Distinct functional properties of the vertical and horizontal saccadic network in Health and Parkinson's disease: An eye-tracking and fMRI study

Performance of antisaccades in patients with cerebral small vessel disease accompanied by white matter hyperintensities

OBJECTIVES

The antisaccades (AS) task is considered a reliable indicator of inhibitory control of eye movements in humans. Achieving good AS performance requires efficient cognitive processes that are sensitive to changes in brain structure. White matter hyperintensities (WMH) can cause subcortical-cortical dysconnectivity, affecting diverse cognitive domains. Thus, the AS task was investigated in patients with WMH in cerebral small vessel disease (CSVD).

METHODS

In this retrospective study, 75 participants with WMH, determined by neuroimaging standards for CSVD research, were admitted to the Department of Neurology of Beijing Luhe Hospital, Capital Medical University from January 2021 to December 2022. All subjects underwent the AS task, Montreal Cognitive Assessment (MoCA), Mini-Mental State Examination (MMSE), and 3.0T brain MRI. Additionally, 61 healthy subjects were recruited to characterize WMH profiles.

RESULTS

Compared to the control group, patients with WMH had a significantly increased AS error rate (49.81%, p = 0.001) and lower gain (76.00%, p = 0.042). The AS error rate was significantly higher in patients with WMH in the frontal lobe than in those without WMH (p = 0.004). After adjusting for confounders (age), a positive correlation was found between the AS error rate and MoCA scores for patients with WMH (coefficient = 0.262, p = 0.024).

CONCLUSIONS

Patients with WMH due to CSVD exhibited abnormal AS performances, particularly in the frontal lobe. The eye movement paradigms, the new diagnostic forms in neurology, can be utilized to investigate the distributed cortical and subcortical systems involved in cognitive control processes, offering simple, well-tolerated and highly sensitive advantages over traditional measures.

Abnormal eye movements in Parkinson's disease: From experimental study to clinical application

Parkinson's disease (PD) is a neurodegenerative disease that integrates a series of motor symptoms and non-motor symptoms, making early recognition challenging. The exploration of biomarkers is urgently required. Abnormal eye movements in PD have been reported to appear in a variety of ways since eye tracking technology was developed, such as decreased saccade amplitude, extended saccade latency, and unique saccade patterns. Non-invasive, objective and simple eye tracking has the potential to provide effective biomarkers for the PD diagnosis, progression and cognitive impairment, as well as ideas for research into the occurrence and treatment strategy of motor symptoms. In this review, we introduced the fundamental eye movement patterns and typical eye movement paradigms (such as fixation, pro-saccade, anti-saccade, smooth tracking, and visual search), summarized the symptoms of various ocular motor abnormalities in PD, and discussed the research implications of oculomotor investigation to the pathogenesis of PD and related motor symptoms, as well as the clinical implications as biomarkers and its inspiration on treatment.

Abnormal saccade profiles in hereditary spinocerebellar degeneration reveal cerebellar contribution to visually guided saccades

OBJECTIVE

To study how the pathophysiology underlying hereditary spinocerebellar degeneration (spinocerebellar ataxia; SCA) with pure cerebellar manifestation evolves with disease progression using saccade recordings.

METHODS

We recorded visually- (VGS) and memory-guided saccade (MGS) task performance in a homogeneous population of 20 genetically proven SCA patients (12 SCA6 and eight SCA31 patients) and 19 normal controls.

RESULTS

For VGS but not MGS, saccade latency and amplitude were increased and more variable than those in normal subjects, which correlated with cerebellar symptom severity assessed using the International Cooperative Ataxia Rating Scale (ICARS). Parameters with significant correlations with cerebellar symptoms showed an aggravation after disease stage progression (ICARS > 50). The saccade velocity profile exhibited shortened acceleration and prolonged deceleration, which also correlated with disease progression. The main sequence relationship between saccade amplitude and peak velocity as well as saccade inhibitory control were preserved.

CONCLUSIONS

The cerebellum may be involved in initiating VGS, which was aggravated acutely during disease stage progression. Dysfunction associated with disease progression occurs mainly in the cerebellum and brainstem interaction but may also eventually involve cortical saccade processing.

