Impairment in shifting attention in autistic and cerebellar patients

MRI and autopsy evidence of early maldevelopment of cerebellar vermis and hemispheres in autism raise the question of how cerebellar maldevelopment contributes to the cognitive and social deficits characteristic of autism. Compared with normal controls, autistic patients and patients with acquired cerebellar lesions were similarly impaired in a task requiring rapid and accurate shifts of attention between auditory and visual stimuli. Neurophysiologic and behavioral evidence rules out motor dysfunction as the cause of this deficit. These findings are consistent with the proposal that in autism cerebellar maldevelopment may contribute to an inability to execute rapid attention shifts, which in turn undermines social and cognitive development, and also with the proposal that the human cerebellum is involved in the coordination of rapid attention shifts in a fashion analogous to its role in the coordination of movement.

Cross-sectional area of the posterior hippocampus in autistic patients with cerebellar and corpus callosum abnormalities

Using MRI methods previously shown to optimize visualization of cytoarchitectonic details in the body of the hippocampal formation caudal to the pes hippocampi, we imaged and quantified the hippocampus proper including the subiculum and the dentate gyrus in 33 autistic patients between the ages of 6 and 42 years and in 23 age-matched normal healthy volunteers. Measures of these structures in autistic patients and normal healthy volunteers differed nonsignificantly, by less than 1.4%, regardless of whether or not the autistic patients were retarded or had a history of seizure episodes. By contrast, measures of vermian lobules VI and VII and the posterior portion of the corpus callosum in these same autistic and normal volunteers differed significantly, by more than 9.9%. The lack of a significant difference in the cross-sectional size of the posterior hippocampal formation between autistic and normal 6- to 42-year-olds is discrepant with predictions based on some, but not all, autopsy studies. This suggests that there is a need for additional quantitative autopsy study of the hippocampal formation and quantitative MRI study of rostral hippocampal regions that we did not explore in the present report. Also, quantitative autopsy and MRI studies have yet to examine hippocampal development in autistic patients younger than 6 years of age; whether early stages of growth are normal or not is unknown.

Impairment in shifting attention in autistic and cerebellar patients

MRI and autopsy evidence of early maldevelopment of cerebellar vermis and hemispheres in autism raise the question of how cerebellar maldevelopment contributes to the cognitive and social deficits characteristic of autism. Compared with normal controls, autistic patients and patients with acquired cerebellar lesions were similarly impaired in a task requiring rapid and accurate shifts of attention between auditory and visual stimuli. Neurophysiologic and behavioral evidence rules out motor dysfunction as the cause of this deficit. These findings are consistent with the proposal that in autism cerebellar maldevelopment may contribute to an inability to execute rapid attention shifts, which in turn undermines social and cognitive development, and also with the proposal that the human cerebellum is involved in the coordination of rapid attention shifts in a fashion analogous to its role in the coordination of movement.

ERP Evidence for a Shifting Attention Deficit in Patients with Damage to the Cerebellum

In a previous study, we found that patients with damage to the neocerebellum were significantly impaired in the ability to rapidly shift their attention between ongoing sequences of auditory and visual stimuli (Akshoomoff & Courchesne, 1992). In the present study, young patients with damage to the neoccrebelluni were found to be impaired in rapidly shifting their mention between visual stimuli that occurred within a single location. Event-related potentials recorded during the shifting attention experiment suggested that this reflects a deficit in the. covert ability to selectively activate and deactivate attention. These results lend Further support to the hypothesis that the neocerebellum plays a role in the ability to rapidly shift attention.

