Marco Neppi-Modona

The anatomy of anosognosia for hemiplegia: a meta-analysis.

Anosognosia for hemiplegia is the denial of the contralesional motor deficits that may follow brain damage. Although this disturbance has been reported in the neurological literature since the beginning of the last century, only few longitudinal studies have addressed the issue of the anatomical substrate of the disorder. Here we present a comprehensive review of the literature on anosognosia for hemiplegia from 1938 to 2001, taking into account some of its clinical, epidemiological and anatomical aspects. In particular, an attempt has been made to identify the intra-hemispheric lesion locations most frequently associated to the denial behaviour. Our review shows that anosognosia for hemiplegia most frequently occurs in association to unilateral right-sided or bilateral lesions of different brain areas (cortical and/or subcortical). It seems to be equally frequent when the damage is confined to frontal, parietal or temporal cortical structures, and may also emerge as a consequence of subcortical lesions. Interestingly, the probability of occurrence of anosognosia is highest when the lesion involves parietal and frontal structures in combination, if compared to other combinations of lesioned areas. This pattern of lesions suggests the existence of a complex cortico-subcortical circuit underlying awareness of motor acts that, if damaged, can give raise to the anosognosic symptoms.

Anisometry of space representation in unilateral neglect: empirical test of a former hypothesis.

When left-neglect patients are required to extend horizontal segments to double their original length, relative left overextension is frequently observed. Less frequently, relative left underextension may also be found. It was hypothesized that this contrast could depend on the degree of horizontal anisometry of the medium for the representation of spatial properties. The present paper reports an experiment conducted in order to test that hypothesis, on the basis of which left overextension should be larger with shorter than with longer segments and with segments lying in the right rather than in the left hemispace. Although supportive, the results unveiled unexpected complications: the expected effect of line length was found only in neglect patients with frontal damage, while the expected effect of side of presentation was found only in neglect patients without frontal damage.

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.

Spatial Coding of the Predicted Impact Location of a Looming Object

Avoiding or intercepting looming objects implies a precise estimate of both time until contact and impact location. In natural situations, extrapolating a movement trajectory relative to some egocentric landmark requires taking into account variations in retinal input associated with moment-to-moment changes in body posture. Here, human observers predicted the impact location on their face of an approaching stimulus mounted on a robotic arm, while we systematically manipulated the relation between eye, head, and trunk orientation. The projected impact point on the observers face was estimated most accurately when the target originated from a location aligned with both the head and eye axes. Eccentric targets with respect to either axis resulted in a systematic perceptual bias ipsilateral to the trajectorys origin. We conclude that (1) predicting the impact point of a looming target requires combining retinal information with eye position information, (2) that this computation is accomplished accurately for some, but not all, possible combinations of these cues, (3) that the representation of looming trajectories is not formed in a single, canonical reference frame, and (4) that the observed perceptual biases could reflect an automatic adaptation for interceptive/defensive actions within near peripersonal space.

Verbal-response and manual-response versions of the Milner Landmark task: normative data.

The paper reports normative data relative to a shortened form of two versions of the Milner Landmark task, involving verbal and manual response, respectively, which have been found to provide crucial information to discriminate perceptual from response bias in unilateral neglect. Normative data based on a large group of subjects were believed to be necessary because the Landmark task is held to be worth further investigation: (a) in comparison with other tasks devised for similar purposes, (b) in elucidating clinico-anatomical correlations, and (c) in planning selective remediation programmes. The results of the investigation provide criteria on the basis of which a patients bias can be classified as being normal, borderline, or pathological.

