Neuronal plasticity: historical roots and evolution of meaning

In this paper, we outline some important milestones in the history of the term "plasticity" in reference to the nervous system. Credit is given to William James for first adopting the term to denote changes in nervous paths associated with the establishment of habits; to Eugenio Tanzi for first identifying the articulations between neurons, not yet called synapses, as possible sites of neural plasticity; to Ernesto Lugaro for first linking neural plasticity with synaptic plasticity; and to Cajal for complementing Tanzi's hypothesis with his own hypothesis of plasticity as the result of the formation of new connections between cortical neurons. Cajal's early use of the word plasticity is demonstrated, and his subsequent avoidance of the term is tentatively accounted for by the fact that other authors extended it to mean neuronal reactions partly pathological and no doubt quite different from those putatively associated with normal learning. Evidence is furnished that in the first two decades of the twentieth century the theory was generally accepted that learning is based on a reduced resistance at exercized synapses, and that neural processes become associated by coactivation. Subsequently the theory fell in disgrace when Lashley's ideas about mass action and functional equipotentiality of the cortex tended to outmode models of the brain based on orthodox neural circuitry. The synaptic plasticity theory of learning was rehabilitated in the late 1940s when Konorski and particularly Hebb argued successfully that there was no better alternative way to think about the modifiability of the brain by experience and practice. Hebb's influential hypothesis about the mechanism of adult learning contained elements strikingly similar to the early speculations of James, Tanzi and Cajal, but Hebb did not acknowledge specifically these roots of his thinking about the brain, though he was fully aware that he had resurrected old ideas wrongly neglected for a long time. Lately the concept of neural plasticity has been complicated by attributing considerably different meanings to it. A scholarly paper by Paillard is used to show how an analysis in depth can clarify some confusion engendered by an unrestricted use of the concept and term of neural plasticity.

Behavioural and electrophysiological analysis of strabismus in cats: modern context

We studied the effects of paralytic strabismus on visual behaviour and binocularity of cortical visual mechanisms by immobilization of one eye in adult cats. Visual discrimination abilities of the immobilized eye were significantly diminished despite extensive training with one or both eyes. The deficits are not caused by the immobilization itself but appear to reflect an adaptive mechanism to deal with double vision. The deficits with the immobilized eye persisted even after section of the optic chiasm, which effectively removes the direct cortical competition of the two eyes. Single-cell electrophysiological recordings showed that cortical cellular responses are modified by the immobilization, with loss of binocularity in some cells and shifting of receptive fields of other cells that continued to respond to both eyes.

The contribution of general and specific motor inhibitory sets to the so-called auditory inhibition of return

The detection of sounds that come from a region of space recently exposed to acoustic stimulation is often slower than the detection of sounds coming from regions of space previously unexposed to acoustic stimulation. The relative increase in reaction time (RT) to targets in recently stimulated locations is generally termed "inhibition of return" (IOR). This term alludes to the possibility that spatial attention is biased against returning to recently visited locations, thus favoring the sampling of new sources of information. However, auditory IOR effects found in paradigms where subjects have to detect a first sound (cue) without making an overt response to it, and then respond as fast as possible to a second sound (target), may be due to a purely motor inhibition carried over from cue to target. Such motor inhibition has been shown to be maximal when cue and target belong to the same category, such as when they occupy the same spatial position. We have assessed the possible contribution of this motor inhibition to auditory IOR effects by having subjects respond to both cues and targets randomly presented in a right location and a left location. Reaction time to targets preceded by cues at the same location was longer than reaction times to targets preceded by cues at the opposite location (IOR effect). Compared to a condition in which subjects responded only to targets, the IOR effect was smaller, but still significant, in the double response condition, suggesting that such an effect depends on both motor inhibition and other factors, possibly related to covert spatial orienting and oculomotor control. A second experiment indicated that the IOR effect component independent of motor inhibition was slightly but significantly greater when space was relevant to the task because subjects had to report the positions of both cues and targets, compared to when space was irrelevant to the task because subjects were not required to report stimulus positions.

