction, slice-timing correction, anatomical coregistration, spatial normalization, and spatial smoothing using an isotropic Gaussian kernel [full-width at halfmaximum = 6 mm (resliced to 2 × 2 × 2 mm)]. The Functional Connectivity Toolbox (CONN), version 14n (http://www.nitrc.org/projects/conn) was used for rsfMRI connectivity (rsFC) analyses. Denoising of data involved band-pass filtering (0.01, 0.10 Hz), removal of physiological noise using the aCompCor strategy [14], and removal of confounds including white matter, cerebrospinal fluid, and motion parameters. In addition, the Artifact Detection Tool was used to identify and regress out scan-to-scan movement greater than 1 mm and global intensity greater than 3 SD. No participant had greater than 3 mm of head movement during the resting scan.
Statistical analysis
Initially, after the removal of confounds, the residual blood oxygen level dependent signal timecourse from the bilateral thalamic seed regions were extracted and Pearson correlations were computed with all other voxels in the brain to derive connectivity maps. These z-score transformed connectivity maps were then entered into a general linear model regression analysis at the second level to evaluate the relationship between ESS scores and the strength of thalamic functional connectivity with the rest of the brain. Two seed regions were placed corresponding to the left and right thalamus as defined by the Automated Anatomical Labeling (AAL) atlas [15]. In addition to seed-to-voxel connectivity in CONN, we also conducted region of interest (ROI)-to-ROI analysis, with target regions defined by the 88 other cerebral ROIs in the AAL atlas. In accord with the standard procedures for CONN (http://www.nitrc.org/projects/conn) for seed-tovoxel analyses, a combination of height and extent thresholds was used to control for false positives within the connectivity maps. Whole brain connectivity maps were height thresholded for two-sided tests at P less than 0.05 [false discovery rate (FDR) corrected], whereas spatial extent (i.e. cluster size) was simultaneously thresholded at P less than 0.05 (FDR corrected). Furthermore, ROI-to-ROI maps were FDR corrected at these thresholds at the analysis-level to account for the number of ROIs included in the analysis. Anatomical specificity of the findings was first determined by the location in the AAL atlas [15], with further refinement by cross-reference with more detailed brain atlases [16,17].
Results Daytime sleepiness scores on the ESS ranged from 0 to 16 (M = 5.98, SD = 3.65). Analysis of the rsFC data revealed that greater ESS scores were associated with greater anticorrelated thalamocortical connectivity that involved primarily somatosensory and motor regions. Specifically, higher ESS scores were associated with decreased thalamic connectivity to areas of the primary somatosensory cortex, including superior regions of the right postcentral gyrus [i.e. Brodmann’s area (BA) 2 and BA 3]. Higher ESS scores were also associated with decreased thalamic connectivity bilaterally within the precentral sulcus, and areas considered to be motor and premotor regions, including regions of the precentral gyrus (BA 4) and superior precentral sulcus/superior frontal gyrus (BA 6). On the superior medial surface of the brain, ESS scores were inversely correlated with thalamocortical connectivity with bilateral areas of the paracentral lobule (medial aspect of BA 4, posterior to the paracentral sulcus) and the supplementary motor area (medial aspect of BA 6, anterior to the paracentral sulcus). Notably, the region most significantly decreased in connectivity with higher ESS was the junction between the right middle temporal gyrus and anterior regions of the right occipital lobe (BA 37). Finally, reduced connectivity
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Sleepiness and functional connectivity Killgore et al. 781
at higher ESS scores was observed bilaterally for the ventromedial prefrontal cortex (vmPFC)/gyrus rectus as well. This connectivity pattern, depicted in Fig. 1a, presents ROI-to-ROI connectivity maps showing that the connectivity of the left thalamus to seven of 88 cerebral
