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Biomedical subjects

D Rogers-Ramachandran

Publications and source records attributed to D Rogers-Ramachandran.

7 recordsLinked to original sources
PubMed1996

Synaesthesia in phantom limbs induced with mirrors.

Although there is a vast clinical literature on phantom limbs, there have been no experimental studies on the effects of visual input on phantom sensations. We introduce an inexpensive new device--a 'virtual reality box'--to resurrect the phantom visually to study inter-sensory effects. A mirror is placed vertically on the table so that the mirror reflection of the patient's intact had is 'superimposed' on the felt position of the phantom. We used this procedure on ten patients and found the following results. 1. In six patients, when the normal hand was moved, so that the phantom was perceived to move in the mirror, it was also felt to move; i.e. kinesthetic sensations emerged in the phantom. In D.S. this effect occurred even though he had never experienced any movements in the phantom for ten years before we tested him. He found the return of sensations very enjoyable. 2. Repeated practice led to a permanent 'disappearance' of the phantom arm in patient D.S. and the hand became telescoped into the stump near the shoulder. 3. Using an optical trick, impossible postures--e.g. extreme hyperextension of the fingers--could be induced visually in the phantom. In one case this was felt as a transient 'painful tug' in the phantom. 4. Five patients experienced involuntary painful 'clenching spasms' in the phantom hand and in four of them the spasms were relieved when the mirror was used to facilitate 'opening' of the phantom hand; opening was not possible without the mirror. 5. In three patients, touching the normal hand evoked precisely localized touch sensations in the phantom. Interestingly, the referral was especially pronounced when the patients actually 'saw' their phantom being touched in the mirror. Indeed, in a fourth patient (R.L.) the referral occurred only if he saw his phantom being touched: a curious form of synaesthesia. These experiments lend themselves readily to imaging studies using PET and fMRI. Taken collectively, they suggest that there is a considerable amount of latent plasticity even in the adult human brain. For example, precisely organized new pathways, bridging the two cerebral hemispheres, can emerge in less than three weeks. Furthermore, there must be a great deal of back and forth interaction between vision and touch, so that the strictly modular, hierarchical model of the brain that is currently in vogue needs to be replaced with a more dynamic, interactive model, in which 're-entrant' signalling plays the main role.

Adult↗
PubMed1996

Rapid adaptive camouflage in tropical flounders.

Despite the commonly held view that flatfish can change their surface markings to match their background pattern, there have been few systematic studies and it has recently been claimed that their capacity for such adaptive changes is minimal. Here we show that the tropical flatfish Bothus ocellatus can achieve pattern-matching with surprising fidelity. By adjusting the contrast of different sets of 'splotches' of different grain size (or spatial frequency) on the skin, the fish can blend into a wide range of background textures in just 2-8 seconds.

Adaptation, Physiological↗
PubMed1994

On the perception of illusory contours.

Illusory contours are invoked by the visual system to account for otherwise inexplicable gaps in the image. We report three sets of novel observations on illusory contours. First, when an illusory square is superimposed on a checkerboard pattern there is a considerable enhancement of the contours so long as they are exactly coincident with the borders of the checks. If the checks are misaligned, on the other hand, the illusory contours associated with the pacman edges disappear and a novel percept emerges: the contours of the checks nearest to the illusory square appear enhanced. This result implies that subjective contours are generated by intermediate-level contour interactions rather than the top-down processes of three-dimensional interpretation. Second, we find that steady fixation for as little as 4 sec leads to a complete disappearance of the enhanced illusory contours caused, presumably, by adaptation or "fatigue" of cells that signal these contours. Such adaptation occurred even when the illusory contours were rendered invisible by displaying them on a misaligned checkerboard, suggesting that the adaptation occurs prior to the vetoing of the signal by the checks. Third, we found that illusory contours persist for a surprisingly long time (0.3 sec) after the inducing elements have been switched off. These results suggest that the stimuli we have designed ("enhanced illusory contours") might provide a novel probe for dissecting different stages involved in the processing of illusory contours and for understanding how the visual system combines different types of contours to construct object boundaries.

Adaptation, Ocular↗