Enhanced GABA(A) inhibition enhances synchrony coding in human perception

Atemporal equilibria: pro- and retroactive coding in the dynamics of cognitive microstructures

Synchronization of spatially distributed neural assemblies at frequencies in the range 30-70 Hz (the "gamma" band) may be instrumental in grouping stimulus features. In agreement with this we have shown that detection reaction times to a grouping target stimulus are expedited when the stimulus is preceded by repeated presentation of a priming stimulus, presented below detection thresholds in a matrix that flickers at particular frequencies in the 27-68 Hz range. This dynamic priming effect can be partly explained as a function of the return phase of the priming stimulus relative to the premask matrix, indicating one of the primary consequences of repeating stimulation is pre-activation of a priming response relative to prime-stimulus presentation. However, this cannot entirely explain the relationship that develops between the timing of stimulus events (in this instance the time of target relative to priming-stimulus presentations) and response. By varying the frequency and phase of priming-stimulus and target presentations we discovered that given a particular relationship between the phase of target presentation relative to the return phase of the prime, target coding is expedited by a prime that achieves its maximum activation at a phase that would precede priming-stimulus presentation by several tens of milliseconds. However, and in addition, the cognition concerned is flexible enough to be able to achieve an identical prime retroactively, that is to say at a phase during or subsequent to priming-stimulus presentation. This occurs because of a different relationship between the phase of target presentation (defined relative to prime frequency) and the frequency of premask-matrix presentation. On this basis, it can be concluded that by virtue of the relationship between its dynamics and the timing of stimulus events, microstructural cognition functions in a temporal context that can shift from past to future states. Consequently and at the lowest level of psychological function, the conventional, one-dimensional model of time flow-from future to past states does not fully explain how cognition can function. In fact depending upon the interaction in phase between different coding frequencies, the same form of cognition can anticipate or retroactively code events. Consequently, and in so far as our cognition at this level provides a content structure for consciousness, our psychological lives may be fundamentally based upon the ability of our cognitive states to travel backwards and forwards across very short intervals of time.

Associations of regional GABA and glutamate with intrinsic and extrinsic neural activity in humans—a review of multimodal imaging studies

The integration of multiple imaging modalities is becoming an increasingly well used research strategy for studying the human brain. The neurotransmitters glutamate and GABA particularly lend themselves towards such studies. This is because these transmitters are ubiquitous throughout the cortex, where they are the key constituents of the inhibition/excitation balance, and because they can be easily measured in vivo through magnetic resonance spectroscopy, as well as with select positron emission tomography approaches. How these transmitters underly functional responses measured with techniques such as fMRI and EEG remains unclear though, and was the target of this review. Consistently shown in the literature was a negative correlation between GABA concentrations and stimulus-induced activity within the measured region. Also consistently found was a positive correlation between glutamate concentrations and inter-regional activity relationships, both during tasks and rest. These findings are outlined along with results from populations with mental disorders to give an overview of what brain imaging has suggested to date about the biochemical underpinnings of functional activity in health and disease. We conclude that the combination of functional and biochemical imaging in humans is an increasingly informative approach that does however require a number of key methodological and interpretive issues be addressed before can meet its potential.

Evidence for impaired visuoperceptual organisation in developmental dyslexics and its relation to temporal processes

An analysis of normal and dyslexic readers' reaction-time (RT) performance in a standard visualdetection task (Experiment A) and in temporally primed visual detection (Experiment B) reveals a tendency for significantly longer search and detection RTs for dyslexic relative to the performance of normal readers. Consistent with previous studies, the RTs of normal readers and fast dyslexic responders exhibited target-specific priming effects. In contrast, in addition to increased but statistically insignificant target priming, a set of slower dyslexic responders showed strong negative priming on target-absent trials. In spite of the longer detection latencies produced by these dyslexic participants, no evidence was found to suggest that negative priming occurred as a general function of increasing difficulty in task performance (Experiment C). The enhanced positive and the negative priming effects are both interpreted in the context of the possible deployment of attentional mechanisms to the priming stimulus. The extent to which this strategy is characteristic of dyslexic performance as a whole may relate to the degree to which the dyslexic responder concerned experiences some general temporal processing impairment: Attentional deployment in this instance serving to compensate a lack of the requisite temporal resolution required for coding the spatiotemporal structure of the prime.