Taste laterality in the split brain

Two patients with corpus callosum resection, one complete and the other sparing the genu and the rostrum, were tested for discrimination of three basic taste stimuli (sour, bitter, salty) applied to the right or left sides of the tongue. Responses were made by pointing with either hand to written words or images of visual objects corresponding to the stimuli, a language-based discrimination. In both patients, response accuracy was significantly above chance for both hemitongues but there was a significant advantage for the left side. Reaction time was shorter for left stimuli than for right stimuli but the difference was not significant. Eight normal controls matched for age with the patients performed equally well with right and left hemitongue stimuli and so did a third callosotomy patient with sparing of the posterior callosum, including the splenium. Tactile and visual tests showed that the left hemisphere was responsible for language-based responses in the first two patients. The results confirm and extend previous findings in another callosotomy patient, indicating that: (i) taste information from either side of the tongue can reach the left hemisphere in the absence of the corpus callosum; (ii) the ipsilateral input from the tongue to the left hemisphere is more potent functionally than the contralateral input and (iii) in the normal brain, the corpus callosum, specifically its posterior part including the splenium, appears to equalize the effects of the ipsilateral and contralateral gustatory inputs on the left hemisphere. Taken together with evidence about lateralized taste deficits following unilateral cortical lesions, the results also suggest that the gustatory pathways from tongue to cortex are bilaterally-distributed with an ipsilateral predominance that may be subject to individual variations.

Volitional covert orienting to a peripheral cue does not suppress cue-induced inhibition of return

Detection reaction time (RT) at an extrafoveal location can be increased by noninformative precues presented at that location or ipsilaterally to it. This cue-induced inhibition is called inhibition of return or ipsilateral inhibition. We measured detection RT to simple light targets at extrafoveal locations that could be designated for covert orienting by local or distant cues. We found that cue-induced inhibition cooccurred in an additive fashion with the direct effects of covert orienting, i.e., it detracted from facilitation at attended locations and increased the disadvantage for unattended locations. Thus, cue-induced inhibition cannot be suppressed by a volitional covert orienting to the cued location; the co-occurrence of different facilitatory and inhibitory effects confirms the simultaneous operation of multiple independent attentional mechanisms during covert orienting.

Incomplete gustatory lateralization as shown by analysis of taste discrimination after callosotomy

The lateral organization of the gustatory pathway in man is incompletely understood. Majority of the studies support an uncrossed projection from each side of the tongue to the cortex, but reports of an opposite crossed organization continue to appear in the neurological literature. We studied the lateral organization of the gustatory pathway in normal controls, a man with a complete callosal agenesis, and a man with a complete section of the corpus callosum, a right anterior-frontal lesion, and language in the left hemisphere. Sapid solutions were applied to one or the other side of the tongue, and subjects reported the taste of the stimulus either verbally or by manually pointing to the name of the taste. There were no differences in accuracy and reaction time between the right and left hemitongues of the controls and the genetically acallosal observer. By contrast, the callosotomy subject showed a constant marked advantage of the left hemitongue over the right for both accuracy and speed of response, though performance with right stimuli was clearly above chance. The left advantage can be attributed to the left hemisphere being favored by the essentially verbal nature of the task, or to the presence of a lesion in cortical gustatory areas in the right hemisphere, or to both factors. Whichever of these hypotheses turns out to be correct, the results unequivocally reject the notion of an exclusively crossed organization of the gustatory pathway from the tongue to the cortex, and favor the notion of a bilaterally distributed organization of this pathway with a marked predominance of the uncrossed over the crossed component.

Unconscious letter discrimination is enhanced by association with conscious color perception in visual form agnosia

Adaptive behavior guided by unconscious visual cues occurs in patients with various kinds of brain damage as well as in normal observers, all of whom can process visual information of which they are fully unaware [1] [2] [3] [4] [5] [6] [7] [8]. Little is known on the possibility that unconscious vision is influenced by visual cues that have access to consciousness [9]. Here we report a 'blind' letter discrimination induced through a semantic interaction with conscious color processing in a patient who is agnosic for visual shapes, but has normal color vision and visual imagery. In seeing the initial letters of color names printed in different colors, it is normally easier to name the print color when it is congruent with the initial letter of the color name than when it is not [10]. The patient could discriminate the initial letters of the words 'red' and 'green' printed in the corresponding colors significantly above chance but without any conscious accompaniment, whereas he performed at chance with the reverse color-letter mapping as well as in standard tests of letter reading. We suggest that the consciously perceived colors activated a representation of the corresponding word names and their component letters, which in turn brought out a partially successful, unconscious processing of visual inputs corresponding to the activated letter representations.

The neurological basis of conscious color perception in a blind patient

We have studied patient PB, who, after an electric shock that led to vascular insufficiency, became virtually blind, although he retained a capacity to see colors consciously. For our psychophysical studies, we used a simplified version of the Land experiments [Land, E. (1974) Proc. R. Inst. G. B. 47, 23-58] to learn whether color constancy mechanisms are intact in him, which amounts to learning whether he can assign a constant color to a surface in spite of changes in the precise wavelength composition of the light reflected from that surface. We supplemented our psychophysical studies with imaging ones, using functional magnetic resonance, to learn something about the location of areas that are active in his brain when he perceives colors. The psychophysical results suggested that color constancy mechanisms are severely defective in PB and that his color vision is wavelength-based. The imaging results showed that, when he viewed and recognized colors, significant increases in activity were restricted mainly to V1-V2. We conclude that a partly defective color system operating on its own in a severely damaged brain is able to mediate a conscious experience of color in the virtually total absence of other visual abilities.

