Altered Sleep Brain Functional Connectivity in Acutely Depressed Patients

Transcription

1 Human Bain Mapping 30: (2009) Alteed Sleep Bain Functional Connectivity in Acutely Depessed Patients Samuël J.J. Leistedt, 1,2 * Nathalie Coumans, 1,2 Matine Dumont, 3 Jean-Pol Lanquat, 1 Conelis J. Stam, 4 and Paul Linkowski 1 1 Sleep Unit and Laboatoy of Psychiatic Reseach, Depatment of Psychiaty, Easme Academic Hospital, Univesité Libe de Buxelles (U.L.B.), Bussels, Belgium 2 National Fund fo Scientific Reseach (F.N.R.S.), Reseach Fellow, Bussels, Belgium 3 Depatment of Biological Physics, Univesité de Mons Hainaut, Mons, Belgium 4 Depatment of Clinical Neuophysiology, VU Univesity Medical Cente, Amstedam, The Nethelands Abstact: Recent evidence suggests that poblems in infomation pocessing within neual netwoks may undelie depessive disease. In this study, we investigated whethe sleep functional bain netwoks ae abnomally oganized duing a majo depessive episode (MDE). We chaacteized spatial pattens of functional connectivity by computing the synchonization likelihood (SL) of 19 sleep EEG channels in 11 acutely depessed patients [42 (20 51) yeas] and 14 healthy contols [32.9 (27 42) yeas]. To test whethe disupting an optimal patten [ small-wold netwok (SWN)] of functional bain connectivity undelies MDE, gaph theoetical measues wee then applied to the esulting synchonization matices, and a clusteing coefficient (C, measue of local connectedness) and a shotest path length (L, measue of oveall netwok integation) wee detemined. In the depessed goup, the mean SL was lowe in the delta, theta and sigma fequency bands. Acutely depessed patients showed a significantly lowe path length in the theta and delta fequency bands, wheeas the cluste coefficient showed no significant changes. The pesent study povides futhe suppot that sleep functional bain netwoks exhibit small-wold popeties. Sleep neuonal functional netwoks in depessed patients ae chaacteized by a functional eoganization with a lowe mean level of global synchonization and loss of SWN chaacteistics. These esults ague fo consideing an MDE as a poblem of neuonal netwok oganization and a poblem of infomation pocessing. Hum Bain Mapp 30: , VC 2008 Wiley-Liss, Inc. Key wods: neuonal netwoks; synchonization; small-wold netwok; sleep; depession; insomnia INTRODUCTION Contact gant sponso: National Fund fo Scientific Reseach (F.N.R.S.). *Coespondence to: Samuël J.J. Leistedt, Sleep Laboatoy, Depatment of Psychiaty, Easme Academic Hospital, Univesité Libe de Buxelles (U.L.B.), Route de Lennik, 808, 1070 Bussels, Belgium. samuel.leistedt@ulb.ac.be Received fo publication 1 May 2008; Revised 3 July 2008; Accepted 30 July 2008 DOI: /hbm Published online 20 Octobe 2008 in Wiley InteScience (www. intescience.wiley.com). Neuons fom a dense netwok of connectivity spanning the entie thalamocotical system in the ceebal cotex [Spons, 2002]. These bain netwoks ae not andom, but fom highly specific patten [Spons, 2002]. A pedominant featue of bain netwoks is that neuons tend to connect with othe neuons in local goups. Because of the close elationship between neual connectivity and neual activity thoughout the bain, it is impotant to conside stuctual connection pattens within the context of the specific pattens of dynamic ( functional ) inteactions that they suppot [Spons, 2002]. Recent beakthoughs in moden VC 2008 Wiley-Liss, Inc.

2 Leistedt et al. TABLE I. Clinical and demogaphic measues Healthy contols Depessed patients (n 5 14) (n 5 11) P value Demogaphic measues Sex, F/M 4/14n 2/11n 0.17 Age, y* 32.9 (27 42) 42 (20 51) 0.20 Maital status, No (%) Maied 4 (28.57) 4 (36.36) NA Single-neve maied 1 (7.14) 1 (9.09) NA Divoced 5 (35.71) 4 (36.36) NA Cohabiting 4 (28.57) 2 (18.18) NA Employment status, No (%) Manage 7 (50) 4 (36.36) NA Employed 4 (28.57) 4 (36.36) NA Wokman 3 (21.43) 3 (27.27) NA Clinical measues* 24-Item Hamilton Depession scoe 0 (0 2) 27 (17 56) < Pittsbugh Sleep Quality Index 1.5 (1 8) 11 (1 20) < NA means Non Applicable. *Values ae indicated as median with maximum and minimum ange. netwok theoy now allow a bette chaacteization and undestanding of complex netwoks that is also elevant fo undestanding bain netwoks. An impotant concept is that of a small-wold netwok (SWN), which is a type of mathematical gaph in which most nodes ae not neighbos of one anothe (the netwok is spasely connected), but most nodes can be eached fom evey othe node by a small numbe of steps [Watts and Stogatz, 1998]. Social netwoks, spatial games, excitable media, the connectivity of Intenet connectivity, and gene netwoks exhibit SWN chaacteistics [Watts and Stogatz, 1998]. Thee ae empiical and theoetical a pioi easons why SWNs pesent an attactive model fo bain netwok connectivity [Watts and Stogatz, 1998]: (i) The bain is a complex netwok on multiple spatial and time scales [Watts and Stogatz, 1998]; (ii) The bain suppots both segegated and distibuted infomation pocessing; and (iii) The bain likely evolved to maximize efficiency and/o minimize the costs of infomation pocessing. Netwok achitectue is egaded as a key substate fo sensoimoto and cognitive pocessing, which may be localized discetely in specialized egions o epesented by coheent oscillations in lage-scale distibuted systems. Small-wold topology compises both high clusteing (compatible with segegated o modula pocessing) and shot path length (compatible with distibuted o integated pocessing), and is associated with high global and local efficiency of paallel infomation pocessing, spase connectivity between nodes, and low wiing costs. SWNs can opeate dynamically in a citical state, facilitating apid adaptive econfiguation of neuonal assemblies in suppot of changing cognitive states [Watts and Stogatz, 1998]. In this study, ou goal is to use computational models to exploe how depessive disease behavios