Neural correlates of conscious and unconscious vision in parietal extinction

Brain areas activated by stimuli in the left visual field of a right parietal patient suffering from left visual extinction were identified using event-related functional magnetic resonance imaging. Left visual field stimuli that were extinguished from awareness still activated the ventral visual cortex, including areas in the damaged right hemisphere. An extinguished face stimulus on the left produced robust category-specific activation of the right fusiform face area. On trials where the left visual stimulus was consciously seen rather than extinguished, greater activity was found in the ventral visual cortex of the damaged hemisphere, and also in frontal and parietal areas of the intact hemisphere. These findings extend recent observations on visual extinction, suggesting distinct neural correlates for conscious and unconscious perception.     

Neural Correlates of Conscious and Unconscious Vision in Parietal Extinction

Brain areas activated by stimuli in the left visual field of a right parietal patient suffering from left visual extinction were identified using event-related functional magnetic resonance imaging. Left visual field stimuli that were extinguished from awareness still activated the ventral visual cortex, including areas in the damaged right hemisphere. An extinguished face stimulus on the left produced robust category-specific activation of the right fusiform face area. On trials where the left visual stimulus was consciously seen rather than extinguished, greater activity was found in the ventral visual cortex of the damaged hemisphere, and also in frontal and parietal areas of the intact hemisphere. These findings extend recent observations on visual extinction, suggesting distinct neural correlates for conscious and unconscious perception.     

Neural response to emotional faces with and without awareness: event-related fMRI in a parietal patient with visual extinction and spatial neglect

This study examined whether differential neural responses are evoked by emotional stimuli with and without conscious perception, in a patient with visual neglect and extinction. Stimuli were briefly shown in either right, left, or both fields during event-related fMRI. On bilateral trials, either a fearful or neutral left face appeared with a right house, and it could either be extinguished from awareness or perceived. Seen faces in left visual field (LVF) activated primary visual cortex in the damaged right-hemisphere and bilateral fusiform gyri. Extinguished left faces increased activity in striate and extrastriate cortex, compared with right houses only. Critically, fearful faces activated the left amygdala and extrastriate cortex both when seen and when extinguished; as well as bilateral orbitofrontal and intact right superior parietal areas. Comparison of perceived versus extinguished faces revealed no difference in amygdala for fearful faces. Conscious perception increased activity in fusiform, parietal and prefrontal areas of the left-hemisphere, irrespective of emotional expression; while a differential emotional response to fearful faces occurring specifically with awareness was found in bilateral parietal, temporal, and frontal areas. These results demonstrate that amygdala and orbitofrontal cortex can be activated by emotional stimuli even without awareness after parietal damage; and that substantial unconscious residual processing can occur within spared brain areas well beyond visual cortex, despite neglect and extinction.     

The electrophysiology of tactile extinction: ERP correlates of unconscious somatosensory processing

We examined the electrophysiological correlates of left-sided tactile extinction in a patient with right-hemisphere damage. Computer-controlled punctate touch was presented to the left, right or both index fingers in an unpredictable sequence. The patient reported his conscious tactile percept (“left”, “right” or “both”). He showed extinction on 75% of bilateral trials, reporting only right stimulation for these. Somatosensory evoked potentials for unilateral stimulation showed early components over contralateral somatosensory areas (P60 and N110) for either hand. In contrast to the results observed for age-matched controls, the patient’s P60 was smaller in amplitude for left-hand touch over the right hemisphere than for right-hand touch over the intact hemisphere. Bilateral trials with extinction revealed residual P60 and N110 components over the right hemisphere in response to the extinguished left touch. These results demonstrate residual unconscious somatosensory processing of extinguished touch. They also suggest that tactile extinction can be caused by attenuation rather than elimination of somatosensory responses in the damaged hemisphere, with an underlying deficit even on unilateral trials.     

