Human parietal "reach region" primarily encodes intrinsic visual direction, not extrinsic movement direction, in a visual motor dissociation task

Posterior parietal cortex (PPC) participates in the planning of visuospatial behaviors, including reach movements, in gaze-centered coordinates. It is not known if these representations encode the visual goal in retinal coordinates, or the movement direction relative to gaze. Here, by dissociating the intrinsic retinal stimulus from the extrinsic direction of movement, we show that PPC employs a visual code. Using delayed pointing and event-related functional magnetic resonance imaging, we identified a cluster of PPC regions whose activity was topographically (contralaterally) related to the direction of the planned movement. We then switched the normal visual-motor spatial relationship by adapting subjects to optical left/right reversing prisms. With prisms, movement-related PPC topography reversed, remaining tied to the retinal image. Thus, remarkably, the PPC region in each hemisphere now responded more for planned ipsilateral pointing movements. Other non-PPC regions showed the opposite world- or motor-fixed pattern. These findings suggest that PPC primarily encodes not motor commands but movement goals in visual coordinates.     

A cortico-subcortical model for generation of spatially accurate sequential saccades.

This article provides a systems framework for the analysis of cortical and subcortical interactions in the control of saccadic eye movements, A major thesis of this model is that a topography of saccade direction and amplitude is preserved through multiple projections between brain regions until they are finally transformed into a temporal pattern of activity that drives the eyes to the target. The control of voluntary saccades to visual and remembered targets is modeled in terms of interactions between posterior parietal cortex, frontal eye fields, the basal ganglia (caudate and substantia nigra), superior colliculus, mediodorsal thalamus, and the saccade generator of the brainstem. Interactions include the modulation of eye movement motor error maps by topographic inhibitory projections, dynamic remapping of spatial target representations in saccade motor error maps, and sustained neural activity that embodies spatial memory. Models of these mechanisms implemented in our Neural Simulation Language simulate behavior and neural activity described in the literature, and suggest new experiments.     

Smooth pursuit-related information processing in frontal eye field neurons that project to the NRTP

The cortical pursuit system begins the process of transforming visual signals into commands for smooth pursuit (SP) eye movements. The frontal eye field (FEF), located in the fundus of arcuate sulcus, is known to play a role in SP and gaze pursuit movements. This role is supported, at least in part, by FEF projections to the rostral nucleus reticularis tegmenti pontis (rNRTP), which in turn projects heavily to the cerebellar vermis. However, the functional characteristics of SP-related FEF neurons that project to rNRTP have never been described. Therefore, we used microelectrical stimulation (ES) to deliver single pulses (50-200 microA, 200-micros duration) in rNRTP to antidromically activate FEF neurons. We estimated the eye or retinal error motion sensitivity (position, velocity, and acceleration) of FEF neurons during SP using multiple linear regression modeling. FEF neurons that projected to rNRTP were most sensitive to eye acceleration. In contrast, FEF neurons not activated following ES of rNRTP were often most sensitive to eye velocity. In similar modeling studies, we found that rNRTP neurons were also biased toward eye acceleration. Therefore, our results suggest that neurons in the FEF-rNRTP pathway carry signals that could play a primary role in initiation of SP.     

Goal-selection and movement-related conflict during bimanual reaching movements

Conflict during bimanual movements can arise during the selection of movement goals or during movement planning and execution. We demonstrate a behavioral and neural dissociation of these 2 types of conflict. During functional magnetic resonance imaging scanning, participants performed bimanual reaching movements with symmetric (congruent) or orthogonal (incongruent) trajectories. The required movements were indicated either spatially, by illuminating the targets, or symbolically, using centrally presented letters. The processing of symbolic cues led to increased activation in a left hemisphere network including the intraparietal sulcus, premotor cortex, and inferior frontal gyrus. Reaction time cost for incongruent movements was substantially larger for symbolic than for spatial cues, indicating that the cost was primarily associated with the selection and assignment of movement goals, demands that are minimized when goals are directly specified by spatial cues. This goal-selection conflict increased activity in the pre-supplementary motor area and cingulate motor areas. Both cueing conditions led to larger activation for incongruent movements in the convexity of the superior parietal cortex, bilaterally, making this region a likely neural site for conflict that arises during the planning and execution of bimanual movements. These results suggest distinct neural loci for 2 forms of constraint on our ability to perform bimanual reaching movements.     

Posterior cingulate cortex: sensory and oculomotor properties of single neurons in behaving cat.