The brain in infantile autism: posterior fossa structures are abnormal

Infantile autism is a neurologic disorder of social, cognitive, and language development. Earlier MRI studies found hypoplasia of posterior vermal lobules VI and VII and cerebellar hemispheres in the majority of autistic patients, and recent autopsy analyses find severe Purkinje neuron loss in the posterior vermis (lobules VI and VII and VIII to X) and hemispheres. A second type of cerebellar pathology in infantile autism was recently found: hyperplasia (excessive enlargement) of posterior vermal lobules VI and VII. If the autistic samples in some MRI studies that did not detect cerebellar hypoplasia were actually composed of both the hypoplasia and hyperplasia subtypes, then the autistic mean size reported in such studies would have appeared to be near the normal mean size only because it would be the sum of the two opposite subtypes. To test this possibility, we statistically reanalyzed previously published vermal area measures of 78 autistic patients from four separate studies. The results revealed that the autistic patient samples from these four studies were indeed composed of both the hypoplasia subtype (87%, 92%, 89%, and 84% of patients) and the hyperplasia subtype (13%, 8%, 11%, and 16% of patients). Cerebellar abnormalities have been found in 15 autopsy and quantitative MRI reports from nine laboratories involving a total of 226 autistic cases. Autism may be one of the first developmental neuropsychiatric disorders for which substantial concordance exists among several independent microscopic and macroscopic studies as to the location and type of neuroanatomic maldevelopment. Onset might be as early as the second trimester. Discovery of the etiologies underlying cerebellar maldevelopment may be the key to uncovering some of the causes of infantile autism.

Abnormality of cerebellar vermian lobules VI and VII in patients with infantile autism: identification of hypoplastic and hyperplastic subgroups with MR imaging

OBJECTIVE

Infantile autism is a neurobehavioral disorder that is widely believed to have etiologically distinct subtypes, including subtypes with a genetic basis, but no neuroanatomic evidence firmly supports this belief. To date, only one type of cerebellar abnormality has been identified in patients with autism: hypoplasia of the vermis and hemispheres. By using a large sample of autistic patients and healthy volunteers along with precise MR imaging and quantitative procedures, we sought to replicate previous reports of cerebellar vermian hypoplasia in autism and to identify additional subtypes of cerebellar abnormality.

MATERIALS AND METHODS

Using MR technology, we imaged and measured posterior and anterior vermian regions in 50 autistic patients (2-40 years old) and 53 healthy control subjects (3-37 years old). The autistic patients had social, language, cognitive, behavioral, and medical history characteristics that were typical of the general autistic population. By using precise procedures for positioning and aligning MR slices, we obtained comparable MR images within and across subject groups.

RESULTS

Statistical analyses showed two subgroups of autistic patients, one (86% of the patients) with findings consistent with vermian hypoplasia and another (12% of the patients) with evidence of vermian hyperplasia. The hypoplasia subgroup included 43 patients whose mean midsagittal area for vermian lobules VI and VII was 237 +/- 38 mm2, and the hyperplasia subgroup included six patients whose mean area was 377 +/- 12 mm2. Thus, the area of lobules VI and VII in the hypoplasia subgroup was 16% smaller than the mean area in the control subjects (282 +/- 42 mm2) (p < .0001), whereas that in the hyperplasia subgroup was 34% larger (p < .0001). Analyses showed that these two subtypes of vermian abnormalities were present across all ages of autistic patients studied.

CONCLUSION

Two different subtypes of autistic patients can be identified on the basis of the presence of vermian hypoplasia or hyperplasia as seen on MR images. Possible origins for vermian hypoplasia include environmental trauma and genetic factors.

Contextual probability evaluation in autistic, receptive developmental language disorder, and control children: event-related brain potential evidence

Compared 8- to 14-year-old children with either autism or receptive developmental language disorder (RDLD) to age- and IQ-matched normal controls in their ability to detect both frequent (p = .70) and infrequent (p = .30) randomly presented auditory stimuli under task and no-task conditions. Event-related brain potentials (ERPs), behavioral reaction times, and target detection accuracy rates were measured. Although the three groups of children performed in a similar manner on behavioral measures, only the children with autism demonstrated an abnormally small amplitude of the P3b, a component of the ERP. This result is interpreted in terms of (a) the consistency of this finding with other ERP studies involving older individuals with autism; and (b) its significance with respect to the difficulty children with autism have in modifying their expectancies to contextually relevant sequences of auditory information.

