Placing a tool in the spotlight: spatial attention modulates visuomotor responses in cortex

Motor and Visuospatial Attention and Motor Planning After Stroke: Considerations for the Rehabilitation of Standing Balance and Gait

Attention and planning can be altered by stroke, which can influence motor performance. Although the influence of these factors on recovery from stroke has been explored for the upper extremity (UE), their impact on balance and gait are unknown. This perspective article presents evidence that altered motor and visuospatial attention influence motor planning of voluntary goal-directed movements poststroke, potentially affecting balance and gait. Additionally, specific strategies for rehabilitation of balance and gait poststroke in the presence of these factors are discussed. Visuospatial attention selects relevant sensory information and supports the preparation of responses to this information. Motor attentional impairments may produce difficulty with selecting appropriate motor feedback, potentially contributing to falls. An original theoretical model is presented for a network of brain regions supporting motor and visuospatial attention, as well as motor planning of voluntary movements. Stroke may influence this functional network both locally and distally, interfering with input or output of the anatomical or functional regions involved and affecting voluntary movements. Although there is limited research directly examining leg function, evidence suggests alterations in motor and visuospatial attention influence motor planning and have a direct impact on performance of gait and balance. This model warrants testing comparing healthy adults with individuals with stroke.

The oculomotor resonance effect in spatial-numerical mapping

We investigated automatic Spatial-Numerical Association of Response Codes (SNARC) effect in auditory number processing. Two experiments continually measured spatial characteristics of ocular drift at central fixation during and after auditory number presentation. Consistent with the notion of a spatially oriented mental number line, we found spontaneous magnitude-dependent gaze adjustments, both with and without a concurrent saccadic task. This fixation adjustment (1) had a small-number/left-lateralized bias and (2) it was biphasic as it emerged for a short time around the point of lexical access and it received later robust representation around following number onset. This pattern suggests a two-step mechanism of sensorimotor mapping between numbers and space - a first-pass bottom-up activation followed by a top-down and more robust horizontal SNARC. Our results inform theories of number processing as well as simulation-based approaches to cognition by identifying the characteristics of an oculomotor resonance phenomenon.

Grounding grammatical categories: attention bias in hand space influences grammatical congruency judgment of Chinese nominal classifiers

Embodied cognitive theories predict that linguistic conceptual representations are grounded and continually represented in real world, sensorimotor experiences. However, there is an on-going debate on whether this also holds for abstract concepts. Grammar is the archetype of abstract knowledge, and therefore constitutes a test case against embodied theories of language representation. Former studies have largely focussed on lexical-level embodied representations. In the present study we take the grounding-by-modality idea a step further by using reaction time (RT) data from the linguistic processing of nominal classifiers in Chinese. We take advantage of an independent body of research, which shows that attention in hand space is biased. Specifically, objects near the hand consistently yield shorter RTs as a function of readiness for action on graspable objects within reaching space, and the same biased attention inhibits attentional disengagement. We predicted that this attention bias would equally apply to the graspable object classifier but not to the big object classifier. Chinese speakers (N = 22) judged grammatical congruency of classifier-noun combinations in two conditions: graspable object classifier and big object classifier. We found that RTs for the graspable object classifier were significantly faster in congruent combinations, and significantly slower in incongruent combinations, than the big object classifier. There was no main effect on grammatical violations, but rather an interaction effect of classifier type. Thus, we demonstrate here grammatical category-specific effects pertaining to the semantic content and by extension the visual and tactile modality of acquisition underlying the acquisition of these categories. We conclude that abstract grammatical categories are subjected to the same mechanisms as general cognitive and neurophysiological processes and may therefore be grounded.

Altered visual-spatial attention to task-irrelevant information is associated with falls risk in older adults

Executive cognitive functions play a critical role in falls risk—a pressing health care issue in seniors. In particular, intact attentional processing is integral for safe mobility and navigation. However, the specific contribution of impaired visual–spatial attention in falls remains unclear. In this study, we examined the association between visual–spatial attention to task-irrelevant stimuli and falls risk in community-dwelling older adults. Participants completed a visual target discrimination task at fixation while task-irrelevant probes were presented in both visual fields. We assessed attention to left and right peripheral probes using event-related potentials (ERPs). Falls risk was determined using the valid and reliable Physiological Profile Assessment (PPA). We found a significantly positive association between reduced attentional facilitation, as measured by the N1 ERP component, and falls risk. This relationship was specific to probes presented in the left visual field and measured at ipsilateral electrode sites. Our results suggest that fallers exhibit reduced attention to the left side of visual space and provide evidence that impaired right hemispheric function and/or structure may contribute to falls.

