Functional frontalisation with age: mapping neurodevelopmental trajectories with fMRI

A review of the biological bases of ADHD: what have we learned from imaging studies?

Attention Deficit Hyperactivity Disorder (ADHD) is a common and impairing neuropsychiatric disorder with onset at preschool age. Although a significant amount of progress has been made investigating the neurobiology of this disorder, its precise etiology still remains unclear. Converging evidence from studies of the neuropharmacology, genetics, neuropsychology, and neuroimaging of ADHD imply the involvement of fronto-striatal circuitry in ADHD. However, while it does appear that poor inhibitory control and the deficits in fronto-striatal circuitry associated with it are central, there is evidence to suggest that more posterior cerebral areas are also implicated in this disorder. Anatomical studies suggest widespread reductions in volume throughout the cerebrum and cerebellum, while functional imaging studies suggest that affected individuals activate more diffuse areas than controls during the performance of cognitive tasks. The future impact of new MR imaging methodologies on the field is discussed.

A functional MRI study of simple arithmetic—a comparison between children and adults

The purpose of this study was to examine brain areas involved in simple arithmetic, and to compare these areas between adults and children. Eight children (four girls and four boys; age, 9-14 years) and eight adults (four women and four men; age, 40-49 years) were subjected to this study. Functional magnetic resonance imaging (fMRI) was performed during mental calculation of addition, subtraction, and multiplication of single digits. In each group, the left middle frontal, bilateral inferior temporal and bilateral lateral occipital cortices were activated during each task. The adult group showed activation of the right frontal cortex during addition and multiplication tasks, but the children group did not. Activation of the intraparietal cortex was observed in the adult group during each task. Although, activation patterns were slightly different among tasks, as well as between groups, only a small number of areas showed statistically significant differences. The results indicate that cortical networks involved in simple arithmetic are similar among arithmetic operations, and may not show significant changes in the structure during the second decade of life.

Tracing the Neuroanatomical Profiles of Reward Pathways with Markers of Neuronal Activation

Functional neuroanatomical tools have played an important role in proposing which structures underlie brain stimulation reward circuitry. This review focuses on studies employing metabolic markers of neuronal and glial activation, including 2-deoxyglucose, cytochrome oxidase, and glycogen phosphorylase, and a marker of cellular activation, the immediate early gene c-fos. The principles underlying each method, their application to the study of brain stimulation reward, and their strengths and limitations are described. The usefulness of this strategy in identifying candidate structures, and the degree of overlap in the patterns of activation arising from different markers is addressed in detail. How these data have contributed to an understanding of the organization of reward circuitry and directed our thinking towards an alternative framework of neuronal arrangement is discussed in the final section.

Brain activity correlates differentially with increasing temporal complexity of rhythms during initialisation, synchronisation, and continuation phases of paced finger tapping

Activity in parts of the human motor system has been shown to correlate with the complexity of performed motor sequences in terms of the number of limbs moved, number of movements, and number of trajectories. Here, we searched for activity correlating with temporal complexity, in terms of the number of different intervals produced in the sequence, using an overlearned tapping task. Our task was divided into three phases: movement selection and initiation (initiate), synchronisation of finger tapping with an external auditory cue (synchronise), and continued tapping in absence of the auditory pacer (continue). Comparisons between synchronisation and continuation showed a pattern in keeping with prior neuroimaging studies of paced finger tapping. Thus, activation of bilateral SMA and basal ganglia was greater in continuation tapping than in synchronisation tapping. Parametric analysis revealed activity correlating with temporal complexity during initiate in bilateral supplementary and pre-supplementary motor cortex (SMA and preSMA), rostral dorsal premotor cortex (PMC), basal ganglia, and dorsolateral prefrontal cortex (DLPFC), among other areas. During synchronise, correlated activity was observed in bilateral SMA, more caudal dorsal and ventral PMC, right DLPFC and right primary motor cortex. No correlated activity was observed during continue at P<0.01 (corrected, cluster level), though left angular gyrus was active at P<0.05. We suggest that the preSMA and rostral dorsal PMC activities during initiate may be associated with selection of timing parameters, while activation in centromedial prefrontal cortex during both initiate and synchronise may be associated with temporal error monitoring or correction. The absence of activity significantly correlated with temporal complexity during continue suggests that, once an overlearned timed movement sequence has been selected and initiated, there is no further adjustment of the timing control processes related to its continued production in absence of external cues.