Parietal lobe abnormalities detected with MR in patients with infantile autism

OBJECTIVE

Infantile autism is a neurologic disorder that severely disrupts the development of many higher cognitive functions. The most consistent abnormal neuroanatomic findings in autism are loss of Purkinje neurons in the posterior cerebellum as detected by autopsy studies and hypoplasia of the posterior cerebellar vermis and hemispheres as detected by in vivo neuroimaging. Evidence of developmental arrest has also been detected in limbic structures in autopsy studies of autistic patients with mental retardation. Neither in vivo neuroimaging nor autopsy studies of autistic persons have reported abnormalities in the cerebrum. Because the cerebrum mediates many higher cognitive functions, such as social communication, language, abstract reasoning, planning, and organization, that are known to be deficient in patients with autism, a closer examination of the neuroanatomy of the cerebrum in infantile autism is warranted.

MATERIALS AND METHODS

MR images of 21 healthy autistic patients (6-32 years old) were mixed with MR images of control subjects and reviewed on four separate occasions by a neuroradiologist for any neuroanatomic abnormalities. Autism was diagnosed on the basis of criteria for autism as defined by the Diagnostic and Statistical Manual of Mental Disorders, and the autistic patients did not have any other concurrent neurologic disorders. To control for systematic bias in judging the type and location of abnormalities in the autistic population, three control groups were used: a normal control group of 12 subjects, a control group of 23 nonautistic patients with a variety of brain abnormalities for the first review, and another control group of 17 nonautistic patients for the second review. Control patients with brain abnormalities were selected from patients' files on the basis of MR findings of a variety of brain abnormalities. All MR images were coded for anonymity, randomly mixed, and examined by a neuroradiologist blinded to the purpose of the study and to the group membership of each subject. All normal and abnormal findings seen on the MR images of each subject were described on a standard form listing all major brain structures to ensure an examination of each structure in turn. To test for reliability, three subsequent reviews were performed by the same neuroradiologist.

RESULTS

Parietal lobes were abnormal in appearance in 43% (9/21) of autistic patients. Cortical volume loss in the parietal lobes was seen in seven autistic patients; in four of these cases, cortical volume loss extended either into the adjacent superior frontal or occipital lobe. Additional abnormalities detected with MR in these nine patients included white matter volume loss in the parietal lobes (three patients) and thinning of the corpus callosum, especially along the posterior body (two patients). Abnormalities were bilateral. The mesial, lateral, and orbital regions of the frontal lobes; temporal lobes; limbic structures; basal ganglia; diencephalon; and brainstem were normal in all autistic patients. No abnormalities were found in the 12 normal control subjects. The control subjects with neurologic abnormalities had various abnormal findings consistent with their medical conditions.

CONCLUSION

Our results indicate that the parietal lobes are reduced in volume in a portion of the autistic population. Possible origins for this localized cerebral abnormality include early-onset altered development and late-onset progressive atrophy.

A new role for the cerebellum in cognitive operations

Over the last 2 centuries, the predominant view of the cerebellum has been that it is part of a motor control system. Evidence is now presented that the neocerebellum, the evolutionarily newest region of the cerebellum, may also be involved in a key mental operation: the voluntary shift of selective attention between sensory modalities. It is theorized that this newly recognized function may operate via previously described sensory modulation properties of the cerebellum and its many connections with areas known to be important for selective attention, such as the pulvinar, the superior colliculus, and the parietal and frontal cortices.