elate to the geneal synchonization and connectivity of the undelying neuonal netwok duing sleep. We popose to detemine the pattens of neuonal functional connectivity duing sleep using two methods that allow fo the study of neuonal netwok behavio: the synchonization likelihood (SL) and the gaph theoy analysis. We hypothesized that acute depessive status will intefee with the optimal achitectue of bain functional netwoks and could show functional connectivity eoganization as a disconnection state of the neual netwok. In this way, we want to confim the fact that depession can be pimaily viewed as an infomation pocessing poblem in the bain. MATERIALS AND METHODS Paticipants: Patient and Contol Selection The pesent study included 25 subjects matched on age and education level: 14 healthy contols and 11 unmedicated inpatients with acute majo depession accoding to DSM- IV-TR citeia [Amecian Psychiatic Association, 2000]. Desciptive clinical featues of the acutely depessed patients and healthy contol subjects ae pesented in Table I. Healthy contols wee ecuited fom the community by advetisements. On the basis of an extensive clinical inteview, they wee detemined to be fee of DSM-IV-TR axis I o evident axis II diagnoses and they had no family histoy of majo psychiatic disodes. They epoted a egula sleep wake schedule and no cuent o past sleep disodes. Patients wee ecuited fom both the Sleep Laboatoy and an inpatient psychiaty wad at the Easme Academic Hospital, whee they wee hospitalized fo both MDD and sleep distubances. Distubances included difficulty falling asleep, difficulty staying asleep, and ealy moning awakening. Sleep self-epots wee obtained by the Pittsbugh Sleep Quality Index (PSQI). The PSQI (ange 0 21, highe values indicating geate sleep distubances) is a self-ated questionnaie that assesses sleep quality and distubances duing the pevious 1-month time inteval [Buysse et al., 1989]. The 19 self-ated questions assess a wide vaiety of 2208

3 Functional Connectivity in Depession factos elating to sleep quality, including estimates of sleep duation and latency, and the fequency and seveity of specific sleep-elated poblems. A global PSQI scoe of 5 was found to coectly disciminate between good and poo sleepes [Buysse et al., 1989]. Patients wee initially evaluated fo a MDD by a psychiatist using DSM- IV-TR citeia. Depessive symptom seveity was assessed with the 24-item Hamilton Rating Scale fo Depession (HAM-D). Patients wee included in the pesent study if they fulfilled the following five citeia: (1) they suffeed fom majo depession (unipola without psychotic featues); (2) they wee fee of all pesciption and nonpesciption psychotopic medications; (3) they had a HAM-D scoe of 20 o geate [Hamilton, 1960]; (4) they had a PSQI of five o geate; and (5) they did not suffe fom unteated o pooly contolled conditions that may have confounded the sleep EEG esults (e.g., Cushing s disease), o equie teatment with agents that may affect esults (i.e., b-blockes o coticosteoids). Both contols and patients wee medically sceened by way of physical examination (pefomed by an intenist), chest X-ay, electocadiogaphy, electoencephalogaphy, and laboatoy tests such as live and kidney function tests, hematology pofile, thyoid function tests, and uinalysis. They did not show cadiovascula o endocine abnomalities, o othe systemic illness. Subjects with a BMI geate than 29 wee excluded. We also excluded contols o patients who showed pimay sleep disodes such as apnea hypopnea syndome, peiodic leg movement syndome, o paasomnia. Befoe signing an infomed consent, each subject eceived a detailed desciption and demonstation of the pocedue involved in the study, and was deemed capable. The study potocol was appoved by the local ethics committee of the Easme Academic Hospital-Fee Univesity of Bussels. EEG Recodings and Expeimental Conditions EEG sleep studies wee pefomed in the Sleep Laboatoy of the Easme Academic Hospital. In the patient goup, sleep studies wee conducted afte a psychotopic medication-fee evaluation peiod of at least 2 weeks. Polysomnogaphic ecodings wee obtained duing thee consecutive nights, of which only the latte two wee examined because of the well-ecognized fist night effect on sleep measues [Agnew et al., 1966]. Fo the puposes of this analysis, one atifact-fee night was chosen fom the latte two nights. If both nights showed low levels of atifact, then one was andomly selected. Atifacts wee detected by visual obsevation using the softwae Endymion (Endymion , Sleep laboatoy, Easme Hospital), which was developed in ou laboatoy fo data analysis. Patients and contols wee instucted not to dink alcohol o coffee o use ove-the-counte sleep aids duing the same time fame. Subjects went to bed and got up at thei usual times. Duing bedtime hous, contols and patients wee supine with lights off. They awoke spontaneously in the moning, and daytime naps wee stictly pohibited. Both contols and patients had a minimum of seven consecutive hous of ecoded time in bed. Polysomnogaphy was ecoded with a 19-channel digital polygaph (Alice5 TM, Respionics, Muysville, PA). The 19 electoencephalogams wee ecoded, accoding to the intenational standad system, with a contalateal efeence to the A1 o A2 mastoid deivation. Simultaneously, two electooculogams (EOG), one submental electomyogam (EMG), and electocadiogaphic activity (ECG) wee ecoded. Oxyhemoglobin satuation was measued using pulse-oximety (Masimo MS-7, Ivine CA), oo-nasal aiflow was detected with themistos (Infinity TM, Sleepmate Technologies, Midlothian, VA), thoacic and abdominal espiatoy movements wee ecoded with piezoelectic sensos (Resp-EZ TM, Sleepmate Technologies), and leg movements wee detected with ankle piezoelectic movement stain gauges (Moving Images TM, Sleepmate Technologies). All channels wee sampled at 2,000 Hz 16 bits. Fo subsequent analyses, the