Biomedical subjects

Nicholas P Holmes

Publications and source records attributed to Nicholas P Holmes.

10 recordsLinked to original sources

PubMed2006

Direction-dependent integration of vision and proprioception in reaching under the influence of the mirror illusion.

Recent models of multisensory integration predict differential weighting of information from different sensory modalities in different spatial directions. This direction-dependent weighting account suggests a heavier weighting for vision in the azimuthal (left-right) direction and a heavier weighting for proprioception in the radial (near-far) direction. Visually induced reaching errors, as demonstrated in previous 'mirror illusion' reaching experiments, should therefore be greater under visual-proprioceptive conflict in the azimuthal direction than in the radial direction. We report two experiments designed to investigate the influence of direction-dependent weighting on the visual bias of reaching movements under the influence of a mirror-illusion. In Experiment 1, participants made reaches straight forward, and showed terminal reaching errors that were biased by vision in both directions, but this bias was significantly greater in the azimuthal as compared to the radial direction. In Experiment 2, participants made reaches from right to left, and showed a significant bias only in the azimuthal direction. These results support the direction-dependent weighting of visual and proprioceptive information, with vision relatively more dominant in the azimuthal direction, and proprioception relatively stronger in the radial direction.

Adult↗

PubMed2006

Multisensory interactions follow the hands across the midline: evidence from a non-spatial visual-tactile congruency task.

Crossing the hands over, whether across the body midline or with respect to each other, leads to measurable changes in spatial compatibility, spatial attention, and frequently to a general decrement in discrimination performance for tactile stimuli. The majority of multisensory crossed hands effects, however, have been demonstrated with explicit or implicit spatial discrimination tasks, raising the question of whether non-spatial discrimination tasks also show spatial effects when the hands are crossed. We designed a novel, non-spatial tactile discrimination task to address this issue. Participants made speeded discriminations of single- versus double-pulse vibrotactile targets, while trying to ignore simultaneous visual distractor stimuli, in both hands uncrossed and hands crossed postures. Tactile discrimination performance was significantly affected by the visual distractors (demonstrating a significant crossmodal congruency effect) and was affected most by visual distractors in the same external location as the tactile target (i.e., spatial modulation), regardless of the posture (uncrossed or crossed) of the hands (i.e., spatial 'remapping' of visual-tactile interactions). Finally, crossing the hands led to a general performance decrement with visual distractors, but not in a control task with unimodal visual or tactile judgements. These results demonstrate, for the first time, significant spatial and postural modulations of crossmodal congruency effects in a non-spatial discrimination task.

Adaptation, Physiological↗

PubMed2006

Reaching with alien limbs: visual exposure to prosthetic hands in a mirror biases proprioception without accompanying illusions of ownership.

In five experiments, we investigated the effects of visual exposure to a real hand, arubber hand, or a wooden block on reaching movements made with the unseen left hand behind a parasagittal mirror. Participants reached from one of four starting positions, corresponding to four levels of conflict between the proprioceptively and visually specified positions of the reaching hand. Reaching movements were affected most by exposure to the real hand, intermediately by the rubber hand, and least of all by the wooden block When the posture and/or movement of the visible hand was incompatible with that of the reaching hand, the effect on reaching was reduced. A "rubber hand illusion" questionnaire revealed that illusions of ownership of the rubber hand were not strongly correlated with reaching performance. This research suggests that proprioception is recalibrated following visual exposure to prosthetic hands and that this recalibration is independent of the rubber hand illusion.

Adolescent↗

PubMed2005

Touching a rubber hand: feeling of body ownership is associated with activity in multisensory brain areas.

In the "rubber-hand illusion," the sight of brushing of a rubber hand at the same time as brushing of the person's own hidden hand is sufficient to produce a feeling of ownership of the fake hand. We shown previously that this illusion is associated with activity in the multisensory areas, most notably the ventral premotor cortex (Ehrsson et al., 2004). However, it remains to be demonstrated that this illusion does not simply reflect the dominant role of vision and that the premotor activity does not reflect a visual representation of an object near the hand. To address these issues, we introduce a somatic rubber-hand illusion. The experimenter moved the blindfolded participant's left index finger so that it touched the fake hand, and simultaneously, he touched the participant's real right hand, synchronizing the touches as perfectly as possible. After approximately 9.7 s, this stimulation elicited an illusion that one was touching one's own hand. We scanned brain activity during this illusion and two control conditions, using functional magnetic resonance imaging. Activity in the ventral premotor cortices, intraparietal cortices, and the cerebellum was associated with the illusion of touching one's own hand. Furthermore, the rated strength of the illusion correlated with the degree of premotor and cerebellar activity. This finding suggests that the activity in these areas reflects the detection of congruent multisensory signals from one's own body, rather than of visual representations. We propose that this could be the mechanism for the feeling of body ownership.

Adult↗

PubMed2005

Multisensory integration: space, time and superadditivity.

The superior colliculus generates and controls eye and head movements based on signals from different senses. The latest research on this structure enhances our understanding of the mechanisms of multisensory integration in the brain.

Eye Movements↗

PubMed2005

Visual bias of unseen hand position with a mirror: spatial and temporal factors.

