Do Individuals with Down's Syndrome Possess Right Hemisphere Language Dominance?

A dichotic digits task with selective listening and attention-switching instructions, was administered to 10 mentally retarded individuals with Down's Syndrome between the ages of 10 and 37 years. Attention was focused on one ear for 20 trials before switching attention to the opposite ear after a 5-minute interval. A majority displayed a right ear-left hemisphere advantage for correct responses, although this advantage failed to reach significance. Comparison of left and right ear intrusion errors when each ear was attended first showed significantly more intrusions from the unattended right ear. No priming effects were found. The results do not support the hemispheric reversed specialization hypothesis. Rather, it is suggested that while Down's Syndrome individuals do possess an inherent left hemisphere advantage for language, such individuals may suffer from a left hemisphere information processing disorder that can produce functional reversals in dichotic listening.     

Perceptual asymmetry in schizophrenia and affective disorder: implications from a right hemisphere task

The patterns of perceptual asymmetry exhibited by normal, schizophrenic, and affectively-disordered subjects on a dichotic tonal discrimination task, were compared. Affectively-disordered subjects' performances differed significantly from those of normal subjects, with normals demonstrating the expected left ear advantage, and affectively-disordered subjects showing no lateral advantage. The performance of the schizophrenic subjects fell between those of the normal and affective groups along a laterality continuum, with paranoid schizophrenic subjects tending to show a larger left ear advantage than non-paranoid schizophrenic subjects. The results do not support the hypothesis that schizophrenic subjects inappropriately transfer processing of right hemisphere stimuli to the left hemisphere, but do suggest that subgroup distinctions may be relevant to hypotheses of lateralized dysfunction in schizophrenia. Further, the performance of the affective group supports previous findings of right hemisphere abnormalities in affective disorder.     

Sex and Hemispheric Differences for Rapid Auditory Processing in Normal Adults

Previous research suggests that left hemisphere specialisation for processing speech may specifically depend on rate-specific parameters, with rapidly successive or faster changing acoustic stimuli (e.g. stop consonant-vowel syllables) processed preferentially by the left hemisphere. The current study further investigates the involvement of the left hemisphere in processing rapidly changing auditory information, and examines the effects of sex on the organisation of this function. Twenty subjects participated in an auditory discrimination task involving the target identification of a two-tone sequence presented to one ear, paired with white noise to the contralateral ear. Analyses demonstrated a right ear advantage for males only at the shorter interstimulus interval durations (mean = 20msec) whereas no ear advantage was observed for women. These results suggest that the male brain is more lateralised for the processing of rapidly presented auditory tones, specifically at shorter stimulus durations.     

Sex and hemispheric differences for rapid auditory processing in normal adults

Previous research suggests that left hemisphere specialisation for processing speech may specifically depend on rate-specific parameters, with rapidly successive or faster changing acoustic stimuli (e.g. stop consonant-vowel syllables) processed preferentially by the left hemisphere. The current study further investigates the involvement of the left hemisphere in processing rapidly changing auditory information, and examines the effects of sex on the organisation of this function. Twenty subjects participated in an auditory discrimination task involving the target identification of a two-tone sequence presented to one ear, paired with white noise to the contralateral ear. Analyses demonstrated a right ear advantage for males only at the shorter interstimulus interval durations (mean = 20 msec) whereas no ear advantage was observed for women. These results suggest that the male brain is more lateralised for the processing of rapidly presented auditory tones, specifically at shorter stimulus durations.     

Enhanced right hemisphere involvement during cognitive processing may relate to intellectual precocity

In the present study, intellectually precocious and average ability youths performed a dichotic listening task (Experiment 1) and a free-vision chimeric face task (Experiment 2). Patterns of hemispheric lateralization and the relative involvement of the left and right hemispheres during cognitive processing were assessed. In Experiment 1, the average ability youths demonstrated a right ear/left hemisphere (re/LH) superiority for identification of CV syllables, while the gifted subjects failed to show any ear/hemisphere advantage. In Experiment 2, members of both groups tended to judge the leftside smile/rightside neutral half-faces as "happier", a pattern indicative of enhanced right hemisphere (RH) arousal. Notably, the degree of RH involvement was significantly greater in the gifted as compared to average ability youths. Moreover, laterality scores of the precocious on the chimeric face task correlated with their performance on the College Board Scholastic Aptitude test (SAT), i.e. the greater the leftward bias, the higher the SAT score. These findings, taken in composite, suggest that a high level of RH involvement during cognitive processing may be related to intellectual precocity.     

