The cognitive neuroscience of visual working memory

Visual working memory allows us to temporarily maintain and manipulate visual information in order to solve a task. The study of the brain mechanisms underlying this function began more than half a century ago, with Scoville and Milner’s (1957) seminal discoveries with amnesic patients. This timely collection of papers brings together diverse perspectives on the cognitive neuroscience of visual working memory from multiple fields that have traditionally been fairly disjointed: human neuroimaging, electrophysiological, behavioural and animal lesion studies, investigating both the developing and the adult brain

Chronic Stress Effects on Prefrontal Cortical Structure and Function

Stressful life events have been implicated clinically in the pathogenesis of major depression, but the neural substrates that may account for this observation remain poorly understood. Attentional impairments symptomatic of depression are associated with structural and functional abnormalities in the prefrontal cortex. In three parallel rodent and human neuroimaging studies, this project assessed the effects of chronic stress on prefrontal cortical structure and function and the behavioral correlates of these changes. The first study used fMRI to elucidate the precise computational contributions of frontoparietal circuitry to attentional control in human subjects, using a task that could be adapted for rats. The results confirmed that the contributions of dorsolateral frontoparietal areas to visual attentional shifts could be dissociated from the regulatory influences of more ventrolateral areas on stimulus/response mappings, in a manner consistent with studies in animal models. They also indicated that anterior cingulate and posterior parietal cortex may act in concert to detect dissociable forms of information processing conflicts and signal to dorsolateral prefrontal cortex the need for increased attentional control. Stress-induced alterations in these regions and in the connections between them may therefore contribute to attentional impairments. The second study tested this hypothesis in rats by examining whether chronic stress effects on medial prefrontal (mPFC) and orbitofrontal (OFC) dendritic morphology underlie impairments in the behaviors that they subserve. Chronic stress induced a selective impairment in attentional control and a corresponding retraction of apical dendritic arbors in mPFC. By contrast, stress did not adversely affect reversal learning or OFC dendritic arborization. These results suggest that prefrontal dendritic remodeling may underlie the attentional deficits that are symptomatic of stress-related mental illness. The third study was designed to extend these findings to human subjects, using the techniques developed in Study 1. Accordingly, chronic stress predicted selective attentional impairments and alterations in prefrontal functional coupling that were reversible after four weeks. Together, these studies outline in broad strokes a mechanistic model by which chronic stress may predispose susceptible persons to the attentional impairments that are characteristic of major depression. Future studies will assess the roles of serotonin and neurotrophins in mediating these changes

On the Neural Mechanisms Subserving Consciousness and Attention

Consciousness, as described in the experimental literature, is a multi-faceted phenomenon, that impinges on other well-studied concepts such as attention and control. Do consciousness and attention refer to different aspects of the same core phenomenon, or do they correspond to distinct functions? One possibility to address this question is to examine the neural mechanisms underlying consciousness and attention. If consciousness and attention pertain to the same concept, they should rely on shared neural mechanisms. Conversely, if their underlying mechanisms are distinct, then consciousness and attention should be considered as distinct entities. This paper therefore reviews neurophysiological facts arguing in favor or against a tight relationship between consciousness and attention. Three neural mechanisms that have been associated with both attention and consciousness are examined (neural amplification, involvement of the fronto-parietal network, and oscillatory synchrony), to conclude that the commonalities between attention and consciousness at the neural level may have been overestimated. Last but not least, experiments in which both attention and consciousness were probed at the neural level point toward a dissociation between the two concepts. It therefore appears from this review that consciousness and attention rely on distinct neural properties, although they can interact at the behavioral level. It is proposed that a “cumulative influence model,” in which attention and consciousness correspond to distinct neural mechanisms feeding a single decisional process leading to behavior, fits best with available neural and behavioral data. In this view, consciousness should not be considered as a top-level executive function but should rather be defined by its experiential properties

