Patrick Haggard

When feeling is more important than seeing in sensorimotor adaptation

Perception and action are based on information from multiple sensory modalities. For instance, both vision and proprioception provide information about hand position, and this information is integrated to generate a single estimate of where the hand is in space. Classically, vision has been thought to dominate this process, with the estimate of hand position relying more on vision than on proprioception. However, an optimal integration model that takes into account the precision of vision and proprioception predicts that the weighting of the two senses varies with direction and that the classical result should only hold for specific spatial directions. Using an adaptation paradigm, we show that, as predicted by this model, the visual-proprioceptive integration varies with direction. Variation with direction was so strong that, in the depth direction, the classical result was reversed: the estimate relies more on proprioception than on vision. These results provide evidence for statistically optimal integration of information from multiple modalities.

On the relation between brain potentials and the awareness of voluntary movements

Abstract We investigated the relation between neural events and the perceived time of voluntary actions or the perceived time of initiating those actions using the method of Libet. No differences were found in either movement-related potentials or perceived time of motor events between a fixed movement condition, where subjects made voluntary movements of a single finger in each block, and a free movement condition, in which subjects chose whether to respond with the left or the right index finger on each trial. We next calculated both the readiness potential (RP) and lateralised readiness potential (LRP) for trials with early and late times of awareness. The RP tended to occur later on trials with early awareness of movement initiation than on trials with late awareness, ruling out the RP as a cause of our awareness of movement intiation. However, the LRP occurred significantly earlier on trials with early awareness than on trials with late awareness, suggesting that the processes underlying the LRP may cause our awareness of movement initiation.

What is embodiment? A psychometric approach.

What is it like to have a body? The present study takes a psychometric approach to this question. We collected structured introspective reports of the rubber hand illusion, to systematically investigate the structure of bodily self-consciousness. Participants observed a rubber hand that was stroked either synchronously or asynchronously with their own hand and then made proprioceptive judgments of the location of their own hand and used Likert scales to rate their agreement or disagreement with 27 statements relating to their subjective experience of the illusion. Principal components analysis of this data revealed four major components of the experience across conditions, which we interpret as: embodiment of rubber hand, loss of own hand, movement, and affect. In the asynchronous condition, an additional fifth component, deafference, was found. Secondary analysis of the embodiment of runner hand component revealed three subcomponents in both conditions: ownership, location, and agency. The ownership and location components were independent significant predictors of proprioceptive biases induced by the illusion. These results suggest that psychometric tools may provide a rich method for studying the structure of conscious experience, and point the way towards an empirically rigorous phenomenology.

Having a body versus moving your body: How agency structures body-ownership

We investigated how motor agency in the voluntary control of body movement influences body awareness. In the Rubber Hand Illusion (RHI), synchronous tactile stimulation of a rubber hand and the participants hand leads to a feeling of the rubber hand being incorporated in the participants own body. One quantifiable behavioural correlate of the illusion is an induced shift in the perceived location of the participants hand towards the rubber hand. Previous studies showed that the induced changes in body awareness are local and fragmented: the proprioceptive drift is largely restricted to the stimulated finger. In the present study, we investigated whether active and passive movements, rather than tactile stimulation, would lead to similarly fragmented body awareness. Participants watched a projected image of their hand under three conditions: active finger movement, passive finger movement, and tactile stimulation. Visual feedback was either synchronous or asynchronous with respect to stimulation of the hand. A significant overall RHI, defined as greater drifts following synchronous than asynchronous stimulation, was found in all cases. However, the distribution of the RHI across stimulated and non-stimulated fingers depended on the kind of stimulation. Localised proprioceptive drifts, specific to the stimulated finger, were found for tactile and passive stimulation. Conversely, during active movement of a single digit, the proprioceptive drifts were not localised to that digit, but were spread across the whole hand. Whereas a purely proprioceptive sense of body-ownership is local and fragmented, the motor sense of agency integrates distinct body-parts into a coherent, unified awareness of the body.

Noninformative vision improves the spatial resolution of touch in humans

Research on sensory perception now often considers more than one sense at a time. This approach reflects real-world situations, such as when a visible object touches us. Indeed, vision and touch show great interdependence: the sight of a body part can reduce tactile target detection times [1], visual and tactile attentional systems are spatially linked [2], and the texture of surfaces that are actively touched with the fingertips is perceived using both vision and touch [3]. However, these previous findings might be mediated by spatial attention [1, 2] or by improved guidance of movement [3] via visually enhanced body position sense [4--6]. Here, we investigate the direct effects of viewing the body on passive touch. We measured tactile two-point discrimination thresholds [7] on the forearm while manipulating the visibility of the arm but holding gaze direction constant. The spatial resolution of touch was better when the arm was visible than when it was not. Tactile performance was further improved when the view of the arm was magnified. In contrast, performance was not improved by viewing a neutral object at the arms location, ruling out improved spatial orienting as a possible account. Controls confirmed that no information about the tactile stimulation was provided by visibility of the arm. This visual enhancement of touch may point to online reorganization of tactile receptive fields.

