Beyond the word and image: II- Structural and functional connectivity of a common semantic system.

Understanding events requires interplaying cognitive processes arising in neural networks whose organisation and connectivity remain subjects of controversy in humans. In the present study, by combining diffusion tensor imaging and functional interaction analysis, we aim to provide new insights on the organisation of the structural and functional pathways connecting the multiple nodes of the identified semantic system -shared by vision and language (Jouen et al., 2015). We investigated a group of 19 healthy human subjects during experimental tasks of reading sentences or seeing pictures. The structural connectivity was realised by deterministic tractography using an algorithm to extract white matter fibers terminating in the selected regions of interest (ROIs) and the functional connectivity by independent component analysis to measure correlated activities among these ROIs. The major connections link ventral neural stuctures including the parietal and temporal cortices through inferior and middle longitudinal fasciculi, the retrosplenial and parahippocampal cortices through the cingulate bundle, and the temporal and prefrontal structures through the uncinate fasciculus. The imageability score provided when the subject was reading a sentence was significantly correlated with the factor of anisotropy of the left parieto-temporal connections of the middle longitudinal fasciculus. A large part of this ventrally localised structural connectivity corresponds to functional interactions between the main parietal, temporal and frontal nodes. More precisely, the strong coactivation both in the anterior temporal pole and in the region of the temporo-parietal cortex suggests dual and cooperating roles for these areas within the semantic system. These findings are discussed in terms of two semantics-related sub-systems responsible for conceptual representation.

Beyond the word and image: characteristics of a common meaning system for language and vision revealed by functional and structural imaging.

This research tests the hypothesis that comprehension of human events will engage an extended semantic representation system, independent of the input modality (sentence vs. picture). To investigate this, we examined brain activation and connectivity in 19 subjects who read sentences and viewed pictures depicting everyday events, in a combined fMRI and DTI study. Conjunction of activity in understanding sentences and pictures revealed a common fronto-temporo-parietal network that included the middle and inferior frontal gyri, the parahippocampal-retrosplenial complex, the anterior and middle temporal gyri, the inferior parietal lobe in particular the temporo-parietal cortex. DTI tractography seeded from this temporo-parietal cortex hub revealed a multi-component network reaching into the temporal pole, the ventral frontal pole and premotor cortex. A significant correlation was found between the relative pathway density issued from the temporo-parietal cortex and the imageability of sentences for individual subjects, suggesting a potential functional link between comprehension and the temporo-parietal connectivity strength. These data help to define a "meaning" network that includes components of recently characterized systems for semantic memory, embodied simulation, and visuo-spatial scene representation. The network substantially overlaps with the "default mode" network implicated as part of a core network of semantic representation, along with brain systems related to the formation of mental models, and reasoning. These data are consistent with a model of real-world situational understanding that is highly embodied. Crucially, the neural basis of this embodied understanding is not limited to sensorimotor systems, but extends to the highest levels of cognition, including autobiographical memory, scene analysis, mental model formation, reasoning and theory of mind.

Otolith signals contribute to inter-individual differences in the perception of gravity-centered space.

The aim of the present study was to investigate (1) the relative contribution of the egocentric reference as well as body orientation perception to visual horizon percept during tilt or during increased gravito-inertial acceleration (GiA, hypergravity environment) conditions and (2) the role of vestibular signals in the inter-individual differences observed in these perceptual modalities. Perceptual estimates analysis showed that backward tilt induced (1) an elevation of the visual horizon, (2) an elevation of the egocentric estimation (visual straight ahead) and (3) an overestimation of body tilt. The increase in the magnitude of GiA induced (1) a lowering of the apparent horizon, (2) a lowering of the straight ahead and (3) a perception of backward tilt. Overall, visual horizon percept can be expressed as the combination of body orientation perception and egocentric estimation. When assessing otolith reactivity using off-vertical axis rotation (OVAR), only visual egocentric estimation was significantly correlated with horizontal OVAR performance. On the one hand, we found a correlation between a low modulation amplitude of the otolith responses and straight ahead accuracy when the head axis was tilted relative to gravity. On the other hand, the bias of otolith responses was significantly correlated with straight ahead accuracy when subjects were submitted to an increase in the GiA. Thus, straight ahead sense would be dependent to some extent to otolith function. These results are discussed in terms of the contribution of otolith inputs in the overall multimodal integration subtending spatial constancy.

