Categorization in single neurons.
Explore the source record for details and available documents.
Journal Article↗
Biomedical subjects
G di Pellegrino
Publications and source records attributed to G di Pellegrino.
Searching for actions.
Explore the source record for details and available documents.
Journal Article↗
Vision and touch in parietal area 5.
Explore the source record for details and available documents.
Journal Article↗
Seeing or not seeing where your hands are.
Previous findings have demonstrated the existence of a visual peripersonal space centered on the hand in humans and its modulatory effects on tactile perception. A strong modulatory effect of vision on touch perception was found when a visual stimulus was presented near the hand. In contrast, when the visual stimulus was presented far from the hand, only a weak modulatory effect was found. The aim of the present study was to verify whether such cross-modal links between touch and vision in the peripersonal space centered on the hand could be mediated by proprioceptive signals specifying the current hand positions or if they directly reflect an interaction between two sensory modalities, i.e., vision and touch. To this aim, cross-modal effects were studied in two different experiments: one in which patients could see their hands and one in which vision of their hands was prevented. The results showed strong modulatory effects of vision on touch perception when the visual stimulus was presented near the seen hand and only mild effects when the vision of the hand was prevented. These findings are explained by referring to the activity of bimodal neurons in premotor and parietal cortex of macaque, which have tactile receptive fields on the hand, and corresponding visual receptive fields in the space immediately adjacent to the tactile fields. One important feature of these bimodal neurons is that their responsiveness to visual stimuli delivered near the body part is reduced or even extinguished when the view of the body part is prevented. This implies that, at least for the hand, the vision of the hand is crucial for determining the spatial mapping between vision and touch that takes place in the peripersonal space. In contrast, the proprioceptive signals specifying the current hand position in space do not seem to be relevant in determining the cross-modal interaction between vision and touch.
Aged↗
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↗
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↗
A model that accounts for activity in primate frontal cortex during a delayed matching-to-sample task.
A fully recurrent neural network model was optimized to perform a spatial delayed matching-to-sample task (DMS). In DMS, a stimulus is presented at a sample location, and a match is reported when a subsequent stimulus appears at that location. Stimuli elsewhere are ignored. Computationally, a DMS system could consist of memory and comparison components. The model, although not constrained to do so, worked by using two corresponding classes of neurons in the hidden layer: storage and comparator units. Storage units form a dynamical system with one fixed point attractor for each sample location. Comparator units constitute a system receiving input from these storage units as well as from current input stimuli. Both unit types were tuned directionally. These two sources of information combine to create unique patterns of activity that determine whether a match has occurred. In networks with abundant hidden units, the storage and comparator functions were distributed so that individual units took part in both. We compared the model with single-neuron recordings from premotor (PM) and prefrontal (PF) cortex. As shown previously, many PM and PF neurons behaved like storage units. In addition, both regions contain neurons that behave like the comparator units of the model and appear to have dual functionality similar to that observed in the model units. No neuron in either area had properties identical to those of the match output neuron of the model. However, four PF neurons and one PM neuron resembled the output signal more closely than any of the hidden units of the model.
Animals↗
Prosopagnosia and alexia without object agnosia.
Following a trauma causing bilateral posterior brain damage, a patient complained of dyslexia and prosopagnosia, but not object agnosia. On testing she showed intact recognition of object drawings, even when it was assessed with perceptually demanding tasks such as Ghent's overlapping figures and Street completion test. This pattern of deficit is inconsistent with Farah's (1990) prediction that the simultaneous occurrence of alexia and prosopagnosia is invariably associated with object agnosia. The patient's reading performance had the features typically found in letter-by-letter readers. On face tests, she showed a discrepancy between the impairment exhibited in familiarity recognition and famous face naming and the correct (though slow) performance in matching the names of famous persons with their photographs. This apparent contradiction was clarified by showing that the patient had maintained the ability to generate the mental images of famous faces in response to the presentation of their names. We assume that face recognition units were intact, but partially disconnected from the output of perceptual processing.