ROIs was negatively correlated with ESS, whereas the connectivity of the right thalamus was negatively correlated with ESS for 21 of the 88 ROIs. In addition, the figures show this association in the seed-to-voxel connectivity maps in terms of the maximum intensity
Fig. 1
(a)
(b)
(c)
(d) Superior precentral sulcus
Postcentral gyrus
Occipital-temporal junction
Paracentral lobule/ SMA
vmPFC
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782 NeuroReport 2015, Vol 26 No 13
Higher scores on the Epworth Sleepiness Scale (ESS) were correlated with reduced (i.e. inverse) thalamocortical functional connectivity. (a) The region of interest (ROI)-to-ROI pattern of connectivity between the left and right thalami (green circles) and other cortical regions is displayed (height threshold P < 0.05, FDR corrected, with an FDR correction at the analysis level to account for the number of ROIs in the analysis). Dark blue circles correspond to regions showing significant connectivity to the left thalamus, and light blue circles correspond to regions showing connectivity to the right thalamus. The darker the line, the more negative the connectivity value. Thalamic functional connectivity was most prominent for somatosensory and motor regions. Seed-to-voxel connectivity for the combined left and right thalami are shown as (b) a standard three-orientation maximum intensity projection (MIP) through the plane containing the paracentral fossa (MNI: − 2, − 16, 54; red carets), which clearly shows the predominant activation in sensory and motor cortex (height threshold P < 0.05, FDR corrected and extent threshold of P < 0.05, FDR corrected), (c) a standard three-view SPM activation map highlighting the sensory and motor strip activation (MNI coordinates: − 2, − 16, 54; height threshold P < 0.05, FDR corrected and extent threshold of P < 0.05, FDR corrected), and d) a cortical inflation map showing the major clusters of activation on the cortical surface (height threshold P < 0.05, FDR corrected and extent threshold of P < 0.05, FDR corrected). The red arrows highlight major regional clusters of activation. FDR, false discovery rate; vmPFC, ventromedial prefrontal cortex.
Table 1
Cortical voxel clusters showing inverse thalamic functional connectivity with greater sleepiness on the ESS MNI coordinates
Target region
Cluster size
x
y
z
Pearson r
T
1691 3186 250 1437 97 680
60 − 50 −2 32 4 22
− 62 −4 − 16 − 40 34 −2
2 30 54 46 − 20 60
− 0.614 − 0.589 − 0.555 − 0.543 − 0.528 − 0.519
5.92 5.55 5.08 4.92 4.74 4.63
Right anterior occipital lobe/middle temporal gyrus (BA37) Left precentral gyrus (BA 4)/superior frontal gyrus/superior precentral sulcus (BA 6) Left/right paracentral fossa (medial BA 4, 6)/supplementary motor area Right parietal/postcentral gyrus Right gyrus rectus/vmPFC Right precentral gyrus (BA 4)/superior frontal gyrus/superior precentral sulcus (BA 6)
ESS, Epworth Sleepiness Scale; vmPFC, ventromedial prefrontal cortex. All voxels significant at P < 0.05 (height) for false discovery rate (FDR), and extent corrected at P < 0.05 (FDR).
Fig. 2
L Sensory-motor
R Sensory-motor
R Occip-temp
Functional connectivity
0.40
0.40
0.20
0.20
0.20
0.00
0.00
0.00
−0.20
−0.20
−0.20
−0.40
−0.40 −0.60
−0.40
−0.60 0
5
10
15
20
−0.80 0
vmPFC
5
10
15
20
5
10
15
20
10
15
20
L Paracen Fossa
R Motor
0.40 Functional connectivity
0
0.40 0.25
0.20
0.20
0.00
0.00
0.00
−0.25
−0.20
−0.20
−0.40
−0.50
−0.40
−0.60
−0.75
−0.60
0
5
10 ESS total
15
20
0
5
10 ESS total
15
20
0
5
ESS total
Scatterplots show the inverse linear association between Epworth Sleepiness Scale (ESS) scores and thalamic connectivity values (Fisher transformed correlation coefficients) for the six-major clusters including: left sensory-motor cortex (top left; r2 = 0.34), right occipito-temporal cortex (top middle; r2 = 0.26), right sensory-motor cortex (top right; r2 = 0.24), ventromedial prefrontal cortex (vmPFC; bottom left; r2 = 0.33), right motor cortex (bottom middle; r2 = 0.25), and left paracentral fossa (bottom right; r2 =0.31). vmPFC, ventromedial prefrontal cortex.
