Biomedical subjects
V S Ramachandran
Publications and source records attributed to V S Ramachandran.
Visual attention modulates metacontrast masking.
How does the human visual system 'bind' different fragments in the visual scene to create enduring representations of objects? A visual illusion known as 'metacontrast' or backward masking provides compelling evidence that perception is not instantaneous and that it occurs sequentially in distinct stages. If a solid white target square is displayed for 50 ms in a tachistoscope, switched off, and followed by a 50 ms display of two flanking mask squares, remarkably, subjects report seeing only the two flanking squares: the first square is simply not 'seen'. By plotting the magnitude of masking as a function of the delay between the target and mask (the stimulus onset asynchrony), one can obtain a characteristic 'U'-shaped function with optimum masking occurring at about 50 ms, and no masking with synchronous target and mask presentations or at delays higher than 300 ms. The illusion is also highly sensitive to elementary stimulus dimensions such as colour, orientation and spatial frequency, and it has been suggested that it is based on 'low level' autonomous visual mechanisms rather than cognitive processes. Here we describe a novel visual stimulus that demonstrates that metacontrast can be strongly modulated by 'top down' influence such as voluntary visual attention.
Anosognosia in parietal lobe syndrome.
Patients with right parietal lesions often deny their paralysis (anosognosia), but do they have "tacit" knowledge of their paralysis? I devised three novel tests to explore this. First, the patients were given a choice between a bimanual task (e.g., tying shoe laces) vs a unimanual one (e.g., threading a bolt). They chose the former on 17 of 18 trials and, surprisingly, showed no frustration or learning despite repeated failed attempts. I conclude that they have no tacit knowledge of paralysis (or, if such knowledge exists, it is not available for this particular task). Second, I used a "virtual reality box" to convey the optical illusion to the patient that she was moving her paralyzed left hand up and down to the rhythm of a metronome, and yet she showed no sign of surprise. Third, I irrigated patient BM's left ear canal with cold water, a procedure that is known to shift that patient's spatial frame of reference by stimulating the vestibular system. Surprisingly, this allowed her "repressed" memory of the paralysis to come to the surface; she said she had been paralyzed continuously for several days. I suggest that the vestibular stimulation produces these remarkable effects by mimicking REM sleep. These patients also employ a whole arsenal of grossly exaggerated Freudian "defense mechanisms" to account for their paralysis. To explain this, I propose that in normal individuals the left hemisphere ordinarily deals with small, local anomalies by trying to impose consistency but, when the anomaly exceeds threshold, an interaction with the right hemisphere forces a "paradigm shift." A failure of this process, in patients with right hemisphere damage, might partially account for anosognosia. Finally, I present a new conceptual framework that may help link several psychological and neurological phenomena such as Freudian defense mechanisms, vestibular stimulation, anosognosia, memory repression, visual illusions, anterograde amnesia, REM sleep, dreaming, and humor.
Monocular double vision in strabismus.
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The neural locus of binocular rivalry and monocular diplopia in intermittent exotropes.
Patients with intermittent exotropia (strabismus) can either fixate normally or allow one eye to deviate outward by as much as 60 degrees. Two such patients (D.N. and K.C.) were studied and it was found that during eye deviation, binocular correspondence is maintained by completely 'remapping' egocentric space for the deviating eye alone using extraretinal signals from that eye. Also, by using foveal afterimages we showed that binocular rivalry occurs at a site earlier than this egocentric remapping, probably in area 17 itself. And finally, consistent with the neural remapping hypothesis, patient K.C. also experienced monocular diplopia; objects appeared double when viewed with the deviating eye.
Sensory maps in the human brain.
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Noninvasive detection of cerebral plasticity in adult human somatosensory cortex.
After upper limb deafferentation, adult macaques exhibit substantial reorganization of cortical somatosensory topography with enlargement of cortical areas responsive to facial stimuli. In the present study non-invasive magnetic source imaging technology has been used to map in detail the bilateral somatosensory homunculi in four neurologically normal controls and two upper arm amputees. Bilateral homuncular maps of normals and of the unaffected hemisphere of both amputees showed a wide hand area. The affected hemisphere of both amputees showed marked intrusion of facial representations into the digit and hand area consistent with the earlier observations in macaques. Our findings provide the first evidence of massive somatosensory plasticity in human adults and suggest a mechanism for post-amputation perceptual changes.
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.
