The negative neural correlates of consciousness

For a long time, scientists and philosophers alike have been speculating about the neural correlates of consciousness (NCC). Although definitions differ, usually an NCC denotes neural events whose occurrence correlates with our phenomenal experiences. In this essay, we motivate the introduction of the concept of a 'negative neural correlate of consciousness' (NNCC). The NNCC can be conceptually understood as neural events whose occurrence inversely correlates with a given phenomenal experience. We wish to introduce this concept for two reasons. First, it is an elegant solution to the problem of sufficiency that so far has unduly neglected in the NCC debate. Secondly, based on the Integrated Information Theory of Consciousness, we argue that only by pairing each NCC with a corresponding NNCC will scientists ever be able to predict conscious experience from brain data. We discuss empirical implications of and potential objections to the suggested NNCC framework

FMRI Effective Connectivity and TMS Chronometry: Complementary Accounts of Causality in the Visuospatial Judgment Network

BACKGROUND: While traditionally quite distinct, functional neuroimaging (e.g. functional magnetic resonance imaging: fMRI) and functional interference techniques (e.g. transcranial magnetic stimulation: TMS) increasingly address similar questions of functional brain organization, including connectivity, interactions, and causality in the brain. Time-resolved TMS over multiple brain network nodes can elucidate the relative timings of functional relevance for behavior ("TMS chronometry"), while fMRI functional or effective connectivity (fMRI EC) can map task-specific interactions between brain regions based on the interrelation of measured signals. The current study empirically assessed the relation between these different methods. METHODOLOGY/PRINCIPAL FINDINGS: One group of 15 participants took part in two experiments: one fMRI EC study, and one TMS chronometry study, both of which used an established cognitive paradigm involving one visuospatial judgment task and one color judgment control task. Granger causality mapping (GCM), a data-driven variant of fMRI EC analysis, revealed a frontal-to-parietal flow of information, from inferior/middle frontal gyrus (MFG) to posterior parietal cortex (PPC). FMRI EC-guided Neuronavigated TMS had behavioral effects when applied to both PPC and to MFG, but the temporal pattern of these effects was similar for both stimulation sites. At first glance, this would seem in contradiction to the fMRI EC results. However, we discuss how TMS chronometry and fMRI EC are conceptually different and show how they can be complementary and mutually constraining, rather than contradictory, on the basis of our data. CONCLUSIONS/SIGNIFICANCE: The findings that fMRI EC could successfully localize functionally relevant TMS target regions on the single subject level, and conversely, that TMS confirmed an fMRI EC identified functional network to be behaviorally relevant, have important methodological and theoretical implications. Our results, in combination with data from earlier studies by our group (Sack et al., 2007, Cerebral Cortex), lead to informed speculations on complex brain mechanisms, and TMS disruption thereof, underlying visuospatial judgment. This first in-depth empirical and conceptual comparison of fMRI EC and TMS chronometry thereby shows the complementary insights offered by the two methods

Position representations of moving objects align with real-time position in the early visual response

When interacting with the dynamic world, the brain receives outdated sensory information, due to the time required for neural transmission and processing. In motion perception, the brain may overcome these fundamental delays through predictively encoding the position of moving objects using information from their past trajectories. In the present study, we evaluated this proposition using multivariate analysis of high temporal resolution electroencephalographic data. We tracked neural position representations of moving objects at different stages of visual processing, relative to the real-time position of the object. During early stimulus-evoked activity, position representations of moving objects were activated substantially earlier than the equivalent activity evoked by unpredictable flashes, aligning the earliest representations of moving stimuli with their real-time positions. These findings indicate that the predictability of straight trajectories enables full compensation for the neural delays accumulated early in stimulus processing, but that delays still accumulate across later stages of cortical processing

The Timing of Feedback to Early Visual Cortex in the Perception of Long-Range Apparent Motion

When 2 visual stimuli are presented one after another in different locations, they are often perceived as one, but moving object. Feedback from area human motion complex hMT/V5+ to V1 has been hypothesized to play an important role in this illusory perception of motion. We measured event-related responses to illusory motion stimuli of varying apparent motion (AM) content and retinal location using Electroencephalography. Detectable cortical stimulus processing started around 60-ms poststimulus in area V1. This component was insensitive to AM content and sequential stimulus presentation. Sensitivity to AM content was observed starting around 90 ms post the second stimulus of a sequence and most likely originated in area hMT/V5+. This AM sensitive response was insensitive to retinal stimulus position. The stimulus sequence related response started to be sensitive to retinal stimulus position at a longer latency of 110 ms. We interpret our findings as evidence for feedback from area hMT/V5+ or a related motion processing area to early visual cortices (V1, V2, V3)

Cerebellar transcranial magnetic stimulation in psychotic disorders: intermittent, continuous, and sham theta-burst stimulation on time perception and symptom severity