Oscillatory priming and form complexity

Detection of a target grouping is expedited when the target display is preceded by presentation of an oscillating premask that includes priming elements presented intraphasically and below detection threshold at the display locations subsequently occupied by the target. Five experiments were performed to investigate how priming is affected by both the complexity and the geometry of prime/target forms. Experiment 1 showed that the subjective complexity of different polygons was coded in accord with an objective measure of form complexity. Experiments 2 and 3 revealed oscillatory priming to increase as a function of the subjective complexity of prime/target forms when those forms were regular and predictable. However, Experiments 4 and 5 showed that this relation did not hold when the prime/target forms were irregular and unpredictable. Taken together, it is argued that both subjective complexity and the Prägnanz quality of the prime/target forms come to determine the magnitude of priming. These results are discussed with reference to current physiological hypotheses regarding perceptual organization.

The Dynamics of Operations in Visual Memory:

Abstract. Evidence from neurophysiological studies indicates that the synchronization of distributed neuronal assemblies in the gamma frequency range is responsible for the integration of discrete stimulus components into coherent wholes (e.g., see Singer, 1999 for review). Psychophysical support for this hypothesis has been reported in experiments that demonstrate that the presentation of a synchronous-premask frame within a 40-Hz flickering premask matrix primes the subsequent detection of a target Kanizsa-type square by generation of a 40-Hz prime ( Elliott & Müller, 1998 ). Psychopharmacological and electrophysiological evidence suggests that this priming mechanism is related to activity in interneuronal networks and relies on the combined function of prefrontal and posterior circuits. In addition, psychophysical experiments demonstrate the existence of a prime-specific visual short-term memory that oscillates at 40 Hz and remains functional for up to 300 ms post-stimulus offset. These results are consistent with a view of the prime as a form of oscillatory mechanism, related to the persistence of visual information ( Coltheart, 1980 ) and in the capacity guided by (prefrontal) top-down influences upon visual-cortical function.

Four facets of a single brain: behaviour, cerebral blood flow/metabolism, neuronal activity and neurotransmitter dynamics

Is functional neuroimaging a royal way to understand brain function or is it a new phrenology without an exact understanding what we measure? After two decades of imaging revolution, more and more authors ask this question. Brain functions are multidimensional, which can be approached from the point of (1) behavioural measures, (2) brain activation as reflected by blood flow and metabolic changes, (3) electrical activity of cells and cell-populations, and (4) neurotransmitter dynamics (release, receptor binding and reuptake). Using imaging techniques, we must take into consideration that even during the simplest task all of these processes operate in a closely interacting manner. Therefore, before drawing final conclusions about brain functions on the basis of a single aspect of these mechanisms, we must clarify the exact relationship among them. In this paper, we address this issue in order to draw attention to a number of uncertainties and controversies in the relationship of the four facets of brain functions.

Different effects of lorazepam and diazepam on perceptual integration

Recent research has established the detrimental effect of lorazepam, a benzodiazepine, on both implicit and explicit memory. Furthermore, lorazepam is known to affect perceptual integration. Diazepam, on the other hand, though being a benzodiazepine too, only impairs explicit memory, leaving implicit memory fairly intact. Little is known about the effect of diazepam on perceptual integration. The present study aimed at filling in this gap, by comparing the effects of lorazepam and diazepam on the detection of discontinuities in random-shaped outlines. In line with previous findings, the results in a lorazepam-treated group were quite different from the results in a placebo-treated group. The results in a diazepam-treated group were analogous to the results in the placebo-treated group and different from the results in the lorazepam-treated group. This shows that lorazepam and diazepam differ, not only with respect to their effect on implicit memory, but also with respect to their effect on perceptual integration. It is argued that this bears important consequences for memory research that makes use of a pharmacological dissociation rationale.