Some aspects of the history of the law of dynamic polarization of the neuron. From William James to Sherrington, from Cajal and van Gehuchten to Golgi

William James was the first to suggest that propagation of impulses in the nervous system proceeds in one direction, from sensory to motor neurons, but not viceversa. His law of forward direction preceded the formulation of the law of dynamic polarization of van Gehuchten and Cajal, which assumed that nerve impulses are conducted cellulipetally along dendrites and cellulifugally along axons, based on different anatomo-functional properties of these neuronal components. Golgi did not accept the law of dynamic polarization because he believed that dendrites are involved in the nutrition of the neuron rather than in impulse propagation, and that impulses can travel in any direction in the axonal components of the diffuse nerve network. Sherrington in turn experimentally demonstrated that intraneuronic conduction is reversible, whereas, in accord with James's law, propagation of impulses along neuronal chains is irreversible, due to the valve-like action of synapses. The story of the law of dynamic polarization shows that neither Golgi nor Cajal paid much heed to Sherrington's findings and to neurophysiological studies in general, probably because they felt that histology alone could provide the key for understanding the general functioning of the nervous system. It is argued here that this attitude was detrimental to the progress of the neurosciences, because a multidisciplinary approach based on different techniques is inevitably called for in order to develop a plausible theory of the nervous system.

Possible recoding of visual space in covert orienting tasks

Reaction time to lateralized light targets is longer if targets are preceded by light stimuli in the same visual hemifield compared to when they are preceded by light stimuli in the opposite visual hemifield. The effect is probably caused by interactions between implicit oculomotor tendencies and covert shifts of attention. We show here that a similar, but much smaller, ipsilateral RT inhibition can be observed when all stimuli are presented in a display completely lateralized to one hemifield, where ipsilateral and contralateral are defined with respect to the midpoint of the display. The persistence of ipsilateral inhibition with unilateral stimulus displays can be accounted for by a recoding of visual space predicated on the centering of covert attention on the display midpoint rather than on the fixation point. The recoding seems to affect the control of covert attention and perhaps oculomotor control as well.

Paradoxically greater interhemispheric transfer deficits in partial than complete callosal agenesis

Symptoms of interhemispheric disconnection are typically much less severe in callosal agenesis than after surgical section of the corpus callosum. Sperry [Sperry, R. W., Plasticity of neural maturation. Developmental Biology, 1968, 2 (Suppl.), 306-327.] has attributed this difference to two interconnected factors: (1) the callosal section is usually performed after the brain has lost the maximal degree of functional plasticity associated with the early stages of development and (2) the removal of an already formed structure is more disruptive for functional brain organization than the failure of the same structure to develop. It has been suggested that functional compensation is less efficient if callosal agenesis is partial rather than complete [Dennis, M., Impaired sensory and motor differentiation with corpus callosum agenesis: A lack of callosal inhibition during ontogeny? Neuropsychologia, 1976, 14, 455-469.]. This suggestion is supported by the present findings of partial left-hand anomia, partial left-field alexia and poor tactile cross-localization in a subject with a congenital absence of the posterior part of the corpus callosum due to an arteriovenous malformation. In agreement with many previous studies, similar, though more severe, symptoms of interhemispheric disconnection were found in a subject with a complete section of the corpus callosum, but not in a subject with complete callosal agenesis. Praxic control of the left hand on verbal commands was severely deficient in the callosotomy subject, but it was normal in the subject with callosal hypogenesis. The lesser degree of compensation in partial compared to complete callosal agenesis may be explained by a reduced pressure to develop extracallosal means of interhemispheric communication, contingent on the partial existence of callosal connections, as well as by the later occurrence in development of the causes of callosal hypogenesis compared to those of total callosal agenesis.

Spatio-temporal properties of the pattern of evoked phantom sensations in a left index amputee patient

In a left index finger amputee, appropriate stimulation of skin areas of the remnant left fingers or left lower face evoked veridical sensations as well as sensations localized to the phantom finger. Five months after the amputation, there was a systematic correspondence between positions of digital and facial stimuli and positions of stimuli felt on the phantom. More than 3 years after the amputation, orderly maps of the phantom index on the ipsilateral fingers were still detected. By contrast, poorly organized facial maps were present only contralaterally to the amputation. The maps on the remnant fingers are likely to acquire stability because they are systematically activated during manipulations performed with the mutilated hand. The disorganization of facial maps may be related to their irrelevance for behavioral control in everyday life conditions.

