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 effect of hemidecortication on the inhibitory interactions in the superior colliculus of the cat.

Single neurons were recorded extracellularly from the superficial layers of the superior colliculus (SC) in 21 curarized cats. Four animals were normal unoperated cats, 17 were animals in which all cortical visual areas were ablated on one side from 7 to 69 days before the electrophysiological experiments. After cortical ablation all animals were blind in the visual field contralateral to the ablated side. In both normal and hemianopsic cats the effect of a visual stimulus located very far from the excitatory part of the unit receptive field, on the neuron responses to visual stimuli was studied. The remote stimulus (extra-field stimulus) was a hand moved black spot 10 degrees in diameter. In normal animals the introduction of the extra-field stimulus in the hemifield contralateral or ipsilateral to the recorded SC produced a marked reduction of unit responses to visual stimuli presented in their receptive field. This effect was particularly strong when the extra-field stimuli were introduced in the hemifield contralateral to the recorded side. In the hemianopsic animals the neurons of the SC ipsilateral to the lesion (receptive fields in the behaviorally blind hemifield) responded well to visual stimuli, but were only weakly inhibited by the extra-field stimuli presented in the blind hemifield. The neurons of this colliculus with the exception of those in the upper part of stratum griseum superficiale were normally inhibited by stimuli presented in the normal hemifield. The neurons of the SC contralateral to the lesion responded well to visual stimuli and were normally inhibited by stimuli presented in the normal hemifield; they were virtually not affected by stimuli presented in the blind hemifield. Mechanisms responsible for the abnormal inhibitory interactions between and within colliculi after cortical lesions and the possible behavioral implications of the findings are discussed.

Absence of normal activation of the left anterior fusiform gyrus during naming in left temporal lobe epilepsy.

Patients with left temporal lobe epilepsy (TLE) often have impaired naming. We studied 13 patients with left TLE and 10 healthy control subjects with [(15)O]H2O PET during visual confrontation naming. Statistical mapping detected multiple regions of significant cerebral blood flow increases within individuals. The left fusiform gyrus was activated in nine healthy subjects, but only in two patients with TLE (a significant difference, p < 0.001). Other activation sites were more variable in healthy subjects and those with TLE. Impaired naming ability may be associated with a lack of increased cerebral blood flow in the left fusiform gyrus in TLE.

Spatial attentional shifts: implications for the role of polysensory mechanisms.

Simple reaction times to lateralized visual (Experiment 1) or auditory (Experiment 2) targets were studied in normal subjects. The targets were preceded by a visual or auditory cue located on the same (valid cue), or opposite (invalid cue) side as the subsequent target, or on both sides (neutral cue), with one of four cue target intervals. The validity of visual and auditory cues influenced the speed of response to the visual target but not to the auditory target. It is hypothesized that cross-modal cueing of spatial position works only with modalities for which a movement (e.g. saccade) leads to improved sensory analysis.

Spatial attentional shifts: further evidence for the role of polysensory mechanisms using visual and tactile stimuli.

Normal subjects performed simple reaction time responses to lateralized visual target stimuli (Experiment 1) and lateralized tactile target stimuli (Experiment 2). In each experiment, the lateralized targets were preceded at one of four intervals by a visual or tactile cue located on the same (valid cue), or opposite (invalid cue) side, or on both sides (neutral cue). The validity of the visual and tactile cues influenced the speed of response to either target stimulus. These findings, together with those previously reported (Buchtel and Butter, Neuropsychologia 26, 499-509, 1988), are consistent with the view that intra- and inter-modal spatial cueing is effective with modalities that are linked to orienting systems in which movements of the sensory array serve to improve sensory analysis.

Effects of stimulus source and intensity on covert orientation to auditory stimuli.

Normal subjects showed costs and benefits of informative auditory spatial cues with auditory targets in RT tasks when stimulus intensity was low, or when the stimuli were presented monaurally through headphones. These findings imply that attention to auditory stimuli, like attention to visual or tactile stimuli, can be shifted spatially in detection tasks, and that covert orienting to auditory stimuli occurs in conditions favoring the intention to orient the head to a sound source. According to this view, orienting of attention to auditory stimuli, as well as to visual and tactile stimuli, is linked functionally to mechanisms controlling overt orienting movements that increase stimulus identification.

Tactile rod bisection: hemispheric activation and sex differences.

