The nature of hemispheric specialization in man

Manual exploratory procedures and asymmetries for a haptic search task: a comparison between capuchins (Cebus apella) and humans

Left-hand preferences in monkeys (capuchins, macaques) for haptic discriminations have been taken to indicate that the right hemisphere is more involved than the left hemisphere in solving these tasks. We confirmed this left-hand bias in a group of 21 capuchins haptically searching for sunflower seeds located in crevices in 12 different objects (Lacreuse & Fragaszy, 1996). In an attempt to specify the relations of the left-hand bias to hand performance and hemispheric lateralisation, we analysed the manual exploratory procedures used by the subjects when exploring the objects, and compared them to those of humans confronted with the same task. All the hand-movement patterns displayed by humans were also observed in capuchins, but humans performed exhaustive explorations, whereas capuchins restricted their haptic investigations to limited portions of the objects. Both species adopted several ''exploratory procedures'' to investigate particular objects. Although capuchins showed a left-hand preference to perform the task, finer analyses of haptic exploratory procedures revealed no difference in the way the left and right hands explored the objects, nor in the efficiency with which each hand solved the task. Nor was any measurable difference in manual exploratory procedure and efficiency found in humans. The discussion emphasises the need to complement measures of manual preference with finer indices of hemispheric lateralisation, for a better understanding of functional asymmetries in primates.

Integrated contextual representation for objects' identities and their locations

Visual context plays a prominent role in everyday perception. Contextual information can facilitate recognition of objects within scenes by providing predictions about objects that are most likely to appear in a specific setting, along with the locations that are most likely to contain objects in the scene. Is such identity-related ("semantic") and location-related ("spatial") contextual knowledge represented separately or jointly as a bound representation? We conducted a functional magnetic resonance imaging (fMRI) priming experiment whereby semantic and spatial contextual relations between prime and target object pictures were independently manipulated. This method allowed us to determine whether the two contextual factors affect object recognition with or without interacting, supporting a unified versus independent representations, respectively. Results revealed a Semantic x Spatial interaction in reaction times for target object recognition. Namely, significant semantic priming was obtained when targets were positioned in expected (congruent), but not in unexpected (incongruent), locations. fMRI results showed corresponding interactive effects in brain regions associated with semantic processing (inferior prefrontal cortex), visual contextual processing (parahippocampal cortex), and object-related processing (lateral occipital complex). In addition, activation in fronto-parietal areas suggests that attention and memory-related processes might also contribute to the contextual effects observed. These findings indicate that object recognition benefits from associative representations that integrate information about objects' identities and their locations, and directly modulate activation in object-processing cortical regions. Such context frames are useful in maintaining a coherent and meaningful representation of the visual world, and in providing a platform from which predictions can be generated to facilitate perception and action.

Functional hemispheric differences for the categorization of global and local information in naturalistic stimuli

Up to now functional hemispheric asymmetries for global/local processing have mainly been investigated with hierarchical letters as stimuli. In the present study, three experiments were conducted to examine whether corresponding visual-field (VF) effects can also be obtained with more naturalistic stimuli. To this end, images of animals with a pattern placed on their body were displayed as stimuli. The task for the global level and for the local level was to categorize the animals and the patterns, respectively. As a result, VF-effects were also found for these stimuli and tasks. It is concluded that the hemispheric differences observed for hierarchical letters also hold for naturalistic stimuli.

Lateralization of visual learning in the honeybee

Lateralization is a well-described phenomenon in humans and other vertebrates and there are interesting parallels across a variety of different vertebrate species. However, there are only a few studies of lateralization in invertebrates. In a recent report, we showed lateralization of olfactory learning in the honeybee (Apis mellifera). Here, we investigate lateralization of another sensory modality, vision. By training honeybees on a modified version of a visual proboscis extension reflex task, we find that bees learn a colour stimulus better with their right eye.

Multiple priming of lexically ambiguous and unambiguous targets in the cerebral hemispheres: the coarse coding hypothesis revisited

The coarse coding hypothesis postulates that the cerebral hemispheres differ in their breadth of semantic activation, with the left hemisphere activating a narrow, focused semantic field and the right weakly activating a broader semantic field. In support of coarse coding, studies investigating priming for multiple senses of a lexically ambiguous word have reported a right hemisphere benefit. However, studies of mediated priming have failed to find a right hemisphere advantage for processing distantly linked, unambiguous words. To address this debate, the present study made use of a multiple priming paradigm in which two primes either converged onto the single meaning of an unambiguous, lexically associated target (LION-STRIPES-TIGER) or diverged onto different meanings of an ambiguous target (KIDNEY-PIANO-ORGAN). In two experiments, participants either made lexical decisions to lateralized targets (Experiment 1) or made a semantic relatedness judgment between primes and targets (Experiment 2). In both tasks, for both ambiguous and unambiguous triplets we found equivalent priming strengths and patterns across the two visual fields, counter to the predictions of the coarse coding hypothesis. Priming patterns further suggested that both hemispheres made use of lexical level representations in the lexical decision task and semantic representations in the semantic judgment task.