SIGNIFICANCE

Saccade recording can reveal cerebellar pathophysiology underlying SCA with disease progression.

The effects of age and sex on the incidence of multiple step saccades and corrective saccades

Objective

Although multiple step saccades (MSS) is occasionally observed in healthy subjects, it is more pronounced in patients with aging-related neurodegenerative diseases, particularly Parkinson’s disease (PD). Thus, MSS has been treated as a complementary biomarker for diagnosing PD. Despite the aforementioned knowledge, several questions remain unexplored: (1) How does aging affect MSS? (2) Is there a sex difference in MSS? (3) Are there differences in MSS between vertical and horizontal saccades? (4) Are MSS and corrective saccade (CS) the same behavior? (5) How do age and sex affect CS? The objectives of the present study are to address these questions.

Method

Four hundred eighty healthy participants were recruited to perform a visually guided reactive saccade task. Participants were divided into six groups according to their ages. Each group consisted of 40 male and 40 female participants. Eye movements were recorded with infrared eye trackers.

Results

The incidence of MSS increased as a function of age, whereas the incidence of CS first increased with age 20–49 and then decreased with age 50–79. The incidences of both MSS and CS did not show sex differences. The incidence of MSS in vertical saccades was significantly higher than that in horizontal saccades, and their difference increased with increasing age, whereas the incidence of CS showed a reversed pattern.

Conclusion

Age and saccadic direction affect the occurrences of MSS and CS differently, indicating that MSS and CS are different saccadic behaviors. In addition, measuring saccades could reliably reflect the function of human’s brain which is affected by aging.

Multiple step saccades in simply reactive saccades could serve as a complementary biomarker for the early diagnosis of Parkinson’s disease

Objective

It has been argued that the incidence of multiple step saccades (MSS) in voluntary saccades could serve as a complementary biomarker for diagnosing Parkinson’s disease (PD). However, voluntary saccadic tasks are usually difficult for elderly subjects to complete. Therefore, task difficulties restrict the application of MSS measurements for the diagnosis of PD. The primary objective of the present study is to assess whether the incidence of MSS in simply reactive saccades could serve as a complementary biomarker for the early diagnosis of PD.

Materials and methods

There were four groups of human subjects: PD patients, mild cognitive impairment (MCI) patients, elderly healthy controls (EHCs), and young healthy controls (YHCs). There were four monkeys with subclinical hemi-PD induced by injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) through the unilateral internal carotid artery and three healthy control monkeys. The behavioral task was a visually guided reactive saccade.

Results

In a human study, the incidence of MSS was significantly higher in PD than in YHC, EHC, and MCI groups. In addition, receiver operating characteristic (ROC) analysis could discriminate PD from the EHC and MCI groups, with areas under the ROC curve (AUCs) of 0.76 and 0.69, respectively. In a monkey study, while typical PD symptoms were absent, subclinical hemi-PD monkeys showed a significantly higher incidence of MSS than control monkeys when the dose of MPTP was greater than 0.4 mg/kg.

Conclusion

The incidence of MSS in simply reactive saccades could be a complementary biomarker for the early diagnosis of PD.

Antisaccades in Parkinson's Disease: A Meta-Analysis

The usefulness of eye-tracking tasks as potential biomarkers for motor or cognitive disease burden in Parkinson's disease (PD) has been subject of debate for many years. Several studies suggest that the performance in the antisaccade task may be altered in patients with PD and associated with motor disease severity or executive dysfunction. In this meta-analysis, random effects models were used to synthesize the existing evidence on antisaccade error rates and latency in PD. Furthermore, meta-regressions were performed to assess the role of motor and cognitive disease severity, dopaminergic medication and methodological factors. Additionally, the impact of acute levodopa administration and activation of deep brain stimulation was evaluated in two separate sub-analyses.This meta-analysis confirms that antisaccade latency and error rate are significantly increased in PD. Disease duration, Unified Parkinson's disease rating scale score and Hoehn and Yahr stage mediate the effect of PD on antisaccade latency with higher motor burden being associated with increased antisaccade latency.Acute administration of levodopa had no significant effects on antisaccade performance in a small number of eligible studies. Deep brain stimulation in the subthalamic nucleus, on the other hand, may alter the speed accuracy trade-off supporting an increase of impulsivity following deep brain stimulation in PD.According to the results of the meta-analysis, antisaccade latency may provide a potential marker for disease severity and progression in PD which needs further confirmation in longitudinal studies.