Practical research-based guidance for motor imagery practice in neurorehabilitation

PURPOSE

The purpose of this appraisal is to offer guidance to clinicians on applying motor imagery in neurorehabilitation and provide guidance to support this process.

METHOD

We used evidence from a variety of fields as well as clinical experience with motor imagery to develop guidance for employing motor imagery during neurorehabilitation.

RESULTS

Motor imagery is a relatively new intervention for neurorehabilitation supported by evidence from areas such as cognitive neuroscience and sports psychology. Motor imagery has become a very popular intervention modality for clinicians but there is insufficient information available on how to administer it in clinical practice and make deliberate decisions during its application.

CONCLUSIONS

We provide evidence-based guidance for employing motor imagery in neurorehabilitation and use the principles of motor learning as the framework for clinical application.

Mental practice with motor imagery in stroke recovery: randomized controlled trial of efficacy

This randomized controlled trial evaluated the therapeutic benefit of mental practice with motor imagery in stroke patients with persistent upper limb motor weakness. There is evidence to suggest that mental rehearsal of movement can produce effects normally attributed to practising the actual movements. Imagining hand movements could stimulate restitution and redistribution of brain activity, which accompanies recovery of hand function, thus resulting in a reduced motor deficit. Current efficacy evidence for mental practice with motor imagery in stroke is insufficient due to methodological limitations. This randomized controlled sequential cohort study included 121 stroke patients with a residual upper limb weakness within 6 months following stroke (on average <3 months post-stroke). Randomization was performed using an automated statistical minimizing procedure. The primary outcome measure was a blinded rating on the Action Research Arm test. The study analysed the outcome of 39 patients involved in 4 weeks of mental rehearsal of upper limb movements during 45-min supervised sessions three times a week and structured independent sessions twice a week, compared to 31 patients who performed equally intensive non-motor mental rehearsal, and 32 patients receiving normal care without additional training. No differences between the treatment groups were found at baseline or outcome on the Action Research Arm Test (ANCOVA statistical P=0.77, and effect size partial η2=0.005) or any of the secondary outcome measures. Results suggest that mental practice with motor imagery does not enhance motor recovery in patients early post-stroke. In light of the evidence, it remains to be seen whether mental practice with motor imagery is a valid rehabilitation technique in its own right.

Are impairments in visual-spatial attention a critical factor for increased falls risk in seniors? An event-related potential study

Falls are experienced annually by approximately one third of community dwellers over the age of 65, and while neuro-cognitive deficits have been shown to increase falls risk, the specific nature of these deficits remain unspecified. Here we examined whether visual-spatial attention may be a core neuro-cognitive system showing abnormal function in fallers. Using a between-groups design, we recorded event-related potentials in a canonical spatial cuing task performed by two groups of senior (aged 65+ years old) participants: those with a recent history of falls and those with no such history. In terms of attentional control systems in cortex, we found no significant differences in function between groups. However, in terms of attentional facilitation of cortical processing, we found that fallers manifest specific abnormalities in the sensory/perceptual processing of targets in the left visual field. Our findings thus suggest that fallers have specific deficits in visuocortical systems associated with attentional enhancement of events on the left side of visual space.

Embodied language: a review of the role of the motor system in language comprehension

A growing body of research suggests that comprehending verbal descriptions of actions relies on an internal simulation of the described action. To assess this motor resonance account of language comprehension, we first review recent developments in the literature on perception and action, with a view towards language processing. We then examine studies of language processing from an action simulation perspective. We conclude by discussing several criteria that might be helpful with regard to assessing the role of motor resonance during language comprehension.