Two experiments examined the integration of visual and proprioceptive information concerning the location of an unseen hand, using a mirror positioned along the midsagittal plane. In experiment 1, participants tapped the fingers of both hands in synchrony, while viewing the mirror-reflection of their left hand. After 6 s, participants made reaching movements to a target with their unseen right hand behind the mirror. Reaches were accurate when visually and proprioceptively specified hand positions were congruent prior to the reach, but significantly biased by vision when the visual location conflicted with the real location. This effect was independent of the target location and depended strongly upon the relative position of the mirror-reflected hand. In experiment 2, participants made reaching movements following 4, 8, or 12 s active visuomotor or passive visual exposure to the mirror, or following passive exposure without the mirror. Reaching was biased more by the visual location following active visuomotor compared to passive visual exposure, and this bias increased with the duration of visual exposure. These results suggest that the felt position of the hand depends upon an integrated, weighted sum of visual and proprioceptive information. Visual information is weighted more strongly under active visuomotor than passive visual exposure, and with increasing exposure duration to the mirror reflected hand.

Adolescent↗

PubMed2004

Extending or projecting peripersonal space with tools? Multisensory interactions highlight only the distal and proximal ends of tools.

The effects of tool-use on the brain's representation of the body and of the space surrounding the body ('peripersonal space') has recently been studied within a number of disciplines in cognitive neuroscience, and is also of great interest to philosophers and behavioural ecologists. To date, most experimental findings suggest that tool-use extends the boundary of peripersonal space-visual stimuli presented at the tips of tools interact more with simultaneous tactile stimuli presented at the hands than visual stimuli presented at the same distance, but not associated with the tools. We studied the proposed extension of peripersonal space by tool-use by measuring the effects of three different tool-use tasks on the integration of visual and tactile stimuli at three distances from participants' hands along two hand-held tools. When the tool-use task required using the shafts or the tips of the tools, visuotactile interactions were stronger at the tips of the tools than in the middle of the shaft. When the handles of the tools were used, however, visuotactile interactions were strongest near the hands and decreased with distance along the tools. These results suggest that tools do not simply 'extend' peripersonal space, but that just the tips of tools actively manipulated in extrapersonal space are incorporated into the brain's visuotactile representations of the body and of peripersonal space.

Adolescent↗

PubMed2004

Functional topography of converging visual and auditory inputs to neurons in the rat superior colliculus.

We have used a slice preparation of the infant rat midbrain to examine converging inputs onto neurons in the deeper multisensory layers of the superior colliculus (dSC). Electrical stimulation of the superficial visual layers (sSC) and of the auditory nucleus of the brachium of the inferior colliculus (nBIC) evoked robust monosynaptic responses in dSC cells. Furthermore, the inputs from the sSC were found to be topographically organized as early as the second postnatal week and thus before opening of the eyes and ear canals. This precocious topography was found to be sculpted by GABAA-mediated inhibition of a more widespread set of connections. Tracer injections in the nBIC, both in coronal slices as well as in hemisected brains, confirmed a robust projection originating in the nBIC with distinct terminals in the proximity of the cell bodies of dSC neurons. Combined stimulation of the sSC and nBIC sites revealed that the presumptive visual and auditory inputs are summed linearly. Finally, whereas either input on its own could manifest a significant degree of paired-pulse facilitation, temporally offset stimulation of the two sites revealed no synaptic interactions, indicating again that the two inputs function independently. Taken together, these data provide the first detailed intracellular analysis of convergent sensory inputs onto dSC neurons and form the basis for further exploration of multisensory integration and developmental plasticity.

Acoustic Stimulation↗

PubMed2004

The body schema and the multisensory representation(s) of peripersonal space.

In order to guide the movement of the body through space, the brain must constantly monitor the position and movement of the body in relation to nearby objects. The effective 'piloting' of the body to avoid or manipulate objects in pursuit of behavioural goals (Popper & Eccles, 1977, p. 129), requires an integrated neural representation of the body (the 'body schema') and of the space around the body ('peripersonal space'). In the review that follows, we describe and evaluate recent results from neurophysiology, neuropsychology, and psychophysics in both human and non-human primates that support the existence of an integrated representation of visual, somatosensory, and auditory peripersonal space. Such a representation involves primarily visual, somatosensory, and proprioceptive modalities, operates in body part-centred reference frames, and demonstrates significant plasticity. Recent research shows that the use of tools, the viewing of one's body or body parts in mirrors, and in video-monitors, may also modulate the visuotactile representation of peripersonal space.

Journal Article↗

PubMed2004

When mirrors lie: "visual capture" of arm position impairs reaching performance.

If we stand at a mirror's edge, we can see one half of our body reflected in the mirror, as if it were the other half of our body, seen "through" the mirror. We used this mirror illusion to examine the effect of conflicts between visually and proprioceptively specified arm positions on subsequent reaching movements made with the unseen right arm. When participants viewed their static left arm in the mirror (i.e., as if it were their right arm), subsequent right-arm reaching movements were affected significantly more when there was conflict between the apparent visual and the proprioceptively specified right-arm positions than when there was no conflict. This result demonstrates that visual capture of arm position can occur when individual body parts are viewed in the mirror and that this capture has a measurable effect on subsequent reaching movements made with an unseen arm. The result has implications for how the brain represents the body across different sensory modalities.

Adult↗