A link between synaptic plasticity and reorganization of brain activity in Parkinson's disease

The link between synaptic plasticity and reorganization of brain activity in health and disease remains a scientific challenge. We examined this question in Parkinson's disease (PD) where functional up-regulation of postsynaptic D₂ receptors has been documented while its significance at the neural activity level has never been identified. We investigated cortico-subcortical plasticity in PD using the oculomotor system as a model to study reorganization of dopaminergic networks. This model is ideal because this system reorganizes due to frontal-to-parietal shifts in blood oxygen level-dependent (BOLD) activity. We tested the prediction that functional activation plasticity is associated with postsynaptic dopaminergic modifications by combining positron emission tomography/functional magnetic resonance imaging to investigate striatal postsynaptic reorganization of dopamine D₂ receptors (using ¹¹C-raclopride) and neural activation in PD. We used covariance (connectivity) statistics at molecular and functional levels to probe striato-cortical reorganization in PD in on/off medication states to show that functional and molecular forms of reorganization are related. D₂ binding across regions defined by prosaccades showed increased molecular connectivity between both caudate/putamen and hyperactive parietal eye fields in PD in contrast with frontal eye fields in controls, in line with the shift model. Concerning antisaccades, parietal-striatal connectivity dominated in again in PD, unlike frontal regions. Concerning molecular-BOLD covariance, a striking sign reversal was observed: PD patients showed negative frontal-putamen functional-molecular associations, consistent with the reorganization shift, in contrast with the positive correlations observed in controls. Follow-up analysis in off-medication PD patients confirmed the negative BOLD-molecular correlation. These results provide a link among BOLD responses, striato-cortical synaptic reorganization, and neural plasticity in PD.

Visual Search Efficiency in Mild Cognitive Impairment and Alzheimer's Disease: An Eye Movement Study

BACKGROUND

Visual search abilities are essential to everyday life activities and are known to be affected in Alzheimer's disease (AD). However, little is known about visual search efficiency in mild cognitive impairment (MCI), a transitive state between normal aging and dementia. Eye movement studies and machine learning methods have been recently used to detect oculomotor impairments in individuals with dementia.

OBJECTIVE

The aim of the present study is to investigate the association between eye movement metrics and visual search impairment in MCI and AD.

METHODS

127 participants were tested: 43 healthy controls, 51 with MCI, and 33 with AD. They completed an eyetracking visual search task where they had to find a previously seen target stimulus among distractors.

RESULTS

Both patient groups made more fixations on the screen when searching for a target, with longer duration than controls. MCI and AD fixated the distractors more often and for a longer period of time than the target. Healthy controls were quicker and made less fixations when scanning the stimuli for the first time. Machine-learning methods were able to distinguish between controls and AD subjects and to identify MCI subjects with a similar oculomotor profile to AD with a good accuracy.

CONCLUSION

Results showed that eye movement metrics are useful for identifying visual search impairments in MCI and AD, with possible implications in the early identification of individuals with high-risk of developing AD.

Attentional reorientation along the meridians of the visual field: Are there different neural mechanisms at play?