A revised computational neuroanatomy for motor control

We discuss a new framework for understanding the structure of motor control. Our approach integrates existing models of motor control with the reality of hierarchical cortical processing and the parallel segregated loops that characterize cortical-subcortical connections. We also incorporate the recent claim that cortex functions via predictive representation and optimal information utilization. Our framework assumes each cortical area engaged in motor control generates a predictive model of a different aspect of motor behavior. In maintaining these predictive models, each area interacts with a different part of the cerebellum and basal ganglia. These subcortical areas are thus engaged in domain appropriate system identification and optimization. This refocuses the question of division of function among different cortical areas. What are the different aspects of motor behavior that are predictively modelled? We suggest that one fundamental division is between modelling of task and body while another is the model of state and action. Thus, we propose that the posterior parietal cortex, somatosensory cortex, premotor cortex, and motor cortex represent task state, body state, task action, and body action, respectively. In the second part of this review, we demonstrate how this division of labor can better account for many recent findings of movement encoding, especially in the premotor and posterior parietal cortices

Default Mode Contributions to Automated Information Processing

Concurrent with mental processes that require rigorous computation and control, a series of automated decisions and actions govern our daily lives, providing efficient and adaptive responses to environmental demands. Using a cognitive flexibility task, we show that a set of brain regions collectively known as the default mode network play a crucial role in such “autopilot” behavior, i.e. when rapidly selecting appropriate responses under predictable behavioral contexts. While applying learned rules, the default mode network shows both greater activity and connectivity. Furthermore, functional interactions between this network and hippocampal, parahippocampal areas as well as primary visual cortex correlate with the speed of accurate responses. These findings indicate a memory-based “autopilot role” for the default mode network, which may have important implications for our current understanding of healthy and adaptive brain processing

Language and thought are not the same thing: evidence from neuroimaging and neurological patients

Is thought possible without language? Individuals with global aphasia, who have almost no ability to understand or produce language, provide a powerful opportunity to find out. Surprisingly, despite their near-total loss of language, these individuals are nonetheless able to add and subtract, solve logic problems, think about another person's thoughts, appreciate music, and successfully navigate their environments. Further, neuroimaging studies show that healthy adults strongly engage the brain's language areas when they understand a sentence, but not when they perform other nonlinguistic tasks such as arithmetic, storing information in working memory, inhibiting prepotent responses, or listening to music. Together, these two complementary lines of evidence provide a clear answer: many aspects of thought engage distinct brain regions from, and do not depend on, language

