Wikimedia commons
Source: Wikimedia commons

In the two previous posts I elaborated the idea of intra-psychic mentalism, meaning your ability to read your own mind and sense your own reactions and emotions. But now a remarkable new review explores, not how you sense your internal self, but how you sense yourself externally in the space around you: what the authors call peripersonal space (PPS, left).

This is illustrated by the striking rubber hand illusion (RHI) in which the subject sees a lifelike rubber model of a hand stroked at the same time that their own, corresponding hand is stroked. As the authors of this review comment:

After a short period of stimulation, healthy participants report feeling ownership over the fake hand, and demonstrate a bias in the localization of their real hand in the direction of the rubber hand. This so-called proprioceptive drift can be taken as an indirect measure for self- or limb-location, and hence as a proxy measure of the ease or difficulty (and magnitude) with which subjects will alter their bodily representation.

Summarizing the findings, the authors conclude that “there is a growing body of evidence suggestive that the integration of body-related multisensory signals within PPS is a fundamental component of the ability to localize oneself in space and differentiate self from other.”

As the authors also point out, despite the fact that both “schizophrenia (SZ) and autism spectrum disorder (ASD) have long been characterized as disorders of the self,” the diametric model of mental illness proposes that they are opposites, implying that they should also be opposite in relation to factors related to PPS, such as the RHI.

And indeed they are. Two different studies showed that schizophrenic subjects were both more prone to the RHI and showed an earlier onset of the effect by comparison to normal controls. Indeed, in one schizophrenic, it also involved an out-of-body experience. As the authors note, “these results suggest a weaker and more variable representation of the body in space in SZ; and hence, a reduced distinction between bodily self and other (shallower gradient from self to other).”

By contrast, although individuals with ASD also experience the RHI (and thus demonstrate some malleability in their body representation), their susceptibility to this illusion is delayed: “While neurotypical individuals exhibited a proprioceptive drift both three and six min after the start of synchronous stroking, the shift in perceived hand location in the case of the ASD group was only apparent after six min of synchronous visuo-tactile stimulation.”

The reason behind the need for additional stroking in the ASD group is unknown, but:

Regardless of mechanism, these findings strongly point to increased difficulty in disembodying the bodily self and embodying the bodily other in ASD, suggestive of a steeper and less flexible gradient from self-to-other. Furthermore, as the field of embodied cognition would predict, this bodily effect (i.e., less proneness to the RHI in ASD) appears to have social and cognitive ramifications. Cascio et al. (2012) report that children with ASD who displayed the least empathy were also those whom were least likely to experience the illusion.

The authors’ illustration below represents the situation.

Noel, J.-P., et al., Schizophr. Res. (2016)
Schematic representation of the working hypothesis that self-other differentiation may lie on a continuum, and that although individuals with SZ and ASD exhibit distinct symptomatologies, these psychopathological conditions may index extreme cases of difficulties in self-other differentiation. The solid line circles represent the overall size of PPS (not necessarily changing in size as we move from SZ to healthy to ASD), while the color gradient represents the shallowness (SZ)/steepness (ASD) of the gradient with which self and other are differentiated throughout PPS.
Source: Noel, J.-P., et al., Schizophr. Res. (2016)

One of the most important implications of the diametric model is for therapy because, if SZ and ASD have opposite causes (hyper-mentalizing as opposed to mentalistic deficits respectively), they also call for opposite treatments. This does not escape the reviewers, who speculate that

if the steepness of the PPS boundary (steep or shallow) affects the capacity to discriminate between self and other, it would follow that exposure to particular patterns of multisensory stimulation could either sharpen or dampen this boundary between self and other. Capitalizing on neural plasticity, SZ patients may benefit from the exposure to repetitive temporally synchronous and spatially nonrandom multisensory stimulation (thus sharpening the PPS boundary), while ASD patients may benefit from exposure to temporally synchronous and spatially random multisensory stimulation (thus making the PPS boundary more shallow).

In other words, diametrically opposite treatment for diametrically opposite disorders!

Indeed, the co-occurrence of ASD and bipolar disorder (BD) in the same individuals discussed in the previous two posts suggests a highly counter-intuitive prediction where PPS experiments like the RHI are concerned. If the conventional model is to be believed, the deficits in sense of self reflected in measures like the RHI should be compounded in subjects co-morbid for ASD and bipolar disorder. But if the diametric model is correct, they should cancel out and reveal performance indistinguishable from normal: something which only the diametric model predicts and something which only it can explain.

(With thanks to Amar Annus for bringing this to my attention.)

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