Hemispatial neglect, after unilateral lesions to parietal brain areas, is characterized by an inability to respond to unexpected stimuli in contralesional space. As the visual field's horizontal meridian is most severely affected, the brain networks controlling visuospatial processes might be tuned explicitly to this axis. We investigated such a potential directional tuning in the dorsal and ventral frontoparietal attention networks, with a particular focus on attentional reorientation. We used an orientation‐discrimination task where a spatial precue indicated the target position with 80% validity. Healthy participants (n = 29) performed this task in two runs and were required to (re‐)orient attention either only along the horizontal or the vertical meridian, while fMRI and behavioral measures were recorded. By using a general linear model for behavioral and fMRI data, dynamic causal modeling for effective connectivity, and other predictive approaches, we found strong statistical evidence for a reorientation effect for horizontal and vertical runs. However, neither neural nor behavioral measures differed between vertical and horizontal reorienting. Moreover, models from one run successfully predicted the cueing condition in the respective other run. Our results suggest that activations in the dorsal and ventral attention networks represent higher‐order cognitive processes related to spatial attentional (re‐)orientating that are independent of directional tuning and that unilateral attention deficits after brain damage are based on disrupted interactions between higher‐level attention networks and sensory areas.

Cerebellar Contribution to Motor and Non-motor Functions in Parkinson's Disease: A Meta-Analysis of fMRI Findings

Background: Parkinson's disease (PD) results in both motor and non-motor symptoms. Traditionally, the underlying mechanism of PD has been linked to neurodegeneration of the basal ganglia. Yet it does not adequately account for the non-motor symptoms of the disease, suggesting that other brain regions may be involved. One such region is the cerebellum, which is known to be involved, together with the basal ganglia, in both motor and non-motor functions. Many studies have found the cerebellum to be hyperactive in PD patients, a finding that is seldom discussed in detail, and warrants further examination. The current study thus aims to examine quantitively the current literature on the cerebellar involvement in both motor and non-motor functioning in PD. Methods: A meta-analysis of functional neuroimaging literature was conducted with Seed-based D mapping. Only the studies testing functional activation in response to motor and non-motor paradigms in PD and healthy controls (HC) were included in the meta-analysis. Separate analyses were conducted by including only studies with non-motor paradigms, as well as meta-regressions with UPDRS III scores and disease duration. Results: A total of 57 studies with both motor and non-motor paradigms fulfilled our inclusion criteria and were included in the meta-analysis, which revealed hyperactivity in Crus I-II and vermal III in PD patients compared to HC. An analysis including only studies with cognitive paradigms revealed a cluster of increased activity in PD patients encompassing lobule VIIB and VIII. Another meta-analysis including the only 20 studies that employed motor paradigms did not reveal any significant group differences. However, a descriptive analysis of these studies revealed that 60% of them reported cerebellar hyperactivations in PD and included motor paradigm with significant cognitive task demands, as opposed to 40% presenting the opposite pattern and using mainly force grip tasks. The meta-regression with UPDRS III scores found a negative association between motor scores and activation in lobule VI and vermal VII-VIII. No correlation was found with disease duration. Discussion: The present findings suggest that one of the main cerebellar implications in PD is linked to cognitive functioning. The negative association between UPDRS scores and activation in regions implicated in motor functioning indicate that there is less involvement of these areas as the disease severity increases. In contrast, the lack of correlation with disease duration seems to indicate that the cerebellar activity may be a compensatory mechanism to the dysfunctional basal ganglia, where certain sub-regions of the cerebellum are employed to cope with motor demands. Yet future longitudinal studies are needed to fully address this possibility.

Vertical saccades and antisaccades: complementary markers for motor and cognitive impairment in Parkinson's disease

Previous studies provide partly contradictory results about the characteristics of saccades in PD and the possible effects of levodopa, which may be attributed to different study design regarding disease stages, medication state or cognitive functioning. We studied horizontal and vertical visually guided saccades (VGS) and antisaccades (AS) in 40 patients with PD with and without postural instability in On and Off medication state as well as in 20 healthy controls (HC). Motor and cognitive performance were assessed using UPDRS, Montreal Cognitive Assessment (MoCA) and Frontal Assessment Battery (FAB). The PD group showed decreased VGS amplitudes and increased vertical VGS and AS latencies. Only relatively few studies had assessed vertical saccades in PD so far. However, our results indicate that vertical saccadic amplitude may be a supportive marker in diagnosing PD since upwards gain demonstrated an AUC of 0.85 for the discrimination of PD and HC. Only more advanced patients in Hoehn & Yahr stage 3 executed higher numbers of AS errors than HC. Since the AS error rate correlated with FAB and MoCA scores, AS performance seems to reflect cognitive ability in PD. Furthermore, the correlation of AS latency with the UPDRS axial subscore promotes the recently highlighted connection between postural control and executive function in PD. Levodopa did not alter saccade amplitudes and had opposing effects on the initiation of VGS and AS: Levodopa intake prolonged VGS latency, but decreased AS latency. Possible mechanisms by which levodopa may be capable of partially reversing the impaired balance between voluntary and reflexive cortical saccade initiation of PD are discussed.