The effects of hemispatial and asymmetrically focused attention on dichotic listening with normal and learning-disabled children

The effects of hemispatial and focused attention were examined with 50 normal and learning-disabled children to determine the extent of these two attentional strategies influenced perceptual laterality as reflected by the dichotic listening right-ear advantage (REA). Twenty-five normal children (8 females, 17 males, mean age 9.10 yr) matched with 25 learning-disabled children (8 females, 17 males, mean age 10.1 yr) were administered a dichotic consonant-vowel (CV) and consonant-vowel-consonant (CVC) syllable task. The two types of stimuli were compared across focused attention (free report, focused left, focused right) and hemispatial (central, left hemispace, right hemispace) conditions implemented independently and in systematic combinations. A four-factorial analysis of variance (groups x stimuli x conditions x ears) resulted in a significant REA for normal children across all attentional conditions whereas learning-disabled did not produce a consistent REA across all attentional conditions, and in several instances, produced equivalent left and right hemisphere processing. Right hemispatial orientation increased the magnitude of the REA (i.e., left hemisphere processing) for both groups, whereas left hemispatial orientation increased the magnitude of the left ear report (i.e., right hemisphere processing) only in learning-disabled subjects. Focused attention to the right ear also increased left hemisphere efficiency for both groups of children; however, focused attention to the left ear produced symmetrical functioning by learning-disabled subjects. Congruent combinations of focused attention and hemispatial orientation were not found to enhance the REA beyond its magnitude when each strategy was assessed independently. When focused attention and hemispatial conditions were employed in opposing directions, normal children were more susceptible to the "rightward" direction regardless of the strategy whereas learning-disabled subjects were more susceptible to the "verbal" nature of the strategy. Higher overall processing performance was exhibited for CVC stimuli when compared to CV stimuli. These findings lend support to the hypothesis that hemispatial and asymmetrically focused attention strategies interact with structural mechanisms in producing the observed REA in dichotic listening and do so differentially for normal and learning-disabled children.     

Hemispheric processing of semantic information: the effects of the semantic priming task and working memory capacity

In a classic semantic priming study (Beeman et al., 1994), participants showed a naming advantage for strongly related targets compared to weakly related targets in the left hemisphere, whereas no difference in naming advantage was found between strongly and weakly related targets in the right hemisphere. However, it is unclear how the type of task and individual differences influence this hemispheric activation. In the current study participants completed a lexical decision task when presented with strongly, weakly, and unrelated words in each visual field-hemisphere. A left hemisphere advantage was evident for strongly and weakly related words compared to unrelated words and a right hemisphere advantage was evident for strongly related words compared to weakly related and unrelated words. Additionally, high working memory capacity participants responded more accurately to strongly related words than weakly or unrelated words in the right hemisphere, whereas low working memory capacity participants showed no difference between these conditions in the right hemisphere. Thus, the type of semantic priming task and working memory capacity seem to influence the hemispheric processing of strongly and weakly related information.     

Commissurotomy subjects show lateralized difference between manual and oral responding

This study investigates the influence of the mode of response on the right ear advantage in verbal dichotic testing. Four complete commissurotomy subjects were tested on an ascending digits tape with oral response and manual response by the dominant hand with the volume equal and raised in each ear. Oral response elicited right ear report in all tests under all conditions at all levels of task difficulty. Pointing and writing responses elicited left ear report when the volume was raised in the left ear and the number of digits to report was small. Right ear performance increased and left ear performance decreased as the number of digits to process increased. Results indicate that response modes differ significantly in the degree to which they engage the left hemisphere or to which they suppress the ipsilateral pathways within the left hemisphere. Use of an oral response may obscure the lateralized contribution of the cognitive activity under investigation.     

Hemispheric differentiation and category width

This study concerns the relationship between a cognitive style dimension, category width, and hemispheric differentiation. When lists of word pairs were presented simultaneously in a dichotic listening task to broad and narrow categorisers (all female, right-handed), both groups of subjects recalled more words presented to the right ear than those presented to the left ear; indicating left hemisphere's superiority in verbal processing. Both broad and narrow categorisers recalled a similar number of words in the right ear (left hemisphere), but the former recalled significantly more words in the left ear than did the latter. This finding is interpreted as meaning that narrow categorisers rely predominantly on the left hemisphere in verbal processing, and that in comparison with narrow categories, there is greater right hemispheric involvement in processing in the case of broad categorisers. The implication of this finding in terms of the differential processing strategies adopted by the two groups of individuals is discussed.     

