Comparison between visual half-field performance and cerebral blood flow changes as indicators of language dominance

The determination of hemispheric language dominance (HLD) can be accomplished in two ways. One approach relies on hemispheric differences in cerebral blood flow velocity (CBFV) changes during language activity, while the other approach makes use of performance differences between the left and right visual field when verbal stimuli are presented in a tachistoscopic visual field paradigm. Since both methodologically different approaches claim to assess functional HLD, it seems plausible to expect that the respective laterality indices (LI) would correspond. To test this expectation we measured language lateralisation in 58 healthy right-handed, left-handed, and ambidextrous subjects with both approaches. CBFV changes were recorded with functional transcranial Doppler sonography (fTCD). We applied a lexical decision task with bilateral visual field presentation of abstract nouns and, in addition, a task of mental word generation. In the lexical decision task, a highly significant right visual field advantage was observed for number of correct responses and reaction times, while at the same time and contrary to expectation the increase of CBFV was significantly higher in the right than left hemisphere. During mental word generation, the acceleration of CBF was significantly higher in the left hemisphere. A comparison between individual LI derived from CBF measurement during mental word generation and from visual field performances in the lexical decision task showed a moderate correspondence in classifying the subjects' HLD. However, the correlation between the corresponding individual LI was surprisingly low and not significant. The results are discussed with regard to the issue of a limited reliability of behavioural LI on the one hand and the possibility of a fundamental difference between the behavioural and the physiological indicators of laterality on the other hand.     

Comparison between visual half-field performance and cerebral blood flow changes as indicators of language dominance

The determination of hemispheric language dominance (HLD) can be accomplished in two ways. One approach relies on hemispheric differences in cerebral blood flow velocity (CBFV) changes during language activity, while the other approach makes use of performance differences between the left and right visual field when verbal stimuli are presented in a tachistoscopic visual field paradigm. Since both methodologically different approaches claim to assess functional HLD, it seems plausible to expect that the respective laterality indices (LI) would correspond. To test this expectation we measured language lateralisation in 58 healthy right-handed, left-handed, and ambidextrous subjects with both approaches. CBFV changes were recorded with functional transcranial Doppler sonography (fTCD). We applied a lexical decision task with bilateral visual field presentation of abstract nouns and, in addition, a task of mental word generation. In the lexical decision task, a highly significant right visual field advantage was observed for number of correct responses and reaction times, while at the same time and contrary to expectation the increase of CBFV was significantly higher in the right than left hemisphere. During mental word generation, the acceleration of CBF was significantly higher in the left hemisphere. A comparison between individual LI derived from CBF measurement during mental word generation and from visual field performances in the lexical decision task showed a moderate correspondence in classifying the subjects' HLD. However, the correlation between the corresponding individual LI was surprisingly low and not significant. The results are discussed with regard to the issue of a limited reliability of behavioural LI on the one hand and the possibility of a fundamental difference between the behavioural and the physiological indicators of laterality on the other hand.     

Cerebral lateralisation at different stages of facial processing

Two experiments are reported which explore the cerebral lateralisation of different stages of facial processing. In Experiment 1 subjects were briefly presented with either upright or inverted Mooney faces to either the left or right visual field. The subjects' task was to decide whether or not the stimulus had the configuration of a face. The data showed a left visual field superiority in performing this task. In Experiment 2 subjects were briefly presented with line drawings of faces to either the left or right visual field. On half the trials all the facial features were intact whilst in the remainder one feature had been replaced by an anomalous feature in the same position. The subjects were required to detect whether an anomalous feature was present or not and this task produced a right visual field superiority. These data argue against a global right hemisphere superiority in facial processing and suggest, instead, that hemispheric superiority will vary depending on the stage of facial analysis investigated. On the basis of these and other recent findings it would appear that the right hemisphere is advantaged for the holistic stages of facial processing whilst the left-hemisphere has superiority for analysis at the level of individual features. However, the data are ambiguous as to whether the hemispheric differences detected reflect face-specific or general processing characteristics of the cerebral hemispheres.     

