Ready ... Go: Amplitude of the fMRI Signal Encodes Expectation of Cue Arrival Time

What happens when the brain awaits a signal of uncertain arrival time, as when a sprinter waits for the starting pistol? And what happens just after the starting pistol fires? Using functional magnetic resonance imaging (fMRI), we have discovered a novel correlate of temporal expectations in several brain regions, most prominently in the supplementary motor area (SMA). Contrary to expectations, we found little fMRI activity during the waiting period; however, a large signal appears after the “go” signal, the amplitude of which reflects learned expectations about the distribution of possible waiting times. Specifically, the amplitude of the fMRI signal appears to encode a cumulative conditional probability, also known as the cumulative hazard function. The fMRI signal loses its dependence on waiting time in a “countdown” condition in which the arrival time of the go cue is known in advance, suggesting that the signal encodes temporal probabilities rather than simply elapsed time. The dependence of the signal on temporal expectation is present in “no-go” conditions, demonstrating that the effect is not a consequence of motor output. Finally, the encoding is not dependent on modality, operating in the same manner with auditory or visual signals. This finding extends our understanding of the relationship between temporal expectancy and measurable neural signals

The notion of contextual locking: Previously learnt items are not accessible as such when appearing in a less common context

We examined the effect of context on the learning of spatial coding in four experiments. Two partially overlapping sets of stimuli, which had the very same stimulus–response spatial coding, were presented in unique contexts. Results show contextual locking—that is, response times to the very same item in a more common context (80%) were significantly shorter than those in a less common context (20%). Contextual locking was obtained both when the context was more salient (Experiments 1 and 2) and less salient (Experiments 3 and 4). In addition, results were obtained even when contextualization seemed less necessary (Experiments 2 and 4). Binding of information to context is discussed in relation to chunking, transfer effects, and practical applications pertaining to professional training

Extrinsic primary afferent signalling in the gut

Visceral sensory neurons activate reflex pathways that control gut function and also give rise to important sensations, such as fullness, bloating, nausea, discomfort, urgency and pain. Sensory neurons are organised into three distinct anatomical pathways to the central nervous system (vagal, thoracolumbar and lumbosacral). Although remarkable progress has been made in characterizing the roles of many ion channels, receptors and second messengers in visceral sensory neurons, the basic aim of understanding how many classes there are, and how they differ, has proven difficult to achieve. We suggest that just five structurally distinct types of sensory endings are present in the gut wall that account for essentially all of the primary afferent neurons in the three pathways. Each of these five major structural types of endings seems to show distinctive combinations of physiological responses. These types are: 'intraganglionic laminar' endings in myenteric ganglia; 'mucosal' endings located in the subepithelial layer; 'muscular–mucosal' afferents, with mechanosensitive endings close to the muscularis mucosae; 'intramuscular' endings, with endings within the smooth muscle layers; and 'vascular' afferents, with sensitive endings primarily on blood vessels. 'Silent' afferents might be a subset of inexcitable 'vascular' afferents, which can be switched on by inflammatory mediators. Extrinsic sensory neurons comprise an attractive focus for targeted therapeutic intervention in a range of gastrointestinal disorders.Australian National Health and Medical Research Counci

Spatiotemporal precision and hemodynamic mechanism of optical point spreads in alert primates

In functional brain imaging there is controversy over which hemodynamic signal best represents neural activity. Intrinsic signal optical imaging (ISOI) suggests that the best signal is the early darkening observed at wavelengths absorbed preferentially by deoxyhemoglobin (HbR). It is assumed that this darkening or “initial dip” reports local conversion of oxyhemoglobin (HbO) to HbR, i.e., oxygen consumption caused by local neural activity, thus giving the most specific measure of such activity. The blood volume signal, by contrast, is believed to be more delayed and less specific. Here, we used multiwavelength ISOI to simultaneously map oxygenation and blood volume [i.e., total hemoglobin (HbT)] in primary visual cortex (V1) of the alert macaque. We found that the hemodynamic “point spread,” i.e., impulse response to a minimal visual stimulus, was as rapid and retinotopically specific when imaged by using blood volume as when using the initial dip. Quantitative separation of the imaged signal into HbR, HbO, and HbT showed, moreover, that the initial dip was dominated by a fast local increase in HbT, with no increase in HbR. We found only a delayed HbR decrease that was broader in retinotopic spread than HbO or HbT. Further, we show that the multiphasic time course of typical ISOI signals and the strength of the initial dip may reflect the temporal interplay of monophasic HbO, HbR, and HbT signals. Characterizing the hemodynamic response is important for understanding neurovascular coupling and elucidating the physiological basis of imaging techniques such as fMRI

Transfer von Luftfahrttechnologie in andere Industriebereiche Beispiele aus dem Gebiet der Aerodynamik. Kurzfassung

Copy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman