Rare Missense Variants in KCNJ10 Are Associated with Paroxysmal Kinesigenic Dyskinesia.

BACKGROUND: Although the group of paroxysmal kinesigenic dyskinesia (PKD) genes is expanding, the molecular cause remains elusive in more than 50% of cases. OBJECTIVE: The aim is to identify the missing genetic causes of PKD. METHODS: Phenotypic characterization, whole exome sequencing and association test were performed among 53 PKD cases. RESULTS: We identified four causative variants in KCNJ10, already associated with EAST syndrome (epilepsy, cerebellar ataxia, sensorineural hearing impairment and renal tubulopathy). Homozygous p.(Ile209Thr) variant was found in two brothers from a single autosomal recessive PKD family, whereas heterozygous p.(Cys294Tyr) and p.(Thr178Ile) variants were found in six patients from two autosomal dominant PKD families. Heterozygous p.(Arg180His) variant was identified in one additional sporadic PKD case. Compared to the Genome Aggregation Database v2.1.1, our PKD cohort was significantly enriched in both rare heterozygous (odds ratio, 21.6; P = 9.7 × 10-8) and rare homozygous (odds ratio, 2047; P = 1.65 × 10-6) missense variants in KCNJ10. CONCLUSIONS: We demonstrated that both rare monoallelic and biallelic missense variants in KCNJ10 are associated with PKD. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Motor neuron pathology in CANVAS due to RFC1 expansions.

CANVAS caused by RFC1 biallelic expansions is a major cause of inherited sensory neuronopathy. Detection of RFC1 expansion is challenging and CANVAS can be associated with atypical features. We clinically and genetically characterized 50 patients, selected based on the presence of sensory neuronopathy confirmed by EMG. We screened RFC1 expansion by PCR, repeat-primed PCR, and Southern blotting of long-range PCR products, a newly developed method. Neuropathological characterization was performed on the brain and spinal cord of one patient. Most patients (88%) carried a biallelic (AAGGG)n expansion in RFC1. In addition to the core CANVAS phenotype (sensory neuronopathy, cerebellar syndrome and vestibular impairment), we observed chronic cough (97%), oculomotor signs (85%), motor neuron involvement (55%), dysautonomia (50%), and parkinsonism (10%). Motor neuron involvement was found for 24 of 38 patients (63.1%). First motor neuron signs, such as brisk reflexes, extensor plantar responses, and/or spasticity, were present in 29% of patients, second motor neuron signs, such as fasciculations, wasting, weakness, or a neurogenic pattern on EMG in 18%, and both in 16%. Mixed motor and sensory neuronopathy was observed in 19% of patients. Among six non-RFC1 patients, one carried a heterozygous AAGGG expansion and a pathogenic variant in GRM1. Neuropathological examination of one RFC1 patient with an enriched phenotype, including parkinsonism, dysautonomia, and cognitive decline, showed posterior column and lumbar posterior root atrophy. Degeneration of the vestibulospinal and spinocerebellar tracts was mild. We observed marked astrocytic gliosis and axonal swelling of the synapse between first and second motor neurons in the anterior horn at the lumbar level. The cerebellum showed mild depletion of Purkinje cells, with empty baskets, torpedoes, and astrogliosis characterized by a disorganization of the Bergmann's radial glia. We found neuronal loss in the vagal nucleus. The pars compacta of the substantia nigra was depleted, with widespread Lewy bodies in the locus coeruleus, substantia nigra, hippocampus, entorhinal cortex, and amygdala. We propose new guidelines for the screening of RFC1 expansion, considering different expansion motifs. Here, we developed a new method to more easily detect pathogenic RFC1 expansions. We report frequent motor neuron involvement and different neuronopathy subtypes. Parkinsonism was more prevalent in this cohort than in the general population, 10% versus the expected 1% (P < 0.001). We describe, for the first time, the spinal cord pathology in CANVAS, showing the alteration of posterior columns and roots, astrocytic gliosis and axonal swelling, suggesting motor neuron synaptic dysfunction.

Eye movement abnormalities in neurodegenerative langerhans cell histiocytosis.

Langerhans cell histiocytosis (LCH) is a rare inflammatory myeloid neoplasm characterized by proliferation of tumor histiocytes that involves multiple organs including central nervous system. The physiopathologic process underlying degenerative neuro-LCH (i.e., DN-LCH) remains imperfectly settled. Since the main clinical features of DN-LCH are cerebellar ataxia and dysexecutive syndrome, eye movements might be disrupted and may help in disease diagnosis and monitoring. We retrospectively analyzed the medical records of twenty DN-LCH patients investigated using eye movement recording (EMR) in our hospital between 2015 and 2018. DN-LCH patients exhibited (i) abnormal gain in visually guided saccades including hypermetric saccades and excessive gain variability -45.0%-, (ii) increased mean antisaccade error rates -66.7%-, (iii) altered smooth pursuit -50.0%-, and (iv) excessive number of square wave jerks-25%- and gaze-evoked nystagmus. Our study suggests that DN-LCH patients present a peculiar pattern of eye movement impairments supporting cerebellar and prefrontal dysfunctions. As a non-invasive method, EMR could therefore be a useful tool for quantitative monitoring of DN-LCH patients. Further studies are warranted to support our findings.

Ipsilateral Saccade Hypometria and Contralateral Saccadic Pursuit in a Focal Brainstem Lesion: a Rare Oculomotor Pattern.

Eye movement examination may be used to rapidly differentiate peripheral and central vestibular syndromes in patients with acute unsteadiness. The analysis of oculomotor impairments may also support the accurate localization of cerebral lesions, particularly those in the brainstem, that are often loosely defined by cerebral MRIs. Saccades, smooth pursuit, and nystagmus were recorded with video-oculography in a patient who had developed sudden vertigo as a consequence of a focal lesion in the depth of the brachium pontis. The patient had shown a previously unreported pattern of eye movement impairments consisting of (i) ipsilesional hypometric saccades, (ii) contralesional saccadic smooth pursuit, and (iii) unilateral gaze-evoked nystagmus. These symptoms enabled the precise localization of the trajectory of pontocerebellar saccadic tracts in the depth of the brachium pontis. We propose that this rare association resulted from a disruption of cerebellar afferents of saccadic pathways and of cerebellar efferents of horizontal smooth pursuit pathways. This reported case emphasizes the crucial role of careful bedside oculomotor examination in order to discriminate between peripheral and central vestibular syndromes in the diagnosis of sudden vertigo. Moreover, it reveals an exceptional pattern of oculomotor impairments that may allow for the precise localization of the trajectory of cerebellar saccadic afferent pathways in the depth of the brachium pontis.

Longitudinal Changes of Clinical, Imaging, and Fluid Biomarkers in Preataxic and Early Ataxic Spinocerebellar Ataxia Type 2 and 7 Carriers.

BACKGROUND AND OBJECTIVES: Brain MRI abnormalities and increases in neurofilament light chain (NfL) have mostly been observed in cross-sectional studies before ataxia onset in polyglutamine spinocerebellar ataxias. Our study aimed to identify longitudinal changes in biological, clinical, and/or imaging biomarkers in spinocerebellar ataxia (SCA) 2 and SCA7 carriers over 1 year. METHODS: We studied SCA2 and SCA7 carriers and controls (expansion-negative relatives) at the Paris Brain Institute. Inclusion criteria included Scale for the Assessment and Rating of Ataxia (SARA) scores between 0 and 15. Assessments at baseline, 6 months, and 12 months comprised neurologic, quality of life, orofacial motor, neuropsychological, and ophthalmologic examinations, along with gait and oculomotor recordings, brain MRI, CSF, and blood sampling. The primary outcome was the longitudinal change in these assessments over 1 year. RESULTS: We included 15 SCA2 carriers, 15 SCA7 carriers, and 10 controls between May 2020 and April 2021. At baseline, the ages were similar (41 [37, 46] for SCA2, 38 [28.5, 39.8] for SCA7, and 39.5 [31, 54.5] for controls, p = 0.78), as well the sex (p = 0.61); SARA scores were low but different (4 [1.25, 6.5] in SCA2, 2 [0, 11.5] in SCA7, and 0 in controls, p < 0.01). Pons and medulla volumes were smaller in SCAs (p < 0.05) and cerebellum volume only in SCA2 (p = 0.01). Plasma NfL levels were higher in SCA participants (SCA2: 14.2 pg/mL [11.52, 15.89], SCA7: 15.53 [13.27, 23.23]) than in controls (4.88 [3.56, 6.17], p < 0.001). After 1-year follow-up, in SCA2, there was significant pons (-144 ± 60 mm3) and cerebellum (-1,508 ± 580 mm3) volume loss and a worsening of gait assessment; in SCA7, SARA score significantly increased (+1.3 ± 0.4) and outer retinal nuclear layer thickness decreased (-15.4 ± 1.6 µm); for both SCA groups, the orofacial motor assessment significantly worsened. For preataxic and early ataxic carriers, the strongest longitudinal deterioration on outcome measures was orofacial motility in SCA2 and retinal thickness in SCA7. DISCUSSION: Despite the limitation of the small sample size, we detected annual changes in preataxic and early ataxic SCA individuals across brain MRI imaging, clinical scores, gait parameters, and retinal thickness. These parameters could serve as potential end points for future therapeutic trials in the preataxic phase. TRIAL REGISTRATION INFORMATION: ClinicalTrials.gov NCT04288128.

Clinical and genetic keys to cerebellar ataxia due to FGF14 GAA expansions.

BACKGROUND: SCA27B caused by FGF14 intronic heterozygous GAA expansions with at least 250 repeats accounts for 10-60% of cases with unresolved cerebellar ataxia. We aimed to assess the size and frequency of FGF14 expanded alleles in individuals with cerebellar ataxia as compared with controls and to characterize genetic and clinical variability. METHODS: We sized this repeat in 1876 individuals from France sampled for research purposes in this cross-sectional study: 845 index cases with cerebellar ataxia and 324 affected relatives, 475 controls, as well as 119 cases with spastic paraplegia, and 113 with familial essential tremor. FINDINGS: A higher frequency of expanded allele carriers in index cases with ataxia was significant only above 300 GAA repeats (10.1%, n = 85) compared with controls (1.1%, n = 5) (p < 0.0001) whereas GAA250-299 alleles were detected in 1.7% of both groups. Eight of 14 index cases with GAA250-299 repeats had other causal pathogenic variants (4/14) and/or discordance of co-segregation (5/14), arguing against GAA causality. We compared the clinical signs in 127 GAA≥300 carriers to cases with non-expanded GAA ataxia resulting in defining a key phenotype triad: onset after 45 years, downbeat nystagmus, episodic ataxic features including diplopia; and a frequent absence of dysarthria. All maternally transmitted alleles above 100 GAA were unstable with a median expansion of +18 repeats per generation (r2 = 0.44; p < 0.0001). In comparison, paternally transmitted alleles above 100 GAA mostly decreased in size (-15 GAA (r2 = 0.63; p < 0.0001)), resulting in the transmission bias observed in SCA27B pedigrees. INTERPRETATION: SCA27B diagnosis must consider both the phenotype and GAA expansion size. In carriers of GAA250-299 repeats, the absence of documented familial transmission and a presentation deviating from the key SCA27B phenotype, should prompt the search for an alternative cause. Affected fathers have a reduced risk of having affected children, which has potential implications for genetic counseling. FUNDING: This work was supported by the Fondation pour la Recherche Médicale, grant number 13338 to JLM, the Association Connaître les Syndrome Cérébelleux - France (to GS) and by the European Union's Horizon 2020 research and innovation program under grant agreement No 779257 ("SOLVE-RD" to GS). DP holds a Fellowship award from the Canadian Institutes of Health Research (CIHR). SK received a grant (01GM1905C) from the Federal Ministry of Education and Research, Germany, through the TreatHSP network. This work was supported by the Australian Government National Health and Medical Research Council grants (GNT2001513 and MRFF2007677) to MB and PJL.