Working memory specific activity in auditory cortex: potential correlates of sequential processing and maintenance

Working memory (WM) tasks involve several interrelated processes during which past information must be transiently maintained, recalled, and compared with test items according to previously instructed rules. It is not clear whether the rule-specific comparisons of perceptual with memorized items are only performed in previously identified frontal and parietal WM areas or whether these areas orchestrate such comparisons by feedback to sensory cortex. We tested the latter hypothesis by focusing on auditory cortex (AC) areas with low-noise functional magnetic resonance imaging in a 2-back WM task involving frequency-modulated (FM) tones. The control condition was a 0-back task on the same stimuli. Analysis of the group data identified an area on right planum temporale equally activated by both tasks and an area on the left planum temporale specifically involved in the 2-back task. A region of interest analysis in each individual revealed that activation on the left planum temporale in the 2-back task positively correlated with the task performance of the subjects. This strongly suggests a prominent role of the AC in 2-back WM tasks. In conjunction with previous findings on FM processing, the left lateralized effect presumably reflects the complex sequential processing demand of the 2-back matching to sample task.

Cerebral lateralisation for facial processing: Gender-related cognitive styles determined using Fourier analysis of mean cerebral blood flow velocity in the middle cerebral arteries

Facial processing was studied in 16 (eight men and eight women) right-handed healthy participants using a new functional transcranial Doppler technique called functional transcranial Doppler spectroscopy (fTCDS). MFV was recorded simultaneously in both right and left middle cerebral arteries in dark condition and during visual processing of object and facial tasks. fTCDS used Fourier analysis of mean flow velocity (MFV) time series to derive spectral density estimates that correlate with expected mental activity. Men were right lateralised for object and facial perception, while women were left lateralised for facial tasks but showed a right tendency or no lateralisation for object perception. For facial perception, men used a category-specific process-mapping system for right cognitive style, but women used same for the left.

Lateralization of olfaction in the honeybee Apis mellifera

Lateralization of function is a well-known phenomenon in humans. The two hemispheres of the human brain are functionally specialized such that certain cognitive skills, such as language or musical ability, conspecific recognition, and even emotional responses, are mediated by one hemisphere more than the other [1, 2]. Studies over the past 30 years suggest that lateralization occurs in other vertebrate species as well [3-11]. In general, lateralization is observed in different sensory modalities in humans as well as vertebrates, and there are interesting parallels (reviewed in [12]). However, little is known about functional asymmetry in invertebrates [13, 14] and there is only one investigation in insects [15]. Here we show, for the first time, that the honeybee Apis mellifera displays a clear laterality in responding to learned odors. By training honeybees on two different versions of the well-known proboscis extension reflex (PER) paradigm [16, 17], we demonstrate that bees respond to odors better when they are trained through their right antenna. To our knowledge, this is the first demonstration of asymmetrical learning performance in an insect.

Mechanisms of hemispheric specialization: insights from analyses of connectivity

Traditionally, anatomical and physiological descriptions of hemispheric specialization have focused on hemispheric asymmetries of local brain structure or local functional properties, respectively. This article reviews the current state of an alternative approach that aims at unraveling the causes and functional principles of hemispheric specialization in terms of asymmetries in connectivity. Starting with an overview of the historical origins of the concept of lateralization, we briefly review recent evidence from anatomical and developmental studies that asymmetries in structural connectivity may be a critical factor shaping hemispheric specialization. These differences in anatomical connectivity, which are found both at the intra- and inter-regional level, are likely to form the structural substrate of different functional principles of information processing in the two hemispheres. The main goal of this article is to describe how these functional principles can be characterized using functional neuroimaging in combination with models of functional and effective connectivity. We discuss the methodology of established models of connectivity which are applicable to data from positron emission tomography and functional magnetic resonance imaging and review published studies that have applied these approaches to characterize asymmetries of connectivity during lateralized tasks. Adopting a model-based approach enables functional imaging to proceed from mere descriptions of asymmetric activation patterns to mechanistic accounts of how these asymmetries are caused.

The differential effects of thalamus and basal ganglia on facial emotion recognition

This study examined if subcortical stroke was associated with impaired facial emotion recognition. Furthermore, the lateralization of the impairment and the differential profiles of facial emotion recognition deficits with localized thalamic or basal ganglia damage were also studied. Thirty-eight patients with subcortical strokes and 19 matched normal controls volunteered to participate. The participants were individually presented with morphed photographs of facial emotion expressions over multiple trials. They were requested to classify each of these morphed photographs according to Ekman's six basic emotion categories. The findings indicated that the clinical participants had impaired facial emotion recognition, though no clear lateralization pattern of impairment was observed. The patients with localized thalamic damage performed significantly worse in recognizing sadness than the controls. Longitudinal studies on patients with subcortical brain damage should be conducted to examine how cognitive reorganization post-stroke would affect emotion recognition.

Does a causal relation exist between the functional hemispheric asymmetries of visual processing subsystems?

Past research indicates that specific shape recognition and spatial-relations encoding rely on subsystems that exhibit right-hemisphere advantages, whereas abstract shape recognition and spatial-relations encoding rely on subsystems that exhibit left-hemisphere advantages. Given these apparent regularities, we tested whether asymmetries in shape processing are causally related to asymmetries in spatial-relations processing. We examined performance in four tasks using the same stimuli with divided-visual-field presentations. Importantly, the asymmetry observed in any one task did not correlate with the asymmetries observed in the other tasks in ways predicted by extant theories. Asymmetries in shape processing and spatial-relations encoding may not be due to a common causal force influencing multiple subsystems.