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.

Spatio-temporal properties of the pattern of evoked phantom sensations in a left index amputee patient

In a left index finger amputee, appropriate stimulation of skin areas of the remnant left fingers or left lower face evoked veridical sensations as well as sensations localized to the phantom finger. Five months after the amputation, there was a systematic correspondence between positions of digital and facial stimuli and positions of stimuli felt on the phantom. More than 3 years after the amputation, orderly maps of the phantom index on the ipsilateral fingers were still detected. By contrast, poorly organized facial maps were present only contralaterally to the amputation. The maps on the remnant fingers are likely to acquire stability because they are systematically activated during manipulations performed with the mutilated hand. The disorganization of facial maps may be related to their irrelevance for behavioral control in everyday life conditions.

Visuospatial Attention and the Split Brain

In callosotomy patients, the right hemisphere attends to the entire visual field, whereas the left hemisphere attends to the right field only. The occurence of rightward attentional biases, simulating a hemineglect from right hemisphere damage, suggests that in these patients visuospatial attention tends to be controlled by the left hemisphere.

Rightward attentional bias and left hemisphere dominance in a cue-target light detection task in a callosotomy patient

Six normal subjects and a callosotomized man with a prefrontal lesion, mostly on the right side, were tested in a reaction time (RT) task involving a key-pressing response to an extrafoveal light target preceded by an extrafoveal light cue. Cues and targets were presented along the horizontal meridian at 4 degrees and 12 degrees on the right and left of fixation. Fixation was maintained throughout each trial. The cue signalled the occurrence of the target within a time window extending from 200 to 4000 misec from the cue, but did not predict target location. Normal controls responded faster to medial than to lateral targets in both fields, but showed no between-field difference, and their RT was not affected by cue location. Furthermore, they showed the so-called 'ipsilateral inhibition' or 'inhibition of return' effect, their RT being longer when cues and targets occurred in the same field than when they occurred in opposite fields. The RT of the callosotomized subject showed a left-right gradient for both cue location and target location, being longest for the leftmost location and shortest for the right locations. In addition, he showed a significant advantage for the right hand regardless of cue and target location, as well as a consistent ipsilateral inhibition in the left field, whereas in the right field there was ipsilateral inhibition only at the two longest stimulus onset asynchronies. These results suggest that, at least under these experimental conditions, there was a rightward orientational bias which reflected the taking over of the control of performance by the left hemisphere. This attentional bias was reminiscent of that seen in patients with hemi-inattention from right hemisphere damage, although the callosotomized patient showed no sign of such hemi-inattention in routine clinical tests. On the basis of several considerations the rightward bias could be attributed to the callosal interhemispheric disconnection rather than to the right prefrontal lesion.

One or many arousal systems? Reflections on some of Giuseppe Moruzzi's foresights and insights about the intrinsic regulation of brain activity

Soon after the birth of the hypothesis of the ascending brainstem activating system, Giuseppe Moruzzi considered the possibility that a fractionated and differentiated arousing action of the reticular formation is required for effective behavior and cognition. Current knowledge about the chemically tagged brainstem systems which project diffusely to thalamus, neocortex and limbic structures has justified the assumption of the existence of multiple arousal systems. Combined changes in the activities of these systems are responsible for the sleep-wake cycle and the modulation of the reactivity of the brain to environmental inputs. There remains the physiological problem--one which has always been foremost in Moruzzi's thinking about the intrinsic regulation of brain activity--of how the separate actions of the different arousal systems are brought together into a functional whole. This problem still awaits experimental answers.

Disownership of left hand and objects related to it in a patient with right brain damage

We describe a woman with right brain damage who denied the ownership of her left hand and of extracorporeal objects (e.g. rings) which were worn on the left hand itself. When the same objects were worn on the right hand or were held by the examiner, the patient correctly recognized them as her own. Other personal objects unrelated to the left hand (e.g. pins, earrings, comb) were always correctly recognized as her own. Thus, by inference, the mental image of one's body may include inanimate objects which had been in contact or in close proximity with the body itself. These findings provide, for the first time, experimental support to the speculative notion of an extended body schema.