Neurocognition of auditory sentence comprehension: event related fMRI reveals sensitivity to syntactic violations and task demands

The present study investigates the sensitivity of distinct brain regions to the syntactic processing of running speech. Experimental conditions varied the grammaticality of sentence types (correct vs. incorrect). Moreover, two different groups of subjects listened to the same sentence material, but followed two different task instructions. All participants were asked to listen to the auditory stimuli and to perform in a grammaticality judgment-task, whereas only half of the subjects were instructed to additionally repair incorrect sentences covertly. Significantly increased brain responses occurred in several left temporal areas as a function of sentences' grammaticality, particularly, in the 'pure' judgment-group. Spatial extent as well as the strength of focal brain activation changed as a function of grammaticality and task demand. A generally enhanced pattern of local blood supply to the right peri-sylvian cortex could be observed when individuals additionally realized the repair-task. In particular, the right inferior frontal gyrus (pars opercularis and pars triangularis) and the right temporal transverse gyrus (Heschl's gyrus) were more strongly affected by the repair-task demand. In contrast, an anterior portion of the superior temporal gyrus (planum polare) displayed increased activation bilaterally. Although left hemisphere activation varied clearly as a function of a sentence's grammaticality, the present findings demonstrate an involvement of the right peri-sylvian cortex, in particular, when task demands explicitly require an on-line repair. The results as a whole suggest a reconsideration of the notion that auditory language comprehension is restricted to the left hemisphere. The underlying mechanisms and the respective roles of both the left and the right hemisphere during speech processing are discussed.     

Hemispheric asymmetry in categorical versus coordinate visuospatial processing revealed by temporary cortical deactivation.

Kosslyn (1987) proposed that the left hemisphere is better than the right hemisphere at categorical visuospatial processing while the right hemisphere is better than the left hemisphere at coordinate visuospatial processing. In 134 patients, one hemisphere (and then usually the other) was temporarily deactivated by intracarotid injection of sodium amobarbital. After a hemisphere was deactivated, a cognitive test battery was conducted, which included categorical and coordinate visuospatial tasks. Using this technique, the processing capabilities of the intact hemisphere could be determined, thus directly testing Kosslyn's hypothesis regarding hemispheric specialization. Specifically, if the left hemisphere does preferentially process categorical visuospatial relationships, then its deactivation should result in more errors during categorical tasks than right hemisphere deactivation and vise versa for the right hemisphere regarding coordinate tasks. The pattern of results obtained in both categorical and coordinate tasks was consistent with Kosslyn's hypothesis when task difficulty was sufficiently high. However, when task difficulty was low, a left hemispheric processing advantage was found for both types of tasks indicating that: (1) the left hemisphere may be better at "easy" tasks regardless of the type of task and (2) the proposed hemispheric processing asymmetry may only become apparent during sufficiently demanding task conditions. These results may explain why some investigators have failed to find a significant hemispheric processing asymmetry in visuospatial categorical and coordinate tasks.     

Verbal dichotic listening in right and left-handed adults: laterality effects of directed attention

Dichotic listening performance of consonant vowel stimuli was studied in 51 adult right- and left-handers in three attention conditions: non-directed and directed to either the right or left ear. In the non-directed condition, a significant right-ear advantage was found in both handedness groups with a stronger asymmetry in right-handers. There are at least three explanations for this ear bias. The classic or structural hypothesis suggests that to the right ear projects more strongly to the language dominant left hemisphere. The callosal relay hypothesis is based on the influence of inhibitory connections via the corpus callosum. The attentional hypothesis suggests that each hemisphere primarily directs attention to contralateral space and because the left hemisphere is dominant for language in both groups, and is aroused by speech stimuli, attention is primarily directed to the right ear. Neither hypothesis can explain why greater than 95% of right-handers have left hemisphere language dominance, but only 70-80% have a right ear bias. Our results demonstrate that in the directed attention conditions both groups increased their lateral biases when directed to either the right or left. The classic or structural hypothesis cannot account for these changes, thereby providing support for the attentional hypothesis. In addition, the right-handed subjects exhibited a greater shift of bias than did the left-handed subjects, when directing their attention leftward. This finding suggests that right-handed people are better able to shift their attention than left-handed people.     

