Enhancement of semantic integration reasoning by tRNS.

The right hemisphere is involved with the integrative processes necessary to achieve global coherence during reasoning and discourse processing. Specifically, the right temporal lobe has been proven to facilitate the processing of distant associate relationships, such as generating novel ideas. Previous studies showed a specific swing of alpha and gamma oscillatory activity over the right parieto-occipital lobe and the right anterior temporal lobe respectively, when people solve semantic problems with a specific strategy, i.e., insight problem-solving. In this study, we investigated the specificity of the right parietal and temporal lobes for semantic integration using transcranial Random Noise Stimulation (tRNS). We administered a set of pure semantics (i.e., Compound Remote Associates [CRA]) and visuo-semantic problems (i.e., Rebus Puzzles) to a sample of 31 healthy volunteers. Behavioral results showed that tRNS stimulation over the right temporal lobe enhances CRA accuracy (+12%), while stimulation on the right parietal lobe causes a decrease of response time on the same task (-2,100 ms). No effects were detected for Rebus Puzzles. Our findings corroborate the involvement of the right temporal and parietal lobes when solving purely semantic problems but not when they involve visuo-semantic material, also providing causal evidence for their postulated different roles in the semantic integration process and promoting tRNS as a candidate tool to boost verbal reasoning in humans.

Grasping in wonderland: altering the visual size of the body recalibrates the body schema.

Can viewing our own body modified in size reshape the bodily representation employed for interacting with the environment? This question was addressed here by exposing participants to either an enlarged, a shrunken, or an unmodified view of their own hand in a reach-to-grasp task toward a target of fixed dimensions. When presented with a visually larger hand, participants modified the kinematics of their grasping movement by reducing maximum grip aperture. This adjustment was carried over even when the hand was rendered invisible in subsequent trials, suggesting a stable modification of the bodily representation employed for the action. The effect was specific for the size of the grip aperture, leaving the other features of the reach-to-grasp movement unaffected. Reducing the visual size of the hand did not induce the opposite effect, although individual differences were found, which possibly depended on the degree of subject's reliance on visual input. A control experiment suggested that the effect exerted by the vision of the enlarged hand could not be merely explained by simple global visual rescaling. Overall, our results suggest that visual information pertaining to the size of the body is accessed by the body schema and is prioritized over the proprioceptive input for motor control.

Unconscious letter discrimination is enhanced by association with conscious color perception in visual form agnosia.

Adaptive behavior guided by unconscious visual cues occurs in patients with various kinds of brain damage as well as in normal observers, all of whom can process visual information of which they are fully unaware [1] [2] [3] [4] [5] [6] [7] [8]. Little is known on the possibility that unconscious vision is influenced by visual cues that have access to consciousness [9]. Here we report a 'blind' letter discrimination induced through a semantic interaction with conscious color processing in a patient who is agnosic for visual shapes, but has normal color vision and visual imagery. In seeing the initial letters of color names printed in different colors, it is normally easier to name the print color when it is congruent with the initial letter of the color name than when it is not [10]. The patient could discriminate the initial letters of the words 'red' and 'green' printed in the corresponding colors significantly above chance but without any conscious accompaniment, whereas he performed at chance with the reverse color-letter mapping as well as in standard tests of letter reading. We suggest that the consciously perceived colors activated a representation of the corresponding word names and their component letters, which in turn brought out a partially successful, unconscious processing of visual inputs corresponding to the activated letter representations.

Abnormally speeded saccades to ipsilesional targets in patients with spatial neglect.

We mapped the distribution of saccadic reaction times (SRTs) in the visual field of patients with spatial neglect in order to characterise the topography of the bias in spatial orientation peculiar to this disorder. LED-generated stimuli were lit randomly in one of four positions (+/-5 degrees , +/-10 degrees , +/-20 degrees , +/-30 degrees ) along the horizontal meridian in blocks of either ipsilesional or contralesional presentations. Patients were asked to move the gaze as quickly as possible from central fixation to target upon its appearance. Unlike control subjects, patients with neglect showed an asymmetric distribution of visuo-motor performance in the two hemifields with an increasing impairment in target detection and saccadic reaction at increasing eccentricities in the contralesional field. In contrast, in the ipsilesional field they showed abnormally speeded SRTs at 5 degrees and 10 degrees , outperforming even healthy subjects. Latency of saccades increased again at more peripheral ipsilesional locations (20 degrees and 30 degrees ) where there was also a tendency for a higher omission rate as compared to control groups. These results indicate that in neglect patients the spatial orientation bias, as witnessed by saccadic performance, specifically affects an off-centred sector of the ipsilesional space, and this is in keeping with evidence from a previous study using a manual RT paradigm. The generality of this phenomenon across different types of motor response suggests that it depends upon abnormal mechanisms of spatial coding interfering with perceptual processing and orienting behaviour.

Redundancy gain in the stop-signal paradigm: implications for the locus of coactivation in simple reaction time.