Blindfolded subjects estimated with either hand the center of rods positioned in either left or right hemispace. In one condition, they also performed a concurrent verbal task. Bias and variability of bisection settings were the dependent variables. Bisections performed in left hemispace were biased to the left of true center, more so when the left hand was used. In contrast, bisections performed in right hemispace were biased rightward, more so when the right hand was used. There were no significant differences in variability of bisections in any condition. Interactions of hand with hemispace in which the task was performed differed for the two sexes. Moreover, the secondary verbal task had no effect on either measure. We conclude that of several factors that may underlie bisection biases, attention was the most relevant.

Impairments in orienting to visual stimuli in monkeys following unilateral lesions of the superior sulcal polysensory cortex.

Monkeys were tested for head and eye orientation to illuminated lamps in a hemisphere before and after serial, unilateral lesions of the polysensory superior temporal cortex (STS) or control lesions. Following STS lesions they were impaired in orienting to contralateral lamps; this impairment was more severe and persistent when a ipsilateral stimulus in the mirror-image position was simultaneously presented. These findings, together with deficits in manual reaching and grasping observed following STS lesions, support the view that the STS is part of a polysensory system controlling attention and exploratory movements.

Left and right hemisphere contributions to physiognomic and verbal discrimination.

The relative contributions of the right- and left-temporal lobes in rapid recognition of faces and letters were studied in patients with anterior right- or left-temporal lobe excisions and a matched control group. On the basis of findings in patients with unilateral and bilateral brain damage, it was hypothesized that left hemisphere damage would not change the reaction time of letters analyzed by the right hemisphere and that right hemisphere damage would not change the reaction time of faces analyzed by the left hemisphere. The hypothesis was supported for letters but not for faces. Patients in the right-temporal group, particularly those with large hippocampal removals, were slow to recognize faces in both visual fields. Two possible explanations for the findings with faces are explored: One holds that right hemisphere mechanisms are involved even when a face is presented to the left hemisphere for rapid recognition; the other holds that specialized encoding is carried out by the right hemisphere during learning, with the encoded template then being used by each hemisphere independently.

Hand posture in writing: possible artifacts from self report.

Seven hundred forty undergraduate students were asked to report the hand posture they used during writing and the presence or absence of left-handedness in the immediate family. Drawings were provided of prototypical "normal" and "inverted" posture and subjects were also given the option of reporting that their hand posture was somewhere between the two illustrated positions. The incidence of reporting "between" was 17% for left-handers and 46% for right-handers. These results suggest that experimenters should exercise caution when categorizing subjects on the basis of self-report forms that include only two choices: normal posture and inverted posture.

Hemispheric superiority in reaction time to faces: a sex difference.

In males but not females, reaction time to faces is faster when stimuli are presented to the right hemisphere than when presented to the left hemisphere. The complete lack of a hemispheric difference in females suggests that with brief exposure and immediate judgement, a lateralized mechanism specialized for faces can be activated only in males.

Verbal fluency and positron emission tomographic mapping of regional cerebral glucose metabolism.

Impairment in verbal fluency (VF) has been a consistently reported clinical feature of focal cerebral deficits in frontal and temporal regions. More recent behavioral activation studies with healthy control subjects using positron emission tomography (PET), however, have noted a negative correlation between performance on verbal fluency tasks and regional cortical activity. To see if this negative relationship extends to steady-state non-activation PET measures, thirty-three healthy adults were given a VF task within a day of their 18F-2-fluoro-2-deoxy-D-glucose PET scan. VF was found to correlate positively with left temporal cortical region metabolic activity but to correlate negatively with right and left frontal activity. VF was not correlated significantly with right temporal cortical metabolic activity. Some previous studies with normals using behavioral activation paradigms and PET have reported negative correlations between metabolic activity and cognitive performance similar to that reported here. An explanation for the disparate relationships that were observed between frontal and temporal brain areas and VF might be found in the mediation of different task demands by these separate locations, i.e., task planning and/or initiation by frontal regions and verbal memory by the left temporal area.

Auditory agnosia: apperceptive or associative disorder?

Neuropsychological testing of a patient with auditory agnosia showed that certain difficulties in the initial analysis of sounds may be the cause of his inability to understand spoken words and other sounds. Abnormalities included a slow reaction time to brief auditory stimuli (but not to equally brief visual stimuli or to longer auditory stimuli) and the need for approximately 1/4 sec of silence between two tones before the patient was able to hear them as separate. He could identify words and word associations if he was able to view the object whose name or word associate he was hearing. The findings imply that this patient's deficit in comprehending speech was probably apperceptive rather than associative in origin.

Advance directives: making them more available to hospitals patients.

This article details methods developed to facilitate patients' understanding and execution of an advance directive (e.g., living will, treatment preferences, and durable power of attorney for healthcare). The approach takes advantage of the computerization of patient records, using automatically generated e-mail messages sent to an advance directives consultation team.