Salvatore Aglioti

The body in the brain: neural bases of corporeal awareness

Recent studies have begun to unravel the brain mechanisms that underlie the mental representation of the body. Imitation of movements by neonates suggests an implicit knowledge of the body structure that antedates the adult body schema. This can include inanimate objects that bear systematic relations to the body, as shown by the elimination from self awareness of a body part and its associated paraphernalia after selective brain lesions. Dynamic aspects of the body schema are revealed by spontaneous sensations from a lost body part as well as by orderly phantom sensations elicited by stimulation of body areas away from the amputation line and even by visual stimulation. The mechanisms of the body schema exhibit stability, since some brain regions seem permanently committed to representing the corresponding body parts in conscious awareness, and plasticity, since brain regions deprived of their natural inputs from a body part become reactive to inputs from other body parts.

Mapping implied body actions in the human motor system.

The human visual system is highly tuned to perceive actual motion as well as to extrapolate dynamic information from static pictures of objects or creatures captured in the middle of motion. Processing of implied motion activates higher-order visual areas that are also involved in processing biological motion. Imagery and observation of actual movements performed by others engenders selective activation of motor and premotor areas that are part of a mirror-neuron system matching action observation and execution. By using single-pulse transcranial magnetic stimulation, we found that the mere observation of static snapshots of hands suggesting a pincer grip action induced an increase in corticospinal excitability as compared with observation of resting, relaxed hands, or hands suggesting a completed action. This facilitatory effect was specific for the muscle that would be activated during actual execution of the observed action. We found no changes in responsiveness of the tested muscles during observation of nonbiological entities with (e.g., waterfalls) or without (e.g., icefalls) implied motion. Thus, extrapolation of motion information concerning human actions induced a selective activation of the motor system. This indicates that overlapping motor regions are engaged in the visual analysis of physical and implied body actions. The absence of motor evoked potential modulation during observation of end posture stimuli may indicate that the observation–execution matching system is preferentially activated by implied, ongoing but not yet completed actions.

Distribution in the visual field of the costs of voluntarily allocated attention and of the inhibitory after-effects of covert orienting

By using a simple reaction time (RT) paradigm we have investigated the spatial distribution of the benefits and costs of voluntarily directed attention and of the inhibitory after-effects of covert orienting. In the first experiment subjects deliberately allocated attention to each one of five stimulus positions disposed along the horizontal meridian, while at the same time fixing their eyes on the central position. The separation in visual angle between the central position and the two nearest positions, one on the left and the other on the right, was 10 degrees; that between the central position and the two most eccentric positions was 30 degrees. By comparing RT to brief flashes of light presented at each position during directed attention with RT to identical flashes at the same position during diffuse attention (i.e. in a condition in which subjects paid equal attention to all five positions), it was possible to determine that benefits, that is RT decreases relative to the diffuse-attention condition, were strictly limited to the attended position. Costs, i.e. RT increases relative to the diffuse-attention condition, showed a more diffuse and complex spatial pattern. When attention was directed to one of the noncentral positions, costs were apparent at the two contralateral positions and at the central position, but not at the ipsilateral position. When attention was directed to the central position, costs occurred at all other positions. This suggests a special role for the vertical meridian in delimiting the area of costs when one covertly orients towards the opposite right or left visual half field. Work of others and our preliminary evidence indicate that the area of costs is similarly limited by the horizontal meridian when one orients toward the opposite upper or lower visual field. In the second experiment we studied the inhibitory after-effect of covert orienting. Orienting to a light stimulus without moving the eyes to it may induce a short-lived facilitation of the speed of response to a second stimulus presented at the same position, but this facilitation is followed by a profound and prolonged RT retardation. By using a two-flashes paradigm we observed this RT retardation not only when the two stimuli appeared at the same position, but also when they occurred at different locations in the same altitudinal or lateral visual hemifield. There were no inhibitory after-effects when the two stimuli appeared on opposite sides of the vertical or horizontal meridian.(ABSTRACT TRUNCATED AT 400 WORDS)

Do peripheral non-informative cues induce early facilitation of target detection ?

