Feedforward somatosensory inhibition is normal in cervical dystonia

Background: Insufficient cortical inhibition is a key pathophysiological finding in dystonia. Subliminal sensory stimuli were reported to transiently inhibit somatosensory processing. Here we investigated whether such subliminal feedforward inhibition is reduced in patients with cervical dystonia. Methods: Sixteen cervical dystonia patients and 16 matched healthy controls performed a somatosensory detection task. We measured the drop in sensitivity to detect a threshold-level digital nerve shock when it was preceded by a subliminal conditioning shock, compared to when it was not. Results: Subliminal conditioning shocks reduced sensitivity to threshold stimuli to a similar extent in both patients and controls, suggesting that somatosensory subliminal feedforward inhibition is normal in cervical dystonia. Conclusion: Somatosensory feedforward inhibition was normal in this group of cervical dystonia patients. Our results qualify previous concepts of a general dystonic deficit in sensorimotor inhibitory processing

Subliminal stimulation and somatosensory signal detection

Only a small fraction of sensory signals is consciously perceived. The brain's perceptual systems may include mechanisms of feedforward inhibition that protect the cortex from subliminal noise, thus reserving cortical capacity and conscious awareness for significant stimuli. Here we provide a new view of these mechanisms based on signal detection theory, and gain control. We demonstrated that subliminal somatosensory stimulation decreased sensitivity for the detection of a subsequent somatosensory input, largely due to increased false alarm rates. By delivering the subliminal somatosensory stimulus and the to-be-detected somatosensory stimulus to different digits of the same hand, we show that this effect spreads across the sensory surface. In addition, subliminal somatosensory stimulation tended to produce an increased probability of responding “yes”, whether the somatosensory stimulus was present or not. Our results suggest that subliminal stimuli temporarily reduce input gain, avoiding excessive responses to further small inputs. This gain control may be automatic, and may precede discriminative classification of inputs into signals or noise. Crucially, we found that subliminal inputs influenced false alarm rates only on blocks where the to-be-detected stimuli were present, and not on pre-test control blocks where they were absent. Participants appeared to adjust their perceptual criterion according to a statistical distribution of stimuli in the current context, with the presence of supraliminal stimuli having an important role in the criterion-setting process. These findings clarify the cognitive mechanisms that reserve conscious perception for salient and important signals

Inhibitory impact of subliminal electrical finger stimulation on SI representation and perceptual sensitivity of an adjacent finger

Simultaneous stimulation of two adjacent fingers above sensory perception threshold (supraliminal stimulation) leads to an inhibitory interaction effect on responses in primary somatosensory cortex (SI). Moreover, during electrical finger stimulation closely below threshold for conscious perception (subliminal stimulation) inhibitory interneurons in cortical layer 4 are assumed to be activated preferentially as compared to excitatory interneurons. Using fMRI in humans, here we show that interspersed subliminal electrical stimulation of an adjacent finger reduces the response to target finger stimulation in contralateral SI. This effect was shown in a complementary study to be associated behaviorally with a diminished detectability of test pulses on the target finger. We propose the mechanism underlying this lateral inhibitory effect to be related to a representational overlap of inhibitory interneurons in SI based on the divergence of thalamocortical feedforward projections, or to intracortical lateral inhibitory projections targeting juxtaposed receptive fields, or both

Brain Mechanism for Enhanced Hand Function with Remote Sensory Stimulation

The neurological bases for remote vibration enhanced sensory feedback and motor function are yet poorly understood. The purpose of this dissertation was to identify and examine the effect of vibration on finger tactile sensation in healthy adults and how imperceptible random vibration applied to the wrist changes cortical activity for fingertip sensation and precision grip. In a series of studies on healthy adults, white-noise vibration was applied to one of four locations (dorsum hand by the second knuckle, thenar and hypothenar areas, and volar wrist) at one of four intensities (zero, 60%, 80%, and 120% of the sensory threshold for each vibration location), while the fingertip sensation, the smallest vibratory signal that could be perceived on the thumb and index fingertip pads, was assessed. Vibration intensities significantly affected the fingertip sensation (p.01), all compared with the zero vibration condition. The next step was to examine the cortical activity for this vibration-enhanced fingertip sensation. We measured somatosensory evoked potentials to assess peak-to-peak response to light touch of the index fingertip with applied wrist vibration versus without. We observed increased peak-to-peak somatosensory evoked potentials with wrist vibration, especially with increased amplitude of the later component for the somatosensory, motor, and premotor cortex with wrist vibration. These findings corroborate an enhanced cortical-level sensory response motivated by vibration. It is possible that the cortical modulation observed here is the result of the establishment of transient networks for improved perception. Finally, we examined the effect of imperceptible vibration applied to the wrist on cortical control for precision grip. We measured β-band power to assess peak-to-peak response while subjects performed precision pinch with wrist vibration versus without. We observed increased peak-to-peak β-band power amplitude with wrist vibration, especially with event-related synchronization for the prefrontal, sensorimotor, motor, premotor, and supplementary motor areas with vibration. The enhanced motor function may possibly be a result of higher recalibration following movement and faster motor learning