The posterior cingulate cortex of the cat is strongly linked to cortical areas with sensory and oculomotor functions. We have now recorded from feline posterior cingulate neurons in order to determine whether they are active in conjunction with sensory events and eye movements. The results described here are based on monitoring the electrical activity of 195 single neurons in the posterior cingulate cortex of three cats equipped with surgically implanted scleral search coils and trained to fixate visual targets. Posterior cingulate neurons carry tonic orbital position signals and are phasically active in conjunction with saccadic eye movements. Activity related to eye movements and gaze is attenuated but not abolished by the elimination of visual feedback. Posterior cingulate neurons also are responsive to visual, auditory, and somatosensory stimulation. Systematic testing with visual stimuli revealed that responses are sharply reduced due to refractoriness at rates of stimulation greater than a few per second. These results conform to the theory that posterior cingulate cortex is involved in processes underlying visuospatial cognition.     

Contributions of the PPC to online control of visually guided reaching movements assessed with fMRI-guided TMS

The posterior parietal cortex (PPC) plays an important role in controlling voluntary movements by continuously integrating sensory information about body state and the environment. We tested which subregions of the PPC contribute to the processing of target- and body-related visual information while reaching for an object, using a reaching paradigm with 2 types of visual perturbation: displacement of the visual target and displacement of the visual feedback about the hand position. Initially, functional magnetic resonance imaging (fMRI) was used to localize putative target areas involved in online corrections of movements in response to perturbations. The causal contribution of these areas to online correction was tested in subsequent neuronavigated transcranial magnetic stimulation (TMS) experiments. Robust TMS effects occurred at distinct anatomical sites along the anterior intraparietal sulcus (aIPS) and the anterior part of the supramarginal gyrus for both perturbations. TMS over neighboring sites did not affect online control. Our results support the hypothesis that the aIPS is more generally involved in visually guided control of movements, independent of body effectors and nature of the visual information. Furthermore, they suggest that the human network of PPC subregions controlling goal-directed visuomotor processes extends more inferiorly than previously thought. Our results also point toward a good spatial specificity of the TMS effects.     

Target visibility and visual awareness modulate amygdala responses to fearful faces

The goals of the present study were twofold. First, we wished to investigate the neural correlates of aware and unaware emotional face perception after characterizing each subject's behavioral performance via signal detection theory methods. Second, we wished to investigate the extent to which amygdala responses to fearful faces depend on the physical characteristics of the stimulus independently of the percept. We show that amygdala responses depend on visual awareness. Under conditions in which subjects were not aware of fearful faces flashed for 33 ms, no differential activation was observed in the amygdala. On the other hand, differential activation was observed for 67 ms fearful targets that the subjects could reliably detect. When trials were divided into hits, misses, correct rejects, and false alarms, we show that target visibility is an important factor in determining amygdala responses to fearful faces. Taken together, our results further challenge the view that amygdala responses occur automatically.     

Modulation of motor and premotor activity during imitation of target-directed actions

Behavioral studies reveal that imitation performance and the motor system are strongly influenced by the goal of the action to be performed. We used functional magnetic resonance imaging (fMRI) to assess the effect of explicit action goals on neural activity during imitation. Subjects imitated index finger movements in the absence and presence of visible goals (red dots that were reached for by the finger movement). Finger movements were either ipsilateral or contralateral. The pars opercularis of the inferior frontal gyrus showed increased blood oxygen level-dependent fMRI signal bilaterally for imitation of goal-oriented actions, compared with imitation of actions with no explicit goal. In addition, bilateral dorsal premotor areas demonstrated greater activity for goal-oriented actions, for contralateral movements and an interaction effect such that goal-oriented contralateral movements yielded the greatest activity. These results support the hypothesis that areas relevant to motor preparation and motor execution are tuned to coding goal-oriented actions and are in keeping with single-cell recordings revealing that neurons in area F5 of the monkey brain represent goal-directed aspects of actions.     

Watching your foot move--an fMRI study of visuomotor interactions during foot movement

The objective of this study was to investigate brain areas involved in distinguishing sensory events caused by self-generated movements from similar sensory events caused by externally generated movements using functional magnetic resonance imaging. Subjects performed 4 types of movements: 1) self-generated voluntary movement with visual feedback, 2) externally generated movement with visual feedback, 3) self-generated voluntary movement without visual feedback, and 4) externally generated movement without visual feedback, this design. This factorial design makes it possible to study which brain areas are activated during self-generated ankle movements guided by visual feedback as compared with externally generated movements under similar visual and proprioceptive conditions. We found a distinct network, comprising the posterior parietal cortex and lateral cerebellar hemispheres, which showed increased activation during visually guided self-generated ankle movements. Furthermore, we found differential activation in the cerebellum depending on the different main effects, that is, whether movements were self- or externally generated regardless of visual feedback, presence or absence of visual feedback, and activation related to proprioceptive input.     