Neuromimetic model of saccades for localizing deficits in an atypical eye-movement pathology.

BACKGROUND: When patients with ocular motor deficits come to the clinic, in numerous situations it is hard to relate their behavior to one or several deficient neural structures. We sought to demonstrate that neuromimetic models of the ocular motor brainstem could be used to test assumptions of the neural deficits linked to a patient's behavior. METHODS: Eye movements of a patient with unexplained neurological pathology were recorded. We analyzed the patient's behavior in terms of a neuromimetic saccadic model of the ocular motor brainstem to formulate a pathophysiological hypothesis. RESULTS: Our patient exhibited unusual ocular motor disorders including increased saccadic peak velocities (up to ≈1000 deg/s), dynamic saccadic overshoot, left-right asymmetrical post-saccadic drift and saccadic oscillations. We show that our model accurately reproduced the observed disorders allowing us to hypothesize that those disorders originated from a deficit in the cerebellum. CONCLUSION: Our study suggests that neuromimetic models could be a good complement to traditional clinical tools. Our behavioral analyses combined with the model simulations localized four different features of abnormal eye movements to cerebellar dysfunction. Importantly, this assumption is consistent with clinical symptoms.

Impaired saccadic adaptation in DYT11 dystonia.

BACKGROUND: Recent neuroimaging studies point to a possible pathophysiological role of cerebellar dysfunction in dystonia. The authors investigated the association between sensorimotor adaptation, cerebellar dysfunction and the myoclonus-dystonia phenotype. METHODS: The authors prospectively analysed reactive saccade adaptation in a genetically homogeneous group of 14 patients with DYT11 dystonia owing to a mutation of the SGCE gene. The authors used a backward reactive saccade adaptation task, a well-characterised experimental oculomotor paradigm involving the cerebellum. The principle of this paradigm is to simulate a spatial error in saccade generation by systematically shifting a visual target during saccade execution. Repetition of this systematic error induces a gradual decrease in the initial saccade amplitude, reflecting an adaptive phenomenon. RESULTS: Saccade adaptation was significantly lower in the DYT11 patients than in healthy controls (mean value: 8.9%±4.5% vs 21.6%±4.5%; p=8.3×10(-6)). The time course of adaptation also differed between the patients and controls (p=0.002), reflecting the slower saccadic adaptation in the patients. CONCLUSIONS: This study provides the first neurophysiological evidence of cerebellar dysfunction in DYT11 dystonia and supports a role of cerebellar dysfunction in the myoclonus-dystonia phenotype.

Do the eyes scan dream images during rapid eye movement sleep? Evidence from the rapid eye movement sleep behaviour disorder model.

Rapid eye movements and complex visual dreams are salient features of human rapid eye movement sleep. However, it remains to be elucidated whether the eyes scan dream images, despite studies that have retrospectively compared the direction of rapid eye movements to the dream recall recorded after having awakened the sleeper. We used the model of rapid eye movement sleep behaviour disorder (when patients enact their dreams by persistence of muscle tone) to determine directly whether the eyes move in the same directions as the head and limbs. In 56 patients with rapid eye movement sleep behaviour disorder and 17 healthy matched controls, the eye movements were monitored by electrooculography in four (right, left, up and down) directions, calibrated with a target and synchronized with video and sleep monitoring. The rapid eye movement sleep behaviour disorder-associated behaviours occurred 2.1 times more frequently during rapid eye movement sleep with than without rapid eye movements, and more often during or after rapid eye movements than before. Rapid eye movement density, index and complexity were similar in patients with rapid eye movement sleep behaviour disorder and controls. When rapid eye movements accompanied goal-oriented motor behaviour during rapid eye movement sleep behaviour disorder (e.g. grabbing a fictive object, hand greetings, climbing a ladder), which happened in 19 sequences, 82% were directed towards the action of the patient (same plane and direction). When restricted to the determinant rapid eye movements, the concordance increased to 90%. Rapid eye movements were absent in 38-42% of behaviours. This directional coherence between limbs, head and eye movements during rapid eye movement sleep behaviour disorder suggests that, when present, rapid eye movements imitate the scanning of the dream scene. Since the rapid eye movements are similar in subjects with and without rapid eye movement sleep behaviour disorder, this concordance can be extended to normal rapid eye movement sleep.