The left intraparietal sulcus modulates the selection of low salient stimuli

Neuropsychological and functional imaging studies have suggested a general right hemisphere advantage for processing global visual information and a left hemisphere advantage for processing local information. In contrast, a recent transcranial magnetic stimulation study [Mevorach, C., Humphreys, G. W., & Shalev, L. Opposite biases in salience-based selection for the left and right posterior parietal cortex. Nature Neuroscience, 9, 740-742, 2006b] demonstrated that functional lateralization of selection in the parietal cortices on the basis of the relative salience of stimuli might provide an alternative explanation for previous results. In the present study, we applied a whole-brain analysis of the functional magnetic resonance signal when participants responded to either the local or the global levels of hierarchical figures. The task (respond to local or global) was crossed with the saliency of the target level (local salient, global salient) to provide, for the first time, a direct contrast between brain activation related to the stimulus level and that related to relative saliency. We found evidence for lateralization of salience-based selection but not for selection based on the level of processing. Activation along the left intraparietal sulcus (IPS) was found when a low saliency stimulus had to be selected irrespective of its level. A control task showed that this was not simply an effect of task difficulty. The data suggest a specific role for regions along the left IPS in salience-based selection, supporting the argument that previous reports of lateralized responses to local and global stimuli were contaminated by effects of saliency.     

Hemispheric differences in global and local processing with orientation classification tasks

Hemispheric differences in global and local processing were examined in one experiment with hierarchical stimuli. The figures consisted of large squares with the right or left side missing made up of small squares with the right or left side missing. The subjects were asked to decide the opening (left/right) of the square either at the global level or at the local level. The findings showed that with the task and stimuli used here global judgements were as fast and accurate as local judgements, the interference was bidirectional and symmetrical and, finally, that the right hemisphere and the left hemisphere had the same ability to manage with global and local information. So, the experiment does not provide evidence for hemispheric specialisation in global and local processing.     

Testing neuropsychological hypotheses for cognitive deficits in psychopathic criminals: a study of global-local processing.

Competing hypotheses about neuropsychological mechanisms underlying psychopathy are seldom examined in the same study. We tested the left hemisphere activation hypothesis and the response modulation hypothesis of psychopathy in 172 inmates completing a global-local processing task under local bias, global bias, and neutral conditions. Consistent with the left hemisphere activation hypothesis, planned comparisons showed that psychopathic inmates classified local targets more slowly than nonpsychopathic inmates in a local bias condition and exhibited a trend toward similar deficits for global targets in this condition. However, contrary to the response modulation hypothesis, psychopaths were no slower to respond to local targets in a global bias condition. Because psychopathic inmates were not generally slower to respond to local targets, results are also not consistent with a general left hemisphere dysfunction account. Correlational analyses also indicated deficits specific to conditions presenting most targets at the local level initially. Implications for neuropsychological conceptualizations of psychopathy are considered.     

The interference of local over global information processing in children with attention deficit hyperactivity disorder of the inattentive type

A classic finding in perception of compound patterns is normal individuals cannot skip global analysis in local-oriented processing, but they can successfully resist local analysis in global-oriented processing-the so-called global interference [1]. Recently, studies examining the role of brain hemisphere activity in the Navon task have indicated that the processing of global and local information can be, respectively, attributed to the right and left hemispheres. Moreover, many neuroimaging researches have revealed that certain core symptoms of attention deficit hyperactivity disorder (ADHD) are related to dysfunction of right hemisphere. These findings imply that global interference will be substantially less evident, and possibly even replaced by local interference in ADHD. The present study compared the performance of children with and without attention deficit hyperactivity disorder of the inattentive type (ADHD-I) in the processing of global and local information to examine the local interference hypothesis in ADHD. An ADHD-I group (n=15) and a paired control group (n=19) completed tasks using two versions of the Navon task, one requiring divided attention, in which no information was given to participants regarding the level at which a target would appear, and the other requiring selective attention, in which participants were instructed to attend to either the local or the global level. The results showed that children with ADHD-I exhibited local interference, regardless of which attention procedure was used. These results support the weak right hemisphere hypothesis in ADHD, and provide evidence against the deficit hypotheses for ADHD in the DSM-IV criteria [29], which postulates that inattention symptoms may manifest as a failure to provide close attention to details.     

