The half of the story we did not know about prism adaptation.

Since 1910 (Helmholtz, treatise on physiological optics), it is known that pointing under deviating prisms induces an initial error in the direction of the deviation, immediately followed by a gradual correction of the error, and an after effect (AE) in the opposite direction after prisms removal, the hallmark of prisms adaptation (PA). Several sensorimotor effects are also produced by PA on proprioceptive, visual and visuo-proprioceptive frames of reference, the latter being called total aftereffect shift (TS) of prism adaptation. Yet, after more than one century, we face a puzzling result: while pointing under prisms exposure, people rapidly achieve an optimal performance and reduce their error by 100%. Invariably, though, when AE is measured (TS) people only show at best 50% of the induced optical deviation, as if the other half was lost somewhere. Here we show that the other half of prism adaptation AE is not lost, and actually emerges clearly and consistently across several experiments when assessing for a so far largely neglected component: the shift induced at the level of the adapted hand. Here we report that this effect is robust and highly specific and we suggest calling it hand-centred aftereffect. These findings reveal that, in PA processes, beside visual and proprioceptive frame of reference, also hand centred ones are involved. Consistently with this view, taking into account the hand aftereffect, the total amount of the aftereffect reaches 76-to-94%, depending on the measure and experiment, thus explaining the largest part of optical shift, previously unnoticed. We suggest this novel aspect of PA would be considered in future clinical studies in relation with responder/non-responder patients' profile to inform integrated models of PA that might allow for optimising patient-tailored PA procedures.

Stress Assignment Errors in Surface Dyslexia: Evidence from Two Italian Patients with a Selective Deficit of the Orthographic Input Lexicon.

Surface dyslexia designates a selective impairment in reading irregular words, with spared ability to read regular and novel words, following a cerebral damage usually located in the left dominant hemisphere. In Italian language, which is regular at the segmental level, surface dyslexia is characterized by stress assignment errors. Here we report on two cases of Italian surface dyslexic patients who produced stress assignment errors, mainly in reading irregular words. In reading nonwords they usually applied the regular stress pattern. Both patients were also impaired in lexical decision and in semantic discrimination tasks when the processing of homophones was required. Our patients' performance relied almost exclusively on the phonological coding of the stimulus, revealing a deficit in accessing the orthographical input lexicon. In addition, one patient showed a cerebral lesion limited to the right thalamus, providing evidence of a possible role of the right hemisphere in the reading process.

Far Space Remapping by Tool Use: A rTMS Study Over the Right Posterior Parietal Cortex.

BACKGROUND: In previous studies, rTMS has been successfully employed to interfere with the right posterior parietal cortex (rPPC) inducing neglect-like behavior in healthy subjects. Several studies have shown that the use of tools can modulate the boundaries between near and far space: indeed when far space is reached by the stick, far space can be remapped as near. OBJECTIVE: The aim of the present study was to investigate whether once that rTMS on the rPPC has selectively induced neglect-like bias in the near space (but not in the far space), neglect can appears also in the far space when the subjects used a tool to perform the task. METHODS: Fifteen right-handed healthy subjects executed a line length judgment task in two different spatial positions (60 cm: near space and 120 cm: far space), with or without rPPC on-line rTMS. In the far space condition, subjects performed the perceptual task while holding or not a tool. RESULTS: During rTMS, visuospatial performance significantly shifted toward right when the task was performed in the near space and in the far space when the tool was used. No significant effect was found when rTMS was delivered in the far space condition without tool use. CONCLUSIONS: Our results demonstrate that the application of rTMS on rPPC, specifically affect the representation of near space because it caused neglect both when the subjects acted in the near space and when they acted in a far space that was remapped as near by the use of a tool.

Are movements necessary for the sense of body ownership? Evidence from the rubber hand illusion in pure hemiplegic patients.

A question still debated within cognitive neuroscience is whether signals present during actions significantly contribute to the emergence of human's body ownership. In the present study, we aimed at answer this question by means of a neuropsychological approach. We administered the classical rubber hand illusion paradigm to a group of healthy participants and to a group of neurological patients affected by a complete left upper limb hemiplegia, but without any propriceptive/tactile deficits. The illusion strength was measured both subjectively (i.e., by a self-report questionnaire) and behaviorally (i.e., the location of one's own hand is shifted towards the rubber hand). We aimed at examining whether, and to which extent, an enduring absence of movements related signals affects body ownership. Our results showed that patients displayed, respect to healthy participants, stronger illusory effects when the left (affected) hand was stimulated and no effects when the right (unaffected) hand was stimulated. In other words, hemiplegics had a weaker/more flexible sense of body ownership for the affected hand, but an enhanced/more rigid one for the healthy hand. Possible interpretations of such asymmetrical distribution of body ownership, as well as limits of our results, are discussed. Broadly speaking, our findings suggest that the alteration of the normal flow of signals present during movements impacts on human's body ownership. This in turn, means that movements have a role per se in developing and maintaining a coherent body ownership.