Spatial stimulus-response compatibility in callosotomy patients and subjects with callosal agenesis

Subjects with partial or complete defects of the corpus callosum, either congenital or acquired, performed a choice reaction time (RT) task involving a right or left key-press response to a light presented at random in the right or left hemifield. Like normal subjects, all of them exhibited two additive effects typical of these tasks: the spatial stimulus-response compatibility effect (faster RT for stimuli and responses matched for side), and the hand placement effect (longer RT for responses performed with crossed hands). Two subjects with a complete callosal defect, one acquired and the other congenital, showed a third effect, not present in normal subjects, consisting of a marked advantage for RT of responses with hand anatomically ipsilateral to the stimulus, independent of both stimulus-response compatibility and hand placement. These findings can be interpreted according to a hierarchical model of information processing assuming that, in the absence of the corpus callosum, the matching of the mental codes for the stimulus and response sets takes place solely in the hemisphere receiving the stimulus, with a subsequent rapid-intrahemispheric or slow-interhemispheric transmission of the response command to the appropriate motor centers.

Oculomotor activity and visual spatial attention

Subjects made a horizontal or vertical saccade in response to a non-lateralized auditory stimulus. Simple manual reaction time (RT) for the detection of light targets at extrafoveal locations was modulated by the intention to make the saccade insofar as RT to targets presented at the saccadic goal location or in the hemifield containing that location was faster than RT to targets presented at the opposite, mirror-symmetric location. This RT difference was maximal prior to the beginning of the saccade and vanished after saccade termination, indicating that the effect was caused by the neural activity leading to the saccade rather than to the eye movement or the eye position per se. The results have implications for the understanding of the relations between visual spatial attention and oculomotor control, especially with regard to inhibitory phenomena arising from the non-correspondence between the line of sight and the focus of attention.

Covert orienting to non-informative cues: reaction time studies

Lateralized, non-informative visual cues lengthen reaction time (RT) to successive targets flashed in the same hemified. Early ipsilateral RT facilitation is limited to the co-occurrence of cues and targets. Inhibition from visual cues has sensory components which do not depend on orienting, as well as attentional components which are limited to one side of the vertical meridian. An inhibition of RT to targets ipsilateral to the cues has been found with somatic or auditory cues and targets, and also when somatic targets follow visual cues or visual targets follow somatic cues. The results reviewed in this paper (1) are best accounted for by directional constraints in motor readiness which are induced by the voluntary suppression of an overt orienting toward the location of the cue; (2) indicate that similar mechanisms of covert orienting operate in the whole peripersonal and near extrapersonal space; and (3) point to a common neural substrate mediating both intramodal and cross-modal effects.

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.

Callosotomy for intractable epilepsy from bihemispheric cortical dysplasias

Four patients suffering for severe drug-resistant epilepsy from bihemispheric cortical dysplasias underwent anterior callosotomy. One of these patients also presented mental retardation of mild degree associated with the epileptic syndrome. There were no operative complications in this series. Clinical signs of interhemispheric disconnection were not detectable postoperatively. Twenty-eight to 53 months after surgery, the generalized seizures were completely suppressed in 2 cases, and were reduced by 89-97% in frequency in the other 2 cases. Partial seizures were less affected by callosotomy being reduced by 14-87%. In an additional fifth case of intractable epilepsy from bihemispheric cortical dysplasias with associated severe mental retardation operated upon elsewhere for callosotomy and followed at our institution, the outcome for seizures was completely unsatisfactory. Neurophysiological studies revealed that the interhemispheric transfer (IHT) of visuo-motor responses was functionally impaired after callosotomy only in one patient who harboured bilateral cortical dysplasias in the occipital lobes. This malformation might affect the pattern of axonal projection to the posterior portion of the corpus callosum which is considered of crucial importance for the integration of crossed visuo-motor responses. From this paper the following conclusions can be drawn: a) epileptic patients with severe drug-resistant epilepsy due to bihemispheric cortical dysplasias are good candidates for callosotomy, b) one-stage extensive anterior callosotomy sparing the splenium is the procedure of choice, c) associated severe mental retardation seems to contra-indicate callosotomy, d) the neurophysiological study of the IHT can yield information on the functional status of the corpus callosum.

Interhemispheric integration of simple visuomotor responses in patients with partial callosal defects

Because of the organization of visual and motor pathways, simple 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. Uncrossed responses can be integrated within a single hemisphere, whereas crossed responses require a time-consuming interhemispheric transfer via the corpus callosum which is reflected in the difference between crossed and uncrossed reaction times. We investigated crossed-uncrossed differences (CUDs) in speed of simple visuomotor responses to lateralized flashes in seven subjects with an anterior section of the corpus callosum sparing the splenium and in one subject with an agenetic absence of the splenium due to a cerebrovascular malformation. There was no evidence of an abnormal prolongation of the CUDs in any of these subjects, in sharp contrast with the very long CUDs exhibited by an epileptic subject with a complete callosal section and two subjects with total callosal agenesis tested in the same experimental situation [1]. The normality of the CUDs in the subjects with partial callosal defects was not due to a postoperatory reorganization of interhemispheric communication, since there was no indication of an increased CUD in a patient tested as early as 5 days after the anterior callosotomy. These results are compatible with the assumption that both anterior and posterior callosal routes may subserve the integration of speeded manual responses to a visual stimulus directed to the hemisphere ipsilateral to the responding hand.

