Biomedical subjects

Patrick Haggard

Publications and source records attributed to Patrick Haggard.

At least 37 records · Page 2Linked to original sources

PubMed2005

Spatial consequences of bridging the saccadic gap.

We report six experiments suggesting that conscious perception is actively redrafted to take account of events both before and after the event that is reported. When observers saccade to a stationary object they overestimate its duration, as if the brain were filling in the saccadic gap with the post-saccadic image. We first demonstrate that this illusion holds for moving objects, implying that the perception of time, velocity, and distance traveled become discrepant. We then show that this discrepancy is partially resolved up to 500 ms after a saccade: the perceived offset position of a post-saccadic moving stimulus shows a greater forward mislocalization when pursued after a saccade than during pursuit alone. These data are consistent with the idea that the temporal bias is resolved by the subsequent spatial adjustment to provide a percept that is coherent in its gist but inconsistent in its detail.

Adult↗

PubMed2005

Motor awareness without perceptual awareness.

The control of action has traditionally been described as "automatic". In particular, movement control may occur without conscious awareness, in contrast to normal visual perception. Studies on rapid visuomotor adjustment of reaching movements following a target shift have played a large part in introducing such distinctions. We suggest that previous studies of the relation between motor performance and perceptual awareness have confounded two separate dissociations. These are: (a) the distinction between motoric and perceptual representations, and (b) an orthogonal distinction between conscious and unconscious processes. To articulate these differences more clearly, we propose a new measure of motor awareness, based on subjects' ability to reproduce the spatial details of reaching movements they have just made. Here we focus on the dissociation between motor awareness and perceptual awareness that may occur when subjects make rapid visuomotor adjustments to reaching movements following a target shift. In experiment 1, motor awareness was dissociated from perceptual awareness of a target shift during reaching movement. Participants' reproduction of movement endpoints following visuomotor adjustment was independent of whether they saw the target shift or not. Experiment 2 replicated this result, and further showed that neither motor awareness nor motor performance were disrupted by TMS over the parietal cortex. The neural mechanisms underlying motor awareness, and the implications for theories of consciousness, are discussed.

Adult↗

PubMed2005

Experience modulates automatic imitation.

Action observation gives rise to activation in corresponding areas of the premotor and primary motor cortices. We tested the hypothesis that this activation depends on visual-motor connections established through correlated experience of observing and executing the same action. Previous work has shown that hand opening and hand closing gestures are facilitated when subjects observe the movement they are performing, relative to a condition in which they observe a different movement from the one they are performing. Experiment 1 replicated this finding in a simple reaction time (RT) procedure using stimulus-response (SR) movements in orthogonal planes. This implies that the effect is an example of automatic imitation, an instruction-independent tendency to execute movements that are topologically similar to those observed, and not merely an example of spatially compatible responding. In Experiment 2, the automatic imitation effect found in Experiment 1 was abolished by a brief period of training in which subjects responded to hand opening by closing their hands, and to hand closing by opening their hands. This outcome is consistent with the hypothesis that, rather than being innate, the cortical connections mediating motor activation by action observation are formed through experience.

Adult↗

PubMed2005

Conscious intention and motor cognition.

The subjective experience of conscious intention is a key component of our mental life. Philosophers studying 'conscious free will' have discussed whether conscious intentions could cause actions, but modern neuroscience rejects this idea of mind-body causation. Instead, recent findings suggest that the conscious experience of intending to act arises from preparation for action in frontal and parietal brain areas. Intentional actions also involve a strong sense of agency, a sense of controlling events in the external world. Both intention and agency result from the brain processes for predictive motor control, not merely from retrospective inference.

Brain↗

PubMed2005

Spatial and temporal aspects of oculomotor inhibition as revealed by saccade trajectories.

The spatial and temporal effect of distractor related inhibition on stimulus elicited (reflexive) and goal driven (voluntary) saccades, was examined using saccade trajectory deviations as a measure. Subjects made voluntary and reflexive saccades to a target location on the vertical midline, while the distance of a distractor from the target was systematically manipulated. The trajectory curvature of both voluntary and reflexive saccades was found to be subject to individual differences. Saccade curvature was found to decrease monotonically with increasing distractor distance from target for some subjects while for others no reduction in curvature or even an increase was found. These results could not be explained by latency differences or landing position effects. The different patterns of distractor effects on saccade trajectories suggest the additional influence of a non-spatial inhibitory mechanism.