Evidence of interhemispheric transmission in laterality effects

The study was aimed at testing various models that can explain visual lateral asymmetries due to hemispheric specialization. In Experiments 1-3 the subjects had to perform a lateralized "go-no go" discrimination of words (primary task) either alone or in association with secondary tasks that interfered with the processing of the left hemisphere (ordered tapping) or the right hemisphere (finger flexion). In Experiment 4 the primary task was one of lateralized "go-no go" discrimination of faces while the secondary tasks were again those of ordered tapping and finger flexion. The results showed that in the case of word discrimination the advantage in speed of response in favour of the right visual field/left hemisphere (RVF/LH), which was observed for the primary task alone, did not change when the secondary task was added. This held true irrespective of whether the secondary task loaded the left or right hemisphere. The advantage for the left visual field/right hemisphere (LVF/RH) observed for face discrimination alone, disappeared when the secondary task interfered with the processing of the right hemisphere and did not change when the secondary task concerned the left hemisphere. It was concluded that each hemisphere is able to elaborate in parallel the incoming information, but, in normal conditions, interhemispheric transmission is responsible for the lateral asymmetries in perception (conditional interhemispheric transmission model).     

Lateralisation of perceptual processing of pro- and anti-social emotions displayed in chimeric faces

Previous studies using a restricted range of split field chimeric faces suggest that the processing of facial emotional expressions is a highly lateralised phenomenon. Two theories have emerged with regard to the role the two cerebral hemispheres play in this processing. The "right hemisphere" hypothesis (e.g. Borod, Koff, Lorch, & Nicholas, 1988) suggests that the right hemisphere plays a prominent role in the processing of both positive and negative facial expressions. In contrast, the "valence hypothesis" (e.g. Reuter-Lorenz & Davidson, 1981) proposes that, while the right hemisphere plays the major role in processing negative emotions, the left hemisphere has a special role for positive emotions. Using all six of the human "universal facial expressions" (Ekman &Friesen, 1971) we present evidence that, superimposed onto a left visual field advantage (right hemisphere specialisation) for emotional processing, there is a "left shift" (right visual field advantage) for pro- as opposed to anti-social expression. We propose that facial expressions that are likely to be related to approach for communicative purposes initiate a greater degree of left hemisphere involvement. As a consequence, we suggest that for future laterality studies, the six universal emotional expressions should be reconsidered using a pro- to anti-social dimension rather than positive and negative groupings.     

Prevalent direction of reflective lateral eye movements and ear asymmetries in a dichotic test of musical chords

Subjects who consistently move their gaze either to the right or to the left while reflecting on visuo-spatial or verbal questions are usually called “lateral eye movers”. This study evaluated auditory asymmetries to a dichotic test of musical chords in 23 right-handed females, selected through test-retest as reliable lateral eye movers; 12 were “left movers” (LMs) and 11 were “right movers” (RMs). During the assessment of the prevalent direction of gaze as well as during the dichotic test, the oculomotor activity was controlled through a video camera. The hypothesis was that the left ear advantage usually found with dichotic chords is enhanced in LMs and reduced in RMs, and that this effect is not due to the facilitating influence of lateral eye movements occuring during the task Results show that: (a) left movers exhibit a marked advantage of the left ear while right movers do not exhibit any significant ear advantage; (b) despite the instruction to fix a central point, lateral movers tend to show unwarranted eye movements in their usual direction; (c) the effect of the prevalent direction of gaze on the dichotic advantage is not due to eye movements made during the dichotic test. These findings give further support to the hypothesis that the tendency to consistently shift the gaze to one side is related to hemispheric asymmetries as measured by lateralized tests.     

Lateralisation of perceptual processing of pro- and anti-social emotions displayed in chimeric faces

Previous studies using a restricted range of split field chimeric faces suggest that the processing of facial emotional expressions is a highly lateralised phenomenon. Two theories have emerged with regard to the role the two cerebral hemispheres play in this processing. The "right hemisphere" hypothesis (e.g. Borod, Koff, Lorch, & Nicholas, 1988) suggests that the right hemisphere plays a prominent role in the processing of both positive and negative facial expressions. In contrast, the "valence hypothesis" (e.g. Reuter-Lorenz & Davidson, 1981) proposes that, while the right hemisphere plays the major role in processing negative emotions, the left hemisphere has a special role for positive emotions. Using all six of the human "universal facial expressions" (Ekman &Friesen, 1971) we present evidence that, superimposed onto a left visual field advantage (right hemisphere specialisation) for emotional processing, there is a "left shift" (right visual field advantage) for pro- as opposed to anti-social expression. We propose that facial expressions that are likely to be related to approach for communicative purposes initiate a greater degree of left hemisphere involvement. As a consequence, we suggest that for future laterality studies, the six universal emotional expressions should be reconsidered using a pro- to anti-social dimension rather than positive and negative groupings.