Oscillatory activity in human parietal and occipital cortex shows hemispheric lateralization and memory effects in a delayed double-step saccade task

We applied magnetoencephalography (MEG) to record oscillatory brain activity from human subjects engaged in planning a double-step saccade. In the experiments, subjects (n = 8) remembered the locations of 2 sequentially flashed targets (each followed by a 2-s delay), presented in either the left or right visual hemifield, and then made saccades to the 2 locations in sequence. We examined changes in spectral power in relation to target location (left or right) and memory load (one or two targets), excluding error trials based on concurrent eye tracking. During the delay period following the first target, power in the alpha (8-12 Hz) and beta (13-25 Hz) bands was significantly suppressed in the hemisphere contralateral to the target. When the second target was presented, there was a further suppression in the alpha- and beta-band power over both hemispheres. In this period, the same sensors also showed contralateral power enhancements in the gamma band (60-90 Hz), most significantly prior to the initiation of the saccades. Adaptive spatial filtering techniques localized the neural sources of the directionally selective power changes in parieto-occipital areas. These results provide further support for a topographic organization for delayed saccades in human parietal and occipital cortex.     

Strength of figure-ground activity in monkey primary visual cortex predicts saccadic reaction time in a delayed detection task

When and where are decisions made? In the visual system a saccade, which is a fast shift of gaze toward a target in the visual scene, is the behavioral outcome of a decision. Current neurophysiological data and reaction time models show that saccadic reaction times are determined by a build-up of activity in motor-related structures, such as the frontal eye fields. These structures depend on the sensory evidence of the stimulus. Here we use a delayed figure-ground detection task to show that late modulated activity in the visual cortex (V1) predicts saccadic reaction time. This predictive activity is part of the process of figure-ground segregation and is specific for the saccade target location. These observations indicate that sensory signals are directly involved in the decision of when and where to look.     

Cerebral bases of subliminal and supraliminal priming during reading

Several studies have investigated the neural correlates of conscious perception by contrasting functional magnetic resonance imaging (fMRI) activation to conscious and nonconscious visual stimuli. The results often reveal an amplification of posterior occipito-temporal activation and its extension into a parieto-frontal network. However, some of these effects might be due to a greater deployment of attentional or strategical processes in the conscious condition. Here, we examined the brain activity evoked by visible and invisible stimuli, both of which were irrelevant to the task. We collected fMRI data in a masking paradigm in which subliminal versus supraliminal letter strings were presented as primes while subjects focused attention on another subsequent, highly visible target word. Under those conditions, prime visibility was associated with greater activity confined to bilateral posterior occipito-temporal cortices, without extension into frontal and parietal cortices. However, supraliminal primes, compared with subliminal primes, evoked more extensive repetition suppression in a widely distributed set of parieto-frontal areas. Furthermore, only supraliminal primes caused phonological repetition enhancement in left inferior frontal and anterior insular cortex. Those results suggest a 2-stage view of conscious access: Relative to masked stimuli, unmasked stimuli elicit increased occipito-temporal activity, thus allowing them to compete for global conscious access and to induce priming in multiple distant areas. In the absence of attention, however, their access to a second stage of distributed parieto-frontal processing may remain blocked.     

Eye-pursuit and reafferent head movement signals carried by pursuit neurons in the caudal part of the frontal eye fields during head-free pursuit

Eye and head movements are coordinated during head-free pursuit. To examine whether pursuit neurons in frontal eye fields (FEF) carry gaze-pursuit commands that drive both eye-pursuit and head-pursuit, monkeys whose heads were free to rotate about a vertical axis were trained to pursue a juice feeder with their head and a target with their eyes. Initially the feeder and target moved synchronously with the same visual angle. FEF neurons responding to this gaze-pursuit were tested for eye-pursuit of target motion while the feeder was stationary and for head-pursuit while the target was stationary. The majority of pursuit neurons exhibited modulation during head-pursuit, but their preferred directions during eye-pursuit and head-pursuit were different. Although peak modulation occurred during head movements, the onset of discharge usually was not aligned with the head movement onset. The minority of neurons whose discharge onset was so aligned discharged after the head movement onset. These results do not support the idea that the head-pursuit-related modulation reflects head-pursuit commands. Furthermore, modulation similar to that during head-pursuit was obtained by passive head rotation on stationary trunk. Our results suggest that FEF pursuit neurons issue gaze or eye movement commands during gaze-pursuit and that the head-pursuit-related modulation primarily reflects reafferent signals resulting from head movements.     