Systems-level neural mechanisms of conscious perception in health and schizophrenia

The interplay between senses and actions is one of the most crucial processes that takes place in the brain. The successful course from perception of a stimulus to a meaningful action requires coherent communication between different cortical areas. In humans, these events can be measured non- invasively outside the skull, for example by recording electric or magnetic fields that are produced by neuronal population activity on the cortex, with electroencephalography and magnetoencephalography (EEG and MEG). By combining MEG and EEG with simultaneous behavioural experiments, it is possible to extract neuronal activities that are correlated with perception and action. In this thesis, MEG recordings combined with advanced data-analysis techniques were used to study the role of cortical oscillations –brain rhythms – in coordinating conscious perception and action as well as their deficits in chronic schizophrenia. In Study I and Study II, I investigated what the local and large-scale neuronal correlates of conscious somatosensory perception are, respectively. Healthy subjects were stimulated at their index fingers with somatosensory stimuli, adjusted individually at the threshold of detection, so that around half of the time the stimulus was detected. Concurrent MEG recordings and subsequent source-modelling revealed in Study I that perceived trials were correlated with strengthened evoked responses (ERs), phase-locking to stimulus onset (SL), and induced oscillation amplitude modulations. The most robust and widespread of these was SL that was sustained in the low-alpha (6-10 Hz) band. The strength of SL and to a lesser extent that of ER predicted conscious perception in the somatosensory, lateral and medial frontal, posterior parietal, and in the cingulate cortex. In Study II, I investigated the role of large-scale synchronization in the conscious somatosensory perception. Perceiving and reporting of weak somatosensory stimuli were correlated with sustained strengthening of large-scale synchrony, concurrently in delta/theta- (3-7 Hz) and gamma- (40-60 Hz) frequency bands. In a data-driven network localization, I found this synchronization to dynamically connect the task-relevant, i.e. the frontoparietal, sensory and motor systems. The strength and temporal pattern of interareal synchronization were also correlated with the response times. These data showed that a rapid phase-reorganization and concurrent oscillation amplitude modulations in the specific areas play a key role in the emergence of a conscious decision-making, and subsequent actions. Furthermore, this study showed that perception is dependent on transient large-scale phase synchronization in the delta/theta and gamma bands. In the third study, I investigated whether aberrant large-scale synchronization or dysconnectivity could underlie perceptual deficits in patients suffering from schizophrenia. To this end, I analysed MEG data from chronic schizophrenia patients and healthy control subjects recorded during a visual perception closure task. In schizophrenia patients, a reduction in gamma-band (30–120 Hz) oscillation amplitudes, accompanied by a pronounced deficit in large-scale synchronization at gamma-band frequencies characterized visual processing compared to healthy control subjects. Synchronization was reduced within visual regions, as well as between the visual and frontal cortex. Additionally, the reduction of synchronization correlated positively with clinical disorganization scores. Accordingly, these data imply that schizophrenia is associated with a profound disruption of transient synchronization. This observation provides critical support for the notion that the core aspect in the pathophysiology of schizophrenia arises from an impairment in coordination of