Fake Hands and Tables-The Malleability of the Body Image

In " The Classical Rubber Hand Illusion", we discussed the original experiments of Botvinick and Cohen (1998). Their hypothesis for the rubber hand illusion was that vision has higher reliability and spatial acquity than proprioception, so the brain gives more weight to visual information. People would thus localize a body part to it's apparent visual location, particularly when the visible location falls within the possible range dictated by proprioception. Some support for this theory lies in the fact that placing the fake hand perpendicular to the real occluded hand destroys the illusion that the fake hand is one's own.

Armel and Ramachandran  (2003) reported a closely related but bizarre illusion. The subject is made to place his or her real hand on a table and the hand is hidden from view. However, instead of stroking a fake rubber hand, the researchers simply stroked and tapped the table in precise synchrony for a minute. Astonishingly, subjects reported sensations arising from the table surface, despite the fact that it bears no physical resemblance to a hand. Whereas Botvinick and Cohen interpret their result in terms of resolving incongruities between visual versus proprioceptive location of the hand, Armel and Ramachandran's experiment would lead one to argue that the illusion arises mainly  from Bayesian logic of all perception; the brain's remarkable ability to detect statistical correlations in sensory inputs in constructing useful perceptual representations of the world, including one's own body. It is especially intriguing that the bizarre perceptual representation (assimilating the table into the body image) is resistant to the "top-down" knowledge of the absurdity of the situation!

To measure the extent to which subjects incorporated the external objects into their body image, they were asked to rate the vividness of the illusion. The experimenters also recorded the skin conductance response (SCR), to provide an objective test of whether the table had indeed become informationally coupled with the subject's body image. If the external objects became integrated into the body's image, would they be aroused when the table (or a fake hand for that matter) was 'injured'?

If a finger of the fake hand is bent backwards to seem painful, does the subject register an SCR? To what extent is the fake hand assimilated into the subject's body image? To address this, after ca. 2.5 minutes of the touching procedure, both the real and the fake fingers were lifted, but only the fake finger was bent backwards into a 'painful position'. SCR was recorded at this point and a free response description and intensity rating of the illusion were obtained. The control for this experiment was a 'delayed synchrony' condition wherein touch to the real and fake hands were identical, the only difference being, that the touch to the real hand was delivered 1 sec after the touch on the fake hand. Mean intensity and mean SCRs showed that subjects identified with the fake hand more in the condition where the touch was synchronized rather than the one where the touch was synchronized but delayed (see Fig. 1)

Fig. 1: Mean Intensity ratings (gray bars) and SCR (black circles) in the first experiment where synchronous touch to the real hand was delayed by a second in the control condition. The error bars indicate one SEM (Adapted from Armel and Ramachandran, 2003)

Would subjects still experience the illusion if the form of the external object was manipulated? To explore this, a barren table was stroked  and tapped in the same manner and in the same relative location (see Fig.2 ). 

Fig. 2: Form manipulation where subjects received the table condition (Adapted from Armel and Ramachandran, 2003)

Band-aids were placed on both the real hand and the table and subjects were told that the band-aid would be pulled off the table but not off their real hands. At the end of the 2.5 min touching period, in lieu of pulling back a fake finger, the band-aid on the table was partially pulled off. In the control condition for this experiment, the real hand was made visible by removing the occluder. Subjects were instructed to look back and forth between their real hands and the table. Furthermore, the real hand and the location where the table was touched were close together so that they could be seen simultaneously even while looking at one or the other. The band-aid was pulled off the table while the subject viewed it. To ensure that the subject was only looking at the table, the experimenter occluded the subject's real finger at this time. In a comparison of the conditions in which the table and real hand were touched with the partition in place or removed, intensity ratings and SCR were significantly different (see Fig. 3). However, the same experiment (using a band-aid) carried out with a fake rubber hand in lieu of the table is more effective at inducing the illusion in terms of intensity ratings but only marginally so for SCR (see Fig. 3)

Fig. 3: Mean Intensity ratings (gray bars) and SCR (black circles) in the second experiment where form of the external object was manipulated. The error bars indicate one SEM (Adapted from Armel and Ramachandran, 2003)

Would subjects still experience the illusion if the form of the external object was manipulated? Each subject viewed touch to a fake hand in a 'realistic' location in one condition and then to a distant fake hand in another (see Fig. 4).