In vivo myeloarchitectonic analysis of human striate and extrastriate cortex using magnetic resonance imaging

A primary goal of investigations into the organization of human cerebral cortex is to determine the functional specificity of architectonic regions. This includes the correlation of neurobehavioral deficits with neuropathological data for clinical diagnosis and treatment, and the identification of active brain regions using functional neural imaging techniques, such as positron emission tomography, electroencephalographic and magnetoencephalographic (EEG and MEG) source localization algorithms, and direct cortical stimulation. Currently, the architectonic classification of a cortical region identified by these methods is inferred from the comparison of its cerebral topographic position to cytoarchitectonic brain atlases. However, substantial intersubject variability in the position of cytoarchitectonic regions with respect to cerebral topographic landmarks may lead to errors in this procedure. An alternative method is presented here, which uses magnetic resonance (MR) imaging to identify myeloarchitectonic regions of isocortex directly by estimating the relative concentration of myelin within cortical laminae. This high-resolution MR protocol is used to identify striate cortex (Brodmann's area 17) and extrastriate cortex in vivo. Correspondence of MR signal intensity with myeloarchitectonic data from a postmortem brain confirms this identification. As MR imaging technology improves, this noninvasive method has the potential to identify and discriminate among at least 50 cortical regions in the living human brain.

Cerebellar and cerebral abnormalities in Rett syndrome: a quantitative MR analysis

Rett syndrome is a neurodegenerative disease of young girls that begins in early childhood with autismlike behavior and loss of language skills, and progresses with marked deterioration of the motor system in the second decade of life. The purpose of this study was to determine if neuroanatomic changes detected with MR imaging could help to explain the clinical presentation and progression of signs and symptoms in these patients. Accordingly, computer-assisted planimetry was used to measure various dimensions of cerebral, cerebellar, and brainstem structures on sagittal and transverse MR images of 13 patients with Rett syndrome and 10 healthy volunteers. Dimensions of the cerebrum, basal ganglia, cerebellum, and brainstem were measured on transverse images. Areas of cerebellar vermian lobules, the fourth ventricle, the pituitary gland, and the corpus callosum were measured on sagittal images. Fourteen dimensions and areas were measured in each patient and each control subject; according to two-tailed Student's t tests, all but two values were significantly smaller in the patients with Rett syndrome than in control subjects. Graphing the measurements against age by using simple linear regression revealed progressive cerebellar atrophy without evidence of atrophy of the brainstem or cerebrum. Our results indicate that patients with Rett syndrome have global hypoplasia of the brain and progressive cerebellar atrophy increasing with age. Cerebellar atrophy with age may contribute to the deterioration of the motor system seen in older patients with Rett syndrome.

Contribution of the cerebellum to neuropsychological functioning: evidence from a case of cerebellar degenerative disorder

A detailed neuropsychological evaluation was performed on a patient with an idiopathic cerebellar degenerative disorder. Significant deficits were found in verbal and nonverbal intelligence, verbal associative learning, and visuospatial skills. These deficits were not readily explained by motor control difficulties. In contrast to the patient's moderately impaired language abilities, he was severely impaired on a test of verbal fluency and demonstrated mild naming deficits. Severe cerebellar parenchymal volume loss was demonstrated by magnetic resonance examination. Supratentorial structures showed only minimal posterior parietal and occipital sulcal prominence. On neurological examination, this patient had signs of severe involvement of the cerebellar systems and mild-to-moderate dysfunction of the corticospinal tract. As is characteristic of patients with cerebellar degeneration, there was neurophysiological evidence of subclinical involvement of auditory and somatosensory pathways at the level of the brain stem. Since relatively little cerebral cortical atrophy was noted in this patient, these findings suggest that an intact cerebellum is important for normal cognitive functions.

A new role for the cerebellum in cognitive operations

Over the last 2 centuries, the predominant view of the cerebellum has been that it is part of a motor control system. Evidence is now presented that the neocerebellum, the evolutionarily newest region of the cerebellum, may also be involved in a key mental operation: the voluntary shift of selective attention between sensory modalities. It is theorized that this newly recognized function may operate via previously described sensory modulation properties of the cerebellum and its many connections with areas known to be important for selective attention, such as the pulvinar, the superior colliculus, and the parietal and frontal cortices.