A right hemispheric frontocerebellar network for time discrimination of several hundreds of milliseconds

Debate still surrounds the nature of the role of the dorsolateral prefrontal gyrus (DLPFC) in time perception. This region is frequently associated with working memory and is thus implicated as a so-called "accumulator" within a hypothesized internal clock model. However, we hypothesized that this region may have a more primary role in time perception. To test this hypothesis we used functional magnetic resonance imaging (fMRI) to examine the neural correlates of relatively pure time perception with a temporal discrimination task where intervals of 1 s had to be discriminated from those of 1.3, 1.4, and 1.5 s. Time perception in this particular time domain within the "perceived present" has not previously been investigated using fMRI. By using relatively short time periods to be discriminated and also contrasting activation with an order judgment task, we aimed to minimize the confounding aspects of sustained attention and working memory. In a group of 20 healthy right-handed adult males, neural activation associated with time discrimination was found in a predominantly right hemispheric network of right dorsolateral and inferior prefrontal cortices, right supplementary motor area, and left cerebellum. We conclude that right DLPFC, rather than having a purely working memory function, might be more centrally involved in time perception than previously thought.

Neural development of selective attention and response inhibition

Brain activation differences between 12 children (9- to 12-year-olds) and 12 adults (20- to 30-year-olds) were examined on two cognitive tasks during functional magnetic resonance imaging (fMRI). Spatial selective attention was measured with the visual search for a conjunction target (red triangle) in a field of distracters and response inhibition was measured with a go no-go task. There were small developmental differences in the selective attention task, with children showing greater activation than adults in the anterior cingulate and thalamus. There were large developmental differences in the response inhibition task, with children showing greater activation than adults in a fronto-striatal network including middle cingulate, medial frontal gyrus, medial aspects of bilateral superior frontal gyrus, and the caudate nucleus on the left. Children also showed greater bilateral activation for the response inhibition task in posterior cingulate, thalamus and the hippocampo-amygdaloid region. The extensive developmental differences on the response inhibition task are consistent with the prolonged maturation of the fronto-striatal network.

Right inferior prefrontal cortex mediates response inhibition while mesial prefrontal cortex is responsible for error detection

Inhibitory control and error detection are among the highest evolved human self-monitoring functions. Attempts in functional neuroimaging to effectively isolate inhibitory motor control from other cognitive functions have met with limited success. Different brain regions in inferior, mesial, and dorsolateral prefrontal cortices and parietal and temporal lobes have been related to inhibitory control in go/no-go and stop tasks. The widespread activation reflects the fact that the designs used so far have comeasured additional noninhibitory cognitive functions such as selective attention, response competition, decision making, target detection, and inhibition failure. Here we use rapid, mixed trial, event-related functional magnetic resonance imaging to correlate brain activation with an extremely difficult situation of inhibitory control in a challenging stop task that controls for noninhibitory functions. The difficulty of the stop task, requiring withholding of a triggered motor response, was assured by an algorithm that adjusted the task individually so that each subject only succeeded on half of all stop trials, failing on the other half. This design allowed to elegantly separate brain activation related to successful motor response inhibition and to inhibition failure or error detection. Brain activation correlating with successful inhibitory control in 20 healthy volunteers could be isolated in right inferior prefrontal cortex. Failure to inhibit was associated with activation in mesial frontopolar and bilateral inferior parietal cortices, presumably reflecting an attention network for error detection.

Motor timing deficits in community and clinical boys with hyperactive behavior: the effect of methylphenidate on motor timing

In a previous paper we showed that community children with hyperactive behavior were more inconsistent than controls in the temporal organization of their motor output. In this study we investigated: (1) various aspects of motor timing processes in 13 clinically diagnosed boys with attention deficit hyperactivity disorder (ADHD) who were compared to 11 community boys with hyperactive behavior and to a control group and (2) the effect of methylphenidate on the motor timing processes in the clinical group with ADHD in a double blind, cross-over, medication-placebo design, including 4 weeks of medication. The clinical group with ADHD, like the community group with hyperactivity, showed greater variability in sensorimotor synchronization and in sensorimotor anticipation relative to controls. The clinical group was also impaired in time perception, which was spared in the community group with hyperactivity. The persistent, but not the acute dose, of methylphenidate reduced the variability of sensorimotor synchronization and anticipation, but had no effect on time perception. This study shows that motor timing functions are impaired in both clinical and community children with hyperactivity. It is the first study to show the effectiveness of persistent administration of methylphenidate on deficits in motor timing in ADHD children and extends the use of methylphenidate from the domain of attentional and inhibitory functions to the domain of executive motor timing.