Fig. 4: Location manipulation where subjects received the distant-hand condition (Adapted from Armel and Ramachandran, 2003)

The fake arm was extended so that it lay 3 feet beyond the real hand. In the distant fake hand manipulation, a fake finger was bent back for a painful stimulus. In the control condition, the touch applied to the fake and real hands was asynchronous i.e.  touch was random and there was no correlation (see Table 1 at the end for summary of conditions for all experiments). Mean intensity and mean SCR showed that subjects identified with the fake hand and the distant fake hand, more in the conditions where touch was synchronized than when not synchronized. However, the 'anatomically correct' fake hand condition was more effective than the distant fake hand condition (both with synchronous touch, see Fig. 5)  

Fig. 5: Mean Intensity ratings (gray bars) and SCR (black circles) in the second experiment where the location of the fake hand was manipulated. The error bars indicate one SEM (Adapted from Armel and Ramachandran, 2003)

The so-called body image appears to be highly malleable. Despite  it's appearance of durability, it can be profoundly altered by stimulus contingencies and correlations that one encounters. Taken together, these experiments illustrate an important principle underlying perception: that the mechanisms of perception may be involved in extracting statistical correlations. Further investigations will further our understanding of phenomena such as body-dysmorphic disorder and anorexia nervosa

Table 1: Experimental design for all experiments. For a given experiment, each subject received all conditions in one of different possible orders (Adapted from Armel and Ramachandran, 2003)

Reference:

Armel, K., & Ramachandran, V. (2003). Projecting sensations to external objects: evidence from skin conductance response Proceedings of the Royal Society B: Biological Sciences, 270 (1523), 1499-1506 DOI: 10.1098/rspb.2003.2364

Look Mom, Three Hands -The Classical Rubber Hand Illusion

Most illusions are not only fun to experience but are also interesting to study in depth for what they can reveal about perceptual purposes. One of the most interesting illusions discovered in recent times in the rubber hand illusion. It was first reported in a paper in Nature in 1998, titled "Rubber hands 'feel' touch that eyes see" by Matthew Botvinick and Jonathan Cohen.  So how does it work? A subject is seated with the left harm resting on a table while a standing screen is positioned besides the arm to hide it from the subject's view. A life-sized rubber model of a left hand and arm is placed on the table, directly in front of the patient. The experimenter uses two paintbrushes to stroke the rubber hand and the real hidden hand, synchronizing the timing of strokes as closely as possible.

In the original study by Botvinick and Cohen, subjects were asked to complete a two part questionnaire that asked for an open description of their experience and also to affirm or deny the occurrence of nine specific perceptual effects (see Fig. 1).

Fig. 1: Questionnaire includes nine statements presented in random order. Subjects indicated their response on a seven-step visual analogue scale ranging from 'agree strongly (+++)' to 'disagree strongly (---)'. Points indicate mean response and bars indicate response range. Underlined questions show a significant tendency to evoke an affirmative response (Adapted from Botvinick and Cohen (1998))

Most subjects indicated that they seemed to feel the touch not of the hidden brush, but that of the viewed brush, as though the rubber hand had sensed the touch. It was hypothesized that the illusion may arise due to a spurious reconciliation of visual and tactile inputs while distorting position sense (proprioception). In a second experiment, subjects were exposed to the illusion for a prolonged period and were then probed for distortion in proprioceptive information. Before and after the viewing period, subjects completed a series of three intermanual reaches. With eyes closed, the right index finger was drawn along a straight edge, until it was judged to be aligned with the index finger of the hidden left hand. The authors found that the subjects' reaches after experiencing the illusion were displaced towards the rubber hand, the magnitude of displacement varying in proportion to the reported duration of the illusion (see Fig. 2)

Fig. 2: Results of reaching experiment. x-axis indicates the percentage of 30-min viewing period during which the illusion was experienced. The y-axis indicates displacement of the three reaches made after the viewing period from the three made before. Data is fitted with a least-squares regression line (adapted from Botvinick and Cohen (1998)) 

In more systematic explorations by Tsakiris and Haggard (2005), it was shown that the drift, while indicating the position of the real left hand, in the direction of the fake rubber hand not only depends upon synchrony of strokes, but also on the position of the rubber hand (spatial congruency) as well as visual characteristics of the hand (body top-down effects) (see Fig. 3 for experimental setup)

Fig. 3: Participants saw in different conditions (a) a rubber hand in a congruent position, (b) a rubber hand in  incongruent position, or (c) a wooden stick. The participant's left hand was out of view for the whole duration of the experiment (Adapted from Tsakiris and Haggard (2005))

The congruent posture elicits the maximum proprioceptive drift while having the rubber hand in an incongruent position or replacing the rubber hand by a wooden stick does not (see Fig. 4).