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Sleepiness and functional connectivity Killgore et al. 783
projection (Fig. 1b), activation maps of the primary sensory and motor cortex (Fig. 1c), and a cortical surface map (Fig. 1c). These figures emphasize a pattern of negative connectivity between the bilateral thalamus and somatosensory/motor cortex in association with greater levels of daytime sleepiness. Voxel clusters showing significant negative connectivity with the bilateral thalamic seed that were associated with higher ESS scores are detailed in Table 1. The table presents the location of the cluster maximum for each region where functional connectivity with the thalamus decreased (i.e. became more negative) in association with greater sleepiness. Finally, Fig. 2 presents scatterplots summarizing the inverse linear association between ESS scores and mean functional connectivity (Fisher transformed correlation coefficients) for the six clusters.
Discussion As hypothesized, greater daytime sleepiness scores on the ESS were significantly correlated with lower thalamocortical rsFC, particularly for somatosensory and motor regions. Specifically, with progressively greater levels of self-reported daytime sleepiness, the thalamic ROIs showed greater inverse connectivity with signal intensity in the precentral gyrus, postcentral gyrus, and broad regions of the parietal and occipito-temporal cortex. Given the well-established role of these regions in motor planning and execution, primary somatosensory input, and sensory integration [18], we interpret our findings as reflecting the underlying neurobiological mechanism by which sleepiness leads to disengagement of exteroceptive processing from the ongoing stream of consciousness. The present findings are consistent with prior rsFC results during TSD, which suggest that a single night of sleep loss leads to instability of thalamic functioning [19] and significant reductions in thalamocortical connectivity in some of the same brain regions observed here [8]. For instance, Shao et al. [8] showed that TSD was associated with reduced thalamocortical connectivity with the middle temporal cortex, parahippocampal gyrus, and medial and superior frontal gyri. In contrast, although we also found that sleepiness was associated with thalamocortical connectivity with the middle temporal cortex, we did not observe reduced functional connectivity with dorsal prefrontal regions. This suggests that at the modest levels of general sleepiness we observed, thalamocortical connectivity disruptions were constrained primarily to somatosensory, motor, and sensory integration regions, but this level of sleepiness was not associated with altered connectivity to most higher executive control systems. This raises the possibility that while modest levels of general daytime sleepiness may contribute to some changes in thalamocortical connectivity within sensory-motor systems, TSD may lead to additional connectivity disruptions with other higher order executive systems through an acute exacerbation of sleepiness (i.e.
more severe acute sleepiness) or through other neurobiological processes that may emerge only after normal sleep is substantially curtailed. Although general daytime sleepiness was not associated with dorsal prefrontal connectivity, we did find greater negative thalamocortical connectivity with the gyrus rectus, a region of the vmPFC. This finding is noteworthy, as the vmPFC has previously emerged as a region that is sensitive to sleep loss [20] and plays a critical role in emotional and value-based decision-making during TSD [21,22]. Recent evidence suggests that reduced gray matter volume of the vmPFC is also associated with sleep related problems, such as chronic insomnia [23], sleep apnea [24], and narcolepsy [25], whereas individuals who habitually obtain sleep in excess of their subjective needs tend to have greater cortical volume within this region [26]. The present finding therefore provides further insight into the neurobiological changes that contribute to the cognitive-emotional deficits that emerge during periods of insufficient sleep [2]. It is interesting that we did not find altered thalamocortical connectivity with primary visual cortex, as prior work had suggested that TSD may lead to disengagement of visual processing [27]. However, such deficits are typically observed following a full night or longer without sleep, and the modest levels of sleepiness observed in the present study may have been insufficient to affect visual systems. It should also be kept in mind that the present findings are limited by the resting state nature of the data. We did not collect cognitive task data during the scan, so it is not possible to determine whether the findings are directly related to cognition or behavior. Future studies using task-based connectivity methods may explore the functional consequences of the altered corticothalamic connectivity occurring with greater sleepiness.