Imaging methods for evaluating brain function in man

Imaging of brain function and neurotransmission is an important bridge between basic and clinical research. Regional cerebral energy metabolism and blood flow are normally coupled to regional cerebral function. Positron tomography (PET) studies of cerebral glucose metabolism and blood flow, single photon tomography (SPECT) and MRI studies of cerebral perfusion, have been used to image cerebral development and aging in man. The sensitivity, temporal resolution, spatial resolution and lack of radiation have led to the widespread utilization of blood oxygen level dependent (BOLD) and MRI perfusion techniques. PET and SPECT methods for studying cerebral neurotransmission include studies of dopaminergic, serotonergic, cholinergic, opiate and GABAergic neurotransmission in man. Studies of cerebral neurotransmission in man have helped to delineate the mechanisms of action of antipsychotic and antidepressant drugs, the diagnosis and progression of Parkinson's disease, and to evaluate neuroprotective drugs. The strengths, limitations, and application of these modalities are reviewed. The application of these methods to cerebral development and aging are briefly discussed.

Distinct systems for automatic and cognitively controlled time measurement: evidence from neuroimaging

A recent review of neuroimaging data on time measurement argued that the brain activity seen in association with timing is not influenced by specific characteristics of the task performed. In contrast, we argue that careful analysis of this literature provides evidence for separate neural timing systems associated with opposing task characteristics. The 'automatic' system draws mainly upon motor circuits and the 'cognitively controlled' system depends upon prefrontal and parietal regions.

A review of electrophysiology in attention-deficit/hyperactivity disorder: II. Event-related potentials

OBJECTIVE

This article reviews the event-related potential (ERP) literature in relation to attention-deficit/hyperactivity disorder (AD/HD).

METHODS

ERP studies exploring various aspects of brain functioning in AD/HD are reviewed, ranging from early preparatory processes to a focus on the auditory and visual attention systems, and the frontal inhibition system. Implications of these data for future research and development in AD/HD are considered.

RESULTS

A complex range of ERP deficits has been associated with the disorder. Differences have been reported in preparatory responses, such as the contingent negative variation. In the auditory modality, AD/HD-related differences are apparent in all components from the auditory brain-stem response to the late slow wave. The most robust of these is the reduced posterior P3 in the auditory oddball task. There are fewer studies of the visual attention system, but similar differences are reported in a range of components. Results suggesting an inhibitory processing deficit have been reported, with recent studies of the frontal inhibitory system indicating problems of inhibitory regulation.

CONCLUSIONS

The research to date has identified a substantial number of ERP correlates of AD/HD. Together with the robust AD/HD differences apparent in the EEG literature, these data offer potential to improve our understanding of the specific brain dysfunction(s) which result in the disorder. Increased focus on the temporal locus of the information processing deficit(s) underlying the observed range of ERP differences is recommended. Further work in this field may benefit from a broader conceptual approach, integrating EEG and ERP measures of brain function.

Brain sites of movement disorder: genetic and environmental agents in neurodevelopmental perturbations

In assessing and assimilating the neurodevelopmental basis of the so-called movement disorders it is probably useful to establish certain concepts that will modulate both the variation and selection of affliction, mechanisms-processes and diversity of disease states. Both genetic, developmental and degenerative aberrations are to be encompassed within such an approach, as well as all deviations from the necessary components of behaviour that are generally understood to incorporate "normal" functioning. In the present treatise, both conditions of hyperactivity/hypoactivity, akinesia and bradykinesia together with a constellation of other symptoms and syndromes are considered in conjunction with the neuropharmacological and brain morphological alterations that may or may not accompany them, e.g. following neonatal denervation. As a case in point, the neuroanatomical and neurochemical points of interaction in Attention Deficit and Hyperactivity disorder (ADHD) are examined with reference to both the perinatal metallic and organic environment and genetic backgrounds. The role of apoptosis, as opposed to necrosis, in cell death during brain development necessitates careful considerations of the current explosion of evidence for brain nerve growth factors, neurotrophins and cytokines, and the processes regulating their appearance, release and fate. Some of these processes may possess putative inherited characteristics, like alpha-synuclein, others may to greater or lesser extents be endogenous or semi-endogenous (in food), like the tetrahydroisoquinolines, others exogenous until inhaled or injested through environmental accident, like heavy metals, e.g. mercury. Another central concept of neurodevelopment is cellular plasticity, thereby underlining the essential involvement of glutamate systems and N-methyl-D-aspartate receptor configurations. Finally, an essential assimilation of brain development in disease must delineate the relative merits of inherited as opposed to environmental risks not only for the commonly-regarded movement disorders, like Parkinson's disease, Huntington's disease and epilepsy, but also for afflictions bearing strong elements of psychosocial tragedy, like ADHD, autism and Savantism.