The effect of strategy on pseudoneglect for luminance judgements

When judging the relative magnitude of the left and right sides of a stimulus, normal participants overestimate the leftward features (pseudoneglect). Although pseudoneglect and clinical neglect operate in opposite directions, the two phenomena may have a common cognitive and neural basis. For neglect, two strategies may be employed when inspecting horizontally aligned stimuli: (1) A global strategy where the stimuli are treated as a gestalt and asymmetries are detected or, (2) a comparison strategy where the qualities on the left and right sides of the stimuli are explicitly compared. To investigate the effect of these strategies on pseudoneglect, normal dextrals (n = 25 and 17) made two-alternative, forced-choice luminance discriminations between two mirror-reversed luminance gradients (greyscales). In an unseparated form, the stimuli are amenable to a global strategy. A comparison strategy was imposed by separating the stimuli into halves (Experiment 1) or quarters (Experiment 2). Despite the fact that the stimuli were equiluminant, participants predominantly chose the stimulus that was dark on the left as being darker overall in the unseparated condition. Response times were also faster for leftward responses. When the stimuli were separated into halves or quarters, the leftward bias was reduced, but not eliminated. The results demonstrate that both strategies contribute to pseudoneglect--though the global strategy may produce stronger pseudoneglect.

About hemispheric differences in the processing of temporal intervals

The purpose of the present study was to identify differences between cerebral hemispheres for processing temporal intervals ranging from .9 to 1.4 s. The intervals to be judged were marked by series of brief visual signals located in the left or the right visual field. Series of three (two standards and one comparison) or five intervals (four standards and one comparison), marked by sequences of 4 or 6 signals, were compared. While discrimination, as estimated by d', was significantly better in the 4-standard than in the 2-standard condition when stimuli were presented in the left visual field (LVF), this number-of-standard effect on discrimination varied with the difficulty levels when the signals were presented in the LVF. Moreover, the discrimination levels were constant for the different base durations with stimuli presented in the LVF, but not with stimuli presented in the right visual field. This article discusses the implication of these findings for the study of hemispheric dominance for temporal processing and for a single-clock hypothesis.

Haptic memory and handedness in 2-month-old infants

This study examined the robustness of infant haptic memory, asymmetry between hands, and sex differences in haptic memory in infancy. A total of 96 2-month-old infants (half males, half females) were habituated haptically to an object with their right and their left hand, out of the field of view. Haptic memory was then tested under three conditions: after haptic interference, after a 30-second delay, or after no delay. The results show that haptic habituation occurred for both hands. The girls needed more time to habituate with their left hand than with their right hand, and they habituated more slowly than the boys did. Discrimination was also found in both hands and in both sexes. Haptic delayed recognition memory was only found in young boys mainly after a short delay and under certain conditions after interference. In young girls, recognition memory was found after interference only with the left hand. This result seemed to depend on the information processing speed. Thus, for memory performance, a sex difference was clearly observed. Moreover, the infants' left hand retained better information on object shape than did the right hand for both the sexes. Asymmetries in infancy are discussed in connection with the difference in brain maturation rate.

Left hand preferences in capuchins (Cebus apella): role of spatial demands in manual activity

Sixteen capuchins (Cebus apella) participated in four tasks that differed in their exploration demands and availability of visual cues. The two explorative tasks required the monkeys to discover sunflower seeds hidden in crevices in objects of various shapes, with vision (Haptic-Visual task) or without vision (Haptic task). Two other tasks required the capuchins to grasp sunflower seeds directly on a flat support, with vision (Visual reaching) or without vision (Tactual reaching). The presence or absence of exploration demands had a significant effect on the direction of hand preferences. The group displayed greater left hand preferences for the Haptic and Haptic-Visual tasks than for the two reaching tasks. The strength of manual preferences did not differ significantly among the four tasks. These findings suggest that the manipulo-spatial demands of a task are of particular importance for the expression of left hand bias in a population of capuchins. It is argued that left hand preferences for the haptic tasks may reflect a right hemisphere specialisation to integrate the spatial and motor components of action.

Broad-Perspective Perceptual Disorder of the Right Hemisphere

Traditional accounts of right-posterior brain injury describe a syndrome of low-level perceptual sequelae producing marked acute dependency and transient safety concerns. The syndrome is also held to spare cognition and to carry a generally favorable long-term prognosis. The present paper reviews publications and anecdotal data that challenge this picture. Recent theoretical expositions and empirical studies stipulate three major cognitive functions of the right posterior association cortex: processing novel input, guiding reactions to emergencies, and anticipating consequences. Appearing benign after acute recovery, the impairment of these processes produces vocational, social and marital dysfunctions that increase as a function of chronicity, ultimately becoming more broadly disabling than focal injuries in other cortical loci. The unique symptom picture and serious implications suggest that the long-term syndrome should be labeled (Broad-Perspective Perceptual Disorder) and incorporated in future clinical taxonomies, underscoring the need for extraordinary long-term assistance and specialized therapeutics. Procedures for assessment and differential diagnosis are outlined.