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)

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 interhemispheric 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.

Sensory and attentional components of slowing of manual reaction time to non-fixated visual targets by ipsilateral primes

Reaction time (RT) for detecting extrafoveal targets is lengthened by a non-informative prime at the same location or in the same hemifield (RT inhibition). We assumed that sensory effects at primed locations should be the same for unilateral and bilateral primes, whereas systematic covert orienting to a primed location should occur only with unilateral primes. We found equal RT inhibition for both types of primes at 0.2 sec prime-target intervals (SOA), as contrasted with inhibition for unilateral but not bilateral primes at 0.6 sec SOAs. We conclude that RT inhibition involves a succession of sensory components and orienting-dependent components.

Ipsilateral inhibition and contralateral facilitation of simple reaction time to non-foveal visual targets from non-informative visual cues

Orienting to an extrafoveal light cue without foveating it induces a temporary inhibition of responses to subsequent targets presented in the same visual hemifield, as evinced from the fact that reaction time (RT) to targets ipsilateral to the cue relative to fixation is longer than RT to targets contralateral to the cue. This study has tested the hypothesis that ipsilateral RT inhibition is associated with contralateral RT facilitation by attempting to divide the difference between ipsilateral and contralateral RTs into costs and benefits. A neutral condition suited to this purpose should involve a cue that does not require a lateral orientation. Such neutral condition was provided by measuring RT to lateralized light targets following a central overhead auditory cue (experiment 1) or a foveal visual cue (experiment 2). In both experiments RT in the neutral condition was intermediate between ipsilateral and contralateral RTs, and the differences reaches significance in the second experiment. Benefits over the neutral condition measured in the contralateral condition were thus associated with costs in the ipsilateral condition. These results suggest that a reciprocal antagonism between opposite turning tendencies underlies the organization of covert orienting. They also agree with general multi-channel theories of selective attention according to which the facilitation of given channels is an obligatory accompaniment of the inhibition of other competing channels and vice versa.

Considerable deficits in the detection performance of the cat after lesion of the suprasylvian visual cortex

The ability of two cats to discriminate between two geometrical outline patterns in the presence of superimposed structured background was tested before and after bilateral removal of the lateral suprasylvian visual areas (PMLS, PLLS, AMLS, ALLS, part of area 7). There were mild deficits when patterns and background were kept stationary; these deficits may be due to a partial undercutting of areas 17, 18 and 19. However, there was a severe impairment in performance when the patterns were moving on a stationary background which may be due to loss of the suprasylvian visual areas. Movement of the background relative to the figure resulted in an intermediate detection deficit.

Spatial distribution of the inhibitory effect of peripheral non-informative cues on simple reaction time to non-fixated visual targets

It is known that reaction time (RT) for the detection of a light target at extrafoveal locations is lengthened by a previous non-informative light cue at the same location. We describe an additional inhibitory effect from cues remote from the target but occurring within the same lateral or altitudinal visual hemifield. Subjects made a speeded key-press response to the second of two successive light flashes in a pair while maintaining fixation. Each of the two flashes could appear at random in one of four positions, two in the right and two in the left visual fields, or two in the upper and two in the lower visual fields. We found an RT prolongation not only for cued over uncued positions, but also for within-field non-coincident cue-target pairs over between-fields cue-target pairs. The within-field inhibitory effect, though smaller than the same-location effect, was fully apparent even when the target occurred at 1 degree of visual angle from the midline and at 29 degrees from the cue. Both effects were seen with cue-target asynchronies ranging from 0.2 to 1.5 sec. The results are relevant to the understanding of the neural mechanisms for covert shifts of attention across the main meridians of the visual field.

Lesion of areas 17/18/19: effects on the cat's performance in a binary detection task

The ability of two cats to discriminate between two geometrical outline patterns in the presence of superimposed Gaussian visual noise-i.e. in a binary detection task--was tested before and after bilateral removal of cortical areas 17, 18 and 19. The detection probability PD was measured as a function of the signal-to-noise ratio. After a lesion of areas 17, 18 and 19 both cats were unable to carry out the discrimination tasks. Their detection performance dropped to chance level, but after an extensive phase of retraining (3 months) they regained the ability to discriminate visual patterns. It was thus possible to obtain detection curves and to determine a measure of a performance which is predominantly bound to be mediated by extra-geniculo-cortical systems. The detection capacity was abnormally low with both large and small patterns. However, the detection of stationary small patterns was similar to the performance of cats with 17/18 lesions; the detection of stationary large patterns was only slightly better than the detection of small patterns and much worse than the comparable performance of cats with 17/18 lesions. Furthermore the cats with lesions of areas 17/18/19 were unable to discriminate moving patterns, their performances being at chance level, whereas for the cats with 17/18 lesions the detection of moving and stationary patterns was equal.

