Hands, tools, neglect and the parietal lobe

The title of this blog is Peripersonal Space. Peripersonal space is the space near the body, extending to the area reachable by the limbs. Understanding how the brain codes this space is not only totally cool as a basic science question but may be important knowledge to have for developing simple, noninvasive kinds of neurological rehabilitation after right hemisphere stroke (see the first paragraph of this excellent paper for references).

The brain has other well-organized representations of space; somatosensory space is topographically mapped in cortex, and I’m sure we’ve all marvelled at the exquisite nature of the sensory homunculus at one time or another…

homunculi.jpg

Similarly (but “up a level” representationally), the whole body is oriented in space by integrating proprioceptive information from limbs and other representational information about the body as a whole (representations of whole bodies have been investigated in mental rotation studies, with clear evidence for a role of left parietal areas in imagined body rotation). But how does the brain represent space that lies just outside itself? We’re learning more and more each passing day by looking at evidence from single cell recording in nonhuman primates and by gathering behavioral data from brain-damaged and neurologically-intact participants.

First, to the monkeys; Iriki, Tanaka, and Iwamura (1996, Neuroreport) showed that visual receptive fields of multisensory (tactile and visual) neurons in the postcentral gyrus (in the parietal lobe) get larger as a macaque uses a rake to retrieve objects. You find and record activity from cells that respond to objects near the hand, and then as the macaque uses the rake those cells become responsive to objects near the tip of the rake. Seemingly, the rake acts as an extension of the hand. Thus peripersonal space is flexibly coded with reference to what is happening visually and in tactile sensation at the same time.

Other sensory neurons in (perhaps several) areas of the parietal lobe are even more polymodal; they integrate visual information about the hand’s location with proprioceptive and tactile information. Under normal conditions, this combination of inputs works seamlessly — we easily catch a ball as it approaches us, we rarely overreach for objects.

Consider the phenomenon of unilateral visual neglect. As you probably know, neglect is caused by (in almost all cases) right-sided parietal lobe damage. In neglect the patient feels as if they cannot see the left side of space, denies the contribution of such stimulation to their behavior, yet shows an influence of objects or words there. My all-time favorite example of this effect is the Marshall and Halligan Burning House experiment.

Patients with neglect will often show extinction upon bilateral simultaneous stimulation. When you touch them on both hands at one time they will only report being touched on the right (ipsilesional) hand; touch on the contralesional hand is extinguished. Extinction also takes place visually; two simultaneous flashes are presented to the patient’s left and right hemifield, and they only report the existence of the ipsilesional flash. The reason for this extinction is that the hemispheres are unbalanced in the way they allocate attention to space; the left, undamaged hemisphere competes with and wins over the damaged right hemisphere, to put matters simply. Extinction can also be crossmodal. Crossmodal extinction occurs when, for example, touch is extinguished when the patient is touched on the left hand and presented with a flash of light on the right side of space.

But Alessandro Farnè and Elizabetta Làdavas and their colleagues have been doing such cool work, I just had to present it here. In several papers, these authors extended our conception of the coding of peripersonal space. One of the first things they were able to show was that contralesional touch was extinguished by the presentation of stimuli near the ipsilesional hand. So, the attentional requirements of something touching the ipsilesional hand are similar to those requirements when something is merely nearby the hand, potentially touchable. Crossmodal extinction was also moderated by proximity to the ipsilesional hand – flashes of light close to the right hand extinguished tactile sensation on the left hand more than farther flashes. In another study, use of a tool by the contralesional hand extending into ipsilesional space reduced extinction. In yet another study,

  • a visual stimulus presented near a seen right rubber hand induced strong crossmodal visual-tactile extinction, similar to that obtained by presenting the same visual stimulus near the patient’s right hand. Critically, this specific cross-modal effect was evident when subjects saw the rubber hand as having a plausible posture relative to their own body (i.e. when it was aligned with the subject’s right shoulder).

One note to Rubber Hand Illusion enthusiasts: in this experiment there was no simultaneous stimulation of the unseen right hand and the seen rubber hand at the same time, as is usually required to induce the rubber hand illusion in neurologically intact folks. Futhermore, if I read the original paper correctly, the real right arm is held behind the back! The illusion is more forgiving than is usually presented in the literature…(see Armel and Ramachandran, 2003, for projecting feeling of ownership to a table top)
In addition, Farnè et al. conclude in their abstract that

  • this phenomenon is due to the dominance of vision over proprioception: the system coding peripersonal space can be ‘deceived’ by the vision of a fake hand, provided that its appearance looks plausible with respect to the subject’s body.

OK, I seem to have run way out of time and space for more, but next time I’ll write about performance in line bisection tasks (see below figure — which line or lines are bisected?).

bisection.gif

I’ll ask questions about what line bisection performance can tell us about the distinction (or lack thereof) between peripersonal and extrapersonal space in both neglect and normal participants, and I’ll propose some experiments that I don’t think have been done to get at the issue.

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