Developmental course of ADHD symptomatology during the transition from childhood to adolescence: a review with recommendations

Although historically conceptualized as a disorder that was limited to males during middle childhood, ADHD is currently conceptualized as a chronic disorder that persists into adolescence and adulthood for both sexes. Nonetheless, the veracity of adult ADHD continues to be the source of debate. In order to frame this debate, research leading to the conceptualization of ADHD as a chronic disorder is reviewed. A distinction is made between the developmental outcomes versus the developmental course of ADHD. It is concluded that although childhood ADHD is associated with negative developmental outcomes in adolescence and adulthood, questions about the developmental course of ADHD remain. Although it appears that ADHD diminishes with advancing age, a number of methodological limitations prohibit firm conclusions. Recommendations for future studies are made with an emphasis on 1) overcoming extant methodological limitations in the literature and 2) the need for theoretically derived hypotheses regarding continuity and change in ADHD symptomatology over time.

Drug addiction and its underlying neurobiological basis: neuroimaging evidence for the involvement of the frontal cortex

OBJECTIVE

Studies of the neurobiological processes underlying drug addiction primarily have focused on limbic subcortical structures. Here the authors evaluated the role of frontal cortical structures in drug addiction.

METHOD

An integrated model of drug addiction that encompasses intoxication, bingeing, withdrawal, and craving is proposed. This model and findings from neuroimaging studies on the behavioral, cognitive, and emotional processes that are at the core of drug addiction were used to analyze the involvement of frontal structures in drug addiction.

RESULTS

The orbitofrontal cortex and the anterior cingulate gyrus, which are regions neuroanatomically connected with limbic structures, are the frontal cortical areas most frequently implicated in drug addiction. They are activated in addicted subjects during intoxication, craving, and bingeing, and they are deactivated during withdrawal. These regions are also involved in higher-order cognitive and motivational functions, such as the ability to track, update, and modulate the salience of a reinforcer as a function of context and expectation and the ability to control and inhibit prepotent responses.

CONCLUSIONS

These results imply that addiction connotes cortically regulated cognitive and emotional processes, which result in the overvaluing of drug reinforcers, the undervaluing of alternative reinforcers, and deficits in inhibitory control for drug responses. These changes in addiction, which the authors call I-RISA (impaired response inhibition and salience attribution), expand the traditional concepts of drug dependence that emphasize limbic-regulated responses to pleasure and reward.

Maturation of brain function associated with response inhibition

OBJECTIVE

To investigate the developmental trajectory of response inhibition and, more specifically, whether there is a dissociation of function in the prefrontal cortex over the course of development of executive function and associated response inhibition abilities.

METHOD

Nineteen typically developing subjects, ranging in age from 8 to 20, performed a Go/NoGo task while behavioral and functional magnetic resonance imaging (fMRI) data were collected.

RESULTS

All subjects performed the task with few errors of omission and commission. No relationship between accuracy and age emerged, but the ability to inhibit responses significantly improved with age. Analyses of fMRI data revealed a positive correlation between activation and age in the left inferior frontal gyrus/insula/orbitofrontal gyrus, and a negative correlation between activation and age in the left middle/superior frontal gyri.

CONCLUSION

These data provide the first evidence of dissociable processes occurring in the prefrontal cortex during development of executive functions associated with response inhibition: (1) Younger subjects activate more extensively than older subjects in discrete regions of the prefrontal cortex, presumably due to increased demands and inefficient recruitment of brain regions subserving executive functions including working memory. (2) Older subjects show increasingly focal activation in specific regions thought to play a more critical role in response inhibition.

Neural basis of protracted developmental changes in visuo-spatial working memory

Developmental studies have shown that visuo-spatial working memory (VSWM) performance improves throughout childhood and adolescence into young adulthood. The neural basis of this protracted development is poorly understood. In this study, we used functional MRI (fMRI) to examine VSWM function in children, adolescents, and young adults, ages 7-22. Subjects performed a 2-back VSWM experiment that required dynamic storage and manipulation of spatial information. Accuracy and response latency on the VSWM task improved gradually, extending into young adulthood. Age-related increases in brain activation were observed in focal regions of the left and right dorsolateral prefrontal cortex, left ventrolateral prefrontal cortex (including Broca's area), left premotor cortex, and left and right posterior parietal cortex. Multiple regression analysis was used to examine the relative contributions of age, accuracy, and response latency on activation. Our analysis showed that age was the most significant predictor of activation in these brain regions. These findings provide strong evidence for a process of protracted functional maturation of bilateral fronto-parietal neural networks involved in VSWM development. At least two neural systems involved in VSWM mature together: (i) a right hemisphere visuo-spatial attentional system, and (ii) a left hemisphere phonological storage and rehearsal system. These observations suggest that visually and verbally mediated mnemonic processes, and their neural representations, develop concurrently during childhood and adolescence and into young adulthood.

Altered cortical activity in children with attention-deficit/hyperactivity disorder during attentional load task

OBJECTIVE

To evaluate whether cortical activity recorded during attentional load in children with ADHD is different compared with controls.