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)

Detection performance of normal cats and those lacking areas 17 and 18: a behavioral approach to analyse pattern recognition deficits

The ability of cats to discriminate between two geometrical outline patterns in the presence of superimposed Gaussian visual noise was tested before and after bilateral removal of cortical area 17 and parts of area 18. The detection probability PD was measured as a function of the signal-to-noise ratio for the parameters: noise bandwidth, spatial frequency content and rate of movement of patterns. In both normal and lesioned cats a broadband noise was found to be most effective in masking the large patterns while two other types of noise, a medium frequency noise and a high frequency noise had little or no masking effect. For recognition of the smaller patterns in normal cats the medium frequency noise was found to be more effective than the broadband noise. The performance of the lesional cats was disturbed severely at low signal-to-noise ratios and was significantly inferior to that of normal cats-especially for small patterns. However, at high S/N ratios and for large patterns the performance of the lesioned cats was comparable to that of normals while for the small patterns they reached PD values inferior to those of normal cats. It is concluded that although pattern recognition can be performed successfully by cats lacking areas 17 and 18, these cortical areas probably make an essential contribution to this function under natural conditions in two ways: because of the X-type input of area 17, they increase the acuity of the system by making it more sensitive to higher spatial frequencies, and they permit detection of patterns at much lower S/N ratios i.e. they lower the signal-to-noise ratio at which the system is able to detect the presence of a pattern in a background of statistical visual noise. The latter effect is not limited to the higher spatial frequencies but also affects the very low spatial frequencies which are normally used for pattern detection. Previous failures to demonstrate clear deficits in pattern discrimination after 17/18 lesions in cats may be attributed to the fact that the patterns presented for discrimination were not masked by visual noise. Movement of patterns led to a slight, but not significant improvement of the performance in both normal and lesioned cats, but the deficits found for stationary and moving patterns were more or less equal.

Immediate postoperative retention of visual discriminations following selective cortical lesions in the cat

Cats were trained preoperatively for brightness discrimination, and 7 pattern and form discriminations, and then retested for preoperative retention on each discrimination. Cortical lesions were then placed in areas 17 and 18 in one group (4 cats), in areas 17, 18 and 19 in another group (3 cats), and in suprasylvian cortex (areas 7, 21, and parts of 19, 5 and the lateral suprasylvian cortex) in a third group (4 cats). Results are also reported for a fourth group with extensive suprasylvian lesions, to which was added an unintended undercutting of areas 17 and 18 (4 cats). While during original preoperative learning the training continued until a fixed, stringent criterion of performance was attained, both preoperative and postoperative retention was tested in short sessions, involving a limited number of trials and a less stringent statistical criterion (significant run). After extensive removal of areas 17 and 18, all cats behaved as though following the cortical lesion they could immediately recognize the discriminative stimuli as efficiently as before, with no need for retraining. On the contrary, the group with areas 17, 18 and 19 lesions showed a substantial postoperative loss of all discriminations, and especially for the more difficult form discriminations, the reattainment of a significant level of performance was hard or impossible within the allotted number of trials. Also in the group with limited suprasylvian lesions, postoperative retention was generally impaired, but the reacquisition of efficient performance was superior to that of the previous group. Finally, large suprasylvian lesions encroaching on the white matter under areas 17, 18 and 19 proved disruptive for all discriminative capacities, both in retention and in relearning. The excellent retention of all discriminations following areas 17 and 18 lesions once again shows that these areas are by no means essential for complex vision in the cat. In addition, the results strongly indicate that the high-level visual capacities of destriate cats are not due to reorganization of readaptation processes occurring in extrastriate areas after a 17/18 removal. The clear-cut retention deficits which were present in cats with cortical lesions more extensive than areas 17 and 18 or outside of the latter areas prove the essential participation of extrastriate cortical areas in visual discrimination including form. However, the distribution of functions among the various visual cortical areas in visual discrimination remains poorly understood.(ABSTRACT TRUNCATED AT 400 WORDS)

Interhemispheric transmission of information in manual and verbal reaction-time tasks