Hemispheric specialisation,spatial activity experience,and sex differences on tests of mental rotation ability

A sex difference in mental rotation ability is well established. Among the explanations offered for this difference are that males are more frequently right hemisphere specialised for spatial processing and that they receive more spatial processing experience because of the sex stereotyping of activities involving spatial ability that exists in Western societies. Several studies have shown that males do report more spatial activity experiences on the Spatial Activity Questionnaire of Newcombe, Bandura, and Taylor (1983) and that such experience does correlate with performances on tests of mental rotation. However, two prior lateralised tachistoscopic studies failed to show right hemisphere superiority for spatial task performance or that it was positively associated with better performances on the Vandenberg Mental Rotation Test. The tachistoscopic tasks used in these studies can be criticised on methodological grounds. The present study employed a new tachistoscopic task that showed that males were significantly more right hemisphere specialised (left visual-field superior) for the tachistoscopic task than females. Magnitudes of right hemisphere specialisation on the tachistoscopic task were positively and significantly related to mental rotation ability. Spatial activity experiences were also more common in males. Covariance and partial correlation analyses indicated that the sex difference in spatial ability was primarily due to sex differences in right hemisphere specialisation for mental rotation, while sex differences in spatial activity experiences were only secondarily involved.     

Hemispheric specialisation, spatial activity experience, and sex differences on tests of mental rotation ability

A sex difference in mental rotation ability is well established. Among the explanations offered for this difference are that males are more frequently right hemisphere specialised for spatial processing and that they receive more spatial processing experience because of the sex stereotyping of activities involving spatial ability that exists in Western societies. Several studies have shown that males do report more spatial activity experiences on the Spatial Activity Questionnaire of Newcombe, Bandura, and Taylor (1983) and that such experience does correlate with performances on tests of mental rotation. However, two prior lateralised tachistoscopic studies failed to show right hemisphere superiority for spatial task performance or that it was positively associated with better performances on the Vandenberg Mental Rotation Test. The tachistoscopic tasks used in these studies can be criticised on methodological grounds. The present study employed a new tachistoscopic task that showed that males were significantly more right hemisphere specialised (left visual-field superior) for the tachistoscopic task than females. Magnitudes of right hemisphere specialisation on the tachistoscopic task were positively and significantly related to mental rotation ability. Spatial activity experiences were also more common in males. Covariance and partial correlation analyses indicated that the sex difference in spatial ability was primarily due to sex differences in right hemisphere specialisation for mental rotation, while sex differences in spatial activity experiences were only secondarily involved.     

Hemisphere priming through practice in a musical chords task

In a study of selective hemisphere activation, the performance of 30 subjects given either a "local" or "global" priming activity before and after a monoaurally presented chord analysis task was compared with an unprimed control group. The hypothesis was that ear advantage scores on the chords task would show increased left hemisphere involvement following local priming and increased right hemisphere involvement following global priming. The results failed to support this hypothesis. All subjects, however, regardless of priming condition, showed a very strong practice effect represented by a shift from a weak initial left ear (right hemisphere) advantage towards a significant right ear (left hemisphere) advantage (P less than 0.00006). The findings suggest that although the local/global priming activity did not lead to selective hemisphere activation, repeated exposure to the chords task resulted in increased use of analytic left hemisphere processing strategies as subjects became familiar with its processing requirements.     

Heart rate and fluency performance among high- and low-anxious men following autonomic stress

Given the well-known associations between the right frontal lobe and negative effective states, as well as the involvement of the right posterior region in the modulation of autonomic nervous system arousal, anxiety is thought to be associated with relative right hemisphere activity (or decreased left hemisphere activity). However, recent evidence suggests that the left hemisphere predominantly modulates parasympathetic efferents, while the right hemisphere predominantly modulates sympathetic efferents of the autonomic nervous system. Thus, the left hemisphere should also play a role in anxiety, as anxiety is associated with decreased parasympathetic activity (increased heart rate). In order to investigate this hypothesis, the present experiment examined the influence of a well-known autonomic stressor (cold pressor) on heart rate (HR), and verbal and nonverbal fluency performance. Fifteen high level (HI) anxious and 15 low level (LO) anxious men completed verbal and nonverbal fluency measures after they were asked to place their left hand in ice water for 45 s. HI anxious subjects demonstrated opposing HR patterns during performance of the verbal (increased HR) and nonverbal (decreased HR) fluency tasks following cold pressor stimulation. In addition, HI anxious men generated fewer words per minute on the verbal fluency task than did LO anxious men. Although the data are supportive of functional right hemisphere activity among anxious individuals, the contribution of decreased left hemisphere activity (i.e., reduced parasympathetic activity and diminished verbal fluency performance) is also discussed.     