Hemispheric differences in frontal and parietal influences on human occipital cortex: direct confirmation with concurrent TMS-fMRI

We used concurrent TMS-fMRI to test directly for hemispheric differences in causal influences of the right or left fronto-parietal cortex on activity (BOLD signal) in the human occipital cortex. Clinical data and some behavioral TMS studies have been taken to suggest right-hemisphere specialization for top-down modulation of vision in humans, based on deficits such as spatial neglect or extinction in lesioned patients, or findings that TMS to right (vs. left) fronto-parietal structures can elicit stronger effects on visual performance. But prior to the recent advent of concurrent TMS and neuroimaging, it was not possible to directly examine the causal impact of one (stimulated) brain region upon others in humans. Here we stimulated the frontal or intraparietal cortex in the left or right hemisphere with TMS, inside an MR scanner, while measuring with fMRI any resulting BOLD signal changes in visual areas V1-V4 and V5/MT+. For both frontal and parietal stimulation, we found clear differences between effects of right- versus left-hemisphere TMS on activity in the visual cortex, with all differences significant in direct statistical comparisons. Frontal TMS over either hemisphere elicited similar BOLD decreases for central visual field representations in V1-V4, but only right frontal TMS led to BOLD increases for peripheral field representations in these regions. Hemispheric differences for effects of parietal TMS were even more marked: Right parietal TMS led to strong BOLD changes in V1-V4 and V5/MT+, but left parietal TMS did not. These data directly confirm that the human frontal and parietal cortex show right-hemisphere specialization for causal influences on the visual cortex.     

The sense of touch: embodied simulation in a visuotactile mirroring mechanism for observed animate or inanimate touch

Previous studies have shown a shared neural circuitry in the somatosensory cortices for the experience of one's own body being touched and the sight of intentional touch. Using functional magnetic resonance imaging (fMRI), the present study aimed to elucidate whether the activation of a visuotactile mirroring mechanism during touch observation applies to the sight of any touch, that is, whether it is independent of the intentionality of observed touching agent. During fMRI scanning, healthy participants viewed video clips depicting a touch that was intentional or accidental, and occurring between animate or inanimate objects. Analyses showed equal overlapping activation for all the touch observation conditions and the experience of one's own body being touched in the bilateral secondary somatosensory cortex (SII), left inferior parietal lobule (IPL)/supramarginal gyrus, bilateral temporal-occipital junction, and left precentral gyrus. A significant difference between the sight of an intentional touch, compared to an accidental touch, was found in the left primary somatosensory cortex (SI/Brodmann's area [BA] 2). Interestingly, activation in SI/BA 2 significantly correlated with the degree of intentionality of the observed touch stimuli as rated by participants. Our findings show that activation of a visuotactile mirroring mechanism for touch observation might underpin an abstract notion of touch, whereas activation in SI might reflect a human tendency to "resonate" more with a present or assumed intentional touching agent.     

Orienting attention in time: behavioural and neuroanatomical distinction between exogenous and endogenous shifts

Temporal orienting of attention is the ability to focus resources at a particular moment in time in order to optimise behaviour, and is associated with activation of left parietal and premotor cortex [Coull, J. T., Nobre, A. C. Where and when to pay attention: the neural systems for directing attention to spatial locations and to time intervals as revealed by both PET and fMRI. Journal of Neuroscience, 1998, 18, 7426-7435]. In the present experiment, we explored the behavioural and anatomical correlates of temporal orienting to foveal visual stimuli, in order to eliminate any spatial attention confounds. We implemented a two-way factorial design in an event-related fMRI study to examine the factors of trial validity (predictability of target by cue), length of delay (cue-target interval), and their interaction. There were two distinct types of invalid trial: those where attention was automatically drawn to a premature target and those where attention was voluntarily shifted to a delayed time-point. Reaction times for valid trials were shorter than those for invalid trials, demonstrating appropriate allocation of attention to temporal cues. All trial-types activated a shared system, including frontoparietal areas bilaterally, showing that this network is consistently associated with attentional orienting and is not specific to spatial tasks. Distinct brain areas were sensitive to cue-target delays and to trial validity. Long cue-target intervals activated areas involved in motor preparation: supplementary motor cortex, basal ganglia and thalamus. Invalid trials, where temporal expectancies were breached, showed enhanced activation of left parietal and frontal areas, and engagement of orbitofrontal cortex bilaterally. Finally, trial validity interacted with length of delay. Appearance of targets prematurely selectively activated visual extrastriate cortex; while postponement of target appearance selectively activated right prefrontal cortex. These findings suggest that distinct brain areas are involved in redirecting attention based upon sensory events (bottom-up, exogenous shifts) and based upon cognitive expectations (top-down, endogenous shifts).     

