Neural correlates of stimulus-invariant decisions about motion in depth.

Perceptual decision-making is a complicated, multi-stage process. Recently human neuroimaging studies implicated a set of regions, extending from the medial frontal cortex to the inferior parietal lobule that are involved in various steps of perceptual judgments. However, relatively little is known about the dependence of perceptual decisions on the visual stimulus itself. In the current study, we used functional magnetic resonance imaging to map neural activations while subjects performed a demanding 3D heading estimation task (heading slightly to the left or right of fixation). Subjects (n=13) were presented a constantly expanding optic-flow stimulus, composed of disparate red-blue spheres, viewed stereoscopically through red-blue glasses. We varied task difficulty either by adding incoherently moving spheres to the stimuli, hence reducing the strength of the motion signal and thereby increasing the amount of noise or by reducing the relevant differential information by decreasing the deviation of the average trajectory of the spheres from straight ahead. BOLD signals were compared during "easy" and "hard" trials in both stimulation conditions to isolate the neural mechanisms underlying the decision process. We hypothesized that areas involved in perceptual decisions about motion should exhibit significantly different activation across both stimulus conditions. Our results indicate that during earlier, sensory-stimulation-related phases of decision-making the left dorsolateral prefrontal cortex, posterior cingulate and inferior parietal cortex showed more activation for the "easy" compared to the "hard" trials, while during later, response-related phases the bilateral precuneus and inferior parietal cortex, as well as the bilateral superior medial gyrus showed this pattern of activation. Our results suggest that a large, non-overlapping network of areas is involved in various steps of decisions regarding 3D motion.

Neural correlates of high-level adaptation-related aftereffects.

Prolonged exposure to complex stimuli, such as faces, biases perceptual decisions toward nonadapted, dissimilar stimuli, leading to contrastive aftereffects. Here we tested the neural correlates of this perceptual bias using a functional magnetic resonance imaging adaptation (fMRIa) paradigm. Adaptation to a face or hand stimulus led to aftereffects by biasing the categorization of subsequent ambiguous face/hand composite stimuli away from the adaptor category. The simultaneously observed fMRIa in the face-sensitive fusiform face area (FFA) and in the body-part-sensitive extrastriate body area (EBA) depended on the behavioral response of the subjects: adaptation to the preferred stimulus of the given area led to larger signal reduction during trials when it biased perception than during trials when it was less effective. Activity in two frontal areas correlated positively with the activity patterns in FFA and EBA. Based on our novel adaptation paradigm, the results suggest that the adaptation-induced aftereffects are mediated by the relative activity of category-sensitive areas of the human brain as demonstrated by fMRI.

Short-duration transcranial random noise stimulation induces blood oxygenation level dependent response attenuation in the human motor cortex.

Manipulation of cortical excitability can be experimentally achieved by the application of transcranial random noise stimulation (tRNS). TRNS is a novel method of non-invasive electrical brain stimulation whereby a random electrical oscillation spectrum is applied over the cortex. A previous study recently reported that application of weak 10-min tRNS over primary motor cortex (M1) enhances corticospinal excitability both during and after stimulation in the healthy human brain. Here, blood oxygenation level dependent (BOLD) MRI was used to monitor modulations in human sensorimotor activity after the application of 4-min tRNS. Activation maps for a right hand index-thumb finger opposition task were obtained for nine subjects after sham and 1-mA tRNS in separate sessions. TRNS of the left-hemispheric sensorimotor cortex resulted in a decrease in the mean number of activated pixels by 17%, in the hand area. Our results indicate that tRNS applied with different durations and/or in combination with a task might result in different outcomes. Application of tRNS to the human cortex allows an unnoticeable and thus painless, selective, non-invasive and reversible activity change within the cortex, its main advantage being the direction insensitivity of the stimulation. TRNS also provides a qualitatively new way of producing and interfering with brain plasticity, although, further research is required to optimise stimulation parameters and efficacy.

Position-specific and position-invariant face aftereffects reflect the adaptation of different cortical areas.

Adaptation to faces leads to face aftereffects and currently this topic attracts a lot of attention because it clearly shows that adaptation occurs even at the higher stages of visual cortical processing. Recently it has been found that long-term exposure to a face stimulus results in adaptation of a position-specific population of face sensitive neurons in addition to a position-invariant neural population, the later being also adapted in the case of short-term adaptation. Here we used the fMRI adaptation technique to investigate the neural locus of position-specific and position-invariant face adaptation. We show that in the right fusiform face area adaptation effects are position invariant and can be evoked by short (500 ms) as well as long (4500 ms) adaptation durations. On the other hand adaptation effects in the right occipital face area are position-specific and require long-term adaptation to develop. These findings imply that the behaviourally observed face aftereffects reflect time-dependent adaptation processes of both position-specific and invariant face sensitive neurons at different stages of visual processing.

Cathodal stimulation of human MT+ leads to elevated fMRI signal: a tDCS-fMRI study.

PURPOSE: Transcranial direct current stimulation (tDCS) was reintroduced about a decade ago as a tool for inducing long-lasting changes in cortical excitability. Recently it has been shown that both motor and cognitive functions can be influenced by tDCS. Here, we tested the effect of tDCS on the blood-oxygen level dependent (BOLD) signal evoked by coherent visual motion using functional magnetic resonance imaging (fMRI). METHODS: The subjects underwent 10 min of cathodal and sham tDCS, applied over the right MT+. Following stimulation, random dot kinomatograms (RDK) with different percentages (10%, 30%, 50%) of coherently moving dots were presented. RESULTS: All motion stimuli activated MT+ in both stimulation conditions. However, cathodal stimulation led to an increase in fMRI signal in MT+ when compared to sham stimulation. This effect did not depend on the coherence level of the visual stimulus. CONCLUSIONS: Here, we show for the first time, that cathodal tDCS stimulation leads to elevated fMRI signal in the human visual cortex.