It has been reported that simple reaction time (RT) to a peripheral visual target is faster if the target is presented within about 200 msec from the onset of a non-informative cue flashed at the same location, as compared with RT to a target presented at an uncued location. This period of facilitation is followed by a period of inhibition during which RT is longer if cue and target are shown at the same location or at different locations within the same hemifield, as opposed to contralateral cues and targets. Early facilitation has been explained by an automatic covert orienting towards the cue, while the following inhibition has been regarded as a consequence of such covert orienting. In a series of four experiments, we have investigated the dependency of these effects on the temporal and spatial relationships between cue and target. Normal, right-handed subjects responded to a target displayed for 16 msec simultaneously with, or following at stimulus-onset asynchronies (SOAs) of 60, 130, 300 or 900 msec, the onset of a non-informative cue. Both cues and targets could appear at random in one of four locations (Expts 1-3) or in one of two locations (Expt 4) disposed symmetrically across the fixation point along the horizontal meridian. Duration of the cue varied between experiments. In Expt 1 it was 16 msec. In Expt 2 the cue remained on view throughout the period of the SOA and terminated 300 msec after target onset. In the remaining two experiments cue duration was 130 msec. In the first experiment, at all cue-target SOAs RTs to target flashed either at the same location or in the same hemifield as the cue were significantly slower than RTs to contralateral cue-target combinations (RT inhibition). In the other experiments, there was no RT inhibition with targets in cued locations if the cue remained on during target presentation and outlasted target offset. Since at no SOA was RT to targets in cued locations shorter than RT to targets contralateral to cues, there was no direct evidence for facilitation. However, the facilitatory influence of these cues could be inferred from the fact that they countered and masked inhibition. RT to uncued targets ipsilateral to cues was consistently inhibited in all experimental conditions. These results show that at each cue-target SOA the consequences of a peripheral non-informative cue depend on whether or not the cue remains visible during target processing.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial summation across the vertical meridian in hemianopics: A test of blindsight

Twenty hemianopic patients with retrochiasmatic lesions have been tested for spatial summation of pairs of flashes simultaneously presented either to the same hemifield or to opposite hemifields across the vertical meridian. In such a task normal subjects show summation, i.e. a faster reaction time in response to a pair of stimuli than in response to a single stimulus. Such an effect is present both for pairs of stimuli presented within the same hemifield and for pairs of stimuli in which the two flashes are presented one in the right and the other in the left hemifield. In contrast to normals, hemianopics as a group did not show interfield summation although, like normals, showed summation within one hemifield. A single-case analysis, however, revealed that in one patient there was a reliable overall interfield summation and that in three others there was evidence of summation in at least one testing session. The presence of interfield spatial summation between the normal and the affected hemifield of hemianopics thus provides further evidence of blindsight in a task paradigm in which guessing strategies and stimulus artefacts can be eliminated. The very small proportion of patients showing blindsight can be in part related to the relatively low stimulus intensity and the very brief stimulus exposure duration used.

Corpus callosum and simple visuomotor integration

Malcolm Jeeves was the first to demonstrate lengthened interhemispheric transmission times in subjects with agenesis of the corpus callosum by using a simple reaction time paradigm with lateralized unstructured light stimuli and crossed and uncrossed hand responses. Uncrossed responses can be integrated within one hemisphere, whereas crossed responses require a communication between the two hemispheres. In the normal brain this communication is effected rapidly by the corpus callosum, whereas in the acallosal brain it must occur much more slowly by way of less efficient alternative interhemispheric pathways. Using a similar experimental paradigm we have studied normal subjects, subjects with a complete callosal agenesis and epileptic patients with surgical callosal sections, either complete or partial. All subjects with complete callosal defects showed much lengthened interhemispheric times compared to normal controls. Virtually normal interhemispheric transmission times were found in subjects with partial callosal defects, whether anterior or posterior, suggesting a possible equipotentiality of different portions of the corpus callosum in the mediation of crossed manual responses. In both normals and acallosals there were no crossed-uncrossed differences in reaction time when responses were made unilaterally with lower limb effectors or para-axial upper limb effectors, as well as bilaterally with upper-limb proximal and para-axial effectors. Since these effectors can be controlled directly from either side of the brain via bilaterally distributed motor pathways, crossed responses using them, unlike crossed manual responses, do not require an interhemispheric integration.