EEG was stoed at 200 Hz, the EOG at 100 Hz, the EMG at 100 Hz, and the ECG at 500 Hz. The Alice 5 device included adequate analog filtes to eliminate low fequency atifacts, difts, offsets, and aliasing. Also, adequate antialiasing filtes wee applied befoe down sampling. Fo example, a Bessel filte (Ode 3) is applied to the EEG befoe digital sampling, with a bandpass fom 0.32 to 106 Hz (23 db). To educe the fequency to 200 Hz, two filtes ae applied: a least-squaed FIR (Ode 20) low-pass filte at 160 Hz followed by a Kaise Window FIR (Ode 88) low-pass filte at 95 Hz. The last step involves a 50-Hz notch filte (Ode 30) with stop band at 43 and 57 Hz. The data wee expoted to the EDF fomat using Alice Sleepwae (Respionics). All subsequent analyses, such as stage detemination and spectum calculation, wee caied out on the sampled data, avoiding synchonization poblems between the stages and the othe calculations. Using the Endymion pogam, each 20-s epoch was visually scoed accoding to standad citeia [Rechtschaffen and Kales, 1968]. The Synchonization Likelihood: Theoy and Computation The SL, intoduced by Stam and van Dijk [2002], is a measue of the statistical intedependencies between a time seies (such as an EEG channel) and one o moe othe time seies within a dynamical system. This measue has aleady been used in seveal clinical neuosciences studies [Stam, 2005; Stam and Reijneveld, 2007]. Fo a detailed technical desciption of the method and its popeties, we efe to Stam and van Dijk [2002]. Hee we explain the geneal pinciples. The SL is sensitive to linea as well as nonlinea intedependencies and can be computed fo each time sample, making it suitable fo tacking time-dependent changes in the synchonization level. Hence, moe accuate infoma- 2209

4 Leistedt et al. tion is given about functional inteactions in a dynamical system [Stam, 2005; Stam and van Dijk, 2002]. The SL basic pinciple is that each time seies is divided into a seies of pattens, which ae seached fo ecuing pattens. Hee, a patten can be thought of as a bief pat of the time seies that contains at least one cycle of the chaacteistic fequency. Moe technically, the bief pattens ae vectos constucted by time-delay embedding, using paamete settings [as descibed in Montez et al., 2006]. The SL is the chance that patten ecuence in time seies X coincides with patten ecuence in time seies Y. The SL takes on values between P ef (a paamete that was taken as P ef as in othe SL studies), in the case of independent time seies and one in the case of fully synchonized time seies. The end esult afte computing SL fo all paiwise channels combinations, is a squae N 3 N matix of size 19 (19 is the numbe of the EEG channels used in this study), whee each enty N ij contains the value of the SL fo channels i and j. We computed the aveage synchonization by taking the mean of these values. Stuctue of the Netwok and Connectivity: Computing the Clusteing Coefficient (C) and the Chaacteistic Path Length (L) Figue 1. Schematic explanation of a gaph and gaph theoetical measues. This figue is based upon Stam et al. [2007]. A gaph consists of elements o vetices, denoted by black dots. If two vetices ae connected a line is dawn between them. Such a connection is called an edge. The size of a gaph is equal to the total numbe of vetices. The degee K of a gaph is the aveage numbe of edge pe vetex. In a gaph all vetices need to be connected. The distance between two vetices is expessed by the numbe of edges that have to be taveled to get fom the Vetex 1 to anothe. As an example, the shotest path fom vetex A to vetex F has a length of thee edges. The chaacteistic path length of a gaph is the mean of all shotest paths connecting all pais of vetices. L is a measue of how well connected a gaph is. The cluste coefficient C is a measue of the existence of local densely connected clustes within a netwok. Fo example, to compute the cluste coefficient fo vetex A, we fist detemine the othe vetices to which it is diectly connected. These neighbos ae vetices B, C, and D. Then we detemine how many edges exist in the set of neighbos. In this case, only B and C ae connected. Next, we detemine how many edges could have existed between the neighbos. In ou example, this is thee (B- C, C-D, and B-D). The cluste coefficient of A is the atio of these two numbes: 1/3. The cluste coefficient can be detemined fo all vetices. This esult is an aveage cluste coefficient fo the entie gaph. Optimal netwoks ae chaacteized by a high C and a low L. Such netwoks ae designated small-wold netwoks [Watt and Stogatz, 1998]. An inteesting appoach to study the topogaphical chaacteistics of both local and long distance functional connectivity in complex netwoks is to apply a measue deived fom gaph theoy [Spons, 2002; Stogatz, 2001; Watts and Stogatz, 1998]. A gaph is a basic epesentation of a netwok that is essentially educed to nodes (vetices) and connections (edges) (see Fig. 1). Gaphs ae chaacteized by a clusteing coefficient (C) and a chaacteistic path length (L), among othe measues. C is a measue of the local inteconnectedness of the gaph, wheeas L is an indicato of its oveall connectedness. Watts and Stogatz have shown that gaphs with many local connections and a few andom long distance connections ae chaacteized by a high C and a shot L. Such nea-optimal netwoks ae designated as small-wold netwoks [Watts and Stogatz, 1998]. Since then, many types of eal netwoks have been shown to have small-wold featues [Stogatz, 2001]. Pattens of anatomical connectivity in neuonal netwoks ae paticulaly chaacteized by high C and a small L [Watts and Stogatz, 1998]. It has also been suggested that small wold-like netwok achitectue may be optimal fo synchonizing neual activity between diffeent bain egions [Masuda and Aihaa, 2004]. C and L wee calculated following the methodology descibed in the