Conclusion Greater self-reported daytime sleepiness was associated with altered thalamocortical connectivity during rested wakefulness, particularly within somatosensory, motor, and parietal association regions. These findings extend prior work on TSD to encompass sleepiness in nonsleep deprived individuals and potentially reflect the disengagement of sensory and motor processing from the stream of consciousness with increased levels of general sleepiness.
Acknowledgements This research was supported by a USAMRAA grant (W81XWH-09-1-0730). Conflicts of interest
There are no conflicts of interest.
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784 NeuroReport 2015, Vol 26 No 13
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Daytime sleepiness is associated with altered resting thalamocortical connectivity William D.S. Killgore, John R. Vanuk, Sara A. Knight, Sarah M. Markowski, Derek Pisner, Bradley Shane, Andrew Fridman and Anna Alkozei Thalamocortical connectivity is believed to underlie basic alertness, motor, sensory information processing, and attention processes. This connectivity appears to be disrupted by total sleep deprivation, but it is not known whether it is affected by normal variations in general daytime sleepiness in nonsleep deprived persons. Healthy adult participants completed the Epworth Sleepiness Scale and underwent resting-state functional MRI. Functional connectivity between the thalamus and other regions of the cortex was examined and correlated with Epworth Sleepiness Scale scores. Greater sleepiness was associated with inverse (i.e. lower or more negative) connectivity between the bilateral thalamus and cortical regions involved in somatosensory and motor functions, potentially reflecting the disengagement of sensory
Introduction Acute total sleep deprivation (TSD) and prolonged partial sleep restriction are associated with a host of neurocognitive and emotional deficits [1,2], but the underlying neurobiological causes for these deficits are still being explored. After a night of TSD, the brain shows dramatic declines in metabolic activity within key systems involved in information transfer and attention, including the thalamus and higher order brain regions such as the prefrontal and parietal cortex [3]. These same systems are also adversely affected by TSD when measured by functional MRI (fMRI) during working memory tasks, which show reduced task-related activation in primary sensory and attention-related regions [4]. More recently, it has been shown that TSD also disrupts low-level intrinsic temporal correlations (i.e. functional connectivity) between key regions involved in alertness and cognition, including the default mode network and its anticorrelated networks [5–7]. Moreover, recent evidence suggests that TSD might impair cognition by disrupting functional connectivity between the thalamus (a key structure involved in alertness and sensory information transfer) and higher cortical regions [8,9]. Recently, Tomasi and colleagues also posited that TSD leads to compensatory hyperactivation of the thalamus to sustain alertness during prolonged wakefulness. However, such directed thalamic activation may come at a cost; namely it could lead to reduced functional and cognitive capacity within other attention demanding areas such as the parietal and prefrontal cortex [9]. Thus, TSD 0959-4965 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
and motor processing from the stream of consciousness. NeuroReport 26:779–784 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. NeuroReport 2015, 26:779–784 Keywords: functional connectivity, functional MRI, medial prefrontal cortex, motor cortex, neuroimaging, parietal cortex, sensory cortex, sleepiness Social, Cognitive and Affective Neuroscience Lab, University of Arizona College of Medicine, Tucson, Arizona, USA Correspondence to William D.S. Killgore, PhD, Social, Cognitive, and Affective Neuroscience Lab, Department of Psychiatry, University of Arizona College of Medicine, 1501 N Campbell Ave, Tucson, AZ 85724, USA Tel: + 1 520 621 0605; fax: + 1 520 626 6050; e-mail: [email protected] Received 3 June 2015 accepted 15 June 2015
leads to reduced or inverse patterns of thalamocortical connectivity. At present, it remains unknown whether functional connectivity disturbances seen in TSD might extend to less extreme sleep loss situations routinely encountered outside of the laboratory such as variations in general daytime sleepiness among nonsleep deprived individuals. Building upon the functional connectivity findings observed during TSD [8,9], we hypothesized that individuals reporting greater general daytime sleepiness would tend to show reduced thalamocortical functional connectivity during a brief resting-state fMRI scan relative to individuals with minimal daytime sleepiness.