The authors carried out 2 experiments designed to cast light on the locus of redundancy gain in simple visual reaction time by using a stop-signal paradigm. In Experiment 1, the authors found that single visual stimuli were more easily inhibited than double visual stimuli by an acoustic stop signal. This result is in keeping with the idea that redundancy gain occurs prior to the ballistic stage of the stop-signal task. In Experiment 2, the authors found that the response to an acoustic go signal was more easily inhibited by a double than by a single visual stop signal. This result provides conclusive evidence for a redundancy gain in the stop process--in a process that does not involve a motor response but rather its inhibition.

At what stage of manual visual reaction time does interhemispheric transmission occur: controlled or ballistic?

Interhemispheric transfer (IT) time through the corpus callosum can be measured with a manual reaction time (RT) to lateralized visual stimuli (the so-called Poffenberger paradigm) by subtracting mean RT of faster uncrossed hemifield-hand combinations (not requiring an IT) from slower crossed combinations (requiring an IT). That the corpus callosum is involved in IT has been demonstrated by its dramatic lengthening in patients with a section of the corpus callosum. However, it is still unclear whether the signal transmitted by the corpus callosum concerns perceptual or motor stages of RT. To try and cast light on this question, in a first experiment we tested normal subjects on a partially modified Poffenberger paradigm with stop trials intermingled with go trials. In the former, subjects are supposed to refrain from responding following a stop signal (stop-signal paradigm). This paradigm can tease apart the contribution of the controlled and ballistic stages to overall RT and, used together with the Poffenberger task, enables one to assess the stage at which IT occurs. The controlled stage lies before the point of no return, i.e. the point beyond which the response cannot be inhibited, and concerns perceptual and pre-motor processes, while the ballistic stage occurs after the point of no return and concerns the motoric aspect of the response. We found that the slower responses typically obtained in the crossed conditions were more likely to be inhibited than the faster uncrossed responses and this suggests that IT occurs prior to the point of no return. Since the precise locus of the point of no return is uncertain, in a second experiment we used response force as a dependent variable reflecting the activation of the motor cortex. We found that none of the force parameters studied differed between crossed and uncrossed conditions while the temporal parameters confirmed the presence of an advantage of the uncrossed combinations. Altogether these results suggest that callosal IT of visuomotor information occurs at the stage of controlled (perceptual and pre-motor) processes and rule out the possibility of an IT at the motoric stage.

Experimentally induced retinal projections to the ferret auditory thalamus: development of clustered eye-specific patterns in a novel target.

We have examined the relative role of afferents and targets in pattern formation using a novel preparation, in which retinal projections in ferrets are induced to innervate the medial geniculate nucleus (MGN). We find that retinal projections to the MGN are arranged in scattered clusters. Clusters arising from the ipsilateral eye are frequently adjacent to, but spatially segregated from, clusters arising from the contralateral eye. Both clustering and eye-specific segregation in the MGN arise as a refinement of initially diffuse and overlapped projections. The shape, size, and orientation of retinal terminal clusters in the MGN closely match those of relay cell dendrites arrayed within fibrodendritic laminae in the MGN. We conclude that specific aspects of a projection system are regulated by afferents and others by targets. Clustering of retinal projections within the MGN and eye-specific segregation involve progressive remodeling of retinal axon arbors, over a time period that closely parallels pattern formation by retinal afferents within their normal target, the lateral geniculate nucleus (LGN). Thus, afferent-driven mechanisms are implicated in these events. However, the termination zones are aligned within the normal cellular organization of the MGN, which does not differentiate into eye-specific cell layers similar to the LGN. Thus, target-driven mechanisms are implicated in lamina formation and cellular differentiation.

Preserved functional competence of perilesional areas in drug-resistant epilepsy with lesion in supplementary motor cortex: fMRI and neuropsychological observations.

We report a presurgical fMRI study and a longitudinal behavioral and structural MRI study in a 26-year-old right-handed woman with drug-resistant epilepsy of the supplementary motor region with cytoarchitectural dysplasia and minimal cortico-subcortical gliotic damage. fMRI scans were acquired during a silent phonemic verbal fluency task (VF), an automatic counting task (CT), and a finger-tapping motor task (MT). These were all compared with rest. Presurgical neuropsychological assessment was substantially normal with only a minor deficit in the domain of visuo-constructive and complex motor-planning skills. Noticeably, performance on phonemic verbal fluency was normal. Presurgical fMRI results revealed a normal specialization of left SMA and pre-SMA, including a fine-grained somatotopy for mouth and hand representations despite epilepsy. Immediately after surgical removal of the epileptogenic zone (the posterior third of the superior and middle frontal gyri including pre-SMA and part of SMA, and part of the anterior cingulate region--all of which were active presurgically at the fMRI tests), the patient suffered from transcortical motor aphasia temporarily. One year after surgery, she still showed impaired performance in the verbal fluency tasks while naming and comprehension were recovered. The patient was now free from seizures. This fMRI study supports the case that repeated seizures per se may not be sufficient to alter the distribution of neural representations of cognitive function. Selective behavioral impairment after surgical removal of brain areas that were activated during presurgical fMRI permits us to establish a causal link between these activations and task performance. This link could not have been made on the basis of activation patterns or lesion data taken on their own. These findings support the case that some epileptic patients may represent a unique opportunity for cognitive neuroscience studies.