Sunday, January 16, 2005

I Can't Reach It; It's Inside the Mirror.

V.S. Ramachandran does some really fascinating research. Every time I read one of his papers, I'm surprised. The rubber hand experiment is his most memorable work to date, but he's done really interesting work on things synesthesia and phantom limbs as well. Today, I read a paper of his on a strange syndrome that affects some people who have suffered strokes that caused damage in their right parietal lobes, which resulted in, among other things, left visual field neglect. The paper, titled "Visual Agnosia," describes an experiment in which the researchers placed mirrors on the right side of four patients who had suffered this type of stroke. After the patients had acknowledged that it was, in fact, a mirror that had been placed on their right side, a researcher held an object (either a pen or a piece of candy) out to them on their left side, out of reach of their right arm. The object was reflected in the mirror so that the patients could see it there. The patients were then told to "reach out and grab" the object. When they tried to do this, they reached with their right arms, not towards the object on the left, but towards the mirror. Their hands repeatedly banged into the mirror, and one patient reached behind the mirror (one even tried to get behind the mirror frame). When asked about what they were doing, the patients said things like, "the object is in the mirror," or "it's not within my reach." On other trials, the mirror was placed in front of them, and the object just behind them on the right side. Patients then had no trouble reaching back for the object.

The authors offer two explanations of "mirror agnosia." They write:
Two interpretations of mirror agnosia are possible, that are not mutually exclusive: (a) the syndrome may be a specific consequence of the neglect. it is as though the patient was saying to herself 'Since the reflection is in the mirror, the pen must be on my left. But left does not exist in my world, therefore it must be inside the mirror', however absurd this may seem to us with our intact brains, and (b) alternatively, it may not be a consequence of neglect, even though it is usually accompanied by neglet. Instead, it may be a striking manifestation of the subtle deficits in spatial abilities that occur following parietal lobe lesions. Responding correctly to a mirror image requires the creation of a rather peculari form of dual representation or 'mental diplopia', and this subtle ability may be compromised by the right parietal lesion.
Further, the authors believe that the behavior of the patients with "mirror agnosia" may provide insight into how we represent mirror images:
[T]he syndrome seems to reveal a 'default' localization of the object to the space behind the mirror, the naive visual perception overriding the cognitive knowledge of mirror reflection. Indeed, the most surprising aspect of the syndrome is not just the patient's reluctance to reach to the left, but her repeated attempts to go behind the mirror and her outlandish remarks, 'It is behind the mirror' or 'Inside the miror'--i.e. in spite of being mentally lucid the patient is apparently unable to even intellectually deduce that she should search for the object on the left, and that it is pointless to reach for the reflection in the mirror. (Such confusion ordinarily occurs only in animals and very young babies.) Even the patient's belief systems and ability to reason intellectual about such matters have become selectively distorted to accommodate the strange-looking glass world in which they now find themselves trapped. (emphasis, and inexplicable switch from the singular to the plural, in the original)
The experiment has apparently been replicated by Binkofski et al., who also discovered a syndrom they call "mirror ataxia," in which people eventually learn to reach for the object on the neglected side, but do so ineffectively.