Methods Participants
Sixty (30 male; 30 female) right-handed, healthy adults ranging in age from 18 to 45 years (M = 30.5, SD = 8.1) were recruited from the New England area via posted flyers and Internet advertisements. Participants were initially screened over the telephone by a trained research technician to exclude for any history of severe medical, neurological, or psychiatric conditions, head injury, alcohol or drug treatment, or current use of psychoactive drugs or medications. Participants had obtained an average of 14.9 years of education (SD = 2.2), and described themselves as normal sleepers, averaging 7.3 h (SD = 0.83) on weeknights and 7.7 h (SD = 1.2) on weekends according to self-report. Typical caffeine use DOI: 10.1097/WNR.0000000000000418
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780 NeuroReport 2015, Vol 26 No 13
was reported to be generally low to moderate (M = 138.9 mg/day, SD = 151.4), and caffeine intake on the day of the scan was close to their typical levels (M = 81.9 mg, SD = 103.7). Each participant provided written informed consent and received nominal financial compensation. Other data from a subset of this sample have been reported elsewhere [10–12], but the analyses reported herein linking sleepiness and thalamic connectivity are novel and have not been previously published. The McLean Hospital Institutional Review Board and the US Army Human Research Protections Office approved this research. Materials and procedure
Participants underwent a morning assessment session that involved completing several questionnaires and cognitive tasks. As part of this process, participants completed the Epworth Sleepiness Scale (ESS), a wellvalidated eight-item instrument that measures the severity of general daytime sleepiness on a scale from 0 to 24 [13]. In the early afternoon, participants underwent resting-state functional MRI (rsfMRI) for 6 min, with their eyes open. Magnetic resonance imaging parameters
Data acquisition occurred on a 3.0 T Siemens (Erlangen, Germany) Tim Trio scanner with a 12-channel head coil. First, T1-weighted 3D MPRAGE structural images were collected (TR/TE/flip angle = 2.1 s/2.25 ms/12°) over 128 sagittal slices (256 × 256 matrix) with a voxel size of 1.0 × 1.33 × 1.0 mm. Then, rsfMRI data were collected using a T2*-weighted blood oxygen level dependent echoplanar imaging sequence (TR/TE/flip angle = 2.0 s/30 ms/90°) over 34 transverse interleaved slices, with 180 images per slice (3.5 mm thickness, no skip; 22.4 cm field of view; 64 × 64 acquisition matrix). Image processing
SPM12 (Wellcome Department of Cognitive Neurology, London, UK) was used for preprocessing and statistical analyses. Preprocessing involved standard routines for motion correction, slice-timing correction, anatomical coregistration, spatial normalization, and spatial smoothing using an isotropic Gaussian kernel [full-width at halfmaximum = 6 mm (resliced to 2 × 2 × 2 mm)]. The Functional Connectivity Toolbox (CONN), version 14n (http://www.nitrc.org/projects/conn) was used for rsfMRI connectivity (rsFC) analyses. Denoising of data involved band-pass filtering (0.01, 0.10 Hz), removal of physiological noise using the aCompCor strategy [14], and removal of confounds including white matter, cerebrospinal fluid, and motion parameters. In addition, the Artifact Detection Tool was used to identify and regress out scan-to-scan movement greater than 1 mm and global intensity greater than 3 SD. No participant had greater than 3 mm of head movement during the resting scan.
