Biomedical subjects

F Frassinetti

Publications and source records attributed to F Frassinetti.

9 recordsLinked to original sources

PubMed2000

Direct evidence from parietal extinction of enhancement of visual attention near a visible hand.

Brain areas exist that appear to be specialized for the coding of visual space surrounding the body (peripersonal space). In marked contrast to neurons in earlier visual areas, cells have been reported in parietal and frontal lobes that effectively respond only when visual stimuli are located in spatial proximity to a particular body part (for example, face, arm or hand) [1-4]. Despite several single-cell studies, the representation of near visual space has scarcely been investigated in humans. Here we focus on the neuropsychological phenomenon of visual extinction following unilateral brain damage. Patients with this disorder may respond well to a single stimulus in either visual field; however, when two stimuli are presented concurrently, the contralesional stimulus is disregarded or poorly identified. Extinction is commonly thought to reflect a pathological bias in selective vision favoring the ipsilesional side under competitive conditions, as a result of the unilateral brain lesion [5-7]. We examined a parietally damaged patient (D.P.) to determine whether visual extinction is modulated by the position of the hands in peripersonal space. We measured the severity of visual extinction in a task which held constant visual and spatial information about stimuli, while varying the distance between hands and stimuli. We found that selection in the affected visual field was remarkably more efficient when visual events were presented in the space near the contralesional finger than far from it. However, the amelioration of extinction dissolved when hands were covered from view, implying that the effect of hand position was not mediated purely through proprioception. These findings illustrate the importance of the spatial relationship between hand position and object location for the internal construction of visual peripersonal space in humans.

Adult↗

PubMed2000

When far becomes near: remapping of space by tool use.

Far (extrapersonal) and near (peripersonal) spaces are behaviorally defined as the space outside the hand-reaching distance and the space within the hand-reaching distance. Animal and human studies have confirmed this distinction, showing that space is not homogeneously represented in the brain. In this paper we demonstrate that the coding of space as "far" and "near" is not only determined by the hand-reaching distance, but it is also dependent on how the brain represents the extension of the body space. We will show that when the cerebral representation of body space is extended to include objects or tools used by the subject, space previously mapped as far can be remapped as near. Patient P.P., after a right hemisphere stroke, showed a dissociation between near and far spaces in the manifestation of neglect. Indeed, in a line bisection task, neglect was apparent in near space, but not in far space when bisection in the far space was performed with a projection lightpen. However, when in the far space bisection was performed with a stick, used by the patient to reach the line, neglect appeared and was as severe as neglect in the near space. An artificial extension of the patient's body (the stick) caused a remapping of far space as near space.

Aged↗

PubMed1999

Selective horizontal dysmetropsia following prestriate lesion.

We describe a patient (P.S.) who, following a right prestriate lesion, reported that objects in the left visual field appeared distorted and smaller than those on the right. Other aspects of visual processing were remarkably unaffected. We carried out a series of size comparison tests using simple or complex stimuli and requiring different types of behavioural responses. We found that P.S. significantly underestimated the size of stimuli presented in her left visual field. When comparison tasks involved stimuli placed along the vertical axis or in the right visual field, P.S. performed well. The vertical and horizontal components of size distortion were found to be differentially affected. We conclude that size processing may be dissociated from other aspects of visual processing, such as form or colour processing, and depends critically on part of the occipital, prestriate areas (Brodmann areas 18-19).

Aged↗

PubMed1998

Visual extinction as a spatio-temporal disorder of selective attention.

Studies of normal behaviour have shown that the process of selection takes a finite time, one measure of which is the attentional dwell time, that is the period of interference produced by one attended stimulus on a subsequent one. Here we investigated the time for selection in FB, a neurological patient suffering from a visuospatial disorder of attention (unilateral extinction). FB was asked to identify two letters displayed in rapid succession either to the left (damaged), or to the right (intact) visual hemifield. By varying the interval between stimuli, we measured how long the first letter continued to interfere with accuracy on the second- that is the first letter's attentional demand over time. The results showed that the process of selection has an abnormal duration in the affected visual field, being at least twice as long as in the intact field. We suggest that the slowed visual processing for the contralesional object may contribute to the competitive bias against that object which is the hallmark of unilateral extinction.

Aged↗

PubMed1997

Spatial extinction on double asynchronous stimulation.