METHOD

Quantitative electroencephalography (QEEG) was performed at open eyes and during performance of the Continuous Performance Task.

RESULTS

Children with ADHD showed an altered pattern of QEEG activity, especially during the attentional load task, with increased slow cortical activity (mainly over the frontal areas) and decreased fast cortical activity.

CONCLUSIONS

The findings indicate a different arousal level in children with ADHD, which could be due to a delay in functional cortical maturation. To evaluate the clinical importance of these findings, a longitudinal follow-up will be conducted.

The effect of preceding context on inhibition: an event-related fMRI study

In this study we combined event-related fMRI with a parametric manipulation of the go nogo paradigm to examine the effect of preceding context on inhibitory processes. Nogo trials were preceded by either 1, 3, or 5 go trials and then compared to one another. Two distinct patterns of activation were associated with behavioral inhibition: First, the ventral prefrontal cortex, cingulate gyrus, and superior parietal regions showed a context effect with an increase in MR signal to nogo trials with increasing number of preceding go trials. Second, anterior regions in the supplementary and premotor cortex showed an increase in MR signal on the nogo condition after 5 preceding go trials, but not after only 1 or 3. A model using the BOLD response in our data was used to verify that the effect of context was not an artifact of the randomization scheme used in the design.

A review of adult ADHD: a neuropsychological and neuroimaging perspective

Attention-deficit/hyperactivity disorder (ADHD) is a developmental disorder which effects an estimated 3% to 5% of children. Despite estimates that ADHD persists in 30% to 70% of adults having had the disorder in childhood, ADHD in adulthood remains controversial. This report summarizes current thinking in the diagnosis and etiology of adult ADHD. Most theories posit that ADHD is related to anomalies in frontal lobe function and dopaminergic transmission. However, there is debate as to whether ADHD is a unitary disorder with different manifestations, a syndrome, or multiple disorders. The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, classifies ADHD into inattention, hyperactivity-impulsivity, and combined subtypes. Although problems with cognition are core ADHD symptoms, self-reporting has not been a reliable predictor of neuropsychological test performance. Nevertheless, we suggest that a performance-based diagnosis, including empirically derived, age-sensitive neuropsychological tests, provides the best hope of dissociating ADHD from psychiatric disorders with similar symptoms. We also describe the promise of new neuroimaging technologies, such as functional magnetic resonance imaging, in elucidating the pathophysiology of ADHD and similar psychiatric disorders.

A developmental fMRI study of the Stroop color-word task

We used fMRI to investigate developmental changes in brain activation during a Stroop color-word interference task. A positive correlation was observed between age and Stroop-related activation (n = 30) in the left lateral prefrontal cortex, the left anterior cingulate, and the left parietal and parieto-occipital cortices. No regions showed a negative correlation between activation and age. We further investigated age-related differences by stratifying the sample into three age groups: children (ages 7-11), adolescents (ages 12-16), and young adults (ages 18-22). Young adult subjects (n = 11) displayed significant activation in the inferior and middle frontal gyri bilaterally, the left anterior cingulate, and bilateral inferior and superior parietal lobules. Between-group comparisons revealed that young adults had significantly greater activation than adolescent subjects (n = 11) in the left middle frontal gyrus and that young adults showed significantly greater activation than children (n = 8) in the anterior cingulate and left parietal and parieto-occipital regions, as well as in the left middle frontal gyrus. Compared to children, both adult and adolescent subjects exhibited significantly greater activation in the parietal cortex. Adult and adolescent groups, however, did not differ in activation for this region. Together, these data suggest that Stroop task-related functional development of the parietal lobe occurs by adolescence. In contrast, prefrontal cortex function contributing to the Stroop interference task continues to develop into adulthood. This neuromaturational process may depend on increased ability to recruit focal neural resources with age. Findings from this study, the first developmental fMRI investigation of the Stroop interference task, provide a template with which normal development and neurodevelopmental disorders of prefrontal cortex function can be assessed.

Evidence for a pure time perception deficit in children with ADHD

BACKGROUND

Deficits have been found previously in children with ADHD on tasks of time reproduction, time production and motor timing, implicating a deficit in temporal processing abilities, which has been interpreted as either secondary or primary to core executive dysfunctions. The aim of this study was to explore further the abilities of hyperactive children in skills of time estimation, using a range of time perception tasks in different temporal domains.

METHOD

Time estimation was tested in a verbal estimation task of 10 seconds. Time reproduction was also acquired for two time intervals of 5 and 12 seconds. A temporal discrimination task aimed to determine the idiosyncratic threshold of minimum time interval (in milliseconds) necessary to distinguish two intervals differing by approximately 300 milliseconds. Twenty-two children diagnosed with ADHD were compared to 22 healthy children, matched for age, handedness and working memory skills.