Relationships between neuropsychological and antisaccade measures in multiple sclerosis patients

BACKGROUND

The Stroop test is frequently used to assess deficits in inhibitory control in people with multiple sclerosis (MS). This test has limitations and antisaccade eye movements, that also measure inhibitory control, may be an alternative to Stroop.

OBJECTIVES

The aim of this study was twofold: (i) to investigate if the performance in the antisaccade task is altered in patients with MS and (ii) to investigate the correlation between performances in neuropsychological tests, the Stroop test and the antisaccade task.

METHODS

We measured antisaccades (AS) parameters with an infrared eye tracker (SMIRED 250 Hz) using a standard AS paradigm. A total of 38 subjects diagnosed with MS and 38 age and gender matched controls participated in this study. Neuropsychological measures were obtained from the MS group.

RESULTS

Patients with MS have higher error rates and prolonged latency than controls in the antisaccade task. There was a consistent association between the Stroop performance and AS latency. Stroop performance but not AS latency was associated with other neuropsychological measures in which the MS group showed deficits.

CONCLUSIONS

Our findings suggest that AS may be a selective and independent measure to investigate inhibitory control in patients with MS. More studies are necessary to confirm our results and to describe brain correlates associated with impaired performance in the antisaccade task in people diagnosed with MS.

Structural and Functional Brain Mapping Correlates of Impaired Eye Movement Control in Parkinsonian Syndromes: A Systems-Based Concept

The investigation of the human oculomotor system by eye movement recordings provides an approach to behavior and its alterations in disease. The neurodegenerative process underlying parkinsonian syndromes, including Parkinson’s disease (PD), progressive supranuclear palsy (PSP), and multisystem atrophy (MSA) changes structural and functional brain organization, and thus affects eye movement control in a characteristic manner. Video-oculography has been established as a non-invasive recording device for eye movements, and systematic investigations of eye movement control in a clinical framework have emerged as a functional diagnostic tool in neurodegenerative parkinsonism. Disease-specific brain atrophy in parkinsonian syndromes has been reported for decades, these findings were refined by studies utilizing diffusion tensor imaging (DTI) and task-based/task-free functional MRI—both MRI techniques revealed disease-specific patterns of altered structural and functional brain organization. Here, characteristic disturbances of eye movement control in parkinsonian syndromes and their correlations with the structural and functional brain network alterations are reviewed. On this basis, we discuss the growing field of graph-based network analysis of the structural and functional connectome as a promising candidate for explaining abnormal phenotypes of eye movement control at the network level, both in health and in disease.