Statistical analysis
Initially, after the removal of confounds, the residual blood oxygen level dependent signal timecourse from the bilateral thalamic seed regions were extracted and Pearson correlations were computed with all other voxels in the brain to derive connectivity maps. These z-score transformed connectivity maps were then entered into a general linear model regression analysis at the second level to evaluate the relationship between ESS scores and the strength of thalamic functional connectivity with the rest of the brain. Two seed regions were placed corresponding to the left and right thalamus as defined by the Automated Anatomical Labeling (AAL) atlas [15]. In addition to seed-to-voxel connectivity in CONN, we also conducted region of interest (ROI)-to-ROI analysis, with target regions defined by the 88 other cerebral ROIs in the AAL atlas. In accord with the standard procedures for CONN (http://www.nitrc.org/projects/conn) for seed-tovoxel analyses, a combination of height and extent thresholds was used to control for false positives within the connectivity maps. Whole brain connectivity maps were height thresholded for two-sided tests at P less than 0.05 [false discovery rate (FDR) corrected], whereas spatial extent (i.e. cluster size) was simultaneously thresholded at P less than 0.05 (FDR corrected). Furthermore, ROI-to-ROI maps were FDR corrected at these thresholds at the analysis-level to account for the number of ROIs included in the analysis. Anatomical specificity of the findings was first determined by the location in the AAL atlas [15], with further refinement by cross-reference with more detailed brain atlases [16,17].
Results Daytime sleepiness scores on the ESS ranged from 0 to 16 (M = 5.98, SD = 3.65). Analysis of the rsFC data revealed that greater ESS scores were associated with greater anticorrelated thalamocortical connectivity that involved primarily somatosensory and motor regions. Specifically, higher ESS scores were associated with decreased thalamic connectivity to areas of the primary somatosensory cortex, including superior regions of the right postcentral gyrus [i.e. Brodmann’s area (BA) 2 and BA 3]. Higher ESS scores were also associated with decreased thalamic connectivity bilaterally within the precentral sulcus, and areas considered to be motor and premotor regions, including regions of the precentral gyrus (BA 4) and superior precentral sulcus/superior frontal gyrus (BA 6). On the superior medial surface of the brain, ESS scores were inversely correlated with thalamocortical connectivity with bilateral areas of the paracentral lobule (medial aspect of BA 4, posterior to the paracentral sulcus) and the supplementary motor area (medial aspect of BA 6, anterior to the paracentral sulcus). Notably, the region most significantly decreased in connectivity with higher ESS was the junction between the right middle temporal gyrus and anterior regions of the right occipital lobe (BA 37). Finally, reduced connectivity
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Sleepiness and functional connectivity Killgore et al. 781
at higher ESS scores was observed bilaterally for the ventromedial prefrontal cortex (vmPFC)/gyrus rectus as well. This connectivity pattern, depicted in Fig. 1a, presents ROI-to-ROI connectivity maps showing that the connectivity of the left thalamus to seven of 88 cerebral
ROIs was negatively correlated with ESS, whereas the connectivity of the right thalamus was negatively correlated with ESS for 21 of the 88 ROIs. In addition, the figures show this association in the seed-to-voxel connectivity maps in terms of the maximum intensity
Fig. 1
(a)
(b)
(c)
(d) Superior precentral sulcus
Postcentral gyrus
Occipital-temporal junction
Paracentral lobule/ SMA
vmPFC
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782 NeuroReport 2015, Vol 26 No 13
Higher scores on the Epworth Sleepiness Scale (ESS) were correlated with reduced (i.e. inverse) thalamocortical functional connectivity. (a) The region of interest (ROI)-to-ROI pattern of connectivity between the left and right thalami (green circles) and other cortical regions is displayed (height threshold P < 0.05, FDR corrected, with an FDR correction at the analysis level to account for the number of ROIs in the analysis). Dark blue circles correspond to regions showing significant connectivity to the left thalamus, and light blue circles correspond to regions showing connectivity to the right thalamus. The darker the line, the more negative the connectivity value. Thalamic functional connectivity was most prominent for somatosensory and motor regions. Seed-to-voxel connectivity for the combined left and right thalami are shown as (b) a standard three-orientation maximum intensity projection (MIP) through the plane containing the paracentral fossa (MNI: − 2, − 16, 54; red carets), which clearly shows the predominant activation in sensory and motor cortex (height threshold P < 0.05, FDR corrected and extent threshold of P < 0.05, FDR corrected), (c) a standard three-view SPM activation map highlighting the sensory and motor strip activation (MNI coordinates: − 2, − 16, 54; height threshold P < 0.05, FDR corrected and extent threshold of P < 0.05, FDR corrected), and d) a cortical inflation map showing the major clusters of activation on the cortical surface (height threshold P < 0.05, FDR corrected and extent threshold of P < 0.05, FDR corrected). The red arrows highlight major regional clusters of activation. FDR, false discovery rate; vmPFC, ventromedial prefrontal cortex.