BackgroundThe cerebellum contributes to the precise timing of non-motor and motor functions, and cerebellum abnormalities have been implicated in psychosis pathophysiology. In this study, we explored the effects of cerebellar theta burst stimulation (TBS), an efficient transcranial magnetic stimulation protocol, on temporal discrimination and self-reported mood and psychotic symptoms.MethodsWe conducted a case-crossover study in which patients with psychosis (schizophrenias, schizoaffective disorders, or bipolar disorders with psychotic features) were assigned to three sessions of TBS to the cerebellar vermis: one session each of intermittent (iTBS), continuous (cTBS), and sham TBS. Of 28 enrolled patients, 26 underwent at least one TBS session, and 20 completed all three. Before and immediately following TBS, participants rated their mood and psychotic symptoms and performed a time interval discrimination task (IDT). We hypothesized that cerebellar iTBS and cTBS would modulate these measures in opposing directions, with iTBS being adaptive and cTBS maladaptive.ResultsReaction time (RT) in the IDT decreased significantly after iTBS vs. Sham (LS-mean difference = −73.3, p = 0.0001, Cohen’s d = 1.62), after iTBS vs. cTBS (LS-mean difference = −137.6, p < 0.0001, d = 2.03), and after Sham vs. cTBS (LS-mean difference = −64.4, p < 0.0001, d = 1.33). We found no effect on IDT accuracy. We did not observe any effects on symptom severity after correcting for multiple comparisons.ConclusionWe observed a frequency-dependent dissociation between the effects of iTBS vs. cTBS to the cerebellar midline on the reaction time of interval discrimination in patients with psychosis. iTBS showed improved (adaptive) while cTBS led to worsening (maladaptive) speed of response. These results demonstrate behavioral target engagement in a cognitive dimension of relevance to patients with psychosis and generate testable hypotheses about the potential therapeutic role of cerebellar iTBS in this clinical population.Clinical Trial Registrationclinicaltrials.gov, identifier NCT02642029

The temporal characteristics of motion processing in hMT/V5+: Combining fMRI and neuronavigated TMS

Functional imaging has demonstrated the specific involvement of the human middle–temporal complex (hMT/V5+) during processing of moving stimuli. Some studies applied transcranial magnetic stimulation (TMS) to investigate the causal relevance of hMT/V5+ for motion perception. Although the studies used similar visual stimuli and TMS parameters, the critical time point of functionally relevant hMT/V5+ activity differed by 100 ms and more. The present study aimed to elucidate further the temporal characteristics of motion processing in hMT/V5+ by investigating all critical time windows currently debated in the literature. In contrast to previous studies, we used TMS neuronavigation based on individual fMRI results of five participants to target hMT/V5+, applying single-pulse TMS at 24 different time windows (−50 till +200 ms relative to stimulus onset). We revealed that TMS significantly impaired motion perception when applied over hMT/V5+ at 40 to 30 ms before as well as 130 to 150 ms after onset of the moving stimuli. While the late effective time window conforms to results from previous experiments, we did not find evidence for an early time window around 0 ms that has been reported in other studies. Our neuronavigation approach enabled us to quantify the interindividual variance in the exact location of hMT/V5+ and the respective TMS target position on the skull of the participants. Considering that shifting the TMS coil position only by a few millimeters can already lead to a complete loss of TMS effects, our study clearly demonstrates the utility of neuronavigated TMS when investigating specific neuronal effects as in the case of motion processing

The temporal characteristics of motion processing in hMT/V5+: combining fMRI and neuronavigated TMS

Functional imaging has demonstrated the specific involvement of the human middle–temporal complex (hMT/V5+) during processing of moving stimuli. Some studies applied transcranial magnetic stimulation (TMS) to investigate the causal relevance of hMT/V5+ for motion perception. Although the studies used similar visual stimuli and TMS parameters, the critical time point of functionally relevant hMT/V5+ activity differed by 100 ms and more. The present study aimed to elucidate further the temporal characteristics of motion processing in hMT/V5+ by investigating all critical time windows currently debated in the literature. In contrast to previous studies, we used TMS neuronavigation based on individual fMRI results of five participants to target hMT/V5+, applying single-pulse TMS at 24 different time windows (−50 till +200 ms relative to stimulus onset). We revealed that TMS significantly impaired motion perception when applied over hMT/V5+ at 40 to 30 ms before as well as 130 to 150 ms after onset of the moving stimuli. While the late effective time window conforms to results from previous experiments, we did not find evidence for an early time window around 0 ms that has been reported in other studies. Our neuronavigation approach enabled us to quantify the interindividual variance in the exact location of hMT/V5+ and the respective TMS target position on the skull of the participants. Considering that shifting the TMS coil position only by a few millimeters can already lead to a complete loss of TMS effects, our study clearly demonstrates the utility of neuronavigated TMS when investigating specific neuronal effects as in the case of motion processing

The role of human motion processing complex, MT+, during sustained perception and attention

Thesis advisor: Scott D. SlotnickThe overarching aim of this dissertation is to examine the role of human motion processing complex, MT+ during sustained perception and attention. MT+ is comprised of sub-region MT, which processes motion in the contralateral visual field (i.e., left hemisphere MT processes motion in the right visual field and vice versa), and sub-region MST, which processes motion in both the contralateral and ipsilateral visual fields. Whereas previous transcranial magnetic stimulation (TMS) research has provided compelling evidence that region MT+ is necessary for low-level motion processing, Chapter 1 describes an experiment testing whether the sub-region MT is necessary for contralateral low-level motion processing. Chapter 2 describes an experiment that dissociates low-level sensory attentional modulation in MT+ from high-level attentional control processing in the parietal cortex (i.e., during sustained attention). Chapter 3 describes an experiment investigating the role of MT+ during aesthetic processing when viewing visual art. Importantly, this experiment tests whether the aesthetic is tied to not only low-level motion processing in MT+ but also high-level processing in frontal regions. Taken together, the results across the three experiments provide novel evidence for the role of MT+ during low-level motion processing during sustained perception and attention. Moreover, these low-level motion processing effects together with the observed high-level processes in frontal-parietal regions provide neural mechanisms for the cognitive processes of sustained perception and attention.Thesis (PhD) — Boston College, 2012.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Psychology