Magnetic resonance imaging lesion pattern in Guadeloupean parkinsonism is distinct from progressive supranuclear palsy.

In the Caribbean island of Guadeloupe, patients with atypical parkinsonism develop a progressive supranuclear palsy-like syndrome, named Guadeloupean parkinsonism. Unlike the classical forms of progressive supranuclear palsy, they develop hallucinations and myoclonus. As lesions associated with Guadeloupean parkinsonism are poorly characterized, it is not known to what extent they differ from progressive supranuclear palsy. The aim of the present study was to determine the structural and metabolic profiles of Guadeloupean parkinsonism compared with progressive supranuclear palsy and controls using combined structural and diffusion magnetic resonance imaging and magnetic resonance spectroscopy. We included 9 patients with Guadeloupean parkinsonism, 10 with progressive supranuclear palsy and 9 age-matched controls. Magnetic resonance imaging examination was performed at 1.5 T and included 3D T(1)-weighted and fluid-attenuated inversion recovery images, diffusion tensor imaging and single voxel magnetic resonance spectroscopy in the lenticular nucleus. Images were analysed using voxel-based morphometry, voxel-based diffusion tensor imaging and brainstem region of interest measurements. In patients with Guadeloupean parkinsonism, structural and diffusion changes predominated in the temporal and occipital lobes, the limbic areas (medial temporal, orbitofrontal and cingulate cortices) and the cerebellum. In contrast to patients with progressive supranuclear palsy, structural changes predominated in the midbrain and the basal ganglia and diffusion abnormalities predominated in the frontocentral white matter, the basal ganglia and the brainstem. Compared with controls, the N-acetylaspartate to creatinine ratio was decreased in patients with progressive supranuclear palsy and to a lesser extent in patients with Guadeloupean parkinsonism. The pattern of structural and diffusion abnormalities differed between progressive supranuclear palsy and Guadeloupean parkinsonism. Widespread cortical atrophy was observed in patients with Guadeloupean parkinsonism who presented marked cognitive changes and hallucinations, whereas midbrain lesions were less severe than in progressive supranuclear palsy. Midbrain (progressive supranuclear palsy) or cortical (Guadeloupean parkinsonism) atrophy was a distinctive neuroimaging feature for differential diagnosis.

Predominant cortical dysfunction in Guadeloupean parkinsonism.

Atypical parkinsonism is extremely frequent in Guadeloupe and may have an environmental cause. One-half of the patients with this tauopathy have dopa-resistant parkinsonism, tremor, subcortical dementia and abnormal eye movements suggestive of progressive supranuclear palsy (PSP). They also have hallucinations, dysautonomia, which are not characteristic of PSP. Furthermore, the oculomotor abnormalities and the tremor, which is jerky, differ from what is observed in classical PSP patients. We therefore undertook an electrophysiological study to characterize these features in greater detail. Nine representative Guadeloupean PSP-like (Gd-PSP) patients were selected for electro-oculographic recordings of horizontal eye movements [visually guided saccades (VGS), antisaccades (AS) and smooth pursuit], clinical evaluation of vertical saccade velocity and electrophysiological analysis of abnormal limb movements [electromyographic polygraphy, EEG jerk-locked-back-averaging (JLBA) and long-loop C-reflex]. Vertical saccade velocity was reduced in five patients. The velocity of horizontal VGS was normal, although the latencies were increased and horizontal smooth pursuit (HSP) was mostly saccadic. The AS error rate was above 70% in most patients. Myoclonus was detected in 89% of the Gd-PSP patients. It was mainly small amplitude rest and action myoclonus in the upper limbs, characterized by short arrhythmic 24-76 ms bursts and was of cortical origin, as confirmed by JLBA in five patients. In conclusion, Gd-PSP patients have cortical myoclonus and cortical oculomotor impairments, but only minor signs of brainstem oculomotor dysfunction, suggesting that cortical dysfunction predominates over brainstem impairments. This electrophysiological study, added to previous clinical, neuropsychological and neuroradiological studies, has enriched the characterization of Guadeloupean atypical parkinsonism, which thus appears to be a new clinical entity.

