Basal ganglia neuronal activity during scanning eye movements in Parkinson's disease

Objective assessment of eye alignment and disparity-driven vergence in Parkinson's disease

Background

Self-reported diplopia is described in up to one-third of Parkinson's disease (PD) patients.

Objective

The purpose of our study was to expand our understanding of the mechanistic underpinnings of diplopia in PD. We hypothesize that the time-based control of eye alignment and increased eye deviation under binocular viewing will be related to the fusion-initiating and fusion-maintaining component deficits of disparity-driven vergence in PD.

Methods

We used high-resolution video-oculography to measure eye alignment under binocular and monocular viewing and disparity-driven vergence in 33 PD and 10 age-matched healthy participants. We computed eye deviation and time-based control of eye alignment, occurrence of conjugate saccadic eye movements, latency and gain of vergence (fusion initiation), and variance of eye position at the end of dynamic vergence (fusion maintenance).

Results

We categorized PD subjects into three groups, considering their time-based control of eye alignment as compared to healthy controls in binocular viewing. Group 1 = 45% had good control and spent >80% of the time when the eyes were well-aligned, Group 2 = 26% had intermediate control and spent <80% but greater >5% of the time when the eyes were well-aligned, and Group 3 = 29% had very poor control with increased eye deviation majority of the times (<5% of the time when the eyes were well-aligned). All three groups exhibited greater eye deviation under monocular viewing than controls. PD subjects exhibited fusion-initiating and fusion-maintaining vergence deficits (prolonged latencies, reduced vergence gain, increased variance of fusion-maintaining component) with a greater probability of saccadic movements than controls. Group 2 and Group 3 subjects were more likely to exhibit failure to initiate vergence (>20%) than Group 1 (13%) and controls (0%) trials. No significant difference was found in the Unified Parkinson's Disease Rating Scale (UPDRS-a tool to measure the severity of PD) values between the three PD groups (Group 1 = 33.69 ± 14.22, Group 2 = 38.43 ± 22.61, and Group 3 = 23.44 ± 1, p > 0.05).

Conclusion

The majority of PD subjects within our cohort had binocular dysfunction with increased eye deviation under monocular viewing and disparity-driven vergence deficits. PD subjects with intermediate or poor control of eye deviation under binocular viewing had greater fusion-initiating and fusion-maintaining vergence deficits. The study highlights the importance of assessing binocular dysfunction in PD subjects independent of the severity of motor symptoms.

The Contribution of Oculomotor Functions to Rates of Visual Information Processing in Younger and Older Adults

Oculomotor functions are established surrogate measures of visual attention shifting and rate of information processing, however, the temporal characteristics of saccades and fixations have seldom been compared in healthy educated samples of younger and older adults. Thus, the current study aimed to compare duration of eye movement components in younger (18-25 years) and older (50-81 years) adults during text reading and during object/alphanumeric Rapid Automatic Naming (RAN) tasks. The current study also aimed to examine the contribution of oculomotor functions to threshold time needed for accurate performance on visually-driven cognitive tasks (Inspection Time [IT] and Change Detection [CD]). Results showed that younger adults fixated on individual stimuli for significantly longer than the older participants, while older adults demonstrated significantly longer saccade durations than the younger group. Results also demonstrated that older adults required longer threshold durations (i.e., performed slower) on the visually-driven cognitive tasks, however, the age-group time difference on the CD task was eradicated when the effects of saccade duration were covaried. Thus, these results suggest that age-related cognitive decline is also related to increased duration of saccades and hence, highlights the need to dissociate the age-related motor constraints on the temporal aspects of oculomotor function from visuo-cognitive speed of processing.

Topography of emotional valence and arousal within the motor part of the subthalamic nucleus in Parkinson's disease

Clinical motor and non-motor effects of deep brain stimulation (DBS) of the subthalamic nucleus (STN) in Parkinson's disease (PD) seem to depend on the stimulation site within the STN. We analysed the effects of the position of the stimulation electrode within the motor STN on subjective emotional experience, expressed as emotional valence and arousal ratings to pictures representing primary rewards and aversive fearful stimuli in 20 PD patients. Patients' ratings from both aversive and erotic stimuli matched the mean ratings from a group of 20 control subjects at similar position within the STN. Patients with electrodes located more posteriorly reported both valence and arousal ratings from both the rewarding and aversive pictures as more extreme. Moreover, posterior electrode positions were associated with a higher occurrence of depression at a long-term follow-up. This brain-behavior relationship suggests a complex emotion topography in the motor part of the STN. Both valence and arousal representations overlapped and were uniformly arranged anterior-posteriorly in a gradient-like manner, suggesting a specific spatial organization needed for the coding of the motivational salience of the stimuli. This finding is relevant for our understanding of neuropsychiatric side effects in STN DBS and potentially for optimal electrode placement.