distributed neural activity.Aistien ja liikkeiden välinen vuorovaikutus on yksi aivojen tärkeimmistä toiminnoista. Mielekkäiden liikkeiden tuottaminen vasteena ärsykkeen tietoiselle havainnolle vaatii useiden aivokuoren alueiden välistä toimivaa yhteydenpitoa. Ihmisillä näitä hermostollisia tapahtumia voidaan mitata turvallisesti kallon ulkopuolelta esimerkiksi tallentamalla hermosolujoukkojen tuottamaa sähkö- tai magneettikenttää aivosähkö- ja aivomagneettikäyrinä (EEG ja MEG). Kun aivotoimintaa mitataan EEG:llä tai MEG:llä samaan aikaan kun koehenkilö suorittaa kokeellista tehtävää, on mahdollista eristää ne hermostolliset ilmiöt, jotka liittyvät kiinteästi yhteen tietoiseen havaintoon ja liikkumiseen. Tässä väitöskirjatyössä MEG-mittaukset on yhdistetty edistyneisiin data-analyysimenetelmiin, jotka mahdollistavat aivokuoren hermostollisten oskillaatioiden eli aivorytmien merkityksen selvittämisen tietoisessa aistihavainnossa, sitä seuraavien liikkeiden synnyssä sekä skitsofrenian aiheuttamissa puutteissa. Väitöskirjaani liittyvissä tutkimuksissa I ja II selvitin mitkä aivokuoren paikalliset ja pitkän matkan hermostolliset ilmiöt liittyvät tietoiseen tuntoärsykehavainnointiin. Näihin tutkimuksiin liittyvissä kokeissa terveille koehenkilöille annettiin etusormenpäihin niin heikkoja ärsykkeitä, että toisinaan he havaitsivat ne ja toisinaan eivät, vaikka ärsykkeen vahvuus oli aina sama. Kokeen kanssa yhtäaikaisesti mitattu MEG ja sitä seuraava lähdemallinnus osoittivat, että ärsykkeiden havainnointi oli yhteydessä samanaikaisesti vahvistuneeseen herätevasteeseen ja vaihelukitukseen kuin myös värähtelylaajuudenmuutoksiin. Kaikkein selkein näistä reaktioista oli oskillaatioiden vaihelukittuminen alfa-taajuuskaistassa (6-10 Hz). Vaihelukituksen vahvuus ja vähemmissä määrin myös herätevasteiden suuruus aivokuoren tuntoaisti-, etulohkon lateraali- ja mediaalipinnoilla sekä päälaenlohkon takaosissa että aivovyössä olivat selvästi yhteydessä ärsykkeen tietoisen havaitsemisen kanssa. Lisäksi ärsykkeen havaitseminen ja sen kertominen käden liikkeellä oli olennaisesti yhteydessä pitkän matkan synkronian pysymisessä delta/teeta- (3-7 Hz) ja gamma- (40-60 Hz) taajuuskaistoilla. Datapohjaisen verkostoanalyysin avulla sain selville, että tämä synkronia yhdisti dynaamisesti tehtävissä olennaiset verkostot aivojen etuotsalohkoilla, päälaella ja aisti- sekä liikeaivoalueilla. Lisäksi tämän eri aivokuoren alueiden välisen synkronian vahvuus ja ajallinen muoto korreloivat koehenkilöiden vastausaikojen kanssa. Nämä tulokset näyttivät toteen sen, että nopea oskillaatiovaiheiden uudelleenjärjestäytyminen ja samanaikaiset amplitudien muutokset tietyillä aivoalueilla ovat merkittävässä roolissa tietoisen päätöksenteon ja sitä seuraavien liikkeiden synnyssä. Näiden ohella tuloksista kävi ilmi, että tietoinen aistihavainto on riippuvainen pitkän matkan synkroniasta delta/teeta- ja gamma-taajuuskaistoissa. Tämän väitöskirjan kolmannessa tutkimuksessa tutkin, voisivatko puutteet aivokuoren pitkän matkan synkroniassa olla taustalla skitsofreniasta kärsivien potilaiden vaikeuksissa havaita epäyhtenäisten näköärsykekuvioiden kokonaisia rakenteita. Tätä varten analysoin sekä terveiden koehenkilöiden että skitsofreniasta kärsivien potilaiden MEG-dataa, joka on mitattu epäyhtenäisistä kasvoista koostuvan kuva-arvoitus tehtävän ratkaisemisen yhteydessä. Tiedetään, että skitsofreniasta kärsivien on hankala erottaa kasvoja vaillinaisista piirteistä. Aivokuoren pitkän matkan oskillaatiosynkronia oli potilailla oleellisesti heikompi kuin terveillä koehenkilöillä eritoten gamma-taajuuskaistassa näköaivokuorella sekä etuotsalohkon että päälaen tarkkaavaisuudesta vastaavilla alueilla. Kaiken lisäksi synkronia oli sitä heikompi mitä vakavammasta sairaudenkuvasta oli kyse. Näin ollen nämä väitöskirjani tulokset osoittavat, että suuret puutteet aivokuoren eri