Handedness, hemispheric asymmetries, and joke comprehension

To address the impact of differences in language lateralization on joke comprehension, event-related brain potentials (ERPs) were recorded as 16 left- and 16 right-handed adults read one-line jokes and non-funny control stimuli ("A replacement player hit a home run with my girl/ball,"). In right-handers, jokes elicited a late positivity 500-900 ms post-stimulus onset that was largest over right hemisphere (RH) centro-parietal electrode sites, and a slow sustained negativity over anterior left lateral sites. In left-handers, jokes elicited a late positivity 500-900 ms post-onset that was larger and more broadly distributed than in the right-handers' ERPs. In right-handed women, the late positivity was larger over RH electrode sites. In left-handed women, the late positivity was bilaterally symmetric. The highly asymmetric slow sustained negativity over left anterior electrode sites was absent from left-handers' ERPs to jokes. Differences may reflect more efficient inter-hemispheric communication in the left-handers, as they are reputed to have relatively larger corpus callosal areas than right-handers. Results support the portrait of more bilateral language representation among left-handers, and suggest language lateralization affects high-level language comprehension tasks such as joke comprehension.

Cross-modal recognition of shape from hand to eyes and handedness in human newborns

The present research addresses the question of the generality of the ability to transfer shape information from one hand to the eyes recently evidenced in human newborns. Using an intersensory paired-preference procedure, we confirmed that newborns can visually recognize the shape of an object that they have previously manipulated with their right hand, out of sight. However, the results revealed that this ability is absent when the left hand is involved. Handedness in cross-modal transfer task is discussed in relation to other behavioral asymmetries in newborns. Taken together, the present research confirms the existence in some conditions of an early fragile ability to extract shape information in a tactual format and transfer it to a visual format, independent of common experience.

Recognition memory of serially or simultaneously presented words or figures, of epilepsy patients with or without mesial temporal sclerosis

Previous studies, examining short-term recognition memory in patients with partial seizures as a consequence of mesial temporal sclerosis (MTS) have reported inconsistent findings. Dependent on the paradigms used for measuring recognition memory, some studies have demonstrated that the mesial temporal structures are not critically involved in short-term recognition memory. In addition, other studies found a lateralization effect that is consistent with the generally accepted association between left temporal lobe lesions and verbal memory deficits, and right temporal lobe lesions and non-verbal memory impairments. In the present study verbal and non-verbal recognition memory was tested in 41 patients with left or right temporal lobe epilepsy with MTS (22 left; 19 right) versus 44 patients with left or right temporal lobe epilepsy but without MTS (28 left; 16 right). Verbal and non-verbal recognition tasks were presented in both a serial and simultaneous condition to test a hypothesized local/global paradigm. Multiple analyses of variance (MANOVA) showed that unilateral MTS has no marked effects on verbal or non-verbal recognition memory of patients with seizures. An interaction effect between MTS and the lateralization of epileptic activity was found on memory performance: MTS only leads to deficits in recognition memory in patients with right-sided epileptiform activity. As hypothesized, patients with left temporal lobe abnormalities, have specific deficits with recognizing serially presented information.

The specialized structure of human language cortex: pyramidal cell size asymmetries within auditory and language-associated regions of the temporal lobes

Functional lateralization of language within the cerebral cortex has long driven the search for structural asymmetries that might underlie language asymmetries. Most examinations of structural asymmetry have focused upon the gross size and shape of cortical regions in and around language areas. In the last 20 years several labs have begun to document microanatomical asymmetries in the structure of language-associated cortical regions. Such microanatomic results provide useful constraints and clues to our understanding of the biological bases of language specialization in the cortex. In a previous study we documented asymmetries in the size of a specific class of pyramidal cells in the superficial cortical layers. The present work uses a nonspecific stain for cell bodies to demonstrate the presence of an asymmetry in layer III pyramidal cell sizes within auditory, secondary auditory and language-associated regions of the temporal lobes. Specifically, the left hemisphere contains a greater number of the largest pyramidal cells, those that are thought to be the origin of long-range cortico-cortical connections. These results are discussed in the context of cortical columns and how such an asymmetry might alter cortical processing. These findings, in conjunction with other asymmetries in cortical organization that have been documented within several labs, clearly demonstrate that the columnar and connective structure of auditory and language cortex in the left hemisphere is distinct from homotopic regions in the contralateral hemisphere.

Lateralized cognitive processes and lateralized task control in the human brain

The principles underlying human hemispheric specialization are poorly understood. We used functional magnetic resonance imaging of letter and visuospatial decision tasks with identical word stimuli to address two unresolved problems. First, hemispheric specialization depended on the nature of the task rather than on the nature of the stimulus. Second, analysis of frontal candidate regions for cognitive control showed increased coupling between left anterior cingulate cortex (ACC) and left inferior frontal gyrus during letter decisions, whereas right ACC showed enhanced coupling with right parietal areas during visuospatial decisions. Cognitive control is thus localized in the same hemisphere as task execution.

Lateralization of aggression in fish

Recent research has suggested that lateralization of aggressive behaviors could follow an homogeneous pattern among all vertebrates. A left eye/right hemisphere dominance in eliciting aggressive responses has been demonstrated for all groups of tetrapods but teleost fish for which data is lacking. Here we studied differential eye use during aggressive interactions in three species of teleosts: Gambusia holbrooki, Xenotoca eiseni and Betta splendens. In the first experiment we checked for lateralization in the use of the eyes while the subject was attacking its own mirror image. In order to confirm the results, other tests were performed on two species and eye preference was scored during attacks or displays directed toward a live rival. All three species showed a marked preference for using the right eye when attacking a mirror image or a live rival. Thus, the direction of asymmetry in fish appears the opposite to that shown by all the other groups of vertebrates. Hypotheses on the origin of the difference are discussed.