Empathy or ownership? evidence from corticospinal excitability modulation during pain observation

Recent studies show that motor responses similar to those present in ones own pain (freezing effect) occur as a result of observation of pain in others. This finding has been interpreted as the physiological basis of empathy. Alternatively, it can represent the physiological counterpart of an embodiment phenomenon related to the sense of body ownership. We compared the empathy and the ownership hypotheses by manipulating the perspective of the observed hand model receiving pain so that it could be a first-person perspective, the one in which embodiment occurs, or a third-person perspective, the one in which we usually perceive the others. Motor-evoked potentials (MEPs) by TMS over M1 were recorded from first dorsal interosseous muscle, whereas participants observed video clips showing (a) a needle penetrating or (b) a Q-tip touching a hand model, presented either in first-person or in third-person perspective. We found that a pain-specific inhibition of MEP amplitude (a significantly greater MEP reduction in the “pain” compared with the “touch” conditions) only pertains to the first-person perspective, and it is related to the strength of the self-reported embodiment. We interpreted this corticospinal modulation according to an “affective” conception of body ownership, suggesting that the body I feel as my own is the body I care more about.

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.

Unilateral neglect and perceptual parsing: a large-group study.

Array-centred and subarray-centred neglect were disambiguated in a group of 116 patients with left neglect by means of a modified version of the Albert test in which the central column of segments was deleted so as to create two separate sets of targets grouped by proximity. The results indicated that neglect was more frequent in array- than subarray-centred coordinates and that, in a minority of cases, neglect co-occurred in both coordinate-systems. The two types of neglect were functionally but not anatomically dissociated. Presence of visual field defects was not prevalent in one type of neglect with respect to the other. These data contribute further evidence to previous single-case and small-group studies by showing that neglect can occur in single or multiple reference frames simultaneously, in agreement with current neuropsychological, neurophysiological and computational concepts of space representation.

The Oppel Kundt Illusion is effective in modulating horizontal space representation in humans

A modified version of the Oppel-Kundt illusion (i.e., a filled space is perceived as more expanded than an empty space of the same length) has been previously employed to distort space representation both in patients with neglect (i.e., failure to report/react to stimuli located in the space contralateral to the brain lesion) and in healthy participants. In those experiments, participants had to bisect or extend horizontal segments on backgrounds of exponentially spaced vertical lines. The exclusive use of visuo-motor tasks, however, did not allow excluding that the results were accounted for by a bias occurring at a response level of stimulus processing rather than by a visual illusion. To address this issue, in addition to a traditional line bisection task, a line length estimation task was employed, which allowed separating response and illusion-related factors. The results demonstrated that performance depended on the visual illusion rather than on a response bias. It was concluded that this version of the Oppel-Kundt illusion can be successfully employed to modulate space representation in humans.

Influence of Response Modality on Awareness of Contralesional Tactile Stimuli in Right Brain Damaged Patients

Thirty right brain-damaged (RBD) patients with left tactile extinction (19 of whom also showed neglect) were given a sequence of 240 tactile stimuli--80 right, 80 left, 80 bilateral--across 4 different response conditions: (a) verbal report of stimulated side/s, (b) motor report of stimulated side/s, (c) verbal report of unstimulated side/s, (d) motor report of unstimulated side/s. Earlier experiments based on similar tasks but involving RBD patients with visual extinction and/or neglect have shown that visual awareness of contralesional stimuli can be influenced by manipulation of response conditions. Since neglect and extinction can be double-dissociated both anatomically and behaviourally, the question arises of whether the underlying neuronal mechanisms are different. To answer this question the present work investigated the role of perceptual and premotor factors in generating tactile extinction in response to Double Simultaneous Stimulation (DSS). The hypothesis was that a directional response bias would result in an overall higher frequency of errors for verbal or motor responses indicating the ipsilesional side (right); a perceptual bias would instead result in errors distributed with similar frequency on the ipsilesional and the contralesional (left) side. Results showed that, in RBD patients, contralesional extinction was not influenced by response conditions (verbal/motor; report of stimulated/unstimulated side) and presence/absence of neglect. This suggests that: (1) among RBD patients, directional response biases are unlikely to play a role in extinction of tactile stimuli on DSS; (2) the mechanisms underlying extinction are, at least to some extent, different from those underlying unilateral neglect.