Josephine Semmes

Hemispheric specialization: A possible clue to mechanism☆

Abstract Studies of sensory and motor capacities of the hands in brain-injured subjects indicate that, contrary to the prevailing view, these capacities are represented differently in the two hemispheres, tending to be focally represented in the left hemisphere but diffusely represented in the right. This difference between the hemispheres was found not only for contralateral sensorimotor function, but also for ipsilateral; moreover, such a difference seemed to apply not only to these relatively simple manual capacities, but to more complex abilities as well. The two contrasting modes of neural organization, which appear to be linked to the hemisphere rather than to the particular hand or level of function involved, provide a possible clue to the mechanism of hemispheric specialization. More specifically, it is proposed that focal representation of elementary functions in the left hemisphere favors integration of similar units and consequently specialization for behaviors which demand fine sensorimotor control, such as manual skills and speech. Conversely, diffuse representation of elementary functions in the right hemisphere may lead to integration of dissimilar units and hence specialization for behaviors requiring multimodal coordination, such as the various spatial abilities.

Behavioral consequences of selective subtotal ablations in the postcentral gyrus ofMacaca mulatta

Abstract This study asked the question of whether the anatomical and physiological subdivisions of the postcentral gyrus (Brodmanns areas 3,1, and 2) contribute differentially to the ability of the monkey to discriminate objects by touch. After removal of the hand area of either the posterior bank of the central suclus (area 3), anterior half of the crown of the postcentral gyrus (area 1), or the posterior half of the postcentral gyrus (area 2), animals were trained on a series of tactile discrimination tasks. Those animals receiving the area 3 lesion were severely impaired on all tasks. Those animals whith the area 1 lesion were retarded on learning those tasks which could be characterized as involving discrimination of ‘texture’ whereas those with the area 2 lesion were significantly impaired on tasks involving the discrimination of ‘angels’. On the basis of electrophysiological recordings made in the adjacent remaining cortex, the input from the hand to area 3 was determined to be normal following an area 1 lesion, as was the input to area 1 following an area 3 lesion. Histological examination of the brains suggested that in most cases the boundaries of the lesions were complete as intended, sparing the adjacent areas. The behavioral specificity suggested by the patterns of deficits on the discrimination tasks was discussed both in terms of known anatomical projections and physiological input to each area, as well as in terms of possible physiological interactions between the areas.

A non-tactual factor in astereognosis

Abstract The classical view that astereognosis represents a modality-specific disorder of integration independent of sensory changes was examined in 65 men with penetrating brain injuries. The usual test of verbal identification of familiar objects was supplemented by a series of non-verbal tasks requiring discrimination of tactual object-qualities. Performance was evaluated in relation to basic somatosensory functions, motor status, cognitive abilities, and spatial orientation. The results failed to support the classical view. Although tactual shape perception may be impaired in the absence of somatosensory defect, such impairment is related to a spatial factor which is not specific to somesthesis.

Visual Agnosia: A Case of Reduced Attentional “Spotlight”?

Abstract A case study is presented of a patient with presenile dementia, for whom the dominant clinical feature from onset was a visual agnosia. The characteristics of the patient’s visual agnosia were investigated in light of her apparent use of a “feature-by-feature” strategy to identify objects. Results from various tasks showed that the patient was unable to use global shape information or other grossly defined property cues characteristic of a “wide angle” attentional processing stage (Treisman, 1988) in object recognition. The patient appeared instead to rely on ‘parts’ or identifying features of the objects for object recognition. The patient showed significant improvement when the size of the drawing was reduced in size, thus suggesting that the disorder may be functionally localized to a reduction of the patient’s attentional “spotlight”.

Critical flicker frequency after unilateral temporal lobectomy in man

Abstract Patients with left or right temporal lobectomy were compared with a matched normal group on CFF. Of the patients without field defects, those with left, but not with right, hemisphere removals were impaired (especially in the nasal fields), suggesting that hemispheric specialization may extend even to basic sensory processes. Regardless of the side of the lobectomy, patients with field defects (from optic radiation damage) showed losses not only in the partially anopic quadrant but also in all three “intact” quadrants. This result confirms earlier reports and suggests that interdependence among parts of the visual field is based on interaction between as well as within the hemispheres.

Effects of Isolating Sensorimotor Cortex in Monkeys

Summary This is the second in a series of studies, prompted by results in brain-injured human cases, on the role in manual discrimination of cortex outside the contralateral sensorimotor region. The first dealt with the effects in monkeys of sensorimotor and complementary nonsensorimotor abaltions ipsilateral to the tested hand. The present study investigates the nonsensorimotor region contralateral to the tested hand. Ablation of this region produced a severe retardation in learning the preliminary discriminations but not the later, difficult ones (the reverse of that found earlier after ipsilateral sensorimotor lesions) and an elevation of roughness but not of size thresholds. Subsequent analysis with modified lesions showed that, whereas the roughness deficit resulted from the tissue ablation alone, the severe impairment on the preliminary tasks was due to the tissue ablation plus transection of the corpus callosum (an unintended accompaniment of the cingulate removal in the initial group). The impairment in the early phases of discrimination learning, even though confined to one hand, was interpreted as a difficulty in developing basic tactual discrimination strategies and was attributed to disconnection of the contralateral sensorimotor cortex from other regions of the telencephalon. Observation suggested further that the difficulty in discrimination learning was one aspect of a more general deficiency in the adaptive use of the hand.