To do or not to do: The neural signature of self-control

Voluntary action is fundamental to human existence. Recent research suggests that volition involves a specific network of brain activity, centered on the fronto-median cortex. An important but neglected aspect of intentional action involves the decision whether to act or not. This decision process is crucial in daily life because it allows us to form intentions without necessarily implementing them. In the present study, we investigate the neural correlates of intentionally inhibiting actions using functional magnetic resonance imaging. Our data show that a specific area of the fronto-median cortex is more strongly activated when people prepare manual actions but then intentionally cancel them, compared with when they prepare and then complete the same actions. Our results suggest that the human brain network for intentional action includes a control structure for self-initiated inhibition or withholding of intended actions. The mental control of action has an enduring scientific interest, linked to the philosophical concept of “free will.” Our results identify a candidate brain area that reflects the crucial decision to do or not to do.

Vision modulates somatosensory cortical processing

Over 150 years ago, E.H. Weber declared that experience showed that tactile acuity was not affected by viewing the stimulated body part. However, more recent investigations suggest that cross-modal links do exist between the senses. Viewing the stimulated body site improves performance on tactile discrimination and detection tasks and enhances tactile acuity. Here, we show that vision modulates somatosensory cortex activity, as measured by somatosensory event-related potentials (ERPs). This modulation is greatest when tactile stimulation is task relevant. Visual modulation is not present in the P50 component reflecting the primary afferent input to the cortex but appears in the subsequent N80 component, which has also been localized to SI, the primary somatosensory cortex. Furthermore, we replicate previous findings that noninformative vision improves spatial acuity. These results are consistent with a hypothesis that vision modulates cortical processing of tactile stimuli via back projections from multimodal cortical areas. Several neurophysiological studies suggest that primary and secondary somatosensory cortex (SI and SII, respectively) activity can be modulated by spatial and tactile attention and by visual cues. To our knowledge, this is the first demonstration of direct modulation of somatosensory cortex activity by a noninformative view of the stimulated body site with concomitant enhancement of tactile acuity in normal subjects.

More than Skin Deep: Body Representation beyond Primary Somatosensory Cortex.

The neural circuits underlying initial sensory processing of somatic information are relatively well understood. In contrast, the processes that go beyond primary somatosensation to create more abstract representations related to the body are less clear. In this review, we focus on two classes of higher-order processing beyond somatosensation. Somatoperception refers to the process of perceiving the body itself, and particularly of ensuring somatic perceptual constancy. We review three key elements of somatoperception: (a) remapping information from the body surface into an egocentric reference frame, (b) exteroceptive perception of objects in the external world through their contact with the body, and (c) interoceptive percepts about the nature and state of the body itself. Somatorepresentation, in contrast, refers to the essentially cognitive process of constructing semantic knowledge and attitudes about the body, including: (d) lexical-semantic knowledge about bodies generally and ones own body specifically, (e) configural knowledge about the structure of bodies, (f) emotions and attitudes directed towards ones own body, and (g) the link between physical body and psychological self. We review a wide range of neuropsychological, neuroimaging and neurophysiological data to explore the dissociation between these different aspects of higher somatosensory function.

The What, When, Whether Model of Intentional Action:

The question of how we can intentionally control our behavior has an enduring fascination for philosophers, psychologists, and neurologists. Brain imaging techniques such as functional MRI have recently provided new insights into the functional and brain mechanisms involved in intentional action. However, the literature is rather contradictory and does not reveal a consistent picture of the functional neuroanatomy of intentional action. Here the authors argue that this confusion arises partly because intentional action has been treated as a unitary concept within neuroscience, even though experimental studies may focus on any of a number of different aspects of intentional action. To provide a heuristic framework for the investigation of intentional action, the authors propose a model that distinguishes three major components: a component related to the decision about which action to execute (what component), a component that is related to the decision about when to execute an action (when component), and finally the decision about whether to execute an action or not (whether component). Based on this distinction, the authors review some key findings on intentional action and provide neuroscientific evidence for the What, When, Whether (WWW) model of intentional action. NEUROSCIENTIST 14(4):319–325, 2008. DOI: 10.1177/1073858408317417

The role of the right temporo-parietal junction in maintaining a coherent sense of one's body

We constantly feel, see and move our body, and have no doubt that it is our own. The brain possesses a distinction between the body and the objects in the outside world. This distinction may be based on a process that monitors whether sensations, events and objects should be attributed to ones body or not. We controlled whether an external object was represented as part of the body or not, by experimentally inducing a bodily illusion using correlated visual and tactile stimulation. We then studied the role of right temporo-parietal junction (rTPJ) in the processing of multisensory events that may or may not be attributed to ones body. Disruption of rTPJ using transcranial magnetic stimulation (TMS) made the distinction between what may or may not be part of ones body on the basis of multisensory evidence more ambiguous, suggesting that the rTPJ is actively involved in maintaining a coherent sense of ones body, distinct from external, non-corporeal, objects.