alueiden yhteydenpidossa luonnehtivat skitsofreniaa hermostollisena oskillopaattisena sairautena.Samspelet mellan sinnen och rörelser är en av de mest centrala processerna som sker i hjärnan. Förloppet från varseblivning av en retning till en meningsfull rörelse kräver en sammanhängande förbindelse mellan olika områden i hjärnbarken. Hos människan kan dessa processer uppmätas icke-invasivt utanför skallen, till exempel genom att registrera förändringarna i det magnetfält som alstras av jonrörelser inuti nervcellerna i hjärnbarken. Genom att kombinera hjärnavbildning i form av MEG och EEG med beteendeexperiment som utförs samtidigt, kan man urskilja olika egenskaper hos hjärnvågornas oscillationer, såsom frekvens, amplitud och fas, samt deras grad av synkronisering med olika områden i hjärnbarken. Dessa neurala korrelat som ligger till grund för företeelser som medveten varseblivning och initiering av påföljande rörelser, har oftast olika värden för friska människor jämfört med patienter som lider av en psykisk sjukdom, som till exempel schizofreni. Detaljer rörande dessa korrelat är emellertid dåligt kända och för närvarande föremål för intensiv forskning. För denna avhandling studerades de processer i hjärnbarken som ligger bakom medveten varseblivning och de påföljande rörelserna, i två skilda experiment. I den första delen stimulerades friska försökspersoners pekfingertoppar med somatosensoriska retningar, vilka justerades individuellt vid detektionsgränsen. Detta innebar att försökspersonerna varseblev retningen omkring hälften av tiden. MEG-uppmätningarna och efterföljande källmodellering avslöjade att varseblivning av en retning var förknippad med förstärkta oscillationsmoduleringar. Den kraftigaste och mest utbredda av dessa var faslåsningen till startpunkten för en retning (SL), vilken bibehölls i låg-alfa (6-10 Hz) bandet. Styrkan på SL och i mindre utsträckning också den i amplitud, förebådade medveten varseblivning i den sensomotoriska hjärnbarken, samt i flera områden som bidrar till uppmärksamhet och förnimmelse. Därtill var medveten varseblivning och rapporteringen av svaga, somatosensoriska retningar korrelerad med en fortsatt förstärkning av storskalig synkronisering, såväl i delta/theta- (3-7 Hz) som gamma- (40-60 Hz) frekvensbanden. Resultaten visar att denna synkronisering dynamiskt kopplade samman det kontralaterala sensorimotoriska området och det ipsilaterala frontoparietala nätverk som styr vår objektsigenkänning och varseblivning. Styrkan, respektive det tidsbestämda mönstret i synkronisationen mellan de olika områdena, korrelerades också med responstiderna. Dessa data visar att en snabb fasomsättning och samtidiga oscillationsamplitudmoduleringar i de specifika områdena spelar en nyckelroll i framväxten av en medveten varseblivning och därpåföljande handlingar. Dessutom finns det en koppling till storskalig, dynamisk fassynkronisering i delta/theta- och gammaoscillationsbanden. I den andra delen av studierna associerade med denna avhandling fick patienter drabbade av schizofreni och friska kontrollpersoner utföra en bildtolkningsuppgift, samtidigt som en hjärnavbildning gjordes med MEG. Hjärnvågorna hos schizofrenipatienterna kännetecknades av en minskning av oscillationsamplitud på gamma-bandet (30-120 Hz). Därtill iakttogs en tydlig brist på storskalig synkronisation vid gammabandfrekvenser. Synkroniseringen var svagare inom områden som berör synen, såväl som mellan visuell och frontal hjärnbark. Dessutom fanns en positiv korrelation mellan denna minskade synkronisering och sjukdomens grad. Följaktligen låter dessa data påskina att schizofreni är förknippat med en djup störning av flyktig synkronisering. Denna observation ger viktigt stöd för uppfattningen att den grundläggande aspekten i patofysiologin rörande schizofreni beror på en försämring av koordinationen av utspridd neural aktivitet