Performance of younger and older adults in lateralised right and left hemisphere asymmetry tasks supports the HAROLD model

The population of industrialised societies has increased tremendously over the last century, raising the question of how an enhanced age affects cognition. The relevance of two models of healthy ageing are contrasted in the present study that both target the functioning of the two cerebral hemispheres. The right hemi-ageing model (RHAM) assumes that functions of the right hemisphere decline before those of the left hemisphere. The Hemispheric Asymmetry Reduction in Older Adults (HAROLD) model suggests that the contralateral hemisphere supports the normally superior hemisphere in a given task resulting in a reduced hemispheric asymmetry overall. In a mixed design 20 younger and 20 older adults performed both a task assessing a left (lateralised lexical decisions) and a right (sex decisions on chimeric faces) hemisphere advantage. Results indicated that lateralised performance in both tasks was attenuated in older as compared to younger adults, in particular in men. These observations support the HAROLD model. Future studies should investigate whether this reduced functional hemispheric asymmetry in older age results from compensatory processes or from a process of de-differentiation.     

Qualitatively different organizational structures of lexical knowledge in the left and right hemisphere

The present study investigates and discusses the organization of lexical knowledge in the intact left and right hemisphere within the framework of hemisphere-specific cognitive modes of processing. Using a divided visual field technique, word pairs of concrete nouns had to be judged. Semantic relation was either intraconceptual (coordinates) or interconceptual (locative). The results suggest that the left hemisphere, lexical structures are predominantly based on intraconceptual relationships corresponding to its analytic sequential processing mechanism, whereas in the right hemisphere, lexical entries are exclusively associated by means of interconceptual relationships in accordance with its "gestalthaft" holistic processor.     

Asymmetry in alpha power predicts accuracy of hemispheric lexical decision.

OBJECTIVE: Previous work has shown that individual differences in resting alpha asymmetry are associated with efficacy on a variety of cognitive tasks. Still unresolved is how ongoing alpha asymmetry relates to behavioral asymmetry, explored here using lateralized lexical decision. METHODS: Alpha power immediately preceding lexical decision trials was measured to assess cognitive engagement during word recognition. This was compared with behavioral performance for the task, measured by accuracy and latency of the lexical decision response. RESULTS: Greater relative left hemisphere alpha power (i.e. higher asymmetry) immediately before presentation of a word led to reduced likelihood for its successful identification. Greater alpha asymmetry was also associated with reduced performance for identifying stimuli lateralized to the right visual field. CONCLUSIONS: Word recognition is facilitated by decreased asymmetry in cognitive engagement in the two cerebral hemispheres, particularly when the stimuli are lateralized to the left hemisphere (right visual field). SIGNIFICANCE: Results address the role of cognitive engagement in the two cerebral hemispheres, and its relationship with lexical access.     

Concreteness: nouns, verbs, and hemispheres

The preferential processing of concrete versus abstract nouns, and of active versus static or "quiet" verbs, was investigated using a lateralized lexical decision task in 32 normal and 4 commissurotomized subjects. Both groups of subjects showed the concreteness effect for nouns in both visual fields. The disconnected right hemisphere of two commissurotomized subjects responded with above chance performance only to concrete nouns. Neither group showed an activeness effect for verbs in either visual field. This supports an imageability rather than a multisensory representation interpretation of the concreteness effect. A comparison of responses to words and to nonwords revealed that males had a "no" bias to stimuli in the left visual field, and both males and females showed a slight "yes" bias for stimuli in the right visual field. These data suggest that the lexical decision task is complex and that word and nonword decisions constitute partly independent functional components. We interpret the sex differences as an indication of strategic rather than functional differences in lateralization patterns between males and females.     