studies of Newman [2003], Batolomei et al. [2006], and Ponten et al. [2007]. The fist step in applying gaph theoetical analysis to synchonization matices is to convet the N 3 N synchonization matix into a binay gaph (N is the numbe of EEG channels). A binay gaph is a netwok that consists of elements (also called vetices ) and undiected connections between elements (also called edges ). Edges between vetices eithe exist o do not exist; they do not have gaded values. The synchonization matix can be conveted into a gaph by consideing a theshold (T). If the SL between a pai of channels i and j exceeds T, an edge is said to exist between i and j, othewise, no edge exists between i and j. Once the synchonization matix has been conveted to a binay, unweighted gaph, the next step is to chaacteize the gaph in tems of its C and L. A schematic explanation of gaphs, C, and L is given in Figue

5 Functional Connectivity in Depession To compute the C of a cetain vetex, we fist detemine to which othe vetices it is diectly connected; these othe vetices ae called neighbos. The C is the atio of all existing edges between the neighbos and the maximum possible numbe of edges between the neighbos. It anges between 0 and 1. This C is computed fo all vetices of the gaph and then aveaged, and it measues fo the tendency of netwok elements to fom local clustes. The chaacteistic L is the aveage shotest path that connects any two vetices of the gaph. The length of a path is indicated by the numbe of edges it contains. The L is an emegent popety of the gaph that indicates how well its elements ae integated/inteconnected. Because C and L ae computed as a function of T, the esults may be influenced by diffeences in the mean level of synchonization between the two goups. The SL is expected to be significantly lowe fo acutely depessed patients than fo contols. Theefoe, depessed patient s gaphs will have fewe edges than contol subject s gaphs fo a given T value and this will influence the diffeences in C and L between the two goups. To contol fo this effect, we used an individual-adapted T such that we get a numbe of connections pe vetex K (which is the aveage numbe of edges pe vetex) fo each individual gaph with the same K fo all subjects and filte settings. In this way, gaphs studied in both goups ae guaanteed to have the same numbe of edges, so any emaining diffeences in C and L between the goups eflect only diffeences in gaph oganization. The choice of K is somewhat abitay. We followed the suggestion of Watts and Stogatz fo the minimal K value necessay fo a netwok with size N (hee 19 channels), such that a andom netwok geneated fom it will still be guaanteed to be fully connected: N [dmt] K [dmt] Ln (N) [Watts and Stogatz, 1998]. Hee, K numbe geate than 3 fulfils these conditions. In ou study we used K 5 5, with which we obtained the most stiking findings. K is fixed in ode to compae the topological stuctue of the netwoks without any bias fom diffeences in the mean SL [Fei et al., 2007]. By fixing K, all of the gaphs have the same numbe of vetices and edges; the only diffeences ae in the spatial aangement (which ae eflected by C and L) [Batolomei et al., 2006]. To avoid the poblem of the disconnected gaph, we used an altenative appoach based on the global efficiency [Latoa and Machioi, 2001], whee L is calculated as the ecipocal of the aveage of the ecipocals. Infinite values of L (when two edges ae not connected) contibute nothing to the sum [Newman, 2003]. Fo the clusteing coefficient C of the gaph, disconnected points ae assigned a value of 0. Then, the values of C and L wee compaed to theoetical values of andom netwoks geneated following the pocedue descibed by Spons and Zwi, which peseves the degee distibution exactly [Spons and Zwi, 2004]. Fo a K value of 5, fo each EEG, 50 andom netwoks wee geneated, and the mean C-s and L-s of all of these netwoks wee calculated as a efeence value fo C and L. The atios C/C-s and L/L-s wee consideed. SWNs ae chaacteized by the simultaneous pesence of values of C/C-s values significantly geate than 1.5 and values of L/L-s values aound the value one [Micheloyannis et al., 2006; Stam et al., 2007; Watts and Stogatz, 1998]. In the andomization pocedue, the degee distibutions of the oiginal netwoks wee peseved. The mean L/L-s atio in the diffeent epochs was above 1, wheeas the mean C/C-s atio was geate than 1.5. EEG Signal Analysis All EEG channels wee down sampled to 200 Hz. Using the Endymion pogam, all 20-s windows wee visually scoed accoding to standad citeia [Rechtschaffen and Kales, 1968]. Fo each of the thee sleep stages [Stage 2, SWS (Stages 3 and 4), and REM sleep], we andomly selected 25 atifact-fee (no eye blinks, slow eye movements, excess muscle activity, electocadiogam atifacts, etc.) 20,48-s epochs. The numbe of epochs (25) was abitaily chosen to obtain a common quantum in ode to cay out ou study despite the geat dispaity in the numbe of epochs duing the diffeent sleep stages. EEG epochs wee conveted to ASCII files, esulting in times seies of 4,096 samples fo futhe analysis. Digital, zeo-phase shift filteing of the EEG in thee fequency bands (delta: 1 4 Hz, theta: 4 8 Hz, and sigma: Hz), SL computation and the two gaph theoetical measues [cluste coefficient (C) and chaacteistic path length (L)] wee done off-line with the DIGEEGXP softwae witten by one of the authos [C.J. Stam]. Gaph theoetical analysis was based on the full matix of all possible paiwise combinations of electodes. Statistical Analysis Mann Whitney nonpaametic U-tests wee used to examine goup (contols vs. depessed) diffeences in clinical, demogaphic, and sleep measues as well as in SL values and gaph paametes (C, L, C/C-s, and L/L-s) within the diffeent fequency bands (delta, theta, and sigma). This test was deemed most appopiate because of the sample size and data distibution. Qualitative data wee compaed by means of the Fishe Exact test, because expected fequencies wee less than 5. Coelations wee studied with the Speaman s ank coelation coefficient. All analyses wee pefomed with a (Type I eo) set at 0.05 using the statistical softwae pogam SPSS fo Windows, vesion RESULTS Clinical and Demogaphic Measues Depessed and healthy goups did not diffe on age o sex (Table I). As expected, the depessed subjects showed 2211