Further evidence for a distinctive atypical degenerative parkinsonism in the Caribbean: A new cluster in the French West Indian Island of Martinique.

BACKGROUND: A high prevalence of an atypical levodopa-resistant parkinsonism has been reported in the Caribbean island of Guadeloupe. These seminal observations have not been replicated or extended to neighbouring populations who share genetic and environmental characteristics. METHODS: To further characterise this atypical parkinsonism we prospectively investigated 305 consecutive patients with neurodegenerative parkinsonism in a community-based population from Guadeloupe and Martinique, a neighbouring French Caribbean island where the population has similar environmental and genetic backgrounds. The aims of this study were to confirm the frequency of atypical parkinsonism within this cohort and to precisely define its clinical phenotype. RESULTS: A high frequency (66%) of atypical parkinsonism was identified in both Guadeloupe and Martinique. The clinical phenotype consisted of a levodopa-resistant parkinsonism with postural instability (72%), early dementia (58%), dysautonomia (58%), rapid-eye-movement sleep behavioural disorder (53%), hallucinations (43%), and supranuclear gaze palsy (29%). A low educational level was identified as a major risk factor for developing atypical parkinsonism (p < .001). CONCLUSION: Our findings support the existence of a distinctive atypical parkinsonism - Caribbean Parkinsonism - within the French Caribbean Islands. This could either correspond to a single entity or reflect a propensity for developing more widespread and rapidly progressive lesions in Caribbean patients with parkinsonism. In both cases, genetic susceptibility and/or environmental exposure may be involved.

Visual vector inversion in the posterior parietal cortex.

In the antisaccade task, a saccade must be triggered towards the mirror location of a visual target. The neural basis required for this visual vector inversion remains unclear, although neuronal activities reflecting this process have been recorded in the monkey lateral intraparietal area. We examined a patient with a small, right-sided, posterior parietal stroke who complained of difficulty in manipulating visual information. Antisaccades were markedly hypometric rightwards but normal leftwards. Largely unaffected performances in other saccade tasks revealed that visual and motor processing were not significantly affected. Antisaccade inaccuracy could therefore be ascribed to the impairment of visual vector inversion, a processing specifically required in this task. These findings provide the first evidence in humans that visual vector inversion could be an intrinsic property of the posterior parietal cortex.

Brain volumetric analysis and cortical thickness in adults with saccadic intrusions (ocular flutter or opsoclonus-myoclonus syndrome).

OBJECTIVES: Ocular flutter (OF) and opsoclonus are considered a continuum with a similar pathogenesis. Due to the rarity of this disease in the adult population, little is known about the brain morphological changes in the chronic phase of the disease. PATIENTS AND METHODS: Six magnetic resonance imaging from adults with previous history of OF/Opsoclonus and 12 healthy patients (paired by age and sex) were analyzed in order to identify the long term cortical thickness pattern in this rare disease by using Freesurfer. RESULTS: Patients with OF/Opsoclonus showed reduced cerebellum cortical volume with a subsequent diminution in total cerebellar volume. White mater cerebellum volume was not modified. In addition, we have also identified a significant supratentorial gray matter volume decrease in OF/Opsoclonus patients, involving both the cortical and the subcortical gray matter. CONCLUSIONS: OF/Opsoclonus in adults may be associated with cortical and subcortical gray matter atrophy, as well as decreased cerebellar cortical volume. Further larger prospective studies are necessary to confirm these results.