Unbewusste Modulatoren der somatosensorischen Wahrnehmung

It is intriguing that perception of the same stimulus can vary profoundly from trial to trial. For example, it has been shown in many studies that weak, so-called “near-threshold stimuli” are sometimes consciously perceived and sometimes not. In my thesis, I have been investigating factors which underlie this profound perceptual variability in the somatosensory domain. Together with my colleagues, I performed three studies in which we tested three different types of presumed non-conscious modulators of somatosensory perception. In the first – behavioral - study, we investigated how the presence of subliminal noise during a peripheral somatosensory stimulation influences perception. Counter-intuitively, we found that peripheral noise can even improve perception of weak somatosensory stimuli. In our interpretation, this occurs most likely due to “stochastic resonance” effects (Study I: Iliopoulos et al. 2014). In the second – behavioral and EEG - study, we tested the effect of different forms of pulsed subliminal stimulation (single pulses versus pulse trains) on brain rhythms and somatosensory perception. Following-up on previous results of our group, we tested the hypothesis that subliminal pulsed stimulation impairs perception of subsequent stimuli via centrally enhanced Mu rhythm. Interestingly, the main result of this study was that trains of subliminal stimuli indeed inhibited subsequent somatosensory detection, however, - in contrast to our previous findings for single pulses – trains were associated with decreased Mu rhythm. We conclude that central rhythms most likely play a role in mediating the perceptual modulation of peripheral subliminal stimuli, however, the relationship is more complex than previously assumed (Study II: Iliopoulos et al. 2020). In the third study, we examined the influence of interoceptive signaling, especially from the heart, on somatosensory perception. The hypothesis was that the cardiac phase (systole versus diastole) and the so-called heart-evoked potential (HEP) would modulate somatosensory perception. Indeed, our study showed that somatosensory perception was better during diastole than during systole and detection performance declined as the amplitude of the HEP increased. Our interpretation of the former effect assumes that all events which occur simultaneously with the “pulse” are assumed by the brain to be pulse-synchronous peripheral noise and therefore suppressed. Our interpretation of the latter effect (HEP) assumes that HEP is a marker of the relative balance between interoception and exteroception (Study III: Al et al. 2020). In conclusion, in the studies which form the basis for my thesis, we have shown that somatosensory perception is modulated by peripheral effects (modes of peripheral stimulation, peripheral noise), central effects (Mu rhythm) and interoceptive signals from the heart. The precise interplay between these modulators is an exciting research topic for future studies.Interessanterweise kann die Wahrnehmung desselben Reizes von Augenblick zu Augenblick so stark variieren, dass dieser manchmal bewusst wahrgenommen wird und manchmal nicht. In meiner Dissertation habe ich Faktoren untersucht, die dieser Wahrnehmungsvariabilität im somatosensorischen (SS) System zugrunde liegen. Mit meinen Kollegen habe ich drei Studien durchgeführt, in denen wir verschiedene mutmaßlich unbewusste Modulatoren der SS-Wahrnehmung untersuchten. In der ersten Studie untersuchten wir, wie die Wahrnehmung peripherer SS-Reize durch unterschwelliges Rauschen beeinflusst wird. Wir konnten zeigen, dass peripheres Rauschen die Wahrnehmung schwacher Reize verbessert. Dies ist ein Hinweis auf das Vorliegen von "stochastischen Resonanzeffekten" (Studie I: Iliopoulos et al. 2014). In der zweiten Studie, die neben behavioralen Messungen auch elektroencephalographische (EEG) Messungen umfasste, testeten wir die Auswirkung verschiedener Formen gepulster unterschwelliger elektrischer Fingerstimulationen (Einzelpulse gegen Pulsserien) auf die Wahrnehmung und auf Hirn-rhythmen. Ausgehend von früheren Ergebnissen unserer Arbeitsgruppe überprüften wir, ob repetitive subliminale Stimulationen die Wahrnehmung nachfolgender Reize über einen zentral verstärkten Mu-Rhythmus beeinträchtigen. Das Ergebnis dieser Studie war, dass Serien unterschwelliger Reize tatsächlich die nachfolgende SS-Wahrnehmung hemmten, jedoch - im Gegensatz zu früheren Ergebnissen für Einzelimpulse – die Reizserien mit einem verringerten Mu-Rhythmus verbunden waren. Daraus schließen wir, dass zentrale Rhythmen höchstwahrscheinlich eine Rolle bei der Wahrnehmungsmodulation durch periphere unterschwellige Reize spielen, dass aber der Zusammenhang zwischen beiden komplexer ist als bisher vermutet (Studie II: Iliopoulos et al. 2020). In der dritten Studie untersuchten wir den Einfluss interozeptiver Signale aus dem Herzen auf die SS-Wahrnehmung. Die Hypothese war, dass die Herzphase und das so genannte Herz-evozierte Potenzial (HEP) die SS-Wahrnehmung modulieren. Wir zeigten, dass die SS-Wahrnehmung während der Diastole besser war als während der Systole und dass die Wahrnehmung in umgekehrtem Verhältnis zur Amplitude des vorausgehenden HEP stand. Für den ersten Effekt legen unsere Daten nahe, dass alle Ereignisse, die zusammen mit der Pulswelle auftreten, vom Gehirn als puls-synchrones peripheres Rauschen angenommen und daher unterdrückt werden. Der zweite Befund wird in Übereinstimmung mit der Literatur am besten dadurch erklärt, dass das HEP ein Marker für das relative Gleichgewicht zwischen Interozeption und Exterozeption darstellt (Studie III: Al et al. 2020). Zusammenfassend zeigen die Ergebnisse dieser Arbeit, wie die SS-Wahrnehmung durch periphere Effekte (Art der Stimulation, Rauschen), zentrale Effekte (Mu-Rhythmus) und interozeptive Signale des Herzens moduliert wird. Das genaue Zusammenspiel zwischen diesen Modulatoren ist ein spannendes Forschungsthema für zukünftige Studien