Insights on embodiment induced by visuo-tactile stimulation during robotic telepresence.

Using a simple neuroscience-inspired procedure to beam human subjects into robots, we previously demonstrated by visuo-motor manipulations that embodiment into a robot can enhance the acceptability and closeness felt towards the robot. In that study, the feelings of likeability and closeness toward the robot were significantly related to the sense of agency, independently of the sensations of enfacement and location. Here, using the same paradigm we investigated the effect of a purely sensory manipulation on the sense of robotic embodiment associated to social cognition. Wearing a head-mounted display, participants saw the visual scene captured from the robot eyes. By positioning a mirror in front of the robot, subjects saw themselves as a robot. Tactile stimulation was provided by stroking synchronously or not with a paintbrush the same location of the subject and robot faces. In contrast to the previous motor induction of embodiment which particularly affected agency, tactile induction yields more generalized effects on the perception of ownership, location and agency. Interestingly, the links between positive social feelings towards the robot and the strength of the embodiment sensations were not observed. We conclude that the embodiment into a robot is not sufficient in itself to induce changes in social cognition.

Embodiment into a robot increases its acceptability.

Recent studies have shown how embodiment induced by multisensory bodily interactions between individuals can positively change social attitudes (closeness, empathy, racial biases). Here we use a simple neuroscience-inspired procedure to beam our human subjects into one of two distinct robots and demonstrate how this can readily increase acceptability and social closeness to that robot. Participants wore a Head Mounted Display tracking their head movements and displaying the 3D visual scene taken from the eyes of a robot which was positioned in front of a mirror and piloted by the subjects' head movements. As a result, participants saw themselves as a robot. When participant' and robot's head movements were correlated, participants felt that they were incorporated into the robot with a sense of agency. Critically, the robot they embodied was judged more likeable and socially closer. Remarkably, we found that the beaming experience with correlated head movements and corresponding sensation of embodiment and social proximity, was independent of robots' humanoid's appearance. These findings not only reveal the ease of body-swapping, via visual-motor synchrony, into robots that do not share any clear human resemblance, but they may also pave a new way to make our future robotic helpers socially acceptable.

Motion sickness induced by otolith stimulation is correlated with otolith-induced eye movements.

This article addresses the relationships between motion sickness (MS) and three-dimensional (3D) ocular responses during otolith stimulation. A group of 19 healthy subjects was tested for motion sickness during a 16 min otolith stimulation induced by off-vertical axis rotation (OVAR) (constant velocity 60 degrees /s, frequency 0.16 Hz). For each subject, the MS induced during the session was quantified, and based on this quantification, the subjects were divided into two groups of less susceptible (MS-), and more susceptible (MS+) subjects. The angular eye velocity induced by the otolith stimulation was analyzed in order to identify a possible correlation between susceptibility to MS and 3D eye velocity. The main results show that: (1) MS significantly correlates in a multiple regression with several components of the horizontal vestibular eye movements i.e. positively with the velocity modulation (P<0.01) and bias (P<0.05) of the otolith ocular reflex and negatively with the time constant of the vestibulo-ocular reflex (P<0.01) and (2) the length of the resultant 3D eye velocity vector is significantly larger in the MS+ as compared with the MS- group. Based on these results we suggest that the CNS, including the velocity storage mechanism, reconstructs an eye velocity vector modulated by head position whose length might predict MS occurrence during OVAR.

Vestibular integration in human cerebral cortex contributes to spatial remapping.

The process of visuo-spatial updating is crucial in guiding human behaviour. While the parietal cortex has long been considered a principal candidate for performing spatial transformations, the exact underlying mechanisms are still unclear. In this study, we investigated in a patient with a right occipito-parietal lesion the ability to update the visual space during vestibularly guided saccades. To quantify the possible deficits in visual and vestibular memory processes, we studied the subject's performance in two separate memory tasks, visual (VIS) and vestibular (VES). In the VIS task, a saccade was elicited from a central fixation point to the location of a visual memorized target and in the VEST task, the saccade was elicited after whole-body rotation to the starting position thus compensating for the rotation. Finally, in an updating task (UPD), the subject had to memorize the position of a visual target then after a whole-body rotation he had to produce a saccade to the remembered visual target location in space. Our main findings was a significant hypometria in the final eye position of both VEST and UPD saccades induced during rotation to the left (contralesional) hemispace as compared to saccades induced after right (ipsilesional) rotation. Moreover, these deficits in vestibularly guided saccades correlated with deficits in vestibulo-ocular time constant, reflecting disorders in the inertial vestibular integration path. We conclude that the occipito-parietal cortex in man can provide a first stage in visuo-spatial remapping by encoding inertial head position signals during gaze orientation.