Table 1
Cortical voxel clusters showing inverse thalamic functional connectivity with greater sleepiness on the ESS MNI coordinates
Target region
Cluster size
x
y
z
Pearson r
T
1691 3186 250 1437 97 680
60 − 50 −2 32 4 22
− 62 −4 − 16 − 40 34 −2
2 30 54 46 − 20 60
− 0.614 − 0.589 − 0.555 − 0.543 − 0.528 − 0.519
5.92 5.55 5.08 4.92 4.74 4.63
Right anterior occipital lobe/middle temporal gyrus (BA37) Left precentral gyrus (BA 4)/superior frontal gyrus/superior precentral sulcus (BA 6) Left/right paracentral fossa (medial BA 4, 6)/supplementary motor area Right parietal/postcentral gyrus Right gyrus rectus/vmPFC Right precentral gyrus (BA 4)/superior frontal gyrus/superior precentral sulcus (BA 6)
ESS, Epworth Sleepiness Scale; vmPFC, ventromedial prefrontal cortex. All voxels significant at P < 0.05 (height) for false discovery rate (FDR), and extent corrected at P < 0.05 (FDR).
Fig. 2
L Sensory-motor
R Sensory-motor
R Occip-temp
Functional connectivity
0.40
0.40
0.20
0.20
0.20
0.00
0.00
0.00
−0.20
−0.20
−0.20
−0.40
−0.40 −0.60
−0.40
−0.60 0
5
10
15
20
−0.80 0
vmPFC
5
10
15
20
5
10
15
20
10
15
20
L Paracen Fossa
R Motor
0.40 Functional connectivity
0
0.40 0.25
0.20
0.20
0.00
0.00
0.00
−0.25
−0.20
−0.20
−0.40
−0.50
−0.40
−0.60
−0.75
−0.60
0
5
10 ESS total
15
20
0
5
10 ESS total
15
20
0
5
ESS total
Scatterplots show the inverse linear association between Epworth Sleepiness Scale (ESS) scores and thalamic connectivity values (Fisher transformed correlation coefficients) for the six-major clusters including: left sensory-motor cortex (top left; r2 = 0.34), right occipito-temporal cortex (top middle; r2 = 0.26), right sensory-motor cortex (top right; r2 = 0.24), ventromedial prefrontal cortex (vmPFC; bottom left; r2 = 0.33), right motor cortex (bottom middle; r2 = 0.25), and left paracentral fossa (bottom right; r2 =0.31). vmPFC, ventromedial prefrontal cortex.
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Sleepiness and functional connectivity Killgore et al. 783
projection (Fig. 1b), activation maps of the primary sensory and motor cortex (Fig. 1c), and a cortical surface map (Fig. 1c). These figures emphasize a pattern of negative connectivity between the bilateral thalamus and somatosensory/motor cortex in association with greater levels of daytime sleepiness. Voxel clusters showing significant negative connectivity with the bilateral thalamic seed that were associated with higher ESS scores are detailed in Table 1. The table presents the location of the cluster maximum for each region where functional connectivity with the thalamus decreased (i.e. became more negative) in association with greater sleepiness. Finally, Fig. 2 presents scatterplots summarizing the inverse linear association between ESS scores and mean functional connectivity (Fisher transformed correlation coefficients) for the six clusters.