Neural correlates of covert orienting of visual spatial attention along vertical and horizontal dimensions

Covert orienting of spatial attention along the horizontal meridian of the visual field is mediated by a fronto-parietal neural network. The neural substrates underlying covert orienting of attention along the vertical meridian, however, are less understood. We recorded hemodynamic responses using functional magnetic resonance imaging (fMRI) from healthy volunteers in covert visual orienting tasks that required to detect targets either at the fixation or at peripheral attended locations on the horizontal or vertical meridian in the left (LVF), right (RVF), upper (UVF), and lower (LoVF) visual fields. We found that, relative to when attention was at the fixation, covert orienting of attention along the horizontal and vertical meridia induced enhanced activities in the superior parietal and frontal lobes bilaterally and the cerebellum. In addition, attention to the LoVF and UVF generated stronger activation in the medial frontal cortex, anterior cingulate, precuneus, and the cerebellum relative to attention along the horizontal meridian. The reversed contrast, however, produced stronger activation in the right lingual gyrus and right premotor cortex. The fMRI results suggest that, while a common neural network is engaged in guiding visual spatial attention along the vertical and horizontal dimensions, unique neural correlates are associated with covert attentional orienting along the vertical and horizontal meridia of the visual field.     

Neural correlates of lower limbs proprioception: An fMRI study of foot position matching

Little is known about the neural correlates of lower limbs position sense, despite the impact that proprioceptive deficits have on everyday life activities, such as posture and gait control. We used fMRI to investigate in 30 healthy right‐handed and right‐footed subjects the regional distribution of brain activity during position matching tasks performed with the right dominant and the left nondominant foot. Along with the brain activation, we assessed the performance during both ipsilateral and contralateral matching tasks. Subjects had lower errors when matching was performed by the left nondominant foot. The fMRI analysis suggested that the significant regions responsible for position sense are in the right parietal and frontal cortex, providing a first characterization of the neural correlates of foot position matching.     

Functional MRI of the primary somatosensory cortex in extinction to simultaneous bilateral tactile stimuli due to right temporal lobe stroke

Patients with right posterior temporoparietal cortical lesions often exhibit extinction to tactile double simultaneous stimuli (EDSS). It is not known whether sensory unawareness-extinction results from suppression of sensory input into the somatosensory cortex (SI), inhibition of SI, or interference which prevents SI output from activating and being fully processed by association areas. A patient with EDSS due to a right temporal stroke sparing SI and posterior parietal cortex and eight age-matched healthy controls were studied with fMRI during tactile stimulation. The volume of activation of SI during tactile stimulation of the right hand, the left hand and both hands was measured and the patient's volume of activation was compared to that of the control subjects in each of these stimulus conditions. Although the patient demonstrated behavioral EDSS, during fMRI the patient's activation of SI on both sides was within the range of the control participants' volumes of activation. These findings suggest that EDSS in patients with a right temporal lobe stroke results from processing abnormalities that occur after these afferent tactile stimuli are processed by SI.     

Neural substrates associated with the concurrent performance of dual working memory tasks

The neural substrates of the dual working memory (WM) process were investigated using concurrent performance of auditory and visual n-back WM tasks. Based on the pre-fMRI behavioral testing, a lettered 1-back WM paradigm was implemented for an fMRI examination of healthy volunteers who performed (1) auditory, (2) visual, and (3) simultaneous visual and auditory WM tasks. The behavioral performance, as measured by the reaction time, was deteriorated in the dual task condition compared to the single task condition. Group analysis of the fMRI data revealed that the majority of activation identified during each component task was concurrently activated in the dual task condition. However, several neural substrates such as left middle frontal gyrus, left superior parietal lobule, posterior aspect of right inferior temporal gyrus, and bilateral parahippocampal gyri were selectively activated during the dual WM task. These data suggest that new neural networks come into play to assist in the greater load placed on the WM with the incongruent stimulation modality, which may also have implications in crossmodal integrative processes.     