Reflexive orienting in spatial neglect is biased towards behaviourally salient stimuli

Patients with spatial neglect are impaired when detecting contralesional targets presented shortly after an ipsilesional cue. This "disengagement" deficit is believed to reflect reflexive orienting towards ipsilesional stimuli that is independent of behavioural goals. Here, we show that the extent of this spatial bias depends on the behavioural salience of ipsilesional stimuli. Healthy participants, brain-injured patients without neglect and neglect patients reacted to ipsilesional and contralesional visual targets. Prior to target presentation, a visual cue similar or dissimilar to the target was presented at target position or opposite the target. Although participants did not react to the similar cue, it had high behavioural salience since it shared features with the target stimulus. Neglect patients showed dramatically increased reaction times to contralesional targets, but only when these followed behaviourally relevant ipsilesional cues. No decrease of performance was observed with irrelevant cues. This performance pattern was not due to perceptual similarity, since the same effect was found when cue and target were semantically related but differed perceptually. Importantly, semantically related cues ceased to attract attention when they were defined as behaviourally irrelevant. These results show that neglect patients only orient attention reflexively towards ipsilesional stimuli with high behavioural salience.     

Recoding arm position to learn visuomotor transformations.

There is strong experimental evidence that guiding the arm toward a visual target involves an initial vectorial transformation from direction in visual space to direction in motor space. Constraints on this transformation are imposed (i) by the neural codes for incoming information: the desired movement direction is thought to be signalled by populations of broadly tuned neurons and arm position by populations of monotonically tuned neurons; and (ii) by the properties of outgoing information: the actual movement direction results from the collective action of broadly tuned neurons whose preferred directions rotate with the position of the arm. A neural network model is presented that computes the visuomotor mapping, given these constraints. Appropriate operations are learned by the network in an unsupervised fashion through repeated action- perception cycles by recoding the arm-related proprioceptive information. The resulting solution has two interesting properties: (i) the required transformation is executed accurately over a large part of the reaching space, although few positions are actually learned; and (ii) properties of single neurons and populations in the network closely resemble those of neurons and populations in parietal and motor cortical regions. This model thus suggests a realistic scenario for the calculation of coordinate transformations and initial motor command for arm reaching movements.     

The role of right temporal lobe structures in off-line action: evidence from lesion-behavior mapping in stroke patients

Recent evidence suggests the possibility that not all action modes depend on dorsal visual stream processing but that off-line nontarget-directed actions, such as antipointing, require additional and even distinct neural networks when compared with target-directed online actions. Here, we explored this potential dissociation in a group of 11 patients with left visual neglect, a syndrome characterized by a loss of awareness of the contralesional side of space. Ten healthy participants and 10 right hemisphere-damaged patients without neglect served as controls. Participants had to point either directly toward targets presented on their left or right (i.e., propointing) or to the mirror position in the opposite hemispace (i.e., antipointing). Compared with both control groups, neglect patients showed reduced accuracy when antipointing but not propointing. Lesion-behavior mapping revealed that the areas critically associated with these deficits were located in the middle and superior temporal and parahippocampal gyri. We argue that neglect patients present specific deficits only when the visuomotor task taps into more perceptual representations thought to rely on ventral visual stream processing and that our results indicate that right temporal brain regions are implicated in these off-line actions.     

Human cortical networks for new and familiar sequences of saccades.

Visual exploration is organized in sequences of saccadic eye movements that depend on both perceptual and cognitive context. Using functional magnetic resonance imaging, we studied the neural basis of sequential oculomotor behavior and its dependence on different types of memory by analyzing cerebral activity during performance of newly learned and familiar sequences of eye movements. Compared to a resting condition, both types of sequences activated a common fronto-parietal network, including frontal and supplementary eye fields, and several parietal areas. Within this network, newly learned sequences induced stronger activation than familiar sequences, probably reflecting higher attentional demands. In addition, specific regions were recruited for the performance of new sequences, including pre-supplementary eye fields, the precuneus and the caudate nucleus. This indicates that in addition to attentional modulation, novelty of saccadic sequences requires specific cortical resources, probably related to effortful sequence preparation and coordination as well as to spatial working memory. For familiar sequences, recalled from long-term memory, we observed specific right medial temporo-occipital activation in the vicinity of the boundary between the parahippocampal and lingual gyri, as well as an activation site in the parieto-occipital fissure. We conclude that neuronal resources recruited by the gaze system can change with the familiarity of the scanpath to be executed. This study is important to better understand how the brain implements memorized scanpaths for visual exploration and orienting.     