Adult↗

PubMed2005

The rubber hand illusion revisited: visuotactile integration and self-attribution.

Watching a rubber hand being stroked, while one's own unseen hand is synchronously stroked, may cause the rubber hand to be attributed to one's own body, to "feel like it's my hand." A behavioral measure of the rubber hand illusion (RHI) is a drift of the perceived position of one's own hand toward the rubber hand. The authors investigated (a) the influence of general body scheme representations on the RHI in Experiments 1 and 2 and (b) the necessary conditions of visuotactile stimulation underlying the RHI in Experiments 3 and 4. Overall, the results suggest that at the level of the process underlying the build up of the RHI, bottom-up processes of visuotactile correlation drive the illusion as a necessary, but not sufficient, condition. Conversely, at the level of the phenomenological content, the illusion is modulated by top-down influences originating from the representation of one's own body.

Adult↗

PubMed2005

Altered subjective time of events in schizophrenia.

Time perception has long been known to be impaired in schizophrenia. Moreover, recent neuropsychological theories have postulated abnormalities of cognitive motor control in schizophrenia. However, the intersection of these two topics has rarely been studied, and it is unclear what role subjects' own actions may play in the construction of subjective time representation. The present experiment compared the performance of 19 patients with schizophrenia and 24 normal controls in a timing judgment task. The results show that patients bind two consecutive movements more strongly than controls. This anomaly could play a role in the subjective pathological experiences of patients.

Adult↗

PubMed2005

Viewing the body prepares the brain for touch: effects of TMS over somatosensory cortex.

Viewing the body can improve tactile perception. We investigated whether this could be due to a remodeling of somatosensory cortical areas during vision of the body. Single-pulse transcranial magnetic stimulation (TMS) was delivered over the primary and secondary somatosensory areas of subjects who showed clear visual-tactile enhancement while they performed a tactile grating discrimination task. Before the tactile stimulus, subjects viewed either their right index finger through a semisilvered mirror or an object reflected by the mirror and positioned to appear in the same location as the finger. In a first experiment we observed that TMS over primary somatosensory cortex significantly reduced subjects' accuracy whilst viewing the hand. No such reduction was found when subjects viewed a neutral object. In a second experiment, we disrupted the activity of primary and secondary somatosensory areas in different sessions. When stimulating the primary somatosensory cortex, a reduction in accuracy was again found while viewing the hand, but not a neutral object. TMS over secondary somatosensory cortex had no effect in any condition. Our results show that vision of the body may act at an early stage in stimulus elaboration and perception, allowing an anticipatory tuning of the neural circuits in primary somatosensory cortex that underlie tactile acuity.

Adolescent↗

PubMed2004

A specific role for efferent information in self-recognition.

We investigated the specific contribution of efferent information in a self-recognition task. Subjects experienced a passive extension of the right index finger, either as an effect of moving their left hand via a lever ('self-generated action'), or imposed externally by the experimenter ('externally-generated action'). The visual feedback was manipulated so that subjects saw either their own right hand ('view own hand' condition) or someone else's right hand ('view other's hand' condition) during the passive extension of the index finger. Both hands were covered with identical gloves, so that discrimination on the basis of morphological differences was not possible. Participants judged whether the right hand they saw was theirs or not. Self-recognition was significantly more accurate when subjects were themselves the authors of the action, even though visual and proprioceptive information always specified the same posture, and despite the fact that subjects judged the effect and not the action per se. When the passive displacement of the participants right index finger was externally generated, and only afferent information was available, self-recognition performance dropped to near-chance levels. Differences in performance across conditions reflect the distinctive contribution of efferent information to self-recognition, and argue against a dominant role of proprioception in self-recognition.

Adult↗

PubMed2004

Internally generated and externally triggered actions are physically distinct and independently controlled.