Fig. 4: Mean proprioceptive drift towards the rubber hand. Error bars indicate standard error. Asterix indicates significant difference between synchronous and asynchronous stimulation (Adapted from Tsakiris and Haggard (2005))

The body is distinguished from other objects as belonging to the self by participating in inter-modal perceptual correlations. In the experiments of Botvinick and Cohen, subjects who referred the tactile sensation to the rubber hand also reported experiencing the rubber hand as belonging to themselves. Indeed eight out of ten subjects employed terms of ownership in the free descriptions (Botvinick and Cohen (1998))

The rubber hand illusion presents a very intriguing case. It shows that certain forms of inter-modal correlations may be sufficient for self attribution even in the face of contradicting signals from other sensory modalities.

References

Botvinick M, & Cohen J (1998). Rubber hands 'feel' touch that eyes see. Nature, 391 (6669) PMID: 9486643

Tsakiris M, & Haggard P (2005). The rubber hand illusion revisited: visuotactile integration and self-attribution. Journal of experimental psychology. Human perception and performance, 31 (1), 80-91 PMID: 15709864

Take that leg off-It ain't mine

So I recently read this paper from V.S Ramachandran's group at the Salk Institute and came to realize how weird the field of cognitive neuroscience can really get. The paper is on Apotemnophilia (phew!! saying it is a task in itself), which yours truly had never heard of before. In any case, the disease is characterized by the desire to amputate one's own limb (Now how about that). These patients are otherwise mentally normal.

Individuals suffering from apotemnophilia always date the desire for amputation since childhood and often term the limb as being over-present or intrusive. Most obtain an amputation and paradoxically report feeling much more 'complete' and happier. Traditional explanations for the disease range from it being sexual paraphilia, related to the phallic resemblance of an amputee's stump (Someone get Freud in here!) or perhaps the mere sight of an amputee is permanently imprinted in the malleable psyche of a child as the "ideal body representation"

In this paper (McGeoch et. al (2009)), they demonstrate that the disease actually has a neurological basis based on the following observations:

1) Sufferers have no other psychological disorders

2) They desire an amputation of a limb at a specific level

3) There is a left sided bias.

The last observation points a finger to one particular area in the right parietal lobe called the superior parietal lobe (SPL) which receives connections from a host of other areas, namely the visual, primary somatosensory, secondary somatosensory, premotor and motor cortices. Additionally, the right parietal cortex is known to play a vital role in constructing body image and damage can lead to various disorders like somatoparaphrenia (denial of ownership of left arm) and others. The autors postulate that the right SPL may contain a hardwired representation of the body and if a particular limb were missing from the representation, the consequence may be a desire for amputation.

Right hemisphere of averaged control brain (a,b) and subject brain (c,d). Right SPL is outlined in black. Images a and c show touch to the left foot of control and subject respectively while b and d show touch to the right foot of control and subject respectively . This subject wanted right below-knee amputation (McGeoch et. al (2009)) 

This confirms the hypothesis that there is a congenital failure to represent affected limbs in the body image. Since visual and somatosensory inputs are still intact, but there is no corresponding  limb representation, the result would be a mismatch that manifests itself as an intrusive and over-present limb.

References

McGeoch, P.D., Brang, D., Song, T., Lee, R.R., Huang, M., & Ramachandran, V.S. (2009). Apotemnophilia - the neurological basis of a 'psychological' disorder Nature Precedings : 10101/npre.2009.2954.1

Brang, D., McGeoch, P., & Ramachandran, V. (2008). Apotemnophilia: a neurological disorder NeuroReport, 19 (13), 1305-1306 DOI: 10.1097/WNR.0b013e32830abc4d