Interhemispheric cooperation for face recognition but not for affective facial expressions

Interhemispheric cooperation can be indicated by enhanced performance when stimuli are presented to both visual fields relative to one visual field alone. This "bilateral gain" is seen for words but not pseudowords in lexical decision tasks, and has been attributed to the operation of interhemispheric cell assemblies that exist only for meaningful words with acquired cortical representations. Recently, a bilateral gain has been reported for famous but not unfamiliar faces in a face recognition task [Neuropsychologia 40 (2002) 1841]. In Experiment 1 of the present paper, participants performed familiarity decisions for faces that were presented to the left (LVF), the right (RVF), or to both visual fields (BVF). An advantage for BVF relative to both LVF and RVF stimuli was seen in reaction times (RTs) to famous faces, but this bilateral advantage was absent for unfamiliar faces. In Experiment 2, participants classified the expression (happy or neutral) of unfamiliar faces. No bilateral advantage was seen for expressions, although a right hemisphere superiority was seen in terms of higher accuracy for LVF and BVF trials relative to the RVF. Recognition of famous faces (but not of facial expressions) require access to acquired memory representations that may be instantiated via cortical cell assemblies, and it is suggested that interhemispheric cooperation depends on these acquired cortical representations.     

Neural correlates of recovery from aphasia after damage to left inferior frontal cortex

OBJECTIVE: To determine neural correlates of recovery from aphasia after left frontal injury. METHODS: The authors studied the verbal performance of patients with infarcts centered in the left inferior frontal gyrus (IFG), using a battery of attention-demanding lexical tasks that normally activate the left IFG and a simpler reading task that does not normally recruit the left IFG. The authors used positron emission tomography (PET) and functional MRI (fMRI) to record neural activity in the same group of patients during word-stem completion, one of the attention-demanding lexical tasks. To identify potential neural correlates of compensation/recovery, they analyzed the resulting data for the group as a whole (PET, fMRI) and also for each participant (fMRI). RESULTS: Patients with damage to the left IFG were impaired on all attention-demanding lexical tasks, but they completed the word-reading tasks normally. The imaging studies demonstrated a stronger-than-normal response in the right IFG, a region homologous to the damaged left IFG. The level of activation in the right IFG did not correlate with verbal performance, however. In addition, a perilesional response within the damaged left IFG was localized in the two patients who gave the best performance in the word-stem completion task and showed the most complete recovery from aphasia. CONCLUSIONS: Right-IFG activity may represent either the recruitment of a preexisting neural pathway through alternative behavioral strategies or an anomalous response caused by removal of the left IFG. Perilesional activity in the left IFG may represent sparing or restoration of normal function in peri-infarctual tissue that was inactive early on after injury. This activity may be of greater functional significance than right IFG activity because it was associated with more normal verbal performance.     

The influence of tobacco consumption on the relationship between schizotypy and hemispheric asymmetry

Tobacco use is positively associated with severity of symptoms along the schizophrenia spectrum. Accordingly it could be argued that neuropsychological performance, formerly thought to be modulated by schizotypy, is actually modulated by drug use or an interaction of drug use and schizotypy. We tested whether habitual cigarette smokers as compared to non-smokers would show a neuropsychological profile similar to that observed along the schizophrenia spectrum and, if so, whether smoking status or nicotine dependence would be more significant modulators of behavior than schizotypy. Because hemispheric dominance has been found to be attenuated along the schizophrenia spectrum, 40 right-handed male students (20 non-smokers) performed lateralized left- (lexical decisions) and right- (facial decision task) hemisphere dominant tasks. All individuals completed self-report measures of schizotypy and nicotine dependence. Schizotypy predicted laterality in addition to smoking status: While positive schizotypy (Unusual Experiences) was unrelated to hemispheric performance, Cognitive Disorganization predicted reduced left hemisphere dominant language functions. These latter findings suggest that Cognitive Disorganization should be regarded separately as a potentially important mediator of thought disorganization and language processing. Additionally, increasing nicotine dependence among smokers predicted a right hemisphere shift of function in both tasks that supports the role of the right hemisphere in compulsive/impulsive behavior.     

Neural correlates of the left-visual-field superiority in face perception appear at multiple stages of face processing

Studies in healthy individuals and split-brain patients have shown that the representation of facial information from the left visual field (LVF) is better than the representation of facial information from the right visual field (RVF). To investigate the neurophysiological basis of this LVF superiority in face perception, we recorded event-related potentials (ERPs) to centrally presented face stimuli in which relevant facial information is present bilaterally (B faces) or only in the left (L faces) or the right (R faces) visual field. Behavioral findings showed best performance for B faces and, in line with the LVF superiority, better performance for L than R faces. Evoked potentials to B, L, and R faces at 100- to 150-msec poststimulus showed no evidence of asymmetric transfer of information between the hemispheres at early stages of visual processing, suggesting that this factor is not responsible for the LVF superiority. Neural correlates of the LVF superiority, however, were manifested in a shorter latency of the face-specific N170 component to L than R faces and in a larger amplitude to L than R faces at 220-280 and 400-600 msec over both hemispheres. These ERP amplitude differences between L and R faces covaried across subjects with the extent to which the face-specific N170 component was larger over the right than the left hemisphere. We conclude that the two hemispheres exchange information symmetrically at early stages of face processing and together generate a shared facial representation, which is better when facial information is directly presented to the right hemisphere (RH; L faces) than to the left hemisphere (LH; R faces) and best when both hemispheres receive facial information (B faces).     