In principle, interhemispheric transmission time can be assessed in normal man by measuring simple reaction time (RT) to lateralized light stimuli. Two paradigms have usually been employed: (1) a manual RT paradigm, whereby interhemispheric transfer time is equated with the difference between ipsilateral and contralateral responses, and (2) a vocal RT paradigm whereby interhemispheric transmission time is equated with a right field/left hemisphere advantage. Experiment I demonstrated the expected systematic advantage of the ipsilateral over the contralateral manual responses, but failed to show a right field advantage for four types of verbal responses, including two monosyllabic and two bisyllabic words. The RT of a blowing response was also equal for the two visual fields. In Experiment II the same words that had failed to yield a right field advantage in Experiment I were used for discriminating between digits presented in the right and left visual fields. Digits were discriminated by naming or by association with a given word. In both tasks RT was significantly faster for right field presentations, and the advantage over the left field was about 5 ms. In Experiment III verbal and manual RTs were measured in a task involving the discrimination between single and double light stimuli presented in the right and left visual fields. There was a nonsignificant advantage for the right field, which was equal for manual and verbal responses. It is concluded that the right field superiority observed with verbal stimuli is due to the processing of the input, and is independent of the verbal or non-verbal nature of the output.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in binocular interactions between cortical areas 17 and 18 and superior colliculus of Siamese cats

In Siamese cats most retinal projections are crossed and almost all neurons in cortical areas 17 and 18 respond exclusively to the contralateral eye. However, neurons in visual areas in the suprasylvian sulci (LSA) can be activated from both eyes, since the input from the ipsilateral eye is relayed to them by the corpus callosum (Marzi et al., '80). This study shows that the superior colliculus (SC) is also the site of binocular interactions in Siamese cats with a predominantly monocular organization of areas 17 and 18. In 8 unanesthetized, brainstem-sectioned Siamese cats, identified as "Boston" on the basis of the pattern of visual field representation in areas 17 and 18 (Hubel and Wiesel, '71; Shatz, '77a), only 25 out of 140 neurons in these areas could be driven from both eyes, whereas as many as 124 out of 143 SC neurons showed a clear binocular input. The input from each eye to the ipsilateral SC and the resulting binocular interactions in our Siamese cats might have depended on corticotectal projections from LSA. If so, since a posterior callosal section abolishes the input from each eye to ipsilateral LSA, the SC should similarly be affected by callosotomy. The posterior half of the corpus callosum was sectioned acutely in three Siamese cats after recording from SC; the input from the ipsilateral eye to SC was suppressed by this operation in one cat (which, however, also had an unintended interruption of the posterior, habenular, and anterior intertectal commissures), and drastically reduced in the other two. In a fourth cat, in which callosotomy was performed three weeks before recording from SC, the number of SC neurons responsive to the ipsilateral eye was significantly inferior to that of Siamese cats with an intact corpus callosum. These findings indicate the importance of the corpus callosum for ipsilateral eye-SC relations, as well as for SC binocularity, in Siamese cats. At the same time they imply that noncallosal routes can also transmit information from each eye to the ipsilateral SC in callosotomized Siamese cats.

Behavioral and electrophysiological effects of unilateral optic tract section in ordinary and Siamese cats

In ordinary cats, section of one optic tract produced a complete contralateral hemianopsia in both eyes. Single-unit recordings showed a normal representation of the contralateral nasal retina and ipsilateral temporal retina in the SC on the side of the intact optic tract. In addition, in the rostral portion of this SC there was a representation of a small portion of the contralateral temporal retina. This portion was apposed to the vertical meridian and its width was at most 6 degrees. In the anterior half of the SC on the side of the optic tract section, despite the interruption of any direct optic input, there was an extensive representation of the ipsilateral nasal retina and the contralateral temporal retina. This indirect visual input to the SC ipsilateral to the optic tract section was absent in a cat with a section of the forebrain commissures. In Boston Siamese cats, section of one optic tract led to a virtually complete blindness in the eye contralateral to the section, whereas the other eye retained a full visual field, although the responsiveness of the temporal retina beyond 20 degrees from the vertical meridian was reduced. Similarly, the nasal hemiretina and most of the temporal hemiretina on the side of the section were represented in the opposite SC, whereas stimulation of the eye contralateral to the section could not drive SC units. There was some evidence that the visual field of the eye on the side of the section could at least in part be represented in the SC on the same side. The findings indicate that the crossed projections from temporal hemiretina in the ordinary cat, and the uncrossed projections from temporal hemiretina in the Siamese cat are insufficient by themselves to sustain visual orientation and to drive SC neurons. Each half of the visual field in the ordinary cat, and the field of each eye in the Siamese cat, can be represented in the ipsilateral SC via across-the-midline, indirect connections.