In everyday life we must constantly balance our intentions to act in a certain way with reactions that are imposed upon us by the outside world. Recent neuroimaging studies have examined these classes of movement separately but despite the fundamental requirement for us to efficiently organize our internally generated and externally triggered actions, few studies have examined the relationship between these two classes of movement. We measured EMG activity in the right first dorsal interosseous while subjects performed right index finger key presses either in an internally generated condition or an externally triggered condition. In addition, in an attempt to probe the relationship between the processing underlying these two types of action, we examined the effect on reaction time (RT) and EMG activity in a third "truncation" condition in which subjects were forced to switch from an intentional (internally generated) mode of response production to an externally triggered mode. Results indicated significantly greater muscle activation for actions that were internally generated as compared to externally triggered. Truncation caused responses to be delayed by, on average, 54.7 ms as compared with simple externally triggered responses, suggesting that the motor system cannot take advantage of preexisting levels of preparation when switching between internally generated and externally triggered actions. Interestingly, the unique EMG signatures of internally generated and externally triggered actions were preserved in truncation. Thus, subjects switched between the two types of action rather than simply modifying an ongoing action. The results provide peripheral physiological support for previous neuroimaging work suggesting that internally generated actions are preceded by greater levels of preparation than externally triggered actions. The present findings also raise the interesting possibility that the motor system processes these two classes of action separately even though the motor output required is the same.

Adult↗

PubMed2004

Attention to intention.

Intention is central to the concept of voluntary action. Using functional magnetic resonance imaging, we compared conditions in which participants made self-paced actions and attended either to their intention to move or to the actual movement. When they attended to their intention rather than their movement, there was an enhancement of activity in the pre-supplementary motor area (pre-SMA). We also found activations in the right dorsal prefrontal cortex and left intraparietal cortex. Prefrontal activity, but not parietal activity, was more strongly coupled with activity in the pre-SMA. We conclude that activity in the pre-SMA reflects the representation of intention.

Attention↗

PubMed2004

Keeping the world a constant size: object constancy in human touch.

The perceived size of objects touching different regions of skin varies across the body surface by much less than is predicted from variations in tactile receptor density. Here we show that altering the visual experience of the body alters perceived tactile distances. We propose that the brain attempts to preserve tactile size constancy by rescaling the primary, distorted body-surface representation into object-centered space according to visual experience of the body.

Adult↗

PubMed2004

Distractor modulation of saccade trajectories: spatial separation and symmetry effects.

The trajectories of saccadic eye movements can be modulated by the presence of a competing visual distractor. In the present study the trajectories of vertical saccades curved away from a single visual distractor presented in one visual field, but tended to be straight when two distractors were presented at mirror symmetric locations in both visual fields. The spatial nature of the mirror distractor effect was examined by presenting a second distractor at mirror and non-mirror locations. Saccade trajectories also tended to be straight with both mirror and non-mirror symmetrical distractors. The relationship between the distractor location and saccade curvature was examined in a third experiment by manipulating the distractor-to-target spatial separation. Although there was a tendency for greater curvature when the distractor was presented in the same hemifield as the target there was no clear relationship between curvature and distractor location. The results show that the distractor modulation of saccade trajectory is not highly spatially specific and that it can be balanced by a second bilateral distractor in the opposite visual field. The results are interpreted in terms of a model in which the initial saccade direction and curvature back towards the saccade goal are controlled by separate processes. Initial saccade direction is modulated by the inhibition of distractor locations within a 'motor map' specifying saccade direction. Curvature back towards the saccade goal may be attributed to a feedback system, with a separate representation of the visual target location, that enables an on-line correction of the saccade during mid-flight.

Adult↗

PubMed2004

Persistence of visual-tactile enhancement in humans.

We report two experiments in which non-informative vision of the finger enhanced tactile acuity on the fingertip. The right index finger was passively lifted to contact a grating. Twelve participants judged orientations of tactile gratings while viewing either the fingertip, or a neutral object presented via a mirror at the fingertip's location. In Expt. 1, tactile orientation discrimination for near-threshold gratings was improved when viewing the fingertip, compared to viewing the neutral object. Experiment 2 examined the temporal persistence of this effect, and found significant visual-tactile enhancement when a dark interval of up to 10 s intervened between viewing the finger and tactile stimulation. These results suggest that viewing the body modulates the neural circuitry of primary somatosensory cortex, outlasting visual inputs.