RESULTS

Children with ADHD were significantly impaired in their time discrimination threshold: on average, time intervals had to be 50 ms longer for the hyperactive children in order to be discriminated when compared with controls. Children with ADHD also responded earlier on a 12-second reproduction task, which however only approached significance after controlling for IQ and short-term memory. No group differences were found for the 5-second time reproduction or verbal time estimation tasks.

CONCLUSIONS

The findings suggest that children with ADHD perform poorly on time reproduction tasks which load heavily on impulsiveness and attentional processes and they also suggest that these children may have a perceptual deficit of time discrimination, which may only be detectable in brief durations which differ by several hundred milliseconds. A temporal perception deficit in the range of milliseconds in ADHD may impact upon other functions such as perceptual language skills and motor timing.

The dynamic approach to neurodevelopmental psychiatric disorders: use of fMRI combined with neuropsychology to elucidate the dynamics of psychiatric disorders, exemplified in ADHD and schizophrenia

The paper discusses the application of fMRI in combination with neuropsychology to neurodevelopmental psychiatric disorders, exemplified on the case of attention deficit hyperactivity disorder (ADHD) in comparison with schizophrenia. The view is presented that ADHD, rather than being a compound of unrelated co-existing deficits, is a pervasive disorder of impulsiveness, which manifests at the motor, emotional, social and cognitive domain. Neuropsychology needs to refine the psychological measurements of these impulsivity symptoms and, in combination with fMRI, provide new insights into the interrelationship between brain and dysfunction and its bi-directional causalities. The suitability of the dynamic technique of functional MRI to assess the dynamic nature of developmental neuropsychiatric disorders is discussed. Brain activation can inform about strategy and compensatory mechanisms at a neuroanatomical level, which are not observable at a psychological level, providing insight into the underlying neurocognitive mechanisms of psychiatric disorders. Data are presented and discussed on opposing neurocognitive activation patterns for patients with ADHD and those with schizophrenia while performing a stop task. Comparisons between patient groups will be essential to address the specificity of neurocognitive mechanisms corresponding to specific neurodevelopmental psychiatric disorders.

Pubertal neurodevelopment and the emergence of psychotic symptoms

One of the chief epidemiological hallmarks of schizophrenia is its modal age at onset in early adulthood. Clinical onset is preceded by an adolescent period that is usually characterized by increasing adjustment problems. Recent theorizing about the etiology of schizophrenia has focused on postpubertal brain changes that may be involved in triggering the expression of vulnerability for schizophrenia. In this paper, we further examine the normal neurodevelopmental processes that occur in adolescence and the underlying role of hormonal factors in controlling the expression of genes that govern brain maturation. We then consider how postpubertal hormone changes might serve to trigger the expression of vulnerability genes that code for abnormal brain development.

Immature frontal lobe contributions to cognitive control in children: evidence from fMRI

Event-related fMRI was employed to characterize differences in brain activation between children ages 8-12 and adults related to two forms of cognitive control: interference suppression and response inhibition. Children were more susceptible to interference and less able to inhibit inappropriate responses than were adults. Effective interference suppression in children was associated with prefrontal activation in the opposite hemisphere relative to adults. In contrast, effective response inhibition in children was associated with activation of posterior, but not prefrontal, regions activated by adults. Children failed to activate a region in right ventrolateral prefrontal cortex that was recruited for both types of cognitive control by adults. Thus, children exhibited immature prefrontal activation that varied according to the type of cognitive control required.

Interhemispheric asymmetry of regional cerebral blood flow in prepubescent boys with attention deficit hyperactivity disorder

The prefrontal cortex is asymmetric in both structure and function. In normal subjects, the right prefrontal cortex is activated more than the left during response inhibition. Patients with attention deficit hyperactivity disorder (ADHD) have impaired response inhibition and altered structural interhemispheric asymmetry. This study was conducted to examine the functional interhemispheric asymmetry during response inhibition in children with ADHD. Subjects were divided into three groups according to the level of motor hyperactivity. Blood flow tracer (99m)Tc-ethyl cysteinate dimer was injected while subjects were performing a response inhibition task (RIT), followed by single photon emission computerized tomography (SPECT). After three-dimensional reconstruction, filtering and smoothing, individual scans were morphed to a template. Three average group images were created from individual scans. Each average group image was subtracted voxel-by-voxel from its mirror image to compare the regional cerebral blood flow (rCBF) in the right and left cerebral hemispheres, yielding images of significant interhemispheric rCBF asymmetry. The severe hyperactivity group exhibited most prefrontal left>right rCBF asymmetry and left>right occipitoparietal asymmetry. Reversal of functional prefrontal asymmetry in boys with severe motor hyperactivity supports the hypothesis of right prefrontal cortex dysfunction in ADHD.

