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

Abstract

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.

Figure 1

Selected cortical areas of the Macaque monkey brain. A. Lateral view of the whole brain. Thick black lines represent major cortical boundaries and sulci. Thin black lines represent cortical area boundaries. Area 4/F1 - primary motor cortex; Area 2/SI – primary somatosensory cortex; Area 5/PE – posterior parietal association cortex/superior parietal lobule; Area F5/PMv – where ‘mirror neurons’ were first recorded. Area F4/PMv – ventral premotor cortex. IPS – intraparietal sulcus; CS – central sulcus. B. The intraparietal sulcus has been opened up to reveal multiple and heterogeneous visual and somatosensory posterior parietal areas. Thick lines represent the superficial border of the sulcus; thin black lines mark the fundus of the sulcus. Other lines indicate the boundaries of cortical areas as follows: MIP – medial intraparietal sulcus; LIP – lateral intraparietal sulcus; AIP – anterior intraparietal sulcus; VIP –ventral intraparietal sulcus; PEip – intraparietal portion of area PE. (Redrawn from Rizzolatti et al., 1998).

Figure 2

The stimuli delivered and responses obtained in Iriki et al.’s (1996a) study of tool-use, the body-schema, and peripersonal space. A. Visual stimuli were moved either toward or away from the hand in a centripetal or centrifugal fashion respectively (as shown by the double-headed arrows). Note that the hand area was stimulated much more frequently than any other part of the visual field. B. Filled circles represent the position in the visual field where the visual stimulus was when the neuron increased its firing rate to above 3 action potentials/second. C. The trajectories of stimuli (grey lines) presented after 5 minutes of tool-use activity (experimental condition). The monkey’s hand is on the right, and the tool (dark ‘T’-shape) extends toward the left. D. The trajectories of stimuli presented after 3 minutes of retrieving food using only the unaided hand, but studied with the tool held in the monkey’s hand (control condition). Note that the frequency and density of stimulation in the critical part of the putative visual receptive field (i.e., around the tip of the tool, shown by the black ellipse) does not seem to be comparable across the two conditions. The proposed ‘extension of visual receptive fields’ may in fact therefore be an artifact of the method of stimulus presentation. (Panels C and D were digitized and adapted from Iriki et al., 1996a).

Figure 3

Crossmodal extinction in peripersonal space. The patient sits at a table, and visually fixates a point at the same level as, and equidistant between their two hands (‘F’, filled circle). A. Visual stimuli (V, open circles) presented near the right hand interfere with the detection of simultaneously presented tactile stimuli (‘T’, open triangles) on the left hand. B. This interference is markedly reduced if the right hand is held away from the visual stimulus, behind the patient’s back. See main text and di Pellegrino et al. (1997) for details.

Figure 4

The visuotactile crossmodal congruency task. Human participants hold a ‘stimulus cube’ containing vibrotactile stimulators (open triangles) and visual distractor LEDs (open circles). Participants look at a central fixation point (filled circle) situated midway between the two hands. White noise presented over headphones masks the sound of the vibrotactile stimulators (see text for further details). The inset shows a magnified view of the participant’s left hand holding one of the stimulus cubes.