A picture says more than a thousand words: behavioural and ERP evidence for attentional enhancements due to action affordances

Previous research has demonstrated that, in addition to ventral stream processing of object form, manipulable objects are represented functionally in the dorsal stream. Here, we demonstrate how the two streams interact via attentional selection and consolidation such that objects whose form fits the function of a previously seen object, such as a tool, benefit from attentional enhancements due to the action affordance of the tool. Using the attentional blink (AB) paradigm and event-related potentials (ERPs), we tested whether providing an action relationship between two objects appearing closely together in time counteracts the typical decrement observed for processing of the second item. We used images (experiments 1a and 2) and names (experiment 1b) of common tools, objects that can be acted upon by those tools, and unrelated objects. We found that pictorial presentation of a tool and its action counterpart results in a diminished attentional blink as well as enhanced attentional selection seen as a larger P3, relative to tools and unrelated objects, and that this attentional enhancement is not driven by semantic associations. This means that the action affordance instantiated by the perception of a tool will reduce the functional blindness normally observed when two targets are presented too closely in time, specifically when the physical properties of the tool that elicit an action affordance are perceived.

On-line grasp control is mediated by the contralateral hemisphere

Electrophysiological recordings from monkeys, as well as functional imaging and neuropsychological work with humans, have suggested that a region in the anterior portion of the intraparietal sulcus (aIPS) is involved in prehensile movements. With recent methodological advances using transcranial magnetic stimulation (TMS), we can now causally attribute anatomy with function to more precisely determine the specific involvement of aIPS in grasping. It has recently been demonstrated that aIPS is specifically involved in executing a grasp under conditions of both constant target requirements, as well as in correcting a movement under conditions in which a target perturbation occurs. In the present study, we extend these findings by determining the differential contribution of the left and right hemisphere to executing a grasping movement with the left and right hands. Transient disruption of left aIPS at movement onset impairs grasping with the right but not the left hand, and disruption of right aIPS impairs grasping with the left but not the right hand. We conclude that grasping is a lateralized process, relying exclusively on the contralateral hemisphere, and discuss the implications of these findings in relationship to models of hemispheric dominance for motor control.

Tool use changes multisensory interactions in seconds: evidence from the crossmodal congruency task

Active tool use in human and non-human primates has been claimed to alter the neural representations of multisensory peripersonal space. To date, most studies suggest that a short period of tool use leads to an expansion or elongation of these spatial representations, which lasts several minutes after the last tool use action. However, the possibility that multisensory interactions also change on a much shorter time scale following or preceding individual tool use movements has not yet been investigated. We measured crossmodal (visual-tactile) congruency effects as an index of multisensory integration during two tool use tasks. In the regular tool use task, the participants used one of two tools in a spatiotemporally predictable sequence after every fourth crossmodal congruency trial. In the random tool use task, the required timing and spatial location of the tool use task varied unpredictably. Multisensory integration effects increased as a function of the number of trials since tool use in the regular tool use group, but remained relatively constant in the random tool use group. The spatial distribution of these multisensory effects, however, was unaffected by tool use predictability, with significant spatial interactions found only near the hands and at the tips of the tools. These data suggest that endogenously preparing to use a tool enhances visual-tactile interactions near the tools. Such enhancements are likely due to the increased behavioural relevance of visual stimuli as each tool use action is prepared before execution.

EEG dynamics of the frontoparietal network during reaching preparation in humans

Visuomotor transformation processes are essential when accurate reaching movements towards a visual target have to be performed. In contrast, those transformations are not needed for similar, but non-visually guided, arm movements. According to previous studies, these transformations are carried out by neuronal populations located in the parietal and frontal cortical areas (the so-called "dorsal visual stream"). However, it is still debated whether these processes are mediated by the sequential and/or parallel activation of the frontoparietal areas. To investigate this issue, we designed a task where the same visual cue could represent either the target of a reaching/pointing movement or the go-signal for a similar but non-targeting arm movement. By subtracting the event-related potentials (ERPs) recorded from healthy subjects performing the two conditions, we identified the brain processes underlying the visuomotor transformations needed for accurate reaching/pointing movements. We then localized the generators by means of cortical current density (CCD) reconstruction and studied their dynamics from visual cue presentation to movement onset. The results showed simultaneous activation of the parietal and frontal areas from 140 to 260 ms. The results are interpreted as neural correlates of two critical phases of visuomotor integration, namely target selection and movement selection. Our findings suggest that the visuomotor transformation processes required for correct reaching/pointing movements do not rely on a purely sequential activation of the frontoparietal areas, but mainly on a parallel information processing system, where feedback circuits play an important role before movement onset.