Interaction between cortical control of vestibular function and spatial representation in man.

An asymmetry of the vestibulo-ocular reflex is observed after unilateral posterior cortical lesions in human. The co-occurrence of vestibular deficits and visuo-spatial disorders suggests a functional link between the representation and update of space and the integration of vestibular information in the occipito-parieto-temporal cortex of the right hemisphere.

Double dissociation in neural correlates of visual working memory: a PET study.

Using positron emission tomography (PET), we investigated the organisation of spatial versus object-based visual working memory in 11 normal human subjects. The paradigm involved a conditional colour-response association task embedded within two visual working memory tasks. The subject had to remember a position (spatial) or shape (object-based) and then use this to recover the colour of the matching element for the conditional association. Activation of the nucleus accumbens and the anterior cingulate cortex was observed during the conditional associative task, indicating a possible role of these limbic structures in associative memory. When the 2 memory tasks were contrasted, we observed activation of 2 distinct cortical networks: (1) The spatial task activated a dorsal stream network distributed in the right hemisphere in the parieto-occipital cortex and the dorsal prefrontal cortex, and (2) The non spatial task activated a ventral stream network distributed in the left hemisphere in the temporo- occipital cortex, the ventral prefrontal cortex and the striatum. These results support the existence of a domain-specific dissociation with dorsal and ventral cortical systems involved respectively in spatial and non spatial working memory functions.

Room tilt illusion. A central otolith dysfunction.

BACKGROUND: We report a sudden 90 degrees room tilt illusion (RTI) following vestibular stimulation in 3 patients with persistent skew deviation caused by a brain stem lesion. Room tilt illusion is a transient tilt perception of the visual surrounding, on its side or even upside down, that is often reported with brain stem lesions. Although its pathophysiologic cause is not well known, the RTI suggests an impairment of otolith pathways, as reported in skew deviation. METHODS: The 3 patients with brain stem lesions were reexamined as part of a follow-up of patients with signs of otolith dysfunction. A registration of vestibular function was performed with a rotatory chair, including earth-vertical axis rotation for canal stimulation and off-vertical axis rotation (OVAR) for otolith stimulation. Measurement of the subjective visual vertical (SVV) was also performed. RESULTS: The otolith-ocular reflex registered by OVAR was impaired in the 3 patients with skew deviation and the SVV in 2 patients. After each direction of OVAR stimulation, the 3 patients reported an RTI as the room was illuminated. CONCLUSIONS: The coexistence of otolith oculomotor (skew deviation and impaired otolith-ocular reflex) and perceptual (tilt of SVV and RTI) disorders suggests a common otolith dysfunction. However, an RTI occurred specifically after vestibular stimulation and when the room was illuminated. We thus suggest that RTI reflects a dynamic visuo-otolith mismatch.

Asymmetric influences of pointing on saccade latency in hemi-Parkinson's disease.

The objective of this study was to investigate whether eye-hand coupling was preserved or not in PD. We studied predictive saccade performance during hand pointing in six Parkinson's disease (PD) patients with asymmetrical motor signs compared to nine age-matched healthy subjects. The motor responses (saccades and hand pointing) were elicited under open loop conditions (without vision of the hand), by a visual target stepping at a predictable location (10 degrees right and left from the centre) and time. The subjects had to simultaneously move the eyes and point with the finger to the visual target alternating at one of three fixed frequencies (0.25, 0.5 and 1 Hz), for 30 cycles. This task was performed in two sessions balanced over the subjects: one session of ocular saccades only and another session of combined ocular saccades and manual pointing. In the PD group, motor performance was perturbed particularly in terms of increased latencies of hand movements. Interestingly, during pointing, associated predictive saccade disorders were tightly related to the defects of the pointing hand. Indeed, with respect to the latency of predictive saccades alone, the predictive saccade latency during hand pointing significantly decreased in the control group and in the PD group when using the non-affected hand. In contrast, for the PD group when using the affected hand, the saccade latency was increased from the latency values of predictive saccades induced without pointing. Moreover, in the control and in the PD groups, the correlation between eye and hand latencies was highly significant, suggesting an intact eye-hand coupling. No saccadic amplitude disorders were found in either condition. These results demonstrate that eye-hand coupling is preserved in PD, as revealed by the possible beneficial or adverse effects on the ocular saccades, respectively, of the less- or more-affected hand motor responses. This eye-hand coupling mechanism likely involves regions other than the nigro-striatal pathways affected in PD.