Adult↗
Neuropsychological evidence of an integrated visuotactile representation of peripersonal space in humans.
Current interpretations of extinction suggest that the disorder is due to an unbalanced competition between ipsilesional and contralesional representations of space. The question addressed in this study is whether the competition between left and right representations of space in one sensory modality (i.e., touch) can be reduced or exacerbated by the activation of an intact spatial representation in a different modality that is functionally linked to the damaged representation (i.e., vision). This hypothesis was tested in 10 right-hemisphere lesioned patients who suffered from reliable tactile extinction. We found that a visual stimulus presented near the patient"s ipsilesional hand (i.e., visual peripersonal space) inhibited the processing of a tactile stimulus delivered on the contralesional hand (cross-modal visuotactile extinction) to the same extent as did an ipsilesional tactile stimulation (unimodal tactile extinction). It was also found that a visual stimulus presented near the contralesional hand improved the detection of a tactile stimulus applied to the same hand. In striking contrast, less modulatory effects of vision on touch perception were observed when a visual stimulus was presented far from the space immediately around the patient"s hand (i.e., extrapersonal space). This study clearly demonstrates the existence of a visual peripersonal space centered on the hand in humans and its modulatory effects on tactile perception. These findings are explained by referring to the activity of bimodal neurons in premotor and parietal cortex of macaque, which have tactile receptive fields on the hand and corresponding visual receptive fields in the space immediately adjacent to the tactile fields.
Aged↗
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↗
The premotor cortex and nonstandard sensorimotor mapping.
We often gaze at and attend to an object while preparing to reach toward and grasp it, and continue doing so when the plan is executed. Elaborate machinery, much of it in the brainstem and spinal cord, provides control systems for the spatially congruent guidance of the eyes, limbs, and body toward targets in visual space. We will use the term standard mapping for the sensorimotor transformations that underlie such behavior. Despite the common sense character of standard mapping, the targets of gaze, attention, and reaching can be dissociated from each other. We can attend to stimuli in locations that differ from the target of action. We can gaze in one direction while reaching in another. And we can guide spatial action with nonspatial stimuli, such as when, in conditional motor tasks, the color of an object instructs a movement elsewhere in space. All of these situations, and many others, call for a process that we term nonstandard mapping, wherein the central nervous system must reject the commonplace correspondences among visuospatial stimuli, gaze, attention, and reaching movements. We focus in this article on the possibility that premotor cortex underlies nonstandard mapping and, therefore, the behavioral flexibility that such a process allows.
Animals↗
Frontal lobe mechanisms subserving vision-for-action versus vision-for-perception.
In the typical course of daily events, we often gaze at an object, attend to its features and its place, reach toward it and grasp it, all with an awareness of what we are doing at the time. But behavior is not always thus. Gaze, attention, limb movement direction and awareness can be behaviorally dissociated from each other, and this review focuses on one such dissociation: that between the perception of an object and the use of that object's inherent spatial and nonspatial information for mediating visuomotor control. We review evidence that partially different neuronal systems underlie these two aspects of visual information processing. In neurophysiological studies of the primate frontal lobe, it has been possible to demonstrate that neural signals appearing to be visual responses reflect, at least in part, the motor significance of a stimulus. This finding has been confirmed, in separate studies, for both spatial and nonspatial visual information and supports the hypothesis that some frontal cortex activity reflects the selection and guidance of action rather than the properties of visual stimuli, per se. These findings are discussed in the context of neuropsychological studies indicating that accurate and appropriate movements are possible without perceptual awareness of the information guiding those movements.
Animals↗
Sparing of verbs and preserved, but ineffectual reading in a patient with impaired word production.
