Symmetrical hemispheric priming in spatial neglect: a hyperactive left-hemisphere phenomenon?

Left Amygdala Regulates the Cerebral Reading Network During Fast Emotion Word Processing

Emotion words constitute a special class of verbal stimuli which can quickly activate the limbic system outside the left-hemisphere language network. Such fast response to emotion words may arise independently of the left occipitotemporal area involved in visual word-form analysis and rely on a distinct amygdala-dependent emotion circuit involved in fearful face processing. Using a hemifield priming paradigm with fMRI, we explored how the left and right amygdala systems interact with the reading network during emotion word processing. On each trial, participants viewed a centrally presented target which was preceded by a masked prime flashed either to the left or right visual field. Primes and targets, each denoting negative or positive nouns, could be either affectively congruent or incongruent with each other. We observed that affective congruency produced parallel changes in neural priming between the left frontal and parietotemporal regions and the bilateral amygdala. However, we also found that the left, but not right, amygdala exhibited significant change in functional connectivity with the neural components of reading as a function of affective congruency. Collectively, these results suggest that emotion words activate the bilateral amygdala during early stages of emotion word processing, whereas only the left amygdala exerts a long-distance regulatory influence over the reading network via its strong within-hemisphere connectivity.

Stimulus-driven changes in the direction of neural priming during visual word recognition

Visual object recognition is generally known to be facilitated when targets are preceded by the same or relevant stimuli. For written words, however, the beneficial effect of priming can be reversed when primes and targets share initial syllables (e.g., "boca" and "bono"). Using fMRI, the present study explored neuroanatomical correlates of this negative syllabic priming. In each trial, participants made semantic judgment about a centrally presented target, which was preceded by a masked prime flashed either to the left or right visual field. We observed that the inhibitory priming during reading was associated with a left-lateralized effect of repetition enhancement in the inferior frontal gyrus (IFG), rather than repetition suppression in the ventral visual region previously associated with facilitatory behavioral priming. We further performed a second fMRI experiment using a classical whole-word repetition priming paradigm with the same hemifield procedure and task instruction, and obtained well-known effects of repetition suppression in the left occipito-temporal cortex. These results therefore suggest that the left IFG constitutes a fast word processing system distinct from the posterior visual word-form system and that the directions of repetition effects can change with intrinsic properties of stimuli even when participants' cognitive and attentional states are kept constant.

Mirror-image discrimination in the literate brain: a causal role for the left occpitotemporal cortex

Previous studies show that the primate and human visual system automatically generates a common and invariant representation from a visual object image and its mirror reflection. For humans, however, this mirror-image generalization seems to be partially suppressed through literacy acquisition, since literate adults have greater difficulty in recognizing mirror images of letters than those of other visual objects. At the neural level, such category-specific effect on mirror-image processing has been associated with the left occpitotemporal cortex (L-OTC), but it remains unclear whether the apparent “inhibition” on mirror letters is mediated by suppressing mirror-image representations covertly generated from normal letter stimuli. Using transcranial magnetic stimulation (TMS), we examined how transient disruption of the L-OTC affects mirror-image recognition during a same-different judgment task, while varying the semantic category (letters and non-letter objects), identity (same or different), and orientation (same or mirror-reversed) of the first and second stimuli. We found that magnetic stimulation of the L-OTC produced a significant delay in mirror-image recognition for letter-strings but not for other objects. By contrast, this category specific impact was not observed when TMS was applied to other control sites, including the right homologous area and vertex. These results thus demonstrate a causal link between the L-OTC and mirror-image discrimination in literate people. We further suggest that left-right sensitivity for letters is not achieved by a local inhibitory mechanism in the L-OTC but probably relies on the inter-regional coupling with other orientation-sensitive occipito-parietal regions.

Studies on semantic priming effects in right hemisphere stroke: A systematic review

The role of the right cerebral hemisphere (RH) associated with semantic priming effects (SPEs) must be better understood, since the consequences of RH damage on SPE are not yet well established.

To the Other Side of the Neglected Brain

The human brain is characterized by the lateralization of cognitive functions. Multiple lines of evidence suggest the deployment of visuospatial attention is controlled by a frontoparietal network, with a right hemisphere dominance. Among cortical areas included in the network, the right posterior parietal cortex (PPC) has been proposed to be a crucial node and has also been implicated on clinical grounds. Here, the authors provide an overview of the existent literature giving evidence to a functional asymmetry of the parietal cortices in directing visuospatial attention, focusing on those studies seeking to characterize the causal role of PPC, applying transcranial magnetic stimulation and its combination with imaging techniques, such as electroencephalography and fMRI. First, the role of PPC and how this region exerts its control over remote areas of both hemispheres is discussed. The second part discusses studies involving neglect patients shedding light on the complex interplay between left and right PPC, strongly supporting the hemispheric rivalry theory. Finally, studies demonstrating changes of neglect disorders following the manipulation of the unaffected hemisphere activation will be discussed.