Functional MRI neuroanatomic correlates of the Hooper Visual Organization Test.

The Hooper Visual Organization Test (VOT), a commonly applied neuropsychological test of visual spatial ability, is used for assessing patients with suspected right hemisphere, or parietal lobe involvement. A controversy has developed over whether the inferences of this test metric can be assumed to involve global, lateralized, or regional functionality. In this study, the characteristic visual organization and object naming aspects of the VOT task presentation were adapted to a functional MR imaging (fMRI) paradigm to probe the neuroanatomic correlates of this neuropsychological test. Whole brain fMRI mapping results are reported on a cohort of normal subjects. Bilateral fMRI responses were found predominantly in the posterior brain, in regions of superior parietal lobules, ventral temporal-occipital cortex, and posterior visual association areas, and to a lesser extent, the frontal eye fields bilaterally, and left dorsolateral prefrontal cortex. The results indicate a general brain region or network in which VOT impairment, due to its visuospatial and object identification demands, is possible to be detected. Discussion is made of interpretive limitations when adapting neuropsychological tests to fMRI analysis.     

Passive tactile recognition of geometrical shape in humans: An fMRI study

Tactile shape discrimination involves frontal other than somatosensory cortex (Palva et al., 2005 [48]), but it is unclear if this frontal activity is related to exploratory concomitants. In this study, we investigated topographical details of prefrontal, premotor, and parietal areas during passive tactile recognition of 2D geometrical shapes in conditions avoiding exploratory movements. Functional magnetic resonance imaging (fMRI) was performed while the same wooden 2D geometrical shapes were blindly pressed on subjects’ passive right palm in three conditions. In the RAW condition, shapes were pressed while subjects were asked to attend to the stimuli but were not trained to recognize them. After a brief training, in the SHAPE condition subjects were asked to covertly recognize shapes. In the RECOGNITION condition, they were asked to overtly recognize shapes, using response buttons with their opposite hand. Results showed that somatosensory cortex including contralateral SII, contralateral SI, and left insula was active in all conditions, confirming its importance in processing tactile shapes. In the RAW vs. SHAPE contrast, bilateral posterior parietal, insular, premotor, prefrontal, and (left) Broca's areas were more active in the latter. In the RECOGNITION, activation of (left) Broca's area correlated with correct responses. These results suggest that, even without exploratory movements, passive recognition of tactile geometrical shapes involves prefrontal and premotor as well as somatosensory regions. In this framework, Broca's area might be involved in a successful selection and/or execution of the correct responses.     

Neural substrates of tactile imagery: a functional MRI study

fMRI investigation on the neural substrates involved in tactile imagery is reported. Healthy subjects performed mental imagery of tactile stimulation on the dorsal aspect of the right hand. The results were compared with the regions of activation during the actual tactile stimulation. During imagery, contralateral primary and secondary somatosensory areas were activated along with activation in the left parietal lobe. Activations in left inferior frontal gyri (Brodmann's area 44), left dorsolateral prefrontal area, left precentral gyrus, left insula, and medial frontal gyrus were also observed. In the basal ganglia, activation in the left thalamus (ventral posteromedial nucleus) and putamen was found. Our results suggest that the primary and secondary somatosensory areas are recruited during tactile imagery, and have partially overlapping neural substrates for the perception of tactile stimulation.     