6 Leistedt et al. TABLE II. Electoencephalogaphic sleep measues Healthy contols Depessed patients (n 5 14) (n 5 11) P value Sleep continuity Total sleep time (min)** ( ) ( ) Sleep latency (min) 31.5 (5 51) 25.7 (5 186) 0.58 Sleep efficiency (%) 84.7 ( ) 72.4 ( ) 0.08 No. of awakenings 62 (38 90) 73 (25 115) 0.76 Awake (min) 62.7 (20 90) 89.3 ( ) 0.35 Awake (% SPT) 11.8 ( ) 18.5 (3.7 47) 0.17 Non apid eye movement (NREM) % Stage (3.5 11) 10.4 ( ) 0.71 % Stage 2* 59.5 ( ) 54.5 ( ) % Slow wave sleep ( ) 12.1 (2.3 21) 0.55 Rapid eye movement (REM) % REM 22 ( ) 22.7 ( ) 0.91 REM latency** 71.8 (50 131) 48 (5 98) REM density (unit/min) 1.68 ( ) 1.74 ( ) 0.68 Values ae expessed as median with minimum maximum ange. *P-value <.05 **P-value <.01 significant elevations in depessive symptoms, as measued by the total scoe on the HDRS [Hamilton, 1960]. Accoding to the insomnia questions on the HRSD [Hamilton, 1960], sleep distubances wee modeately sevee and distibuted evenly thoughout the night (initial, middle, and delayed). Sleep distubance, as measued by the PSQI, was significantly highe in the depessed patients who epoted difficulty falling asleep, ealy waking, and many awakenings. Seven depessed patients had ecuent majo depession [aveage age at onset yeas (mean 6 SD)], while fou had single-episode majo depession [aveage age at onset yeas (mean 6 SD)]. Conventional Sleep Chaacteistics A Mann Whitney U-test evealed a significant decease in total sleep time as well as in REM latency onset among the patients goup (Table II). The depessed patient goup also showed seveal of the well-known pathognomonic sleep changes, such as Figue 2. Mean synchonization matices fo healthy contols (n 5 14) and acutely depessed patients (n 5 11) in the delta fequency band. The synchonization matix is a squae matix, whee X axis and the Y axis coespond with the channels numbes, and whee the enties indicate the mean stength of the SL between specific pais of channels. The stength of the SL is indicated with a black-gey scale, fom black (SL 5 0) to light gay (SL? 1). The diagonal unning fom the uppe left to the lowe ight is intentionally left dak gay. The names of the coesponding electodes accoding to the electode placement system have been indicated espectively on the X and Y axis. 2212

7 Functional Connectivity in Depession synchonization matices of Figue 2, in both goups and fo delta fequency band. As shown in Figue 5, the oganization of the neuonal netwok diffeed between the acutely depessed patients and healthy contols. Compaed with the depessed goup, the contol goup gaph shows a lage numbe of edges between the cental, tempoal, paietal, and fontal egions of the delta band (see Fig. 5). The most stiking SWN paamete values in the delta (NREM sleep SWS), theta (REM sleep), and sigma Figue 3. Mean synchonization likelihood (eo ba denote standad deviations) of healthy contols and depessed patients fo the diffeent fequency bands. * indicates P < ** indicates P < deceased sleep efficiency and deceased Stage 2 pecentage. We also obseved inceased sleep onset latency, Stage 1 pecentage, awakenings thoughout the night (in tems of numbe and pecentage), REM sleep pecentage, and REM density. We also obseved a deceased pecentage of SWS. Howeve, the above-mentioned paametes did not diffe significantly between the two goups in ou sample, but wee significantly diffeent in a lage numbe of pevious epots [Kekhofs et al., 1991; Kupfe, 1995]. Synchonization Likelihood Pe Fequency Band As seen in the delta fequency band (see Fig. 2), the synchonization matices of both goups show a complex but, nonetheless, athe simila patten, with vaious egions of high (light gey: SL? 1) and low (black) level of synchonization. These gaphs evealed global SL that occued at all channels. Ou esults show that the mean SL (aveaged ove all paiwise combinations of channels and time points) was significantly lowe in the depessed goup in all of the fequency bands that we have investigated (see Fig. 3): delta (NREM sleep slow wave sleep), theta (REM sleep), and sigma (NREM Stage 2). To ende the diffeences between goups moe clealy, we also computed SL ove all channels, compaing each channel with all othes fo the thee fequency bands of inteest (see Fig. 4). The loss of synchonization obseved in the depessed goup is global in the theta band, wheeas it localized moe specifically in the fontal, paietal, and tempoal aeas of the delta and sigma fequency bands. Small-Wold Netwok Paametes The synchonization N 3 N matices wee conveted to gaphs using an individual-adapted theshold which esulted in a value of K 5 5 fo all gaphs. As an example, Figue 5 shows the gaphs that coespond to the mean Figue 4. Mean synchonization likelihood in the diffeent channels in sigma (a), theta (b), and delta (c) fequency bands. * indicates P < ** indicates P <

8 Leistedt et al. Mean synchonization matices of Figue 2 conveted to gaphs using individual thesholds fo contols and depessed patients. Schematic image of the head seen fom above, with the positions of the electodes indicated by small cicles and numbeed accoding to the electode placement electode (a). Gaph of the contol subjects in the delta fequency band (b). Figue 5. Gaph of the acutely depessed patients in the delta fequency band (c). Diffeences between the two goups in the delta fequency band (d): Co De: edges only pesent in contol goup (solid lines); De Co: edges only pesent in depessed goup (dotted line). 2214

