Tactile Toe Agnosia and Percept of a "Missing Toe" in Healthy Humans.

A disturbance of body representation is central to many neurological and psychiatric conditions, but the mechanisms by which body representations are constructed by the brain are not fully understood. We demonstrate a directional disturbance in tactile identification of the toes in healthy humans. Nineteen young adult participants underwent tactile stimulation of the digits with the eyes closed and verbally reported the identity of the stimulated digit. In the majority of individuals, responses to the second and third toes were significantly biased toward the laterally neighboring digit. The directional bias was greater for the nondominant foot and was affected by the identity of the immediately preceding stimulated toe. Unexpectedly, 9/19 participants reported the subjective experience of a "missing toe" or "missing space" during the protocol. These findings challenge current models of somatosensory localization, as they cannot be explained simply by a lack of distinct representations for toes compared with fingers, or by overt toe-finger correspondences. We present a novel theory of equal spatial representations of digit width combined with a "preceding neighbor" effect to explain the observed phenomena. The diagnostic implications for neurological disorders that involve "digit agnosia" are discussed.

Reward modulates the effect of visual cortical microstimulation on perceptual decisions.

Effective perceptual decisions rely upon combining sensory information with knowledge of the rewards available for different choices. However, it is not known where reward signals interact with the multiple stages of the perceptual decision-making pathway and by what mechanisms this may occur. We combined electrical microstimulation of functionally specific groups of neurons in visual area V5/MT with performance-contingent reward manipulation, while monkeys performed a visual discrimination task. Microstimulation was less effective in shifting perceptual choices towards the stimulus preferences of the stimulated neurons when available reward was larger. Psychophysical control experiments showed this result was not explained by a selective change in response strategy on microstimulated trials. A bounded accumulation decision model, applied to analyse behavioural performance, revealed that the interaction of expected reward with microstimulation can be explained if expected reward modulates a sensory representation stage of perceptual decision-making, in addition to the better-known effects at the integration stage.

Playing the electric light orchestra--how electrical stimulation of visual cortex elucidates the neural basis of perception.

Vision research has the potential to reveal fundamental mechanisms underlying sensory experience. Causal experimental approaches, such as electrical microstimulation, provide a unique opportunity to test the direct contributions of visual cortical neurons to perception and behaviour. But in spite of their importance, causal methods constitute a minority of the experiments used to investigate the visual cortex to date. We reconsider the function and organization of visual cortex according to results obtained from stimulation techniques, with a special emphasis on electrical stimulation of small groups of cells in awake subjects who can report their visual experience. We compare findings from humans and monkeys, striate and extrastriate cortex, and superficial versus deep cortical layers, and identify a number of revealing gaps in the 'causal map' of visual cortex. Integrating results from different methods and species, we provide a critical overview of the ways in which causal approaches have been used to further our understanding of circuitry, plasticity and information integration in visual cortex. Electrical stimulation not only elucidates the contributions of different visual areas to perception, but also contributes to our understanding of neuronal mechanisms underlying memory, attention and decision-making.

Body image among eating disorder patients with disabilities: a review of published case studies.

While individual cases of eating disorder (ED) patients with disabilities have been reported, there has been little synthesis of their experiences of body image and thin idealization. This study reviews 19 published clinical reports of ED patients with sensory, mobility-related, or intellectual disabilities and evaluates the extent to which their experiences align with or challenge current conceptions of body image in ED. ED patients with visual impairment reported a profound disturbance of body image, perceived intersubjectively and through tactile sensations. Reducing dependence in mobility was an important motivation to control body size for ED patients with mobility-related disabilities. ED as a way of coping with and compensating for the psychosocial consequences of disability was a recurrent theme for patients across a range of disabilities. These experiential accounts of ED patients with disabilities broaden current understandings of body image to include touch and kinaesthetic awareness, intersubjective dynamics, and perceptions of normalcy.

Localization of MEG human brain responses to retinotopic visual stimuli with contrasting source reconstruction approaches.

Magnetoencephalography (MEG) allows the physiological recording of human brain activity at high temporal resolution. However, spatial localization of the source of the MEG signal is an ill-posed problem as the signal alone cannot constrain a unique solution and additional prior assumptions must be enforced. An adequate source reconstruction method for investigating the human visual system should place the sources of early visual activity in known locations in the occipital cortex. We localized sources of retinotopic MEG signals from the human brain with contrasting reconstruction approaches (minimum norm, multiple sparse priors, and beamformer) and compared these to the visual retinotopic map obtained with fMRI in the same individuals. When reconstructing brain responses to visual stimuli that differed by angular position, we found reliable localization to the appropriate retinotopic visual field quadrant by a minimum norm approach and by beamforming. Retinotopic map eccentricity in accordance with the fMRI map could not consistently be localized using an annular stimulus with any reconstruction method, but confining eccentricity stimuli to one visual field quadrant resulted in significant improvement with the minimum norm. These results inform the application of source analysis approaches for future MEG studies of the visual system, and indicate some current limits on localization accuracy of MEG signals.

A causal role for V5/MT neurons coding motion-disparity conjunctions in resolving perceptual ambiguity.

Judgments about the perceptual appearance of visual objects require the combination of multiple parameters, like location, direction, color, speed, and depth. Our understanding of perceptual judgments has been greatly informed by studies of ambiguous figures, which take on different appearances depending upon the brain state of the observer. Here we probe the neural mechanisms hypothesized as responsible for judging the apparent direction of rotation of ambiguous structure from motion (SFM) stimuli. Resolving the rotation direction of SFM cylinders requires the conjoint decoding of direction of motion and binocular depth signals [1, 2]. Within cortical visual area V5/MT of two macaque monkeys, we applied electrical stimulation at sites with consistent multiunit tuning to combinations of binocular depth and direction of motion, while the monkey made perceptual decisions about the rotation of SFM stimuli. For both ambiguous and unambiguous SFM figures, rotation judgments shifted as if we had added a specific conjunction of disparity and motion signals to the stimulus elements. This is the first causal demonstration that the activity of neurons in V5/MT contributes directly to the perception of SFM stimuli and by implication to decoding the specific conjunction of disparity and motion, the two different visual cues whose combination drives the perceptual judgment.