Behavioral Aspects of Frontal Lobe Epilepsy

There is growing interest in disorders of behavior, personality, and mood associated with focal epilepsies, though the neuropsychological and behavioral or psychiatric aspects of epilepsy have usually been treated separately. The causes of behavioral disorders in patients with focal epilepsies are multifactorial, though the positive effects of seizure control on behavior suggest that state dependency is a major contributing factor. Patients with temporal lobe epilepsy manifest depression, anxiety, neuroticism, and social limitations, as well as impaired memory. By contrast, studies of cognitive function in patients with frontal lobe epilepsy show executive dysfunctions in response selection/initiation and inhibition, as well as cognitive impairment, hyperactivity, conscientiousness, obsession, and addictive behaviors.

Brain imaging in neurobehavioral disorders

Neuroimaging studies of neurobehavioral disorders are using new imaging modalities. In dyslexia, anatomic imaging studies demonstrate an abnormal symmetry of the planum temporale. Functional imaging supports the hypothesis that developmental dyslexia is frequently the result of deficits in phonologic processing and that normal reading requires a patent network organization of a number of anterior and posterior brain areas. In autism, anatomic imaging studies are conflicting. Functional imaging demonstrates temporal lobe abnormalities and abnormal interaction between frontal and parietal brain areas. In attention-deficit-hyperactivity disorder, imaging studies suggest an abnormality in the prefrontal and striatal regions. Neuroimaging studies are often contradictory, but trends, especially with functional imaging analysis, are evolving. Because neurobehavioral disorders seem to be a result of a dysfunction in brain circuits, no one region will be abnormal in all patients studied. Further studies with well-defined patient populations and appropriate activation paradigms will better elucidate the pathophysiology of these conditions.

An fMRI study of reduced left prefrontal activation in schizophrenia during normal inhibitory function

Functional magnetic resonance imaging (fMRI) was used to investigate the hypothesis that schizophrenia is associated with a dysfunction of prefrontal brain regions during motor response inhibition. Generic brain activation of six male medicated patients with schizophrenia was compared to that of seven healthy comparison subjects matched for sex, age, and education level while performing 'stop' and 'go-no-go' tasks. No group differences were observed in task performance. Patients, however, showed reduced BOLD signal response in left anterior cingulate during both inhibition tasks and reduced left rostral dorsolateral prefrontal and increased thalamus and putamen BOLD signal response during stop task performance. Despite good task performance, patients with schizophrenia thus showed abnormal neural network patterns of reduced left prefrontal activation and increased subcortical activation when challenged with motor response inhibition.

Neuropsychological analyses of impulsiveness in childhood hyperactivity

BACKGROUND

Neuropsychological analyses of impulsiveness are needed to refine assessment of attention-deficit hyperactivity disorder (ADHD).

AIMS

To investigate specific impairments in hyperactive children in a neuropsychological task battery of impulsiveness, the Maudsley Attention and Response Suppression (MARS) task battery, and to identify the neural substrates.

METHOD

Impulsiveness was assessed using different tasks of inhibitory control and time management (MARS) in 55 children with ADHD, other diagnoses and controls. Functional magnetic resonance images were obtained from adolescents with and without ADHD during three of the tasks.

RESULTS

Children with ADHD, but not psychiatric controls, were impaired on tests of response inhibition, but not of motor timing. Reduced right prefrontal activation was observed in hyperactive adolescents during higher level inhibition and delay management, but not during simple sensorimotor coordination.

CONCLUSIONS

Attention-deficit hyperactivity disorder is characterised by specific deficits in tasks of motor response inhibition, but not motor timing, and by dysfunction of frontostriatal brain regions.

Sex-specific developmental changes in amygdala responses to affective faces

It is hypothesized that adolescent development involves a redistribution of cerebral functions from lower subcortical structures to higher regions of the prefrontal cortex to provide greater self-control over emotional behavior. We further hypothesized that this redistribution is likely to be moderated by sex-specific hormonal changes. To examine developmental sex differences in affective processing, 19 children and adolescents underwent fMRI while viewing photographs of faces expressing fear. Males and females differed in the pattern of their amygdala vs prefrontal activation during adolescent maturation. With age, females showed a progressive increase in prefrontal relative to amygdala activation in the left hemisphere, whereas males failed to show a significant age related difference. There appear to be sex differences in the functional maturation of affect-related prefrontal-amygdala circuits during adolescence.