On the nature and evolution of the neural bases of human language

The traditional theory equating the brain bases of language with Broca's and Wernicke's neocortical areas is wrong. Neural circuits linking activity in anatomically segregated populations of neurons in subcortical structures and the neocortex throughout the human brain regulate complex behaviors such as walking, talking, and comprehending the meaning of sentences. When we hear or read a word, neural structures involved in the perception or real-world associations of the word are activated as well as posterior cortical regions adjacent to Wernicke's area. Many areas of the neocortex and subcortical structures support the cortical-striatal-cortical circuits that confer complex syntactic ability, speech production, and a large vocabulary. However, many of these structures also form part of the neural circuits regulating other aspects of behavior. For example, the basal ganglia, which regulate motor control, are also crucial elements in the circuits that confer human linguistic ability and abstract reasoning. The cerebellum, traditionally associated with motor control, is active in motor learning. The basal ganglia are also key elements in reward-based learning. Data from studies of Broca's aphasia, Parkinson's disease, hypoxia, focal brain damage, and a genetically transmitted brain anomaly (the putative "language gene," family KE), and from comparative studies of the brains and behavior of other species, demonstrate that the basal ganglia sequence the discrete elements that constitute a complete motor act, syntactic process, or thought process. Imaging studies of intact human subjects and electrophysiologic and tracer studies of the brains and behavior of other species confirm these findings. As Dobzansky put it, "Nothing in biology makes sense except in the light of evolution" (cited in Mayr, 1982). That applies with as much force to the human brain and the neural bases of language as it does to the human foot or jaw. The converse follows: the mark of evolution on the brains of human beings and other species provides insight into the evolution of the brain bases of human language. The neural substrate that regulated motor control in the common ancestor of apes and humans most likely was modified to enhance cognitive and linguistic ability. Speech communication played a central role in this process. However, the process that ultimately resulted in the human brain may have started when our earliest hominid ancestors began to walk.

Hemispheric differences in stop task performance

This study examined hemispheric specialization for stop task performance. It was found that inhibitory performance was better for stop signals presented in the right visual field. This result provided support for the hypothesis that, during stop task performance, subjects call upon the left-lateralized neural system that is involved in active attention. It was suggested that a stop task requires such a mode of attention because subjects maintain a tonic readiness for inhibitory action while being engaged in the stop task's go routine. Subjects are continuously alert for possible stop signals while discriminating between go stimuli. The stop task may be considered a typical activation task.

Temporal pattern of social aggregation in tadpoles and its influence on the measurement of lateralised response to social stimuli

Tadpoles of several species have been proven to prefer using the left hemifield during fixation of their own mirror images. The lateral bias typically emerges some minutes after the placement of the animals in the test apparatus. Here we checked whether such a temporal pattern was associated with lateralisation per se, or rather reflected temporal variations in social aggregation. We tested the temporal changes in tadpoles' movements directed towards conspecifics and other parts of the environment. We found that the propensity to move to make social aggregation only appears after about 5 min following placement in a novel environment and this corresponded quite well with the appearance of lateralisation, when tadpoles showed a higher probability of approaching a conspecific appearing on their left hemifield rather than on their right hemifield. These findings confirm, using natural conspecifics, evidence that in tadpoles, the left hemifield is better at detecting and directing approach responses to social stimuli.

Differential brain activation patterns during perception of voice and tone onset time series: a MEG study

Evoked magnetic fields were recorded from 18 adult volunteers using magnetoencephalography (MEG) during perception of speech stimuli (the endpoints of a voice onset time (VOT) series ranging from /ga/ to /ka/), analogous nonspeech stimuli (the endpoints of a two-tone series varying in relative tone onset time (TOT), and a set of harmonically complex tones varying in pitch. During the early time window (approximately 60 to approximately 130 ms post-stimulus onset), activation of the primary auditory cortex was bilaterally equal in strength for all three tasks. During the middle (approximately 130 to 800 ms) and late (800 to 1400 ms) time windows of the VOT task, activation of the posterior portion of the superior temporal gyrus (STGp) was greater in the left hemisphere than in the right hemisphere, in both group and individual data. These asymmetries were not evident in response to the nonspeech stimuli. Hemispheric asymmetries in a measure of neurophysiological activity in STGp, which includes the supratemporal plane and cortex inside the superior temporal sulcus, may reflect a specialization of association auditory cortex in the left hemisphere for processing speech sounds. Differences in late activation patterns potentially reflect the operation of a postperceptual process (e.g., rehearsal in working memory) that is restricted to speech stimuli.

A positron emission tomography study of short- and long-term treatment effects on functional brain activation in adults who stutter

Previous studies have shown that fluency-inducing techniques, such as choral speech, result in changes in neural activation as measured by functional neuroimaging. In the present study, positron emission tomography was used to investigate the effects of intensive behavioural treatment, followed by a 1-year maintenance program, on the pattern of cortical and subcortical activation in stuttering adults during silent and oral reading of single words. The results indicate changes in activation lateralisation, as well as a general reduction in over-activation, especially in the motor cortex, following treatment. The results are discussed in light of previous functional imaging studies with stuttering adults.