Evidence for interacting cortical control of vestibular function and spatial representation in man.

The objective of this research was to determine the possible relation between deficits in spatial representation capability and vestibular function following cortical lesions. We thus investigated vestibulo-ocular behaviour in a group of 14 patients with unilateral cortical damage involving the occipito-temporo-parietal junction. Patients were divided in three sub-groups: (1) Group R+: five patients with right sided cortical lesions associated with a left hemi-neglect, (2) Group R-: four patients with right sided cortical lesions with no hemi-neglect and (3) Group L: five patients with left-sided cortical lesions. The patient groups were compared to a group of eight healthy age-matched subjects. The vestibulo-ocular reflex (VOR) was tested in complete darkness by rotating the subject around the vertical axis by sinusoidal rotation at different frequencies, and by steps of acceleration or deceleration. The nystagmus slow phase velocity was measured and plotted as a function of the head velocity and the VOR parameters including gain, bias, time constant and phase were calculated. The cortical lesions induced a significant VOR asymmetry in terms of: a directional preponderance of the VOR gain to the contralesion side, only during sinusoidal rotation, and, in contrast, a VOR bias and a directional preponderance of the VOR time constant and of the nystagmus frequency to the side of the cortical lesion. These latter VOR deficits were the most significant in the R+ group, i.e. in right cortical lesions with hemi-neglect syndrome. These results demonstrate in man, the existence of a cortical influence on vestibular function related to the mechanisms of spatial representation.

Vestibulo-ocular dysfunction induced by cortical damage in man: a case report.

We studied the rare case of a patient presenting with vestibulo-ocular dysfunction and clinical vestibular symptoms after right temporo-parietal cortex infarction. The vestibulo-ocular reflex (VOR) was elicited in the dark, by sinusoidal (0.02; 0.05 and 0.1 Hz) and by step velocity rotation (100 degrees/s2) in clockwise and counterclockwise directions. Horizontal and vertical eye movements were recorded by DC electro-oculography (EOG). When compared to a control group of 8 healthy subjects, this patient presented VOR asymmetry with (1) a significant VOR velocity bias toward the lesioned side revealed as a vestibulo-ocular offset that occurred only under dynamic conditions (2) a significant reduction of the VOR time constant when rotation was directed to the lesioned side. VOR gain was normal. We suggest that the parieto-temporal cortex is implicated in the regulation of vestibulo-ocular symmetry in man. This cortical processing of vestibular integration might involve a multidimensional velocity storage integrator that subserves the maintenance of spatial coordinates along the spatial vertical axis.

Analogical transfer is effective in a serial reaction time task in Parkinson's disease: evidence for a dissociable form of sequence learning.

Several studies of procedural learning in Parkinson's disease (PD) have demonstrated that these patients are impaired with respect to age-matched control subjects. In order to examine more closely the specific impairment, we considered three dimensions along which a procedural learning task could vary. These are: (1) implicit vs explicit learning, (2) instance vs rule learning, and (3) learning with internal vs external error correction. We consider two hypotheses that could explain the impairments observed in PD for different types of explicit motor learning: (H1) an impairment related to the acquisition of rules vs specific instances, and (H2) an impairment in learning when no explicit error feedback is provided. In order to examine the condition of rule learning with external error feedback, we developed a modified version of the serial reaction time (SRT) protocol that tests analogical transfer in sequence learning (ATSL). Reaction times are measured for responses to visual stimuli that appear in several different repeating sequences. While these isomorphic sequences are different, they share a common rule. Verbatim learning of a sequence would result in negative transfer from one sequence to a different one, while rule learning would result in positive transfer. Parkinson's patients and age-matched controls demonstrate significant acquisition and positive transfer of the rule between sequences. Our results demonstrate that PD patients are capable of learning and transferring rule or schema-based representations in an explicit learning format, and that this form of learning may be functionally distinct from learning mechanisms that rely on representations of the verbatim or statistical structure of sequences.