Absence of magnetic resonance imaging evidence of pontine abnormality in infantile autism

In vivo studies involving magnetic resonance imaging and studies of neuropathologic specimens have shown that autism is most consistently associated with developmental hypoplasia of the neocerebellum. We investigated whether the cerebellar hypoplasia was accompanied by gross structural abnormalities in the major input (cerebrocerebellar) and output (cerebrorubral) pathways to the cerebellum by measuring the area of the ventral pons (including the pontine nuclei and the transverse fibers) and the midbrain on midsagittal magnetic resonance images in 34 autistic and 44 subjects. The area of the entire pons and several regions of interest within the midbrain (including the superior and inferior colliculi) were also determined with midsagittal magnetic resonance images. We found no significant difference between measurements of the pons and midbrain in autistic and control subjects. Our data show no evidence of gross anatomic abnormalities in the input and output pathways to the cerebellum in autism, a finding that is consistent with previous studies of neuropathologic specimens; rather, the reduced size of the neocerebellum in autism appears to be the result of maldevelopment within the cerebellum itself.

Neuroanatomic imaging in autism

Based on neuroimaging and autopsy research, in autism no common site or type of abnormality appears in the cerebral hemispheres, thalamus, lenticular nuclei, and caudate nucleus. Nonetheless, further imaging and autopsy studies on this issue can be anticipated. Limbic system abnormalities have been reported at autopsy by one laboratory but not another, and no abnormality was found by the one quantitative MR study to measure a limbic structure. More autopsy and imaging research on the limbic system is needed. The cerebellum is the only anatomical structure for which there is both imaging and autopsy evidence of abnormality based on data gathered by many laboratories. Also, the only autopsy study to conduct statistical analyses of cerebellar cell loss found statistically significant Purkinje cell loss in both the vermis and hemispheres. Despite this, normal findings on routine radiologic examination are not of diagnostic significance at this time. On the one hand, the autopsy data show that most, if not all, autistic individuals have cerebellar cell loss, but on the other, research shows that MR images of the cerebellum in a substantial proportion of autistic individuals (perhaps 20% to 50%) will be indistinguishable from normal. Thus, it is likely that MR technology is not yet sufficiently sensitive to detect cerebellar abnormalities in all autistic persons who have them. Finally, the cerebellum seems an unlikely site of damage for a developmental disorder of higher cognition such as autism. However, new neurophysiologic and neuropsychologic studies of children with hemicerebellar resections and children with hemicerebellar resections and children with autism present an entirely new picture of the role of the cerebellum in normal human cognition in general and in the development of the social and communication deficits in autism in particular. These studies show that autistic subjects and patients with acquired cerebellar damage are unable to rapidly shift their mental focus of attention.

Effects of task relevance and attention on P3 in schizophrenic patients

The P3 component of the event-related potentials (ERPs) to auditory task-relevant and task-irrelevant stimuli in 'Attend Auditory' (i.e., reaction time task) and 'Attend Visual' (i.e., when the auditory stimuli were being ignored) conditions was investigated in 13 RDC/DSM-III diagnosed schizophrenic patients. ERPs were recorded from Fz, Cz, and Pz. Compared to controls, schizophrenics had a significantly smaller P3 in the Attend Auditory than in the Attend Visual condition and to the task-relevant than to the task-irrelevant stimuli. Furthermore, the patients' P3 response to the most salient task-irrelevant stimuli in the Attend visual condition was normal. The results are discussed as suggesting that schizophrenics either allocate relatively more resources to task-irrelevant than task-relevant stimuli or that they fail to habituate to task-irrelevant stimuli.