Phantom limbs, neglect syndromes, repressed memories, and Freudian psychology.
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Behavioral and magnetoencephalographic correlates of plasticity in the adult human brain.
Recent behavioral and physiological evidence suggests that even brief sensory deprivation can lead to the rapid emergence of new and functionally effective neural connections in the adult human brain.
Perceptual correlates of massive cortical reorganization.
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Perceptual correlates of massive cortical reorganization.
Following long-term deafferentation of one upper limb in adult primates, the cortical areas corresponding to that limb become responsive to stimuli applied to the face. To explore this phenomenon, we studied some patients after upper limb amputation. In patient VQ, stimuli applied to the lower face or 7 cm above the stump evoked precisely localized referred sensations in individual digits which were often modality specific. Similarly, in another patient, WK several complete somatotopic representations of the phantom limb were found, on the face, chest and axilla, indicating the emergence of such maps in regions remote from the stump. These effects may be a direct perceptual correlate of the physiological observations of Merzenick et al (1984), Wall (1977) and Pons et al (1991).
On the perception of shape from shading.
The extraction of three-dimensional shape from shading is one of the most perceptually compelling, yet poorly understood, aspects of visual perception. In this paper, we report several new experiments on the manner in which the perception of shape from shading interacts with other visual processes such as perceptual grouping, preattentive search ("pop-out"), and motion perception. Our specific findings are as follows: (1) The extraction of shape from shading information incorporates at least two "assumptions" or constraints--first, that there is a single light source illuminating the whole scene, and second, that the light is shining from "above" in relation to retinal coordinates. (2) Tokens defined by shading can serve as a basis for perceptual grouping and segregation. (3) Reaction time for detecting a single convex shape does not increase with the number of items in the display. This "pop-out" effect must be based on shading rather than on differences in luminance polarity, since neither left-right differences nor step changes in luminance resulted in pop-out. (4) When the subjects were experienced, there were no search asymmetries for convex as opposed to concave tokens, but when the subjects were naive, cavities were much easier to detect than convex shapes. (5) The extraction of shape from shading can also provide an input to motion perception. And finally, (6) the assumption of "overhead illumination" that leads to perceptual grouping depends primarily on retinal rather than on "phenomenal" or gravitational coordinates. Taken collectively, these findings imply that the extraction of shape from shading is an "early" visual process that occurs prior to perceptual grouping, motion perception, and vestibular (as well as "cognitive") correction for head tilt. Hence, there may be neural elements very early in visual processing that are specialized for the extraction of shape from shading.
Perceptual filling in of artificially induced scotomas in human vision.
Patients with scotomas or blind-spots in their visual field resulting from damage to the visual pathways often report that the pattern from the rest of the visual field 'fills in' to occupy the scotoma. Here we describe a novel technique for generating an artificial perceptual scotoma which enabled us to study the spatial and temporal characteristics of this filling-in process. A homogeneous grey square subtending 1.5 degrees was displayed against a background of twinkling two-dimensional noise of equal mean luminance. On steady eccentric fixation for 10 s the square vanished and was filled in by the twinkling noise from the surround. Using this display we found that 'filling in' is an active visual process that probably involves creating an actual neural representation of the surround rather than merely ignoring the absence of information from the scotoma; filling in can occur separately for colour and texture, suggesting separate mechanisms; the filling-in process does not completely suppress information from the scotoma, even after an image has faded completely from consciousness it can nevertheless contribute to motion perception; and the process can be strongly influenced by illusory contours.
Form, motion, and binocular rivalry.
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Transparency and coherence in human motion perception.
When confronted with moving images, the visual system often must decide whether the motion signals arise from a single object or from multiple objects. A special case of this problem arises when two independently moving gratings are superimposed. The gratings tend to cohere and move unambiguously in a single direction (pattern motion) instead of moving independently (component motion). Here we report that the tendency to see pattern motion depends very strongly on the luminance of the intersections (that is, to regions where the gratings overlap) relative to that of the gratings in a way that closely parallels the physics of transparency. When the luminance of these regions is chosen appropriately, pattern motion is destroyed and replaced by the appearance of two transparent gratings moving independently. The observations imply that motion detecting mechanisms in the visual system must have access to tacit 'knowledge' of the physics of transparency and that this knowledge can be used to segment the scene into different objects. The same knowledge could, in principle, be used to avoid confusing shadows with real object boundaries.