Long-lasting amelioration of walking trajectory in neglect after prismatic adaptation.

In the present study we explored the effect of prismatic adaptation (PA) applied to the upper right limb on the walking trajectory of a neglect patient with more severe neglect in far than in near space. The patient was asked to bisect a line fixed to the floor by walking across it before and after four sessions of PA distributed over a time frame of 67 days. Gait path was analyzed by means of an optoelectronic motion analysis system. The walking trajectory improved following PA and the result was maintained at follow-up, 15 months after treatment. The improvement was greater for the predicted bisection error (estimated on the basis of the trajectory extrapolated from the first walking step) than for the observed bisection error (measured at line bisection). These results show that PA may act on high level spatial representation of gait trajectory rather than on lower level sensory-motor gait components and suggest that PA may have a long-lasting rehabilitative effect on neglect patients showing a deviated walking trajectory.

Functional independence between numerical and visual space: evidence from right brain-damaged patients.

What is the relationship between numerical and visual space? Here we tried to shed new light on this debated issue investigating whether and how the two forms of representation are associated or dissociated when co-activated. We carried out a series of visual-numerical bisection experiments on a large group of right brain-damaged patients (N=32) with and without left neglect. We examined (a) the degree of association between the pathological rightward error in the bisection of numerical intervals and left neglect (experiment 1); (b) if the size of the numerical interval modulates spatial errors in bisection tasks in which numerical and visual space representations are co-activated (experiment 2). The results showed that (a) numerical bisection error and left spatial neglect are doubly dissociated and that, when both are present, they are not correlated; (b) the size of the numerical interval did not affect the spatial bisection error but influenced the numerical bisection error. These data suggest that attentional processes involved in the navigation along visual space and numerical internal representations are independent neurocognitive operations. We must emphasize that our findings should be taken with caution because they are based mainly on negative results.

Object-centred pseudoneglect for non-verbal visual stimuli.

The rightward spatial bias shown by left neglect patients and the small leftward bias displayed by healthy subjects (pseudoneglect) have been interpreted as phenomena sharing a common attentional imbalance mechanism. Here we investigated whether pseudoneglect, similarly as neglect, can occur in an object-centred frame of reference. Thirty healthy participants repeatedly bisected the elongated caricature of a basset hound with the head on the left and the tail on the right or viceversa. In the last critical trials, the figure appeared horizontally mirrored. The bisection error reversed from the left to the right space in the critical trials. This result shows that it is possible to induce object-centred pseudoneglect on newly established knowledge about the canonical orientation of non-verbal visual stimuli.

Losing one's hand: visual-proprioceptive conflict affects touch perception.

BACKGROUND: While the sense of bodily ownership has now been widely investigated through the rubber hand illusion (RHI), very little is known about the sense of disownership. It has been hypothesized that the RHI also affects the ownership feelings towards the participant's own hand, as if the rubber hand replaced the participant's actual hand. Somatosensory changes observed in the participants' hand while experiencing the RHI have been taken as evidence for disownership of their real hand. Here we propose a theoretical framework to disambiguate whether such somatosensory changes are to be ascribed to the disownership of the real hand or rather to the anomalous visuo-proprioceptive conflict experienced by the participant during the RHI. METHODOLOGY/PRINCIPAL FINDINGS: In experiment 1, reaction times (RTs) to tactile stimuli delivered to the participants' hand slowed down following the establishment of the RHI. In experiment 2, the misalignment of visual and proprioceptive inputs was obtained via prismatic displacement, a situation in which ownership of the seen hand was doubtless. This condition slowed down the participants' tactile RTs. Thus, similar effects on touch perception emerged following RHI and prismatic displacement. Both manipulations also induced a proprioceptive drift, toward the fake hand in the first experiment and toward the visual position of the participants' hand in the second experiment. CONCLUSIONS/SIGNIFICANCE: These findings reveal that somatosensory alterations in the experimental hand resulting from the RHI result from cross-modal mismatch between the seen and felt position of the hand. As such, they are not necessarily a signature of disownership.

Mental number line disruption in a right-neglect patient after a left-hemisphere stroke.