Mapping motor inhibition: conjunctive brain activations across different versions of go/no-go and stop tasks

Conjunction analysis methods were used in functional magnetic resonance imaging to investigate brain regions commonly activated in subjects performing different versions of go/no-go and stop tasks, differing in probability of inhibitory signals and/or contrast conditions. Generic brain activation maps highlighted brain regions commonly activated in (a) two different go/no-go task versions, (b) three different stop task versions, and (c) all 5 inhibition task versions. Comparison between the generic activation maps of stop and go/no-go task versions revealed inhibitory mechanisms specific to go/no-go or stop task performance in 15 healthy, right-handed, male adults. In the go/no-go task a motor response had to be selectively executed or inhibited in either 50% or 30% of trials. In the stop task, the motor response to a go-stimulus had to be retracted on either 50 or 30% of trials, indicated by a stop signal, shortly (250 ms) following the go-stimulus. The shared "inhibitory" neurocognitive network by all inhibition tasks comprised mesial, medial, and inferior frontal and parietal cortices. Generic activation of the go/no-go task versions identified bilateral, but more predominantly left hemispheric mesial, medial, and inferior frontal and parietal cortices. Common activation to all stop task versions was in predominantly right hemispheric anterior cingulate, supplementary motor area, inferior prefrontal, and parietal cortices. On direct comparison between generic stop and go/no-go activation maps increased BOLD signal was observed in left hemispheric dorsolateral prefrontal, medial, and parietal cortices during the go/no-go task, presumably reflecting a left frontoparietal specialization for response selection.

Probing emotion in the developing brain: functional neuroimaging in the assessment of the neural substrates of emotion in normal and disordered children and adolescents

Virtually all developmental neuropsychiatric disorders involve some dysfunction or dysregulation of emotion. Moreover, many psychiatric disorders with adult onset have early subclinical manifestations in children. This essay selectively reviews the literature on the neuroimaging of affect and disorders of affect in children. Some critical definitional and conceptual issues are first addressed, including the distinctions between the perception and production of emotion and between emotional states and traits. Developmental changes in morphometric measures of brain structure are then discussed and the implications of such findings for studies of functional brain activity are considered. Data on functional neuroimaging and childhood depression are then reviewed. While the extant data in this area are meager, they are consistent with studies in adults that have observed decreased left-sided anterolateral prefrontal cortex activation in depression. Studies in children on the recognition of emotion and affective intent in faces using functional magnetic resonance imaging are then reviewed. These findings indicate that the amygdala plays an important role in such affective face processing in children, similar to the patterns of activation observed in adults. Moreover, one study has reported abnormalities in amygdala activation during a task requiring the judgment of affective intent from the eye region of the face in subjects with autism. Some of the methodological complexities of developmental research in this area are discussed, and directions for future research are suggested.

The acallosal mouse strain I/LnJ: a putative model of ADHD?

ADHD has been sometimes associated to a defective interhemispheric cross-talk caused by hypoplasia of the corpus callosum. The inbred mouse strain I/LnJ shows total callosal agenesis with complete penetrance, and behavioral features which resemble ADHD. In conditioned learning tasks, as well as in paradigms of spontaneous behavior. I/LnJ mice, as compared to other inbred strains, show lower learning scores, impulsiveness, and significantly higher locomotor activity, albeit with considerable individual variations. In order to disentangle the influences of the genetic background from the effects of the callosal agenesis, we undertook crossing studies between I/LnJ and C57BL/6 mice, obtaining hybrids with missing corpus callosum. In comparison to normal C57BL/6 mice, acallosal hybrids exposed to a novel open-field showed a different locomotor pattern, with less short stops and more center crossing during the beginning of the session. In a metabolic mapping study, the tendency of acallosals to stay off the walls was found to be associated to lower 2-deoxyglucose uptake in the left striatum and cerebral cortex, while the number of short stops was correlated to the bilateral levels of 2-deoxyglucose uptake in the frontal and parietal cortex. The results hint at a right hemisphere dominance in impulsiveness and hyperactivity, boosted by the lack of callosal connections.

Synchronization, anticipation, and consistency in motor timing of children with dimensionally defined attention deficit hyperactivity behaviour

We tested the hypothesis that children with hyperactive behaviour are impaired in the temporal organization of their motor output. The performance of 11 boys, scoring above a cut-off on standard scales of overactivity and inattention, was compared to that of controls in progressively more complex Motor-timing tasks. The tasks administered required self-paced and externally paced Sensorimotor Synchronization and Sensorimotor Anticipation. Deficits at a perceptual level were investigated with a Time-discrimination task. As hypothesized, we found that hyperactive children had no deficits in their perception of time but were impaired in timing their motor output. Hyperactive children were more inconsistent than controls in maintaining a freely chosen tapping rhythm, in synchronizing and in anticipating their motor response to external visual stimulation.