Linking pain and the body: neural correlates of visually induced analgesia

The visual context of seeing the body can reduce the experience of acute pain, producing a multisensory analgesia. Here we investigated the neural correlates of this "visually induced analgesia" using fMRI. We induced acute pain with an infrared laser while human participants looked either at their stimulated right hand or at another object. Behavioral results confirmed the expected analgesic effect of seeing the body, while fMRI results revealed an associated reduction of laser-induced activity in ipsilateral primary somatosensory cortex (SI) and contralateral operculoinsular cortex during the visual context of seeing the body. We further identified two known cortical networks activated by sensory stimulation: (1) a set of brain areas consistently activated by painful stimuli (the so-called "pain matrix"), and (2) an extensive set of posterior brain areas activated by the visual perception of the body ("visual body network"). Connectivity analyses via psychophysiological interactions revealed that the visual context of seeing the body increased effective connectivity (i.e., functional coupling) between posterior parietal nodes of the visual body network and the purported pain matrix. Increased connectivity with these posterior parietal nodes was seen for several pain-related regions, including somatosensory area SII, anterior and posterior insula, and anterior cingulate cortex. These findings suggest that visually induced analgesia does not involve an overall reduction of the cortical response elicited by laser stimulation, but is consequent to the interplay between the brain's pain network and a posterior network for body perception, resulting in modulation of the experience of pain.     

Neural correlates of semantic associations in patients with schizophrenia

Patients with schizophrenia have semantic processing disturbances leading to expressive language deficits (formal thought disorder). The underlying pathology has been related to alterations in the semantic network and its neural correlates. Moreover, crossmodal processing, an important aspect of communication, is impaired in schizophrenia. Here we investigated specific processing abnormalities in patients with schizophrenia with regard to modality and semantic distance in a semantic priming paradigm. Fourteen patients with schizophrenia and fourteen demographically matched controls made visual lexical decisions on successively presented word-pairs (SOA = 350 ms) with direct or indirect relations, unrelated word-pairs, and pseudoword-target stimuli during fMRI measurement. Stimuli were presented in a unimodal (visual) or crossmodal (auditory-visual) fashion. On the neural level, the effect of semantic relation indicated differences (patients > controls) within the right angular gyrus and precuneus. The effect of modality revealed differences (controls > patients) within the left superior frontal, middle temporal, inferior occipital, right angular gyri, and anterior cingulate cortex. Semantic distance (direct vs. indirect) induced distinct activations within the left middle temporal, fusiform gyrus, right precuneus, and thalamus with patients showing fewer differences between direct and indirect word-pairs. The results highlight aberrant priming-related brain responses in patients with schizophrenia. Enhanced activation for patients possibly reflects deficits in semantic processes that might be caused by a delayed and enhanced spread of activation within the semantic network. Modality-specific decreases of activation in patients might be related to impaired perceptual integration. Those deficits could induce and increase the prominent symptoms of schizophrenia like impaired speech processing.     

Cognitive influences on the affective representation of touch and the sight of touch in the human brain

We show that the affective experience of touch and the sight of touch can be modulated by cognition, and investigate in an fMRI study where top-down cognitive modulations of bottom-up somatosensory and visual processing of touch and its affective value occur in the human brain. The cognitive modulation was produced by word labels, ‘Rich moisturizing cream’ or ‘Basic cream’, while cream was being applied to the forearm, or was seen being applied to a forearm. The subjective pleasantness and richness were modulated by the word labels, as were the fMRI activations to touch in parietal cortex area 7, the insula and ventral striatum. The cognitive labels influenced the activations to the sight of touch and also the correlations with pleasantness in the pregenual cingulate/orbitofrontal cortex and ventral striatum. Further evidence of how the orbitofrontal cortex is involved in affective aspects of touch was that touch to the forearm [which has C fiber Touch (CT) afferents sensitive to light touch] compared with touch to the glabrous skin of the hand (which does not) revealed activation in the mid-orbitofrontal cortex. This is of interest as previous studies have suggested that the CT system is important in affiliative caress-like touch between individuals.     