Adult↗

PubMed2004

Supplementary motor area provides an efferent signal for sensory suppression.

Voluntary actions produce suppression of neural activity in sensory areas, and reduced levels of conscious sensation. Recent computational models of motor control have linked sensory suppression to motor prediction: an efferent signal from motor areas may cancel the sensory reafferences predicted as a consequence of movement. Direct evidence for the efferent mechanism in sensory suppression has been lacking. We investigated the perceived size of finger-muscle twitches (MEPs) evoked by TMS in eight normal subjects. Subjects freely chose on each trial whether to make or withhold a voluntary flexion of the right index finger, in synchrony with an instructional stimulus. A test MEP occurred at the instructed time of action. The subject then relaxed and a second reference MEP occurred a few seconds later. Subjects judged which of the two MEPs was larger. Subjects perceived the first test MEP to be smaller in trials where they made voluntary actions than on trials where they did not, demonstrating sensory suppression. On randomly selected trials, a conditioning prepulse was delivered over the supplementary motor area (SMA) 10 ms before the pulse producing the test MEP. The SMA prepulse reduced and almost abolished the sensory suppression effect in voluntary action trials. We suggest the SMA may provide an efferent signal which is used by other brain areas to modulate somatosensory activity during self-generated movement.

Adult↗

PubMed2004

Action, arousal, and subjective time.

Saccadic chronostasis refers to the subjective temporal lengthening of the first visual stimulus perceived after an eye movement. It has been quantified using a duration discrimination task. Most models of human duration discrimination hypothesise an internal clock. These models could explain chronostasis as a transient increase in internal clock speed due to arousal following a saccade, leading to temporal overestimation. Two experiments are described which addressed this hypothesis by parametrically varying the duration of the stimuli that are being judged. Changes in internal clock speed predict chronostasis effects proportional to stimulus duration. No evidence for proportionality was found. Two further experiments assessed the appropriateness of the control conditions employed. Results indicated that the chronostasis effect is constant across a wide range of stimulus durations and does not reflect the pattern of visual stimulation experienced during a saccade, suggesting that arousal is not critical. Instead, alternative processes, such as one affecting the onset of timing (i.e., the time of internal clock switch closure) are implicated. Further research is required to select between these alternatives.

Adult↗

PubMed2004

Anomalous control: when 'free-will' is not conscious.

The conscious feeling of exercising 'free-will' is fundamental to our sense of self. However, in some psychopathological conditions actions may be experienced as involuntary or unwilled. We have used suggestion in hypnosis to create the experience of involuntariness (anomalous control) in normal participants. We compared a voluntary finger movement, a passive movement and a voluntary movement suggested by hypnosis to be 'involuntary.' Hypnosis itself had no effect on the subjective experience of voluntariness associated with willed movements and passive movements or on time estimations of their occurrence. However, subjective time estimates of a hypnotically-suggested, 'involuntary' finger movement were more similar to those for passive movements than for voluntary movements. The experience of anomalous control is qualitatively and quantitatively different from the normal conscious experience of a similar act produced intentionally. The experience of anomalous control may be produced either by pathology, or, in our case, by suggestion.

Adolescent↗

PubMed2004

Facilitated processing of visual stimuli associated with the body.

Recent work on tactile perception has revealed enhanced tactile acuity and speeded spatial-choice reaction times (RTs) when viewing the stimulated body site as opposed to viewing a neutral object. Here we examine whether this body-view enhancement effect extends to visual targets. Participants performed a speeded spatial discrimination between two lights attached either to their own left index finger or to a wooden finger-shaped object, making a simple distal--proximal decision. We filmed either the finger-mounted or the object-mounted lights in separate experimental blocks and the live scene was projected onto a screen in front of the participants. Thus, participants responded to identical visual targets varying only in their context: on the body or not. Results revealed a large performance advantage for the finger-mounted stimuli: reaction times were substantially reduced, while discrimination accuracy was unaffected. With this finding we address concerns associated with previous work on the processing of stimuli attributed to the self and extend the finding of a performance advantage for such stimuli to vision.

Adult↗