EDUCATIONAL OBJECTIVES

The reader will learn about and be able to describe the: (1) use of functional neuroimaging PET in the study of stuttering; (2) differences in neural activation between stuttering and non-stuttering adults; and (3) effects of behavioural fluency treatment on cortical and subcortical activations in stuttering speakers.

Gender differences in cerebral glucose metabolism: a PET study

OBJECTIVE

Some studies have examined gender differences in brain function based on cerebral blood flow and cerebral metabolism by using positron emission tomography (PET). However, the findings of these studies are controversial and most of them were analyzed by the regions of interest (ROIs) method. Here, we evaluated gender differences of cerebral glucose metabolism under the resting state in a voxel-based analysis.

METHODS

We studied 44 healthy volunteers (22 females, 63.0+/-6.3 years, and 22 males, 63.1+/-8.4 years). Cerebral glucose metabolic images were obtained with (18)F-fluorodeoxyglucose (FDG) and PET. All individual data were transformed to standard brain space and the male and female groups were compared using statistical parametric mapping (SPM).

RESULTS

The males had significantly higher glucose metabolism in the right insula, middle temporal gyrus, and medial frontal lobe than the females. Glucose metabolism in the hypothalamus was significantly higher in females than in males. There was a significant correlation between aging and glucose metabolism in the left thalamus in males and in the left caudate nucleus and hypothalamus in females. In males, but not females, there was a significant asymmetry between the bilateral hemispheres.

CONCLUSION

We found that there were obvious gender differences in regional cerebral glucose metabolism and this is the first report of higher glucose metabolism in the hypothalamus in females than in males.

Hemispheric asymmetries for visual and auditory temporal processing: an evoked potential study

Lateralization for temporal processing was investigated using evoked potentials to an auditory and visual gap detection task in 12 dextral adults. The auditory stimuli consisted of 300-ms bursts of white noise, half of which contained an interruption lasting 4 or 6 ms. The visual stimuli consisted of 130-ms flashes of light, half of which contained a gap lasting 6 or 8 ms. The stimuli were presented bilaterally to both ears or both visual fields. Participants made a forced two-choice discrimination using a bimanual response. Manipulations of the task had no effect on the early evoked components. However, an effect was observed for a late positive component, which occurred approximately 300-400 ms following gap presentation. This component tended to be later and lower in amplitude for the more difficult stimulus conditions. An index of the capacity to discriminate gap from no-gap stimuli was gained by calculating the difference waveform between these conditions. The peak of the difference waveform was delayed for the short-gap stimuli relative to the long-gap stimuli, reflecting decreased levels of difficulty associated with the latter stimuli. Topographic maps of the difference waveforms revealed a prominence over the left hemisphere. The visual stimuli had an occipital parietal focus whereas the auditory stimuli were parietally centered. These results confirm the importance of the left hemisphere for temporal processing and demonstrate that it is not the result of a hemispatial attentional bias or a peripheral sensory asymmetry.

The effects of interdistracter similarity on search processes in superior parietal cortex

The superior parietal lobe has been associated with the spatial integration of visual features, an important step in the detection of particular form conjunctions. However, behavioral research has indicated that when target items can be segmented from neighboring distracters via similarity grouping, detection may not rely on spatial integration. The question therefore arises as to whether the superior parietal cortex is an integral component of conjunction search or only important in the absence of certain grouping relations. Here, we acquired measures of reaction time and event-related fMRI, while subjects searched for conjunction targets in displays containing either homogeneous or heterogeneous distracters. We confirm that under conditions of low distracter similarity, search involves parietal-motor areas associated with spatial selection. However, we also demonstrate that under conditions of high distracter similarity, search is instead associated with activation of right temporal-parietal cortex. These results suggest that the superior parietal cortex is not a necessary component of visual conjunction search and highlight a new role for the right temporal-parietal cortex in perceptual grouping.

Lateralization of brain activation to imagination and smell of odors using functional magnetic resonance imaging (fMRI): left hemispheric localization of pleasant and right hemispheric localization of unpleasant odors

PURPOSE

Our goal was to use functional MRI (fMRI) of brain to reveal activation in each cerebral hemisphere in response to imagination and smell of odors.

METHOD

FMRI brain scans were obtained in 24 normal subjects using multislice fast low angle shot (FLASH) MRI in response to imagination of banana and peppermint odors and in response to smell of corresponding odors of amyl acetate and menthone, respectively, and of pyridine. Three coronal sections selected from anterior to posterior brain regions were used. Similar studies were obtained in two patients with hyposmia using FLASH MRI and in one patient with hyposmia using echo planar imaging (EPI) both before and after theophylline treatment that returned smell function to or toward normal in each patient and in two patients with birhinal phantosmia (persistent foul odor) and global phantogeusia (persistent foul taste) with FLASH and EPI fMRI before and after treatment with neuroleptic drugs that inhibited their phantosmia and phantogeusia. Activation images were derived using correlation analysis. Ratios of hemispheric areas of brain activation to total hemispheric brain areas were calculated for FLASH fMRI, and numerical counts of pixel clusters in each hemisphere were made for EPI studies. Total pixel cluster counts in localized regions of each hemispheric section were also obtained.