Functional interactions during the retrieval of conceptual action knowledge: an fMRI study

Impaired retrieval of conceptual knowledge for actions has been associated with lesions of left premotor, left parietal, and left middle temporal areas [Tranel, D., Kemmerer, D., Adolphs, R., Damasio, H., & Damasio, A. R. Neural correlates of conceptual knowledge for actions. Cognitive Neuropsychology, 409-432, 2003]. Here we aimed at characterizing the differential contribution of these areas to the retrieval of conceptual knowledge about actions. During functional magnetic resonance imaging (fMRI), different categories of pictograms (whole-body actions, manipulable and nonmanipulable objects) were presented to healthy subjects. fMRI data were analyzed using SPM2. A conjunction analysis of the neural activations elicited by all pictograms revealed ( p<.05, corrected) a bilateral inferior occipito-temporal neural network with strong activations in the right and left fusiform gyri. Action pictograms contrasted to object pictograms showed differential activation of area MT+, the inferior and superior parietal cortex, and the premotor cortex bilaterally. An analysis of psychophysiological interactions identified contribution-dependent changes in the neural responses when pictograms triggered the retrieval of conceptual action knowledge: Processing of action pictograms specifically enhanced the neural interaction between the right and left fusiform gyri, the right and left middle temporal cortices (MT+), and the left superior and inferior parietal cortex. These results complement and extend previous neuropsychological and neuroimaging studies by showing that knowledge about action concepts results from an increased coupling between areas concerned with semantic processing (fusiform gyrus), movement perception (MT+), and temporospatial movement control (left parietal cortex).     

Neural correlates of switching set as measured in fast, event-related functional magnetic resonance imaging

Attentional switching has shown to involve several prefrontal and parietal brain regions. Most cognitive paradigms used to measure cognitive switching such as the Wisconsin Card Sorting Task (WCST) involve additional cognitive processes besides switching, in particular working memory (WM). It is, therefore, questionable whether prefrontal brain regions activated in these conditions, especially dorsolateral prefrontal cortex (DLPFC), are involved in cognitive switching per se, or are related to WM components involved in switching tasks. Functional magnetic resonance imaging (fMRI) was used to examine neural correlates of pure switching using a paradigm purposely designed to minimize WM functions. The switching paradigm required subjects to switch unpredictably between two spatial dimensions, clearly indicated throughout the task before each trial. Fast, event-related fMRI was used to compare neural activation associated with switch trials to that related to repeat trials in 20 healthy, right-handed, adult males. A large cluster of activation was observed in the right hemisphere, extending from inferior prefrontal and pre- and postcentral gyri to superior temporal and inferior parietal cortices. A smaller and more caudal cluster of homologous activation in the left hemisphere was accompanied by activation of left dorsolateral prefrontal cortex (DLPFC). We conclude that left DLPFC activation is involved directly in cognitive switching, in conjunction with parietal and temporal brain regions. Pre- and postcentral gyrus activation may be related to motor components of switching set.     

Tactile-visual integration in the posterior parietal cortex: a functional magnetic resonance imaging study

To explore the neural substrates of visual-tactile crossmodal integration during motion direction discrimination, we conducted functional magnetic resonance imaging with 15 subjects. We initially performed independent unimodal visual and tactile experiments involving motion direction matching tasks. Visual motion discrimination activated the occipital cortex bilaterally, extending to the posterior portion of the superior parietal lobule, and the dorsal and ventral premotor cortex. Tactile motion direction discrimination activated the bilateral parieto-premotor cortices. The left superior parietal lobule, intraparietal sulcus, bilateral premotor cortices and right cerebellum were activated during both visual and tactile motion discrimination. Tactile discrimination deactivated the visual cortex including the middle temporal/V5 area. To identify the crossmodal interference of the neural activities in both the unimodal and the multimodal areas, tactile and visual crossmodal experiments with event-related designs were also performed by the same subjects who performed crossmodal tactile-visual tasks or intramodal tactile-tactile and visual-visual matching tasks within the same session. The activities detected during intramodal tasks in the visual regions (including the middle temporal/V5 area) and the tactile regions were suppressed during crossmodal conditions compared with intramodal conditions. Within the polymodal areas, the left superior parietal lobule and the premotor areas were activated by crossmodal tasks. The left superior parietal lobule was more prominently activated under congruent event conditions than under incongruent conditions. These findings suggest that a reciprocal and competitive association between the unimodal and polymodal areas underlies the interaction between motion direction-related signals received simultaneously from different sensory modalities.