Distilling the neural correlates of conscious somatosensory perception

The ability to consciously perceive the world profoundly defines our lives as human beings. Somehow, our brains process information in a way that allows us to become aware of the images, sounds, touches, smells, and tastes surrounding us. Yet our understanding of the neurobiological processes that generate perceptual awareness is very limited. One of the most contested questions in the neuroscientific study of conscious perception is whether awareness arises from the activity of early sensory brain regions, or instead requires later processing in widespread supramodal networks. It has been suggested that the conflicting evidence supporting these two perspectives may be the result of methodological confounds in classical experimental tasks. In order to infer participants’ perceptual awareness in these tasks, they need to report the contents of their perception. This means that the neural signals underlying the emergence of perceptual awareness often cannot be dissociated from pre- and postperceptual processes. Consequently, some of the previously observed effects may not be correlates of awareness after all but instead may have resulted from task requirements. In this thesis, I investigate this possibility in the somatosensory modality. To scrutinise the task dependence of the neural correlates of somatosensory awareness, I developed an experimental paradigm that controls for the most common experimental confounds. In a somatosensory-visual matching task, participants were required to detect electrical target stimuli at ten different intensity levels. Instead of reporting their perception directly, they compared their somatosensory percepts to simultaneously presented visual cues that signalled stimulus presence or absence and then reported a match or mismatch accordingly. As a result, target detection was decorrelated from working memory and reports, the behavioural relevance of detected and undetected stimuli was equated, the influence of attentional processes was mitigated, and perceptual uncertainty was varied in a controlled manner. Results from a functional magnetic resonance imaging (fMRI) study and an electroencephalography (EEG) study showed that, when controlled for task demands, the neural correlates of somatosensory awareness were restricted to relatively early activity (~150 ms) in secondary somatosensory regions. In contrast, late activity (>300 ms) indicative of processing in frontoparietal networks occurred irrespective of stimulus awareness, and activity in anterior insular, anterior cingulate, and supplementary motor cortex was associated with processing perceptual uncertainty and reports. These results add novel evidence to the early-local vs. late-global debate and favour the view that perceptual awareness emerges at the level of modality-specific sensory cortices.Die Fähigkeit zur bewussten Wahrnehmung bestimmt maßgeblich unser Selbstbild als Menschen. Unser Gehirn verarbeitet Informationen auf eine Weise, die es uns ermöglicht, uns der Bilder, Töne, Berührungen, Gerüche und Geschmäcker, die uns umgeben, bewusst zu werden. Unser Verständnis davon, wie neurobiologische Prozesse diese bewusste Wahrnehmung erzeugen, ist jedoch noch sehr begrenzt. Eine der umstrittensten Fragen in der neurowissenschaftlichen Erforschung des perzeptuellen Bewusstseins besteht darin, ob die bewusste Wahrnehmung aus der Aktivität früher sensorischer Hirnregionen entsteht, oder aber die spätere Prozessierung in ausgedehnten supramodalen Netzwerken erfordert. Eine mögliche Erklärung für die widersprüchlichen Ergebnisse, die diesen beiden Perspektiven zugrunde liegen, wird in methodologischen Störfaktoren vermutet, die in klassischen experimentellen Paradigmen auftreten können. Um auf die Wahrnehmung der Versuchspersonen schließen zu können, müssen diese den Inhalt ihrer Wahrnehmung berichten. Das führt dazu, dass neuronale Korrelate bewusster Wahrnehmung häufig nicht sauber von prä- und postperzeptuellen Prozessen getrennt werden können. Folglich könnten einige der zuvor beobachteten Effekte, anstatt tatsächlich bewusste Wahrnehmung widerzuspiegeln, aus den Anforderungen experimenteller Paradigmen entstanden sein. In dieser Arbeit untersuche ich diese Möglichkeit in der somatosensorischen Modalität. Um zu überprüfen, inwiefern neuronale Korrelate bewusster somatosensorischer Wahrnehmung von den Anforderungen experimenteller Aufgaben abhängen, habe ich ein Paradigma entwickelt, dass die häufigsten experimentellen Störfaktoren kontrolliert. In einer somatosensorisch-visuellen Vergleichsaufgabe mussten die Versuchspersonen elektrische Zielreize in zehn verschiedenen Intensitätsstufen detektieren. Anstatt diese jedoch direkt zu berichten, sollten sie ihre somatosensorischen Perzepte mit gleichzeitig präsentierten visuellen Symbolen vergleichen, die entweder Reizanwesenheit oder -abwesenheit signalisierten. Entsprechend wurde dann eine Übereinstimmung oder Nichtübereinstimmung berichtet. Dadurch wurde die Reizwahrnehmung von Arbeitsgedächtnis und Berichterstattung dekorreliert, die Verhaltensrelevanz detektierter und nicht detektierter Reize gleichgesetzt, der Einfluss von Aufmerksamkeitsprozessen reduziert und die mit der Detektion verbundene Unsicherheit auf kontrollierte Weise variiert. Die Ergebnisse aus einer funktionellen Magnetresonanztomographie (fMRT)-Studie und einer Elektroenzephalographie (EEG)-Studie zeigen, dass die neuronalen Korrelate bewusster somatosensorischer Wahrnehmung auf relativ frühe Aktivität (~150 ms) in sekundären somatosensorischen Regionen beschränkt sind, wenn experimentelle Störfaktoren kontrolliert werden. Im Gegensatz dazu trat späte Aktivität (>300 ms), die auf die Verarbeitung in frontoparietalen Netzwerken hindeutet, unabhängig von der Reizwahrnehmung auf, und Aktivität im anterioren insulären, anterioren cingulären und supplementär-motorischen Kortex war mit der Verarbeitung von Detektionsunsicherheit und der Berichterstattung verbunden. Diese Ergebnisse liefern neue Erkenntnisse zur Debatte um die Relevanz früher, lokaler vs. später, globaler Hirnaktivität und unterstützen die Ansicht, dass perzeptuelles Bewusstsein in modalitätsspezifischen sensorischen Kortizes entsteht