Slower saccadic reading in Parkinson's disease

Idiopathic Parkinson's Disease (PD) is characterized by degeneration of dopaminergic and other neurons, leading to motor and non-motor deficits. Abnormal eye movements in PD, including fixations, saccades, and convergence, are well described. However, saccadic reading, which requires serial and alternating saccades and fixations, is not well studied, despite its obvious impact on the quality of life. In this study, we assessed saccadic reading using variations of the King-Devick (KD) test, a rapid single digit number naming test, as a way to assess the ability to make serial left-to-right ocular motor movements necessary for reading. We recruited 42 treated PD patients and 80 age-matched controls and compared their reading times with a variety of measures, including age, duration of disease, Unified Parkinson's Disease Rating Scale (UPDRS), the National Eye Institute 25-Item Visual Functioning Questionnaire 25 (VFQ-25), and Montreal Cognitive assessment (MoCA) test. The subjects performed 4 trials of reading 120 single digit numbers aloud as fast as possible without making errors. In each trial, they read 3 pages (KD1, KD2, and KD3), and each page contained 40 numbers per page in 8 lines with 5 numbers/line. We found that PD patients read about 20% slower than controls on all tests (KD1, 2, and 3 tests) (p < 0.02), and both groups read irregularly spaced numbers slower than regularly spaced numbers. Having lines between numbers to guide reading (KD1 tests) did not impact reading time in both PD and controls, but increased visual crowding as a result of decreased spacing between numbers (KD3 tests) was associated with significantly slower reading times in both PD and control groups. Our study revealed that saccadic reading is slower in PD, but controls and PD patients are both impacted by visuospatial planning challenges posed by increased visual crowding and irregularity of number spacing. Reading time did not correlate with UPDRS or MoCA scores in PD patients but significantly correlated with age, duration of disease, and VFQ-25 scores. The presence of convergence insufficiency did not significantly correlate with reading time in PD patients, although on average there was slower reading time in those with convergence insufficiency by 8 s (p = 0.2613). We propose that a simple reading task using 120 single-digit numbers can be used as a screening tool in the clinical setting to assess functional ocular motor difficulties in Parkinson's disease that can have a profound impact on quality of life.

Eye Movements in Parkinson's Disease and Inherited Parkinsonian Syndromes

Despite extensive research, the functions of the basal ganglia (BG) in movement control have not been fully understood. Eye movements, particularly saccades, are convenient indicators of BG function. Here, we review the main oculomotor findings reported in Parkinson’s disease (PD) and genetic parkinsonian syndromes. PD is a progressive, neurodegenerative disorder caused by dopaminergic cell loss within the substantia nigra pars compacta, resulting in depletion of striatal dopamine and subsequent increased inhibitory BG output from the internal globus pallidus and the substantia nigra pars reticulata. Eye movement abnormalities are common in PD: anomalies are more evident in voluntary than reflexive saccades in the initial stages, but visually guided saccades may also be involved at later stages. Saccadic hypometria (including abnormally fragmented saccades), reduced accuracy, and increased latency are among the most prominent deficits. PD patients show also unusually frequent and large square wave jerks and impaired inhibition of reflexive saccades when voluntary mirror saccades are required. Poor convergence ability and altered pursuit are common. Inherited parkinsonisms are a heterogeneous group of rare syndromes due to gene mutations causing symptoms resembling those of PD. Eye movement characteristics of some parkinsonisms have been studied. While sharing some PD features, each syndrome has a distinctive profile that could contribute to better define the clinical phenotype of parkinsonian disorders. Moreover, because the pathogenesis and the underlying neural circuit failure of inherited parkinsonisms are often well defined, they might offer a better prospect than idiopathic PD to understand the BG function.

Response inhibition in Parkinson's disease: a meta-analysis of dopaminergic medication and disease duration effects

Parkinson's disease is a neurodegenerative disorder involving the basal ganglia that results in a host of motor and cognitive deficits. Dopamine-replacement therapy ameliorates some of the hallmark motor symptoms of Parkinson's disease, but whether these medications improve deficits in response inhibition, a critical executive function for behavioral control, has been questioned. Several studies of Parkinson's disease patients "on" and "off" (12-h withdrawal) dopaminergic medications suggested that dopamine-replacement therapy did not provide significant response inhibition benefits. However, these studies tended to include patients with moderate-to-advanced Parkinson's disease, when the efficacy of dopaminergic drugs is reduced compared to early-stage Parkinson's disease. In contrast, a few recent studies in early-stage Parkinson's disease report that dopaminergic drugs do improve response inhibition deficits. Based on these findings, we hypothesized that Parkinson's disease duration interacts with medication status to produce changes in cognitive function. To investigate this issue, we conducted a meta-analysis of studies comparing patients with Parkinson's disease and healthy controls on tests of response inhibition (50 comparisons from 42 studies). The findings supported the hypothesis; medication benefited response inhibition in patients with shorter disease duration, whereas "off" medication, moderate deficits were present that were relatively unaffected by disease duration. These findings support the role of dopamine in response inhibition and suggest the need to consider disease duration in research of the efficacy of dopamine-replacement therapy on cognitive function in Parkinson's disease.