Conscious and unconscious somatosensory perception and its modulation by attention

Our brains handle vast amounts of information incoming through our senses. Continuously exposed to sensory input, the sense of touch, however, may miss tactile stimuli, no matter how much attention we pay to them. In four empirical studies, this thesis tested (1) the feasibility of investigating undetectable stimulation by electrical finger nerve pulses, (2) how its neural correlates dissociate from detectable stimulation and (3) whether and how selective somatosensory attention nevertheless affects the neural representation of undetectable stimuli. The first two studies showed that there is a natural range of electrical stimulation intensities that cannot be detected. A rigorous statistical evaluation with Bayes factor analysis indicated that the evidence of chance performance after undetectable stimulation reliably outweighed evidence of above-chance performance. A subsequent study applying electroencephalography (EEG) revealed qualitative differences between the processing of detectable and undetectable stimulation, which is evident in altered event-related potentials (ERP). Specifically, undetectable stimulation evokes a single component that is not predictive of stimulus detectability but lacks a subsequent component, which correlates with upcoming stimulus detection. The final study showed that attention nevertheless affects neural processing of undetectable stimuli in a top-down manner as it does for detectable stimuli and fosters the view of attention and awareness being two separate and mostly independent mechanisms. The influence of the pre-stimulus oscillatory (~10 Hz) alpha amplitude—a putative marker of attentional deployment—on the ERP depended on the current attentional state and indicates that both processes are interacting but not functionally matching.:1 Touch, Consciousness, And Attention – Theoretical Considerations ........ 1-11 1.1 A Neural Account To (Un-) Consciousness ............................................ 1-12 1.2 Controlling detectability of external stimulation ...................................... 1-14 1.3 Thresholds in the light of signal detection theory ................................... 1-17 1.4 Selective attention in touch .................................................................... 1-19 1.5 Research questions ............................................................................... 1-21 2 Empirical Evidence .................................................................................... 2-25 2.1 General methods .................................................................................... 2-25 2.1.1 Stimulation ........................................................................................... 2-25 2.1.2 Threshold assessment procedure ....................................................... 2-25 2.1.3 Behavioral analysis .............................................................................. 2-26 2.1.4 Electrophysiological measurement ...................................................... 2-28 2.1.5 Analysis of event-related potentials ..................................................... 2-30 2.1.6 Spectral Analysis resolved over time ................................................... 2-30 2.2 Psychophysical assessment of subthreshold stimulation ........................ 2-33 2.2.1 A method for assessing the individual absolute detection threshold (ADTH) ......................................................................................................... 2-33 2.2.2 Validation of absolute detection threshold assessment by signal detection theory measures and Bayesian Null-Hypothesis testing ................ 2-39 2.3 Non-invasive neural markers of unconscious perception ....................... 2-47 2.3.1 Neural Correlates of Undetectable Somatosensory Stimulation in EEG and fMRI ...................................................................................................... 2-47 2.3.2 Prediction of stimulus perception by features of the evoked potential for different stimulation intensities along the psychometric function ................. 2-51 2.4 The role of Rolandic Alpha Activity in Somatosensation and its Relation to Attention ................................................................................................. 2-75 3 General Discussion and Conclusions ...................................................... 3-101 3.1 Summary of empirical results ................................................................ 3-101 3.2 Neural processing of undetectable stimulation ..................................... 3-102 3.3 Attention, awareness and neural oscillatory activity ............................. 3-104 3.4 Limits of the current studies and future perspectives ........................... 3-109 References .................................................................................................... 113 Summary ....................................................................................................... 137 Zusammenfassung ........................................................................................ 143 Curriculum Vitae ............................................................................................ 151 Selbständigkeitserklärung ............................................................................. 155 Nachweis über die Anteile der Co-Autoren .................................................... 15