Functional neuroanatomy of proprioception

Proprioception is the sense of body position that is perceived both at the conscious and unconscious levels. Typically, it refers to two kinds of sensations: that of static limb position and of kinesthesia. Static position reflects the recognition of the orientation of the different body parts, whereas kinesthesia is the recognition of rates of movement. Proprioception is based on a multicomponent sensory system. There are various peripheral receptors that detect specific signals and major sensory afferent pathways that carry the information from the spinal cord up to the cortex. There are parallel pathways, some of which serve conscious proprioception, and others that serve subconscious proprioception. Conscious proprioception is relayed mostly by the dorsal column and in part by the spinocervical tract. Finally, the organ of perception for position sense is the sensory cortex of the brain. This review outlines the current understanding of these complex neural pathways, starting from the peripheral receptors and working up toward the center of perception, the brain.     

Surgical targeting accuracy analysis of six methods for subthalamic nucleus deep brain stimulation.

A commonly adopted surgical target in deep brain stimulation (DBS) procedures, the subthalamic nucleus (STN) is located deep within the brain and is surrounded by delicate deep-brain structures. Symptoms of Parkinson's disease can be reduced by precisely implanting a multi-electrode stimulator at a specific location within the STN and delivering the appropriate signal to the target. A number of techniques have recently been proposed to facilitate STN DBS surgical targeting and thereby improve the surgical outcome. This paper presents a retrospective study evaluating the target localization accuracy and precision of six approaches in 55 STN DBS procedures. The targeting procedures were performed using a neurosurgical visualization and navigation system, which integrates normalized and standardized anatomical and functional information into the planning environment. In this study, we employed as the "gold standard" the actual surgical target locations determined by an experienced neurosurgeon using both pre-operative image-guided surgical target/trajectory planning and intra-operative electrophysiological exploration and confirmation. The surgical target locations determined using each of the six targeting methods were compared with the "gold standards". The average displacement between the actual surgical targets and those planned with targeting approaches was 3.0 +/- 1.3 mm, 3.0 +/- 1.3 mm, 3.0 +/- 1.0 mm, 2.6 +/- 1.1 mm, 2.5 +/- 0.9 mm, and 1.7 +/- 0.7 mm for approaches based on T2-weighted MRI, a brain atlas, T1 and T2 maps, an electrophysiological database, a collection of final surgical targets from previous patients, and the combination of these functional and anatomical data, respectively. The technique incorporating both anatomical and functional data provides the most reliable and accurate target position for STN DBS.     

Neural basis for priming of pop-out during visual search revealed with fMRI

Maljkovic and Nakayama first showed that visual search efficiency can be influenced by priming effects. Even "pop-out" targets (defined by unique color) are judged quicker if they appear at the same location and/or in the same color as on the preceding trial, in an unpredictable sequence. Here, we studied the potential neural correlates of such priming in human visual search using functional magnetic resonance imaging (fMRI). We found that repeating either the location or the color of a singleton target led to repetition suppression of blood oxygen level-dependent (BOLD) activity in brain regions traditionally linked with attentional control, including bilateral intraparietal sulci. This indicates that the attention system of the human brain can be "primed," in apparent analogy to repetition-suppression effects on activity in other neural systems. For repetition of target color but not location, we also found repetition suppression in inferior temporal areas that may be associated with color processing, whereas repetition of target location led to greater reduction of activation in contralateral inferior parietal and frontal areas, relative to color repetition. The frontal eye fields were also implicated, notably when both target properties (color and location) were repeated together, which also led to further BOLD decreases in anterior fusiform cortex not seen when either property was repeated alone. These findings reveal the neural correlates for priming of pop-out search, including commonalities, differences, and interactions between location and color repetition. fMRI repetition-suppression effects may arise in components of the attention network because these settle into a stable "attractor state" more readily when the same target property is repeated than when a different attentional state is required.