Effect of Eye Opening on Single-Unit Activity and Local Field Potentials in the Subthalamic Nucleus

BACKGROUND

Subthalamic nucleus deep brain stimulation (STN DBS) is an established treatment in Parkinson's disease (PD). We investigate the effect of eye opening on neuronal activity and local field potentials (LFPs) in the STN.

METHODS

We prospectively enrolled 25 PD patients undergoing STN DBS in our institution. During DBS, single-unit activity (SUA) and LFPs were measured when eyes were open and closed. As movement is known to result in changes in LFPs, we tested response to eye opening in the presence and absence of movement.

RESULTS

Neither eye state nor arm movement has a significant influence on SUA recordings. There is a statistically significant interaction between eye state and arm movement (p < 0.05). In the presence of movement, STN SUA increase when eyes open (p < 0.05). When eyes are closed, STN SUA decrease with movement (p < 0.05). STN theta LFP oscillations decrease when eyes are open compared to closed, irrespective of movement status (p < 0.05).

DISCUSSION

STN activity is influenced by eye state and arm movement. It is unclear whether this is attributed to a change in the STN's role in oculomotor control or from a change in attentional state. Understanding how physiologic normal activity alters neural activity is critical for the optimization of DBS therapy, particularly in closed-loop neuromodulation.

Abnormal eye movement behavior during reading in Parkinson's disease

INTRODUCTION

Reading difficulties are common in Parkinson's disease (PD) but not well studied. We report a case of reading difficulties in a 40-year-old man with 6-year history of PD on dopamine replacement therapy.

METHODS

We performed detailed neuro-ophthalmic examination and assessment of reading with and without infrared oculography.

RESULTS

Clinical examination revealed visual acuity of 20/20, no evidence of vision loss, and normal eye movement and ocular alignment with normal saccades, pursuit, and normal convergence. During King-Devick test, a rapid number reading task performed on a book, patient had normal number reading speed. More detailed study of number and word reading using infrared oculography revealed that while this patient had normal speed and eye movement behavior during number reading, he had dramatic slowing and eye movement abnormality during word reading. The slower reading speed during word reading was due to increased number of progressive saccades, smaller saccade amplitudes, increased number of regressive saccades, and longer fixation durations.

CONCLUSIONS

This case nicely illustrated the importance of comprehensive neuro-ophthalmic evaluations in Parkinson's disease and shows that reading difficulties can arise even when there is good visual acuity, ocular motor abilities necessary to read, and accommodation. In this case, reading difficulty was due to higher order ocular motor planning or cognitive abilities involved in word reading since the patient had no difficulty with ocular motor planning while reading numbers. These findings may have important implications towards our understanding of PD and can serve to spark further research in this important area.

Eye movements and deep brain stimulation

PURPOSE OF REVIEW

Deep brain stimulation (DBS) is an established treatment for several neurological conditions, and is most commonly used to treat Parkinson's disease by implanting electrodes in the basal ganglia. Despite the fact that circuits involved in eye movement control traverse the basal ganglia and are thus likely to be affected by DBS, studies combining DBS with eye movement analysis have been infrequent. This review focuses on recent research studies that examine the relationship between DBS and various types of eye movements and which highlight the potential of this approach.

RECENT FINDINGS

Recent work shows that DBS in the subthalamic nucleus (STN) can improve smooth pursuit in Parkinson's disease. STN DBS has also been shown to modulate visuospatial attention, and has provided experimental evidence backing a Bayesian model of basal ganglia function. DBS in the pallidum can improve antisaccadic performance in Parkinson's disease, suggesting improvement in higher control of oculomotor function, and implying retrograde striatal stimulation as part of the mechanism of action.

SUMMARY

These studies show that the combination of DBS with eye movement analysis is a powerful research tool. It may be used to study oculomotor physiology, basal ganglia pathophysiology, and the mechanism of action of DBS.

STN-DBS Reduces Saccadic Hypometria but Not Visuospatial Bias in Parkinson's Disease Patients

In contrast to its well-established role in alleviating skeleto-motor symptoms in Parkinson's disease, little is known about the impact of deep brain stimulation (DBS) of the subthalamic nucleus (STN) on oculomotor control and attention. Eye-tracking data of 17 patients with left-hemibody symptom onset was compared with 17 age-matched control subjects. Free-viewing of natural images was assessed without stimulation as baseline and during bilateral DBS. To examine the involvement of ventral STN territories in oculomotion and spatial attention, we employed unilateral stimulation via the left and right ventralmost contacts respectively. When DBS was off, patients showed shorter saccades and a rightward viewing bias compared with controls. Bilateral stimulation in therapeutic settings improved saccadic hypometria but not the visuospatial bias. At a group level, unilateral ventral stimulation yielded no consistent effects. However, the evaluation of electrode position within normalized MNI coordinate space revealed that the extent of early exploration bias correlated with the precise stimulation site within the left subthalamic area. These results suggest that oculomotor impairments "but not higher-level exploration patterns" are effectively ameliorable by DBS in therapeutic settings. Our findings highlight the relevance of the STN topography in selecting contacts for chronic stimulation especially upon appearance of visuospatial attention deficits.