Dissociable effects of dopaminergic therapy on spatial versus non-spatial working memory in Parkinson's disease.

There is now evidence for definite and early cognitive deficits in Parkinson's disease (PD), involving, in particular, executive functions and working memory. However, the distinction between visuo-spatial and non-spatial working memory deficits and the impact of dopamine on these deficits are still open to debate. The aim of this study was therefore to investigate cognitive and motor performance in PD patients in two conditional associative learning tasks requiring either spatial or non-spatial visual working memory. The subject had to point to visual targets according to the visual characteristics of memorised visual cues (colour, position and form). To assess the effect of L-dopa therapy, PD patients were studied over two consecutive days: one ON/OFF group of nine PD patients with treatment (ON condition) on the first day and without treatment (OFF condition) on the second day; and another OFF/ON group of nine PD patients tested on reverse. The PD groups were compared to a control group of nine age-matched healthy subjects. Our main data demonstrate that: (1) in PD patients with OFF treatment, the response time of manual pointing is increased mainly in the non-spatial working memory task; and (2) in PD patients with ON treatment, either the response time is normal (on the first day) or is increased in both visuo-spatial and non-spatial tasks. We suggest that this dissociation between spatial versus non-spatial working memory deficits in non-medicated PD might be related to compensatory mechanisms that occur following fronto-striatal dysfunction.

Predictive saccade behavior is enhanced in schizophrenia.

In this study, we investigated the ability of schizophrenic patients to produce predictive saccades in response to a visual target moving with predictable timing and location. The performance of visually guided saccades and predictive saccades (gain and latency) were analyzed in a group of 12 schizophrenic patients as compared with a group of ten control subjects. Our main finding was an enhancement of the predictive tracking ability in schizophrenics. In particular, the predictive tracking built-up and became installed as a steady pattern of highly anticipatory saccades (in terms of percentage of negative latency saccades) much faster in the schizophrenic group than in the control group. These data are discussed in terms of an involvement of the fronto-striatal system in this specific enhancement of predictive saccade behavior in schizophrenia.

Extraocular proprioception is required for spatial localization in man.

We studied the effect of extraocular muscle deafferentation on the performance of manual pointing to foveated visual targets at different craniotopic locations. Five patients who underwent unilateral thermocoagulation of the trigeminal nerve (V) for trigeminal neuralgia were tested before and after surgery. Only the group of patients with postsurgical corneal anaesthesia, attesting to involvement of the ophthalmic branch (which carries the ocular proprioceptive fibres), presented deficits in accuracy of pointing. The position of initial and final pointing was shifted toward the lesioned side predominantly in the ipsilateral hemispace. These data support a model in which balanced ocular proprioceptive inputs are required for accuracy of visually guided movements in egocentric space.

Vestibular information contributes to update retinotopic maps.

In order to investigate the contribution of the vestibular system to spatial orientation, we studied memory-guided saccades in three conditions: visual-memory guided saccades (ViC), saccades to the remembered spatiotopic position of a visual target, after whole-body rotation (SVeC) and saccades to the remembered retinotopic position of a visual target, after whole-body rotation (RVeC). Visual feedback presented after each trial allowed eye position correction. The error was larger in SVeC, but the performance improved throughout the experiment (learning) in that condition only. As learning occurred over the first four trials, we omitted these trials from the average computation, and the significant difference between the conditions disappeared. It is concluded that vestibular information does contribute to update the internal spatial representation of visual information when a visual feedback is provided.

Saccadic eye movements in schizophrenic patients.

The nature of saccadic abnormalities in schizophrenia was investigated in three different paradigms: (1) the visually guided saccade; (2) the antisaccade; and (3) the remembered saccade paradigm. Subjects comprised 14 schizophrenic patients and 14 normal volunteers. Deficits in the schizophrenic group were observed in the antisaccade and remembered saccade tasks, both of which were characterized by increased latency and reduced gain. Moreover, in the antisaccade task, schizophrenic patients showed an increased number of errors compared with control subjects. Saccadic abnormalities in the patients were correlated with impaired performance on the Wisconsin Card Sorting Test. These data suggest that schizophrenic patients have difficulty in inhibiting reflexive saccades and in producing voluntary saccades. The implications of these findings for a prefrontal cortex dysfunction involved in oculomotor control in schizophrenia are discussed.