Despite the fact that visual extinction is widely considered a space-based disturbance of selective attention, there has been little theoretical consensus about the nature of its pathogenic mechanism. A specific disruption in the ability to disengage attention from ipsilesional stimuli, or a loss of weight with which contralesional objects compete for visual selection, have been hypothesized to account for the disorder. We tested the merits of these two explanations in a right-hemisphere-lesioned patient, FB, who failed to recognize a contralesional target only when it was shown concurrently to an ipsilesional target (i.e. visual extinction). His task was to report two target letters presented in rapid succession to the left and right of the fixation point. The order of stimulus presentation (Left-First vs Right-First), and the intertarget interval (stimulus onset asynchrony) were varied systematically. We showed that contralesional extinction may occur for successively presented targets, not just for stimuli displayed at the same time. Of most importance, FB was seriously and equally impaired in dealing with a contralesional stimulus when this either preceded the ipsilesional stimulus or followed it by an interval less than about 600 msec. The data appear to contradict the disengagement hypothesis, which predicted a substantial reduction of extinction when a stimulus was displayed first into the lesioned side of space. We suggest that a competitive model of visual selective attention fits the data quite well.

Aged↗

PubMed1996

Time perception in a neglected space.

We have studied the distortion of perceived time in a patient with left neglect. This patient consistently over-estimated the duration of stimuli in the neglected space. Overestimation was observed both with an interval comparison (300/700 ms) and with a time production (1 s) paradigm. We suggest that encoding duration in the hundreds of milliseconds range is a process based on an internal clock mechanism. The functioning of that clock varies as a function of the processing load.

Attention↗

PubMed1995

Coordinate frames for naming misoriented chimerics: a case study of visuo-spatial neglect.

A persistent line of inquiry for the students of visuo-spatial neglect has involved the perceptual frame of reference respect to which the neglected region of space is defined. On standard testing conditions viewer-centered and object-centered systems of coordinates are confounded. In order to disambiguate these two reference frames FB, a patient with severe left visual neglect consequent upon a right parieto-temporal haemorrhage, was asked to identify chimeric figures presented at different orientations. FB continued to recognize poorly the left side of chimeric figures even when the display was rotated 90 degrees clockwise or anticlockwise so that the 'left' of the chimeric fell on the patient's egocentric up or down, respectively. The result suggests that, at least under the present testing conditions, unilateral neglect is tied to the principal (top-bottom) axis of the object. Object-centered vs. (viewer-centered) representational accounts of this finding are discussed.

Aged↗

PubMed1995

Unilateral neglect can be affected by stimuli in the neglected field.

Neglect patients' attention is usually pathologically captured by stimuli present on the side ipsilateral to the brain damage. In previous research it was shown that not only the mere presence but also the number of "relevant" stimuli on the right side influenced patient's performance. However, the influence of stimuli whose presence was completely irrelevant for the execution of the task was not previously studied. We asked neglect patients to bisect lines of 28 cm of length, which could be presented alone or with line-drawings of objects placed either only on the left, only on the right or bilaterally on the display. We found that the presence of irrelevant left-side stimuli improved the bisection performance in 3 out of 5 patients with respect to the baseline condition, in which, no stimuli were present on either side of the display. Right-side stimuli did not affect the performance, whereas bilateral stimuli tended to worsen the performance for those patients who presented the beneficial effect of left-side stimuli. These findings were discussed with reference to the hypotheses advanced to explain neglect.

Adult↗

PubMed1994

Nonconscious reading? Evidence from neglect dyslexia.

In this study we present evidence which supports the view that reading mechanisms, if implicit assessed, are available also in the presence of a severe deficit of spatial awareness. A Stroop-like task was performed by a right brain-damaged patient affected by severe extrapersonal neglect and neglect dyslexia. In reading words and color words, the patient showed the usual pattern of neglect errors; omission, substitution and addition errors. However, when asked to name the colors in which color words were written, naming time was found to be affected by the meaning of those words he was not able to read correctly. The pattern of results in MD and in a group of normal subjects, who performed a modified version of the Stroop test performed by MD, have been interpreted as evidence of MD's implicit reading of the left-hand letters of color words during the Stroop test. The theoretical implications of this finding are discussed. Moreover, the comparison between the performance of MD and the performance of another group of normal subjects suggested that implicit processing in MD was carried out at a lower level of efficiency than in normals.

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