RESULTS

In normal subjects, activation generally occurred in left (L) > right (R) brain hemisphere in response to banana and peppermint odor imagination and to smell of corresponding odors of amyl acetate and menthone. Whereas there were no overall hemispheric differences for pyridine odor, activation in men was R > L hemisphere. Although absolute activation in both L and R hemispheres in response to banana odor imagination and amyl acetate smell was men > women, the ratio of L to R activation was women > men. In hyposmic patients studied by FLASH fMRI, activation to banana odor imagination and amyl acetate smell was L > R hemisphere both before and after theophylline treatment. In the hyposmic patient studied with EPI before theophylline treatment, activation to banana and peppermint odor imagination and to amyl acetate, menthone, and pyridine smell was R > L hemisphere; after theophylline treatment restored normal smell function, activation shifted completely with banana and peppermint odor imagination and amyl acetate and menthone smell to L > R hemisphere, consistent with responses in normal subjects. However, this shift also occurred for pyridine smell, which is opposite to responses in normal control subjects. In patients with phantosmia and phantogeusia, activation to phantosmia and phantogeusia before treatment was R > L hemisphere; after treatment inhibited phantosmia and phantogeusia, activation shifted with a slight L > R hemispheric lateralization. Localization of all lateralized responses indicated that anterior frontal and temporal cortices were brain regions most involved with imagination and smell of odors and with phantosmia and phantogeusia presence.

CONCLUSION

Imagination and smell of odors perceived as pleasant generally activated the dominant or L > R brain hemisphere. Smell of odors perceived as unpleasant and unpleasant phantosmia and phantogeusia generally activated the contralateral or R > L brain hemisphere. With remission of phantosmia and phantogeusia, hemispheric activation was not only inhibited, but also there was a slight shift to L > R hemispheric predominance. Predominant L > R hemispheric differences in brain activation in normal subjects occurred in the order amyl acetate > menthone > pyridine, consistent with the hypothesis that pleasant odors are more appreciated in L hemisphere and unpleasant odors more in R hemisphere. Anterior frontal and temporal cortex regions previously found activated by imagination and smell of odors and phantosmia and phantogeusia perception accounted for most hemispheric differences.

Invariance of long-term visual priming to scale, reflection, translation, and hemisphere

The representation of shape mediating visual object priming was investigated. In two blocks of trials, subjects named images of common objects presented for 185 ms that were bandpass filtered, either at high (10 cpd) or at low (2 cpd) center frequency with a 1.5 octave bandwidth, and positioned either 5 degrees right or left of fixation. The second presentation of an image of a given object type could be filtered at the same or different band, be shown at the same or translated (and mirror reflected) position, and be the same exemplar as that in the first block or a same-name different-shaped exemplar (e.g. a different kind of chair). Second block reaction times (RTs) and error rates were markedly lower than they were on the first block, which, in the context of prior results, was indicative of strong priming. A change of exemplar in the second block resulted in a significant cost in RTs and error rates, indicating that a portion of the priming was visual and not just verbal or basic-level conceptual. However, a change in the spatial frequency (SF) content of the image had no effect on priming despite the dramatic difference it made in appearance of the objects. This invariance to SF changes was also preserved with centrally presented images in a second experiment. Priming was also invariant to a change in left-right position (and mirror orientation) of the image. The invariance over translation of such a large magnitude suggests that the locus of the representation mediating the priming is beyond an area that would be homologous to posterior TEO in the monkey. We conclude that this representation is insensitive to low level image variations (e.g. SF, precise position or orientation of features) that do not alter the basic part-structure of the object. Finally, recognition performance was unaffected by whether low or high bandpassed images were presented either in the left or right visual field, giving no support to the hypothesis of hemispheric differences in processing low and high spatial frequencies.

Lateralization of cognitive processes in the brain

The lateralization of cognitive processes in the brain is discussed. The traditional view of a language-visuo/spatial dichotomy of function between the hemispheres has been replaced by more subtle distinctions. The use of magnetic resonance imaging (MRI) to study brain morphology has resulted in a renewed focus on the relationship between structural and functional asymmetry. Focus has been on the role played by the planum temporale area in the posterior part of the superior temporal gyrus for language asymmetry, and the possible significance of the larger left planum. The dichotic listening technique is used to illustrate the difference between bottom-up, or stimulus-driven laterality versus top-down, or instruction-driven laterality. It is suggested that the hemispheric dominance observed at any time is the sum result of the dynamic interaction between bottom-up and top-down processing tendencies. Stimulus-driven laterality dominance is always monitored and modulated through top-down cognitive processes, like shifting of attention and changes in arousal. A model of top-down modulation of bottom-up laterality is presented with special reference to the understanding of psychiatric disorders.

Brain hemispheric organization, anxiety, and psychoses

Abnormalities of brain hemispheric organization have been found in a variety of psychiatric disorders. Despite the great amount of data collected and the number of theoretical models elaborated, the role of these abnormalities in the pathogenesis of these disorders remains controversial. This article briefly reviews current concepts of hemispheric functioning, discusses the role of abnormalities of brain hemispheric organization in schizophrenia and in two anxiety disorders (panic disorder and obsessive-compulsive disorder), and outlines a developmental perspective that accounts for the observed abnormalities.