9 Functional Connectivity in Depession TABLE III. Small-wold netwok paametes Healthy contols Depessed patients (n 5 10) (n 5 11) P value C Delta 0.48 ( ) 0.48 ( ) 0.33 Theta 0.55 ( ) 0.52 ( ) 0.1 Sigma 0.54 ( ) 0.54 ( ) 0.29 L Delta* 2.1 ( ) 2.06 ( ) 0.03 Theta* 2.12 ( ) 2.08 ( ) 0.01 Sigma 2.09 ( ) 2.09 ( ) 0.89 C/C-s Delta 2.17 ( ) 2.13 ( ) 0.27 Theta 2.41 ( ) 2.3 ( ) 0.19 Sigma 2.43 ( ) 2.37 ( ) 0.17 L/L-s Delta* 1.24 ( ) 1.21 ( ) 0.03 Theta** 1.26 ( ) 1.21 ( ) Sigma 1.21 ( ) 1.22 ( ) 0.95 Values ae expessed as median with minimum maximum ange. *P-value <.05 **P-value <.01. (NREM sleep Stage 2) fequency bands ae pesented in Table III. Thee wee no significant clusteing coefficient diffeences between the depessed patients and the healthy contols in the fequency bands that wee investigated. Howeve, the C tends to show a geate value in contols than in the patient goup in the theta band. In contast, the chaacteistic L is significantly lowe in the depessed goup in the theta and delta fequency bands. The C and L values wee compaed with the values fo andom gaphs, using the pocedue descibed by Spons and Zwi [2004]. In the andomization pocedue, the degee distibutions of the oiginal netwoks wee peseved. As expected in SWN oganization, the mean L/L-s atio was slightly above 1 in the diffeent fequency bands, wheeas the mean C/C-s atio was significantly geate than 1 (Table III). In the diffeent fequency bands that wee investigated, thee wee no statistically significant C/ C-s diffeences. In the delta, especially, theta bands, L/L-s was significantly lowe in the depessed goup. As a summay, the most obvious changes occued in the depessed goup, in which a decease of both the L and L/L-s atio wee obseved duing delta and theta bands. Thus, alteations in gaph achitectue wee found to only affect L (except in sigma band). Mean SL, Small-Wold Netwok Paametes and the Seveity of Depession We also tested whethe the mean level of SL, C, and L wee elated to the seveity of depession. Linea coelations wee obseved between the global SL, C, L, and the HDRS as: the moe depessed the patient, less ae the global level of SL, C, and L in theta and delta fequency bands. These coelations wee significant fo SL in the sigma and delta fequency bands and fo L in the delta and theta fequency bands. Then, in these fequency bands and with the specific paametes (SL o L), the moe depessed ae the patients, the less ae the values of SL and L. This obsevation was contasted by an invese elation (without statistical significance) in the sigma fequency band fo L (Table IV). DISCUSSION The pincipal findings of the pesent study ae twofold. Fist, we demonstated that the acute state of depessive disease is chaacteized by a decease in the global mean levels of the sleep EEG synchonization. Second, we showed that a majo depessive episode (MDE) displays a significant eoganization of the neuonal bain netwoks. Sleep Bain Rhythms and Cotical Synchonization Studying the functional connectivity of the diffeent fequency bands is impotant, because it has been postulated that synchonous oscillations between the diffeent cotical aeas play a ole in vaious aspects of bain function, specifically cognitive functioning [Klimesch, 1999]. Moeove, because sleep is not simply a suspension of waking activities, thee must be a good eason why complex bains have developed an elaboate dynamic choeogaphy duing sleep [Buzsáki, 2006]. To the best of ou knowledge, with the exception of a few studies pefomed specifically on the slow-wave EEG and cycling altenating patten synchonization [Fei et al., 2005, 2006; Stam, 2004], this is the fist study that shows a global deceased level of ceebal synchonization in the sigma, theta, and delta fequency bands duing sleep in depessed patients. In the sigma and delta bands, this loss of synchonization is moe specifically localized in the fontal, tempoal, and paietal aeas. A geneally ageed upon view among the scientific community is that depession may be a disode that shows distubed lagescale cotical (and subcotical) systems that include a numbe of functionally connected cotical egions, such as the tempoal, fontal, and paietal lobes [Davidson, 2004]. TABLE IV. Coelation values between the scoe in the Hamilton Depession Rating Scale, the global level of SL and the small-wold measues Global synchonization C L Theta Rs P value Sigma Rs P value Delta Rs P value Rs, Speaman s ank coelation coefficient. 2215

10 Leistedt et al. Moeove, a malfunction of limbic-cotical netwoks is cuently the leading systems-level candidate fo mediating majo depession, based on an extensive liteatue on blood flow, metabolism, EEG dynamics, and histological findings [Devets, 2000; Linkenkae-Hansen et al., 2005]. The fontal cotex is also clealy a significant component of the distubed system elated to majo depession [Devets et al., 1997]. Thus, the topogaphic synchonization abnomalities found in this aticle (in both the delta and sigma fequency bands) in tems of loss of synchonization ae in ageement with the cuent liteatue [Devets, 2000; Devets et al., 1997; Linkenkae-Hansen et al., 2005]. Slow-wave activity synchonization duing sleep may be one facto that plays a ole in the ecently hypothesized impotance of EEG slow-waves duing sleep fo cognitive pocessing [Hube et al., 2004]. The oles of sleep slowwave activity and synchonization in memoy consolidation, and leaning pocesses ae the subject of diffeent animal and human studies [Steiade, 2006]. Pesumably, netwoks that ae activated duing the leaning of a new task ae eactivated duing sleep, which impoves pefomance on the same task the next day [Hube et al., 2004]. Moeove, the suface-positive half-waves of sleep slow-waves have been epoted to be associated with distinctly enhanced coheence afte leaning in the slow-oscillatoy, delta, sigma, and gamma bands [Mölle et al., 