A comparison of precentral and postcentral cortical lesions on somatosensory discrimination in the monkey.

Summary Monkeys ( Macaca mulatta ) were prepared with one of four adjacent lesions at the level of the hand representation in the sensorimotor cortex opposite the hand to be trained on a series of somatosensory discrimination tasks. The two precentral lesions, one anterior and the other posterior in the precentral gyrus, failed to impair performance on the tasks. In contrast, both postcentral lesions, one anterior and the other posterior in the postcentral gyrus, produced deficits. The anterior postcentral lesion produced a severe decrement on virtually all tasks, whereas the posterior postcentral removal chiefly affected certain form discriminations, which were the most difficult tasks of the series. The results are discussed in relation to the much less marked effects of postcentral lesions previously reported and to neural mechanisms of somatosensory discrimination.

Interaction between the hemispheres in unimanual somesthetic learning

Abstract The prefrontal and temporal lobes of both hemispheres in monkeys have been found to contribute to unimanual learning of somesthetic discrimination and reversal. To investigate the pathways through which this contribution is made, subjects with bilateral removals of each lobe (B group) were compared with those having corresponding unilateral removals, opposite the hand trained, combined with section of the telencephalic commissures (U+s group). Unoperated monkeys and monkeys with contralateral lesions alone served as controls. Since the deficit was the same for corresponding prefrontal and temporal lesion subgroups, these were combined for further comparisons among the two major types of preparations and the controls. Although monkeys of the B and the U+s groups showed quantitatively similar over-all impairments, further analysis of the course of reversal learning revealed a double dissociation of deficits. The B group subjects were impaired only in extinction of the original habit, whereas those of the U + s group were deficient only in acquisition of the new habit after the acute extinction phase. Differences between these groups were also uncovered by closer examination of their performance on the discrimination tasks. These results were interpreted in terms of the differential roles of “horizontal” and “vertical” connections. It was proposed that the deficit specific to animals of the B group was one of perseverative interference with learning and was due to the loss of vertical connections between nonsensorimotor regions of the endbrain and lower centers; by contrast, the deficit specific to animals of the U+s group was attributed to a sensory and motor “neglect”, resulting from the loss of horizontal connections between nonsensorimotor and sensorimotor regions within the endbrain.

Somesthetic Discrimination Learning after Partial Nonsensorimotor Lesions in Monkeys

Summary Cortical isolation of the sensorimotor region of one hemisphere in the monkey (achieved by combining a unilateral removal of the prefrontal, temporal, and occipital lobes with transection of the corpus callosum) was previously found to produce a severe retardation in learning tactual discriminations with the opposite hand. To determine which part of the three-lobe ablation was critical for this “disconnection syndrome,” callosal transection was combined in the present study with one-or two-lobe removals. The tasks used were the two simple discriminations employed previously and the reversal of the second of these. It was found that either the prefrontal or the temporal lesion produced an impairment, that the effects of these two lesions were roughly equal both for discri-mination and for reversal, and that combined prefrontal and temporal lesion resulted in an impairment comparable in severity to that following the original three-lobe ablation. The occipital lesion appeared to have no effect either alone or in any combination. The attempt to relate these findings to the deficit reported earlier after (orbital) prefrontal or (medial) temporal lesions, both in somesthesis and in other modalities, suggested that the impairment found in the present study was supramodal in nature, although it differed in certain respects from earlier descriptions. It was concluded that the supramodal contribution to somesthetic function from the prefrontal and temporal lobes is mediated by cortico-cortical con-nections with the sensorimotor region governing the hand used.

Further Studies of Anterior Postcentral Lesions in Monkeys

Summary It has recently been reported that monkeys with unilateral ablation of the hand area in the postcertral gyrus opposite the hand tested are severely impaired in somesthetic discrimination tasks. Yet earlier investigations had revealed either moderate or no impairment following complete bilateral removal of the postcentral gyri. The work reported here was an attempt to reconcile these divergent findings by exploring factors which might ameliorate the deficit: preoperative training on all tasks, a six-month recovery period, and complete or bilateral postcentral removals. None of these factors proved to be effective in lessening the severity of the impairment. It is suggested that, in the earlier studies, the representation of the finger tips near the bottom of the posterior bank of the central sulcus may have been spared.