Discussion As hypothesized, greater daytime sleepiness scores on the ESS were significantly correlated with lower thalamocortical rsFC, particularly for somatosensory and motor regions. Specifically, with progressively greater levels of self-reported daytime sleepiness, the thalamic ROIs showed greater inverse connectivity with signal intensity in the precentral gyrus, postcentral gyrus, and broad regions of the parietal and occipito-temporal cortex. Given the well-established role of these regions in motor planning and execution, primary somatosensory input, and sensory integration [18], we interpret our findings as reflecting the underlying neurobiological mechanism by which sleepiness leads to disengagement of exteroceptive processing from the ongoing stream of consciousness. The present findings are consistent with prior rsFC results during TSD, which suggest that a single night of sleep loss leads to instability of thalamic functioning [19] and significant reductions in thalamocortical connectivity in some of the same brain regions observed here [8]. For instance, Shao et al. [8] showed that TSD was associated with reduced thalamocortical connectivity with the middle temporal cortex, parahippocampal gyrus, and medial and superior frontal gyri. In contrast, although we also found that sleepiness was associated with thalamocortical connectivity with the middle temporal cortex, we did not observe reduced functional connectivity with dorsal prefrontal regions. This suggests that at the modest levels of general sleepiness we observed, thalamocortical connectivity disruptions were constrained primarily to somatosensory, motor, and sensory integration regions, but this level of sleepiness was not associated with altered connectivity to most higher executive control systems. This raises the possibility that while modest levels of general daytime sleepiness may contribute to some changes in thalamocortical connectivity within sensory-motor systems, TSD may lead to additional connectivity disruptions with other higher order executive systems through an acute exacerbation of sleepiness (i.e.
more severe acute sleepiness) or through other neurobiological processes that may emerge only after normal sleep is substantially curtailed. Although general daytime sleepiness was not associated with dorsal prefrontal connectivity, we did find greater negative thalamocortical connectivity with the gyrus rectus, a region of the vmPFC. This finding is noteworthy, as the vmPFC has previously emerged as a region that is sensitive to sleep loss [20] and plays a critical role in emotional and value-based decision-making during TSD [21,22]. Recent evidence suggests that reduced gray matter volume of the vmPFC is also associated with sleep related problems, such as chronic insomnia [23], sleep apnea [24], and narcolepsy [25], whereas individuals who habitually obtain sleep in excess of their subjective needs tend to have greater cortical volume within this region [26]. The present finding therefore provides further insight into the neurobiological changes that contribute to the cognitive-emotional deficits that emerge during periods of insufficient sleep [2]. It is interesting that we did not find altered thalamocortical connectivity with primary visual cortex, as prior work had suggested that TSD may lead to disengagement of visual processing [27]. However, such deficits are typically observed following a full night or longer without sleep, and the modest levels of sleepiness observed in the present study may have been insufficient to affect visual systems. It should also be kept in mind that the present findings are limited by the resting state nature of the data. We did not collect cognitive task data during the scan, so it is not possible to determine whether the findings are directly related to cognition or behavior. Future studies using task-based connectivity methods may explore the functional consequences of the altered corticothalamic connectivity occurring with greater sleepiness.
Conclusion Greater self-reported daytime sleepiness was associated with altered thalamocortical connectivity during rested wakefulness, particularly within somatosensory, motor, and parietal association regions. These findings extend prior work on TSD to encompass sleepiness in nonsleep deprived individuals and potentially reflect the disengagement of sensory and motor processing from the stream of consciousness with increased levels of general sleepiness.
Acknowledgements This research was supported by a USAMRAA grant (W81XWH-09-1-0730). Conflicts of interest
There are no conflicts of interest.
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784 NeuroReport 2015, Vol 26 No 13
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