2004]. The loss of delta mean synchonization activity obseved in this study could have a pat in the cognitive dysfunctions that ae classically found in patients suffeing fom depession [Pote et al., 2003]. Sleep spindles ae initiated by a deep bain stuctue, the thalamic eticula nucleus, in connection with pincipal thalamic nuclei and ae synchonized by coticotical, coticothalamic, and thalamocotical loops [Steiade, 2006]. Both intinsic thalamic conductances and eticulothalamic/cotico-thalamo-cotical loops ae involved in spindle geneation, amplification, and synchonization of spindles [Steiade, 2006]. Of impotance, abnomalities in these thalamocotical stuctues have been implicated in the neuobiology of schizophenia [Andeasen et al., 1994]. Neuomodulatoy systems that affect the function of these thalamocotical cicuits and the geneation of these sleep hythms (such as noepinephine, seotonin, acetylcholine, and g-aminobutyic acid) ae alteed in schizophenia as well, but also and mostly in majo depession [Belmake and Agam, 2008]. A few pevious woks have studied the sleep spindle activity in patients suffeing fom majo depession with diffeent and contadictoy esults [de Maetelae et al., 1987; Feaelli et al., 2007]. Because ou study clealy shows a defect in the mean synchonization of the sigma fequency bands in depessed patients, we can ague that thalamocotical system abnomalities may play a ole in MDE. Theta hythm, which is one of seveal electoencephalogaphic wavefoms associated with vaious sleep (REM sleep) and wakefulness states, is one of the main hythms in the hippocampus, and has been associated with cognitive pocesses and memoy [Buzsáki, 2006]. The hippocampus and associated stuctues ae oganized in multiple loops and ae pat of all the allocotex, with ecipocal connections to the neocotex [Buzsáki, 2006]. Synchonous activity in the theta band ove the fontal aea inceases stongly duing tasks that ae demanding fo woking memoy [Stam et al., 2002]. Moeove, Tesche and Kahu succeeded in measuing, with MEG, theta activity in the hippocampus duing pefomance of a woking memoy task. They showed that the woking memoy task stimuli induced bief busts of theta band activity in the hippocampus [Tesche and Kahu, 2000]. These distubances in the global mean synchonization of the theta band could pobably be explained by seize and function modifications of the hippocampus, which is one of the taget stuctues of depession. Loss of the Small-Wold Netwoks Popeties As A Randomization Pocess Thee ae indications that SWNs epesent an optimal oganization in tems of low wiing costs, local independence, global integation, and esilience to eo. Modeling studies have shown that neual netwoks with smallwold configuation facilitate synchonization between distant neuons and efficient infomation pocessing [Masuda and Aihaa, 2004]. In a study dealing with MEG ecodings fom healthy subjects, gaph analysis of synchonization matices evealed small-wold pattens in both lowand high-fequency bands [Stam, 2004]. When C and L ae expessed as atios of C and L on andom gaphs, the esults of the pesent study ae quite compatible with those of seveal pevious studies [Hilgetag et al., 2000; Stam, 2004]. Paticulaly, anothe study that extacted ceebal netwoks fom these EEG data using a method descibed peviously by Stam [2004], showed that netwoks demonstated moe small-wold chaacteistics duing all sleep stages as compaed to the awake state, and that these featues wee even moe ponounced duing cyclic altenating patten sleep phase A1 than B [Fei et al., 2007]. The esults of this study confim that the functional connectivity of the sleep EEG activity has a tendency to show an oganization simila to a SWN duing sleep (with the values of C/C-s clealy highe than 1.5 and the values of L/L-s close to 1) [Fei et al., 2007; Micheloyannis et al., 2006; Stam and Reijneveld, 2007]. The values found duing sleep ae clealy diffeent fom those chaacteizing andom netwoks (both values aound 1) and odeed netwoks (both values significantly highe than 1). Ou analyses showed that depessed patients still had a significantly shote L in the theta and delta bands compaed with contols, with no diffeence in C. These esults eflect the fact that healthy contols exhibit neuonal netwoks close to the odeed pat of the ewiing scale. On the othe hand, patients have bain netwoks close to the andom pat of the scale. 2216

11 Functional Connectivity in Depession These esults ae in line with those obseved in anothe neuopsychiatic disease that has aleady been studied in seveal pevious woks: schizophenia. The disconnection hypothesis poposed by Beakspea in 2003, has been confimed by seveal studies, in which we also obseved distubed connectivity between diffeent bain egions, especially the fontal aea, and paticulaly in the left hemisphee [Beakspea et al., 2003; Micheloyannis et al., 2006; Rubinov et al., in pess]. The geneal patten that emeges fom ou study and those on Alzheime s disease and schizophenia is that netwoks of patients with distubed cognition functions ae moe andom. As a consequence, it seems that this disconnection hypothesis of schizophenia is not specific, but also founded duing MDEs. This andomization pocess of the neuonal netwoks could be viewed as a geneal patten of neuopsychiatic diseases. Possible Functional and Clinical Consequences of Deceased Bain Functional Connectivity Duing MDE Gowing evidence indicates that sleep is impotant in leaning and memoy pocessing [Stickgold, 2005]. Ove the last decade, a multitude of molecula, cellula, integative systems and behavioal findings have demonstated the need fo sleep afte leaning to consolidate of memoies [Walke and Stickgold, 2004, 2006]. Moeove, leaning and memoy depend on bain plasticity pocesses, and sleep-dependent leaning and memoy consolidation must be mediated by such pocesses [Walke and Stickgold, 2004, 