The time course of visuo-motor affordances

To measure dynamic visuo-motor coupling within the two hemispheres, we showed observers an animation of a rotating cup. They indicated with left or right buttons when a fixation dot in the center of the animation changed color. For either hand, response times changed continuously with the irrelevant position of the cup's handle, even when the perceptual asymmetry of the handle was controlled. This spontaneous lateralized motor preparation showed both top-down and bottom-up components and was more pronounced for the right than the left hand. The dynamic affordance method will help understand perception-action coupling in the brain.

The spatial distribution and temporal dynamics of brain regions activated during the perception of object and non-object patterns

Both animal and human studies suggest that the efficiency with which we are able to grasp objects is attributable to a repertoire of motor signals derived directly from vision. This is in general agreement with the long-held belief that the automatic generation of motor signals by the perception of objects is based on the actions they afford. In this study, we used magnetoencephalography (MEG) to determine the spatial distribution and temporal dynamics of brain regions activated during passive viewing of object and non-object targets that varied in the extent to which they afforded a grasping action. Synthetic Aperture Magnetometry (SAM) was used to localize task-related oscillatory power changes within specific frequency bands, and the time course of activity within given regions-of-interest was determined by calculating time-frequency plots using a Morlet wavelet transform. Both single subject and group-averaged data on the spatial distribution of brain activity are presented. We show that: (i) significant reductions in 10-25 Hz activity within extrastriate cortex, occipito-temporal cortex, sensori-motor cortex and cerebellum were evident with passive viewing of both objects and non-objects; and (ii) reductions in oscillatory activity within the posterior part of the superior parietal cortex (area Ba7) were only evident with the perception of objects. Assuming that focal reductions in low-frequency oscillations (<30 Hz) reflect areas of heightened neural activity, we conclude that: (i) activity within a network of brain areas, including the sensori-motor cortex, is not critically dependent on stimulus type and may reflect general changes in visual attention; and (ii) the posterior part of the superior parietal cortex, area Ba7, is activated preferentially by objects and may play a role in computations related to grasping.

Motor experience with graspable objects reduces their implicit analysis in visual- and motor-related cortex

Motor-related regions of parietal and prefrontal cortices have been shown to selectively activate when observers passively view objects that afford manual grasping. Yet, it remains unknown whether these cortical responses depend on prior motor-related experience with the object being observed. To address this question, we asked participants to undergo fMRI scanning while viewing exemplars of two different categories of graspable objects: one associated with extensive motor experience (door knobs) and one associated with no self-reported motor experience (artificial rock climbing holds). Despite participants' lack of experience grasping climbing holds, these objects were found to generate a systematic response in several visuomotor-related regions of cortex-including left PMv and left AIP. Interestingly, however, the response to door knobs did not include activity in any motor-related regions, being limited instead to a comparatively small bilateral area of lateral occipital cortex, relative to the more spatially extensive response in occipital and temporal cortex that was observed for climbing holds. This result suggested that object-specific responses in both visual- and motor-related cortex may in fact negatively correlate with object-specific motor experience. To test this possibility, we repeated the experiment using participants having extensive self-reported experience grasping climbing holds (i.e., veteran indoor rock climbers). Consistent with our hypothesis, both climbing holds and door knobs generated activity limited to lateral occipital cortex. Taken together, these data support the proposal that repeated real-world motor experience with an object category may lead to reduced implicit analysis in both motor- and visual-related regions of cortex.

Attention and sensory gain control: a peripheral visual process?

Attention-related sensory gain control in human extrastriate cortex is believed to improve the acuity of visual perception. Yet given wide variance in the spatial resolution of vision across the retina, it remains unclear whether sensory gain operates homogenously between foveal and nonfoveal retinotopic locations. To address this issue, we used event-related potentials (ERPs) in a variant of the canonical spatial attention task. Participants were cued to expect targets at either fixation (foveal targets) or at a location several degrees above fixation (parafoveal targets). At both target locations, manual reaction times were shorter for cued relative to uncued targets, indicating that attention was consistently oriented to the cued location. Nevertheless, attention-related increases in sensory-evoked cortical activity were only observed at the parafoveal target location, as measured by the amplitude of the lateral occipital P1 ERP component. A second experiment replicated this data pattern using targets with lower stimulus contrast, indicating that the absence of a P1 effect for foveal targets could not be attributed to a saturated P1 response under higher-contrast stimulus conditions. When considered in light of retinogeniculate projections to cortex showing systematic changes in their physiological organization beginning within a degree of visual angle of the fovea, our findings support the proposal that the strategic functions of visual attention may vary with the retinotopic location involved.