We report a patient who, following a left fronto-temporal lesion, showed a complete sparing of the semantic store and a deficit of word production characterized by two types of dissociations. First, oral and written output was severely disrupted in naming and partially impaired in repetition, writing to dictation and oral spelling, with the exception of verbs, which were normally produced in every modality and condition of stimulation. Second, reading was normal for all type of words as well as non-words. This pattern of deficits suggests two functional lesions, one affecting the connections between the semantic store and the phonological lexicon and the other damaging the sublexical route that converts sound to sound and sound to print. It also implies that words are independently organized in the phonological lexicon, based on their grammatical class and have discrete connections with the semantic store. However, CT scan evidence does not support the hypothesis that this functional dissociation finds its anatomical correlate in the specialization of the frontal premotor cortex for verbs and the antero-medial temporal cortex for nouns. In spite of his normal reading performance, both in terms of comprehension and of accuracy and speed in word production, the patient complained that he met with great difficulty in reading newspapers and books, to the point that he had to renounce to this previously favourite activity. It was found that it took him time and effort to grasp the meaning of complex sentences and passages and it was speculated that, contrary to single words and elementary sentences, comprehension of this type of material cannot be achieved by the mere access of orthographic stimuli to semantics, but requires the retrieval of word-forms. It would appear that a patient, whose lexical route is blocked, can only read passages, by first converting print to sound via the sublexical route and then re-entering the semantic store with oral input.
Adult↗
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↗
Effects of attention on visuomotor activity in the premotor and prefrontal cortex of a primate.
We examined neuronal activity in the primate premotor (PM) and prefrontal (PF) areas during a demanding spatial matching task. On each behavioral trial, a rhesus monkey moved its forelimb when a visual stimulus, called the "prime stimulus," reappeared at a previously cued location. Because it triggered a movement, the part of space cued by the prime stimulus had to be either remembered or attended during the time between prime stimulus presentations. Between the first and second appearances of the prime stimulus, behaviorally irrelevant visual stimuli could appear at one or several locations other than that of the prime stimulus. We could thereby examine the activity that followed a stimulus when it was attended versus when it was irrelevant and presumably unattended. We found that visuospatial attention affected neuronal activity in both the motor and "nonmotor" parts of the frontal cortex. The magnitude of attention effects exceeded that previously reported--a finding that probably resulted from the intensive attentional demands of the present task.
Animals↗
Visuospatial versus visuomotor activity in the premotor and prefrontal cortex of a primate.
When visuospatial stimuli instruct a limb movement, the stimulus can be said to have both sensory and sensorimotor aspects. We studied the premotor and prefrontal areas of a rhesus monkey in order to identify neuronal activity related to the motor (or instructional) aspects of such stimuli. A rhesus monkey chose limb-movement targets according to one of two rules: (1) visuospatial stimuli instructed and triggered a limb movement toward their locations or (2) identical stimuli triggered a movement toward a predetermined target regardless of their location. Gaze and head fixation assured that each stimulus appeared at a constant location in both retinocentric and craniocentric coordinates, as well as in allocentric space. The task required that the spatial location cued by certain stimuli had to be either remembered or attended after stimulus presentation and before movement. Thus, the visuospatial information presented under one rule differed from that presented under the other only in its motor (instructional) significance and not in its attentional, spatial, mnemonic, or strictly sensory aspects. We could thereby test and confirm the hypothesis that the motor significance of visuospatial cues should commonly affect neuronal activity in the premotor cortex, but less commonly do so in the prefrontal cortex.
Animals↗
Space coding by premotor cortex.
Many neurons in inferior area 6, a cortical premotor area, respond to visual stimuli presented in the space around the animal. We were interested to learn whether the receptive fields of these neurons are coded in retinotopic or in body-centered coordinates. To this purpose we recorded single neurons from inferior area 6 (F4 sector) in a monkey trained to fixate a light and detect its dimming. During fixation visual stimuli were moved towards the monkey both within and outside the neuron's receptive field. The fixation point was then moved and the neuron retested with the monkey's gaze deviated to the new location. The results showed that most inferior area 6 visual neurons code the stimulus position in spatial and not in retinal coordinates. It is proposed that these visual neurons are involved in generating the stable body-centered frame of reference necessary for programming visually guided movements.
Animals↗