2006]. Significant cognitive impaiments ae usually found in young and eldely people suffeing fom MDE [Castaneda et al., 2008]. Despite diffeent study esults, cognitive impaiments duing depession include executive dysfunctions, attention deficits, shot-tem and woking memoy impaiments in both vebal and visual tasks, psychomoto skill deficits, vebal memoy deficits, and leaning functions impaiments [Castaneda et al., 2008]. In elation to ou esults, authos ague, as a andomization pocess, that these modifications in the functional bain connectivity duing sleep could have a pat in these cognitive dysfunctions that ae classically obseved in acutely depessed patients. Neuonal Netwoks and the Depessive Disease: Implications of the Netwok Hypothesis of Depession The view of mood disodes as a poblem of neuonal netwoks and infomation pocessing in the bain has seveal impotant implications [Castén, 2005]. Neuonal netwoks develop though inteactions with the envionment, and neuotansmission in these netwoks ae constantly being efined though activity-dependent synaptic plasticity to optimal pocess and stoe elevant infomation [Katz and Shatz, 1996]. Like othe bain diseases, depession may epesent distubances in the activity-dependent infomation pocessing of the bain athe than the chemical balance of signaling molecules [Castén, 2005; Fingelkuts et al., 2007]. Pehaps, the most impotant evidence fo this hypothesis is the ecent obsevation that antidepessants incease the poduction of new neuons in the odent hippocampus [Malbeg et al., 2000]. Newly geneated neuons diffeentiate ove time and ae only matue enough to paticipate in infomation pocessing seveal weeks afte bith [van Paag et al., 2002]. The fact that this time couse coelates with the delayed onset of the clinical effects of antidepessants has ceated a lot of excitement among neuophamacologists [Castén, 2005]. Antidepessants may facilitate optimization of ceebal connectivity by (i) inceasing the choice of neuons available fo selection though activity-dependent mechanisms [Castén, 2005], (ii) stimulating the tunove of axonal banches and synaptic contact (theeby poviding moe mateial fo activitydependent selection), and (iii) activating the neuotophin signaling as a bain-deived neuotophic facto [Castén, 2004]. Analogously, psychological theapies (psychotheapy, cognitive behavioal theapy, etc.), electoconvulsive shock teatment could also have theapeutic effects on mood disodes though use-dependent neuonal plasticity [Castén, 2005]. In this scenaio, a combination of dug teatment and psychological ehabilitation would be expected to be moe beneficial than eithe teatment alone [TADS, 2004]. As the netwok hypothesis emphasizes the impotance of envionmental infomation in the pocess of activity-dependent selection of neuons and synapses, it pedicts that full ecovey would not even be possible with dug teatment alone, but that extenal stimuli, such as social communication, would be equied to povide envionmental input fo the selection of the appopiate netwok connections [Castén, 2005]. Though the diffeent data that ae summaized above, it is clea that thee is some evidence that antidepessants activate plasticity pocesses, which ae thought to gadually impove infomation pocessing in the neual netwoks that ae involved in diffeent bain functions such as sleep and mood egulation. Though functional bain netwok models, ou study agues fo consideing MDE as a poblem of neuonal netwoks and a poblem of infomation pocessing as well. CONCLUDING REMARKS AND PERSPECTIVES Limitations of this study ae pimaily elated to methodology. Fist, majo depession is a heteogeneous syndome, and othe souces of vaiance may exist within patients (e.g., seveity, atypicality, seasonality, numbe of pevious episode, etc.). Second, by excluding patients with moe seious foms of psychiatic comobidity, ou conclusions should be intepeted with cae. Thid, the sample size and the psychopathological assessment of the subjects did not allow fo statifying the patient sample by depes- 2217

12 Leistedt et al. sive subtypes. Replicating ou study in a lage goup is clealy equied in ode to futhe examine the neuophysiological aspects that wee evealed in this study. Finally, confiming ou findings will have to await a moe complete undestanding of the neual netwok dynamics in healthy subjects and patients suffeing fom seveal neuopsychiatic diseases. In summay, this study povides futhe suppot fo the pesence of small-wold featues in functional netwoks in the bain duing sleep stages. Patients suffeing fom acute depession have significantly shote path lengths in thei EEG gaphs, even afte coecting fo diffeences in the mean level of synchonization. This suggests a disuption in effective inteactions between and acoss cotical egions, and povides futhe suppot fo the concept of depession as a disconnection disease. Recent epots show that many (but not all) functional abnomalities found duing a depessive episode ecove afte phamacological o psychotheapeutic teatment [Austin et al., 2001; Castén, 2005]. Futue studies should establish whethe the functional connectivity of the sleep EEG inceases with ecovey afte a depessive episode o whethe impaied functional bain connectivity epesents tait abnomalities. Finally, undestanding how moe complicated neuonal netwok topologies influence both human behavios and also mental disodes epesents one paticulaly impotant challenge that needs to be tackled as the mathematical study of neuonal netwoks advances. ACKNOWLEDGMENTS The authos gatefully acknowledge all the individuals who paticipated in this study. They thank Chantal Kempenaes, Stéphanie Baun, Bois Leistedt, and Benad Jacques fo technical assistance. They also acknowledge the Société belge fancophone de medicine du sommeil and the National Fund fo Scientific Reseach (F.N.R.S.). 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