Ipsilesional versus contralesional neglect depends on attentional demands

Right hemisphere injuries often produce contralesional hemispatial neglect (CN). In contrast to CN, some patients with right hemisphere damage can also show so-called ipsilesional neglect (IN). Previous reports found that patients tend to show IN on line bisection tasks but CN on other tasks such as target cancellation. To learn why these two tasks induce different spatial biases in patients with right hemisphere injury, conventional (i.e. solid) line bisection was compared with two novel bisection tasks consisting of horizontally aligned strings of characters. The subjects' task was to mark a target character that was at or closest to the true midpoint of the simulated line. Four of the 5 patients showed a dissociation whereby IN occurred for solid lines while CN was observed on character lines. The two patients assessed with an antisaccade paradigm showed a "visual grasp" for leftward stimuli. The present results suggest that neglect on line bisection may reflect two opposing forces, an approach behavior or "visual grasp" toward left hemispace and an attentional bias toward right hemispace.

Language laterality in English/Welsh bilinguals: Language-acquisitional and language-specific factors in the development of lateralisation

In order to test the hypothesis that monolinguals differ from bilinguals in their pattern of language lateralisation and to examine the relative merits of language-acquisitional versus language-specific factors, two experiments involving divided screen presentation of two languages were conducted using Welsh/English speaking participants. In the first experiment 80 monolingual teenagers were compared to 80 bilingual teenagers on a tachistoscopic "visual half-field" test of Welsh and English nouns and verbs. ANOVA revealed a greater left hemisphere advantage for Welsh-English bilinguals as compared to English monolinguals. Thus, in contrast to previous studies, in our bilinguals there was evidence of greater left hemisphere involvement in the processing of language. In the second experiment, four separate groups of 40 teenagers, varying in the age and manner of acquisition of their languages, were compared on the same test of Welsh and English words. These groups can be viewed as graded from the early to late bilinguals. ANOVA revealed a greater left hemisphere advantage when processing Welsh as compared to English words for all four groups. However no significant difference was observed between the four groups in respect of laterality for Welsh and English, indicating an equally greater left hemisphere bias for all four groups when processing Welsh words. We discuss these results in terms of a language-specific effect and suggest the specific orthography of the Welsh language (for individually presented nouns and verbs) promotes a left hemisphere advantage over and above language-acquisitional factors.

The neuroanatomical correlates of route learning impairment

Recent functional imaging studies of topographical learning point to the participation of a large network of cortical and subcortical regions. Nevertheless, areas which are crucial remain poorly specified due to the absence of group studies of subjects with focal lesions distributed throughout the brain. We assessed the ability of 127 subjects with stable, focal lesions to learn a complex real-life route, a critical aspect of topographical functioning. Results indicated that impairment in route learning was highly associated with damage to medial occipital and posterior parahippocampal cortices in either hemisphere, the right hippocampus, and the right inferotemporal region. Impairment was seen among 86% of the subjects with damage to any these regions, in contrast to impairment among 31% of subjects with lesions in other regions. The importance of medial occipitotemporal cortices bilaterally and right inferotemporal cortex likely reflects the critical role of the ability to quickly and accurately perceive and learn multiple topographical scenes. The importance of the right (and probably left) posterior parahippocampal gyrus and of the right hippocampus likely reflects their critical, distinctive roles forming an integrated representation of the extended topographical environment (i.e., the appearance of places and spatial relationships between specific places), and consolidating that representation into multifaceted contextual knowledge of the environment.

Hemispheric processing deficits in patients with paranoid schizophrenia

The purpose of the present study was to investigate hemispheric deficits in individuals with paranoid schizophrenia on four kinds of tasks: dichoptic viewing tasks involving verbal and nonverbal visual stimuli, and dichotic listening tasks involving verbal and nonverbal auditory stimuli. As dependent measures, both accuracy and speed of (correct) responding were measured. The sample recruited for this study consisted of 18 patients with paranoid schizophrenia, 15 outpatients with anxiety disorders, and 20 controls with no history of psychiatric disorders. Results indicated that, relative to the controls, the paranoid schizophrenic patients were less accurate and less efficient on auditory-verbal tasks requiring right hemisphere processing. Unlike the controls the paranoid schizophrenic patients manifested a lateralized left hemisphere advantage.

Dopamine and the origins of human intelligence

A general theory is proposed that attributes the origins of human intelligence to an expansion of dopaminergic systems in human cognition. Dopamine is postulated to be the key neurotransmitter regulating six predominantly left-hemispheric cognitive skills critical to human language and thought: motor planning, working memory, cognitive flexibility, abstract reasoning, temporal analysis/sequencing, and generativity. A dopaminergic expansion during early hominid evolution could have enabled successful chase-hunting in the savannas of sub-Saharan Africa, given the critical role of dopamine in counteracting hyperthermia during endurance activity. In turn, changes in physical activity and diet may have further increased cortical dopamine levels by augmenting tyrosine and its conversion to dopamine in the central nervous system (CNS). By means of the regulatory action of dopamine and other substances, the physiological and dietary changes may have contributed to the vertical elongation of the body, increased brain size, and increased cortical convolutedness that occurred during human evolution. Finally, emphasizing the role of dopamine in human intelligence may offer a new perspective on the advanced cognitive reasoning skills in nonprimate lineages such as cetaceans and avians, whose cortical anatomy differs radically from that of primates.