Effects of rare non-target stimuli on brain electrophysiological activity and performance

In order to assess the effects of non-target stimuli on task performance and electrophysiological activity, 16 subjects performed reaction time (RT) experiments under 3 conditions. In all conditions, subjects had to press a button upon detection of rare (15%) target stimuli (1600 Hz) presented among frequent (85%) non-target stimuli. The 3 conditions differed based on their non-target stimuli. In one condition, the non-targets consisted of 'standard' stimuli (900 Hz). In the two other conditions, rare and deviant non-target stimuli were randomly added to the standard stimuli. These deviant non-target stimuli consisted of either constant (700-Hz tones) or novel (buzzes, filtered noises and other unusual sounds) stimuli. Both the rare target and non-target stimuli elicited P300, responses. Behavioral (RT) and electrophysiological (event-related potential) data showed that stimuli that followed standard stimuli were processed differently compared to stimuli that followed deviant non-target stimuli. In the conditions containing deviant non-target stimuli, the P3b to the target stimuli was smaller and later, and the mean RT longer than in the condition with no deviant stimuli. These behavioral and electrophysiological changes induced by the deviant non-target stimuli were discussed with reference to two factors, distraction and increased level of task difficulty. It was suggested that each of these factors were differentially sensitive to the novelty of the rare deviant stimuli.

The cerebellum: 3. Anatomic-MR correlation in the coronal plane

Thin (5-mm) coronal high-field (1.5-T) MR images of four human brain specimens and 14 normal volunteers were correlated with myelin-stained microtomic sections of the specimen cerebella. The primary white-matter tracts innervating several hemispheric (posterior quadrangular, superior, and inferior semilunar, gracile, biventer, tonsil) and vermian (declive, folium, tuber) lobules are oriented perpendicularly to the coronal plane of section and are shown well on proton-density-weighted (long TR/short TE) and T2-weighted (long TR/long TE) spin-echo images, which provide excellent contrast between gray and white matter. Several of the surface sulci and fissures of the cerebellar hemispheres (including the superior posterior, horizontal, secondary, and posterolateral fissures) also course perpendicular to the coronal plane and are depicted well on T1-weighted (short TR/short TE) and T2-weighted images, which maximize contrast between CSF and parenchyma. The opportunity for side-to-side comparison of the hemispheres is a distinct advantage of the coronal view. Nevertheless, more obliquely oriented surfaces (preculminate, primary, inferior posterior, inferior anterior, and intrabiventral fissures) and deep hemispheric structures (primary white-matter tracts to central, anterior quadrangular, and floccular lobules) may be obscured by volume-averaging in the coronal plane; moreover, much of the finer anatomy of the vermis is depicted poorly. The constant surface and deep anatomy of the cerebellum revealed on coronal images in normal volunteers encourages detailed mapping. MR imaging in the coronal plane should be especially useful in identifying, localizing, and quantifying normal and abnormal morphologic differences between the cerebellar hemispheres.

Effects of focused selective attention tasks on event-related potentials in autistic and normal individuals

Event-related potentials (ERPs) and behavioral responses were recorded from autistic and normal subjects under two focused selective attention conditions. In each condition, subjects were presented with an identical stimulus paradigm--a random sequence of 50 msec sounds and flashes occurring at interstimulus intervals ranging between 0.5 and 1.5 sec. The sequence consisted of rare auditory (12.5%), rare visual (12.5%), standard auditory (37.5%) and standard visual (37.5%) stimuli. In the focal auditory condition, subjects pressed a button to the rare auditory target, and in a focal visual condition, they pressed a button to the rare visual stimuli. When normal subjects detected target stimuli in a given attended modality, all stimuli in the attended modality produced enhanced negative ERP responses at frontal electrode sites (i.e., auditory Nde, Ndl, and Nc; visual N270 and Nc) and enhanced positive ERP responses at posterior electrode sites (i.e., P3b and visual P400). In the autistic subjects, in contrast, all auditory and visual attention-related negativities recorded in the auditory and visual focused attention tasks were not in evidence. P3b was significantly diminished in size. The results suggest that abnormalities in the neurophysiological mechanisms of selective attention may underlie the cognitive deficits in autism. The present report and its companion papers are the first reports of the neurophysiological correlates of selective attention in autism.