Frames of Reference for Mapping Tactil Stimuli in Brain-Damaged Patients

Twelve normal controls, twelve left-brain-damaged patients, and thirty-six right-brain-damaged patients with or without tactile extinction or tactile neglect were asked to report light touches delivered to the left or the right hand or simultaneously to both hands. The hands could be in anatomic position or one hand could cross over the other. Moreover, the two hands could be in the left or the right hemispace or across the corporeal midline. Controls and nontactile-extinction groups performed better when the hands were in anatomical than in crossed position. By contrast, patients with tactile extinction detected contralesional stimuli with higher accuracy in crossed than in anatomical position. This result suggests that, in these patients, impairments in detecting contralesional stimuli can be due not only to sensory but also to spatial factors contingent upon the position of the hands. There was no interaction between the effect of crossing the hands and the hemispace where the crossing took place. This suggests that coding the position of a hand as left or right does not necessarily occur in relation to the bodily midline, but it may arise from the computation of the position of the other hand.

Sensory and spatial components of somaesthetic deficits following right brain damage

We instructed patients with right brain damage (RBD) and somatosensory extinction, hemispatial neglect, or both to verbally report light touches delivered to the left hand, to the right hand, or simultaneously to both hands in two experimental situations.In the ``anatomic situation, each hand was in its homonymous hemispace; in the ``crossed one, each hand was held across the corporeal midline, in its heteronymous hemispace. Under both single and double stimulation conditions, RBD patients detected stimuli delivered to the contralesional hand with lower accuracy in the anatomic than in the crossed position. This result suggests that somaesthetic deficits can be due not only to sensory but also to attentional factors, depending on the spatial position of the hands. Processing of sensory information in primary areas should not be influenced by the hemispatial position of the stimulated body part. These results suggest that somaesthetic deficits may stem not only from damage of primary sensory areas, as classically held, but also from damage of higher-order areas where information about stimuli, body parts, and extrapersonal space is integrated. Finally, the results show that sensory and attentional components of the deficit can be dissociated by using a very simple clinical test. NEUROLOGY 1995;45: 1725-1730

Hemispheric control of unilateral and bilateral responses to lateralized light stimuli after callosotomy and in callosal agenesis

Normally, simple digital or manual responses to a light stimulus in the right or left visual hemifields are performed faster with uncrossed hand-field combinations than with crossed hand-field combinations. Because of the organization of visual and motor pathways, the integration of uncrossed responses is assumed to occur within a single hemisphere, whereas a time-consuming inter-hemispheric transfer via the corpus callosum is considered to be necessary for the integration of crossed responses. However, callosal transfer may be dispensable for those crossed responses which can be controlled through ipsilaterally descending motor pathways by the hemisphere receiving the visual stimulus. We investigated crossed-uncrossed differences (CUDs) in speed of simple visuomotor responses to lateralized flashes in one subject with total section of the corpus callosum and two subjects with complete callosal agenesis. We recorded the reaction times as well as the premotor times, as indicated by the electromyographic latencies of the prime movers, of three types of responses: a distal response involving a thumb flexion, a proximal response chiefly involving a forearm flexion and an axial response involving a shoulder elevation. Further, the three types of responses to a single lateralised flash were performed both unilaterally and bilaterally. The three acallosal subjects showed CUDs greatly exceeding normal values on distal responses, either unilateral or bilateral, and on unilateral proximal responses. These abnormally long CUDs stood in sharp contrast to the insignificant CUDs exhibited by the same subjects on bilateral proximal responses and on unilateral and bilateral axial responses in agreement with correspondingly insignificant CUDs reported for normal subjects. These results confirm that a callosal contribution is important for the execution of fast distal and unilateral proximal responses to a visual stimulus directed to the hemisphere ipsilateral to the responding hand. By contrast, the other types of crossed responses appear to be efficiently coordinated across the midline without the aid of the corpus callosum. This is in keeping with the hypothesis that they are governed by a bilaterally distributed motor system which is preferentially activated for the execution of symmetrical bilateral movements, employing axial and proximal limb muscles.

Influence of Stimulus Salience and Attentional Demands on Visual Search Patterns in Hemispatial Neglect

Seventy-five left and right brain-damaged patients, with or without hemispatial neglect, and 40 age-matched control subjects were tested on cancellation tasks with two different visual textures modeled after Julesz (1981). In one condition (preattentive), target elements segregated easily from background elements and were perceived effortlessly. In the other (attentive), target elements did not segregate easily and could be detected only after prolonged focal scrutiny. Both controls and patients were more accurate and faster on the preattentive than attentive texture. However, only neglect patients were disproportionately impaired on the attentive texture, thus suggesting that unilateral neglect is exacerbated by the low visual salience of the stimuli and a higher engagement of focal attention. Thus, a simple bedside test may help to tell apart the level of visual information processing maximally impaired in neglect patients.