Antisaccades in Parkinson disease: A new marker of postural control?

OBJECTIVE: To describe the relation between gaze and posture/gait control in Parkinson disease (PD) and to determine the role of the mesencephalic locomotor region (MLR) and cortex-MLR connection in saccadic behavior because this structure is a major area involved in both gait/postural control and gaze control networks. METHODS: We recruited 30 patients with PD with or without altered postural control and 25 age-matched healthy controls (HCs). We assessed gait, balance, and neuropsychological status and separately recorded gait initiation and eye movements (visually guided saccades and volitional antisaccades). We identified correlations between the clinical and physiologic parameters that best characterized patients with postural instability. We measured resting-state functional connectivity in 2 pathways involving the frontal oculomotor cortices and the MLR and sought correlations with saccadic behavior. RESULTS: Patients with PD with postural instability showed altered antisaccade latencies that correlated with the stand-walk-sit time (r = 0.78, p < 0.001) and the duration of anticipatory postural adjustments before gait initiation (r = 0.61, p = 0.001). Functional connectivity between the pedunculopontine nucleus (PPN) and the frontal eye field correlated with antisaccade latency in the HCs (r = -0.54, p = 0.02) but not in patients with PD. CONCLUSIONS: In PD, impairment of antisaccade latencies, a simple and robust parameter, may be an indirect marker correlated with impaired release of anticipatory postural program. PPN alterations may account for both antisaccade and postural impairments.

The prefrontal substrate of reflexive saccade inhibition in humans.

BACKGROUND: Prefrontal dysfunction in neuropsychiatric disorders such as schizophrenia has been shown to impair inhibition of reflexive saccadic eye movements; however, it is unclear whether reflexive saccade inhibition can be attributed to a distinct subregion of the human prefrontal cortex. METHODS: We tested 15 patients with acute unilateral ischemic lesions of the prefrontal cortex and 20 control subjects with an antisaccade task. Lesions were reconstructed using Talairach coordinates, and possible candidate regions for reflexive saccade inhibition were identified. RESULTS: Significantly increased antisaccade error rates were observed in patients with lesions affecting a region in mid-dorsolateral prefrontal cortex or the white matter between this region and the anterior portions of the internal capsule. Antisaccade error rates of patients with lesions outside this region were normal. These findings were largely independent of lesion volume, postlesion delay, and subject age. CONCLUSIONS: Our findings suggest that inhibition of reflexive saccades depends on a circumscribed subregion of the human dorsolateral prefrontal cortex. This region closely corresponds to Brodmann area 46 as defined by recent cytoarchitectonic studies. Increased antisaccade error rates in patients with prefrontal pathology may be explained by dysfunction of this region.

Ketamine-induced distractibility: An oculomotor study in monkeys.

BACKGROUND: Administration of subanesthetic doses of ketamine, a noncompetitive N-methyl-D-aspartate receptor antagonist, induces a spectrum of behavioral disorders that are commonly observed in patients with schizophrenia. Although it has been demonstrated that poor antisaccade performance is a core dysfunction in schizophrenia, the ability of ketamine to induce an increased distractibility has not been demonstrated. The present study aimed to determine whether ketamine administration would reproduce the same antisaccade deficit as that observed in schizophrenic subjects. METHODS: We studied the effect of acute ketamine or saline administration on the performance of two monkeys trained on a reflexive visually guided saccade task and an antisaccade task. RESULTS: The main result is that ketamine administration induced a markedly increased antisaccade error rate and increased antisaccade latency, similar to that seen in schizophrenic subjects. Other impairments consisted of increased reflexive saccade latency and the presence of a gaze-evoked nystagmus. CONCLUSIONS: This study supports the validity of ketamine as a pharmacological model of schizophrenia. Based on the known pharmacological effects of ketamine, further studies should allow the investigation of the pharmacological basis of distractibility.