A right-neglect patient with focal left-hemisphere damage to the posterior superior parietal lobe was assessed for numerical knowledge and tested on the bisection of numerical intervals and visual lines. The semantic and verbal knowledge of numbers was preserved, whereas the performance in numerical tasks that strongly emphasize the visuo-spatial layout of numbers (e.g. number bisection) was impaired. The behavioral pattern of error in the two bisection tasks mirrored the one previously described in left-neglect patients. In other words, our patient misplaced the subjective midpoint (numerical or visual) to the left as function of the interval size. These data, paired with the patient's lesion site are strictly consistent with the tripartite organization of number-related processes in the parietal lobes as proposed by Dehaene and colleagues. According to these authors, the posterior superior parietal lobe on both hemispheres underpins the attentional orientation on the putative mental number line, the horizontal segment of the intraparietal sulcus is bilaterally related to the semantic of the numerical domain, whereas the left angular gyrus subserves the verbal knowledge of numbers. In summary, our results suggest that the processes involved in the navigation along the mental number line, which are related to the parietal mechanisms for spatial attention, and the processes involved in the semantic and verbal knowledge of numbers, are dissociable.

Are drawing perseverations part of the neglect syndrome?

Unilateral neglect patients typically omit to cancel contralesional targets. Moreover, they can repeatedly cancel ipsilesional stimuli exhibiting what is termed 'perseverative behavior'. Two alternative accounts of this behavior have been proposed. According to one of them, it is considered as integral to neglect and due either to a perceptual (allochiria), or a premotor (directional hypokinesia) pathological mechanism leading to the ipsilesional displacement of contralesional responses. According to the other one, perseverations are interpreted as the consequence of motor-control-disinhibition co-occurring with, although independent of, spatial neglect. We compared some crucial predictions of these two hypotheses on a group of 10 right-brain-damaged patients, eight with neglect and two without neglect, showing a perseverative behavior in both conventional and experimental cancellation tasks. In our experiment, the spatial location and the numerosity of targets were manipulated to obtain different degrees of horizontal alignment between targets on the left and on the right of the central vertical axis of the sheet. We found that ipsilesional perseverations were not influenced by left neglected targets and were not correlated to neglect severity. Additionally, perseverative errors were associated with right basal ganglia lesions rather than with presence of neglect. These findings support the view that two different pathological mechanisms might be involved in left spatial neglect and ipsilesional perseverative behavior.

Modular structure of awareness for sensorimotor disorders: evidence from anosognosia for hemiplegia and anosognosia for hemianaesthesia.

In the present paper, we shall review clinical evidence and theoretical models related to anosognosia for sensorimotor impairments that may help in understanding the normal processing underlying conscious self-awareness. The dissociations between anosognosia for hemiplegia and anosognosia for hemianaesthesia are considered to give important clinical evidence supporting the hypothesis that awareness of sensory and motor deficits depends on the functioning of discrete self-monitoring processes. We shall also present clinical and anatomical data on four single case reports of patients selectively affected by anosognosia for hemianaesthesia. The differences in the anatomical localization of lesions causing anosognosia for hemiplegia and anosognosia for hemianaesthesia are taken as evidence that cerebral circuits subserving these monitoring processes are located in separate brain areas, which may be involved both in the execution of primary functions and the emergence of awareness related to the monitoring of the same functions. The implications of these findings for the structure of conscious processes shall be also discussed.

Bisecting lines with different tools in right brain damaged patients: the role of action programming and sensory feedback in modulating spatial remapping.

In a first experiment we studied, through a line bisection task, (a) the frequency of the selective disruption of far or near space representations in a group of 28 right brain-damaged patients and (b) the effect of tool use on line bisection error in far and near space in order to clarify whether the kind of action performed by the subject influences the extension of space representation, as suggested by previous studies. In a second experiment, carried out on two neglect patients, we asked whether the representation of "near" and "far" space depends on the sensory feedback during the execution of the action or whether it is independent on sensory feedback and more related to the action programmed as a consequence of the kind of tool used. Our data show (a) that dissociations between far and near space neglect are a frequent observation in right brain damaged patients and that most of these patients are able to recode space representations when tools change the spatial relation between the agent's body and the target object; (b) that spatial remapping can be elicited by the kind of action associated to the tool used and by the sensory feedback (either visual or proprioceptive) available during the execution of the task. In particular, presence of tactile proprioceptive feedback elicited remapping of far space into near space, whereas absence of visual feedback induced remapping of near space into far space.

Spatial representations, distortions and alterations in the graphic and artistic production of brain-damaged patients and of famous artists.

The authors review the drawing disorders that can be observed in patients with brain damage (in particular, those with constructional apraxia and unilateral spatial neglect), the impact of brain damage on the output of professional artists, and the distortions of body image that characterise the work of many great artists, from Rubens to Lucien Freud. These different perspectives share the basic assumption that the graphic and artistic output of patients, normal subjects, and exceptionally gifted individuals may represent a window onto the neural organisation of body image.