Attentional mechanisms and perceptual asymmetries in tachistoscopic recognition of words and faces.

In simultaneous bilateral tachistoscopic recognition tasks, normal right-handers named more words in the right visual field and recognized more faces in the left visual field. Priming the left hemisphere with a verbal task diminished the left field superiority for faces, and priming the right hemisphere with a face-recognition task, reduced the right visual field superiority for words. These priming effects disappeared when a recognition procedure was used. When words and faces were presented simultaneously, subjects required to attend to the faces and report them first showed a left visual field superiority for faces and a right visual field superiority for words. Subjects told to attend to the words and report them first showed only the right visual field superiority for words.     

HEART RATE AND FLUENCY PERFORMANCE AMONG HIGH- AND LOW-ANXIOUS MEN FOLLOWING AUTONOMIC STRESS

Given the well-known associations between the right frontal lobe and negative effective states, as well as the involvement of the right posterior region in the modulation of autonomic nervous system arousal, anxiety is thought to be associated with relative right hemisphere activity (or decreased left hemisphere activity). However, recent evidence suggests that the left hemisphere predominantly modulates parasympathetic efferents, while the right hemisphere predominantly modulates sympathetic efferents of the autonomic nervous system. Thus, the left hemisphere should also play a role in anxiety, as anxiety is associated with decreased parasympathetic activity (increased heart rate). In order to investigate this hypothesis, the present experiment examined the influence of a well-known autonomic stressor (cold pressor) on heart rate (HR), and verbal and nonverbal fluency performance. Fifteen high level (HI) anxious and 15 low level (LO) anxious men completed verbal and nonverbal fluency measures after they were asked to place their left hand in ice water for 45 s. HI anxious subjects demonstrated opposing HR patterns during performance of the verbal (increased HR) and nonverbal (decreased HR) fluency tasks following cold pressor stimulation. In addition, HI anxious men generated fewer words per minute on the verbal fluency task than did LO anxious men. Although the data are supportive of functional right hemisphere activ ity among anxious individuals, the contribution of decreased left hemisphere activity (i.e., reduced parasympathetic activity and diminished verbal fluency performance) is also discussed.     

Laterality and sex differences for visual recognition of emotional and non-emotional words

Clinical and experimental evidence indicate that the right hemisphere possesses a special capacity for emotional non-verbal material. The possibility that this ability would extend to language was tested with a lexical decision paradigm using tachistoscopic double simultaneous presentation of emotional and non-emotional words. Male subjects showed an overall right visual field superiority with the predicted effect of emotional words in the left visual field. In contrast, the field advantages for females were more variable and showed evidence of a larger effect of emotional words in the right visual field. The imageability potential of words also appeared to influence the female performance.     

Visual hemispace differences reflect hemisphere asymmetries

The relationship between the representation of the extracorporeal visual hemispace and the contralateral hemisphere has been investigated by presenting visual stimuli foveally either in the right or in the left hemispace. In a lexical decision task (Exp. 1) and a face-familiarity decision task (Exp. 2) lateral asymmetries very similar to those obtained using the divided visual field technique have been found. These results suggest that under particular circumstances the hemispace representation in the contralateral hemisphere overrides the retinal representation. Finally, it was found that in a lexical decision task performance with stimuli in the right hemispace was very similar to that with stimuli in a central position. The physiological and adaptive meaning of this phenomenon is discussed.     

Functional asymmetry of the human brain in the recognition of faces

Lateralized cerebral function in the recognition of facial stimuli was tested directly by examining patients before and after they were administered a single, unilateral electroconvulsive treatment. Functional demands imposed by the task (i.e. verbal or non-verbal) were the chief factors influencing the relative importance of the right or the left cerebral hemisphere to task performance.