Brain activation underlying turning in Parkinson's disease patients with and without freezing of gait: a virtual reality fMRI study

BACKGROUND

Freezing of gait is a debilitating symptom affecting many patients with Parkinson's disease (PD), causing severe immobility and decreased quality of life. Turning is known to be the most common trigger for freezing and also causes the highest rates of falls. However, the pathophysiological basis for these effects is not well understood.

METHODS

This study used a virtual reality paradigm in combination with functional magnetic resonance imaging to explore the neural correlates underlying turning in 17 PD patients with freezing of gait (FOG) and 10 PD patients without FOG while off their dopaminergic medication. Participants used foot pedals to navigate a virtual environment, which allowed for blood oxygen level-dependent (BOLD) responses and footstep latencies to be compared between periods of straight "walking" and periods of turning through 90°. BOLD data were then analyzed using a mixed effects analysis.

RESULTS

Within group similarities revealed that overall, PD patients with freezing relied heavily on cortical control to enable effective stepping with increased visual cortex activation during turning. Between groups differences showed that when turning, patients with freezing preferentially activated inferior frontal regions that have been implicated in the recruitment of a putative stopping network. In addition, freezers failed to activate premotor and superior parietal cortices. Finally, increased task-based functional connectivity was found in subcortical regions associated with gait and stopping within the freezers group during turning.

CONCLUSIONS

These findings suggest that an increased propensity towards stopping in combination with reduced sensorimotor integration may underlie the neurobiology of freezing of gait during turning.

Visual Attention and Saccadic Oculomotor Control in Parkinson's Disease

BACKGROUND

In patients with Parkinson's disease (PD) we aimed at differentiating the relation between selective visual attention, deficits of programming and dynamics of saccadic eye movements while searching for a target and hand-reaction time as well as hand-movement time. Visual attention is crucial for concentrating selectively on one aspect of the visual field while ignoring other aspects. Eye movements are anatomically and functionally related to mechanisms of visual attention. Saccadic dysfunction might confound selective visual attention in PD.

METHODS

We studied visual selective attention in 22 medicated PD patients (clinical ON status, mild to moderate disease severity) and 22 age matched controls. We looked for possible interferences through oculomotor deficits. Two tasks were compared: free viewing of photographs and time optimal visual search of a hidden target. Visual search times (VST), task related dynamics of saccades, and hand-reaction and hand-movement times were analyzed.

RESULTS

In the free viewing task mild to moderately affected PD patients did not differ statistically from healthy subjects with respect to saccade dynamics. However, patients differed significantly from healthy subjects in the time optimal visual search task with 25% lower rates of successful searches. Hand movement reaction time did not differ in both groups, whereas hand movement execution time was significantly prolonged in PD patients.

CONCLUSION

Saccadic oculomotor control and hand movement reaction times were intact, whereas in our less severely affected treated PD patients, visual selective attention was not. The highly reduced successful search rate might be related to disturbed programming and delayed execution of saccades during time optimal visual search due to decreased execution of serial-order sequential generation of saccades.

Distinct populations of neurons respond to emotional valence and arousal in the human subthalamic nucleus

Both animal studies and studies using deep brain stimulation in humans have demonstrated the involvement of the subthalamic nucleus (STN) in motivational and emotional processes; however, participation of this nucleus in processing human emotion has not been investigated directly at the single-neuron level. We analyzed the relationship between the neuronal firing from intraoperative microrecordings from the STN during affective picture presentation in patients with Parkinson's disease (PD) and the affective ratings of emotional valence and arousal performed subsequently. We observed that 17% of neurons responded to emotional valence and arousal of visual stimuli according to individual ratings. The activity of some neurons was related to emotional valence, whereas different neurons responded to arousal. In addition, 14% of neurons responded to visual stimuli. Our results suggest the existence of neurons involved in processing or transmission of visual and emotional information in the human STN, and provide evidence of separate processing of the affective dimensions of valence and arousal at the level of single neurons as well.

Eye movements in ephedrone-induced parkinsonism

Patients with ephedrone parkinsonism (EP) show a complex, rapidly progressive, irreversible, and levodopa non-responsive parkinsonian and dystonic syndrome due to manganese intoxication. Eye movements may help to differentiate parkinsonian syndromes providing insights into which brain networks are affected in the underlying disease, but they have never been systematically studied in EP. Horizontal and vertical eye movements were recorded in 28 EP and compared to 21 Parkinson's disease (PD) patients, and 27 age- and gender-matched healthy subjects using standardized oculomotor tasks with infrared videooculography. EP patients showed slow and hypometric horizontal saccades, an increased occurrence of square wave jerks, long latencies of vertical antisaccades, a high error rate in the horizontal antisaccade task, and made more errors than controls when pro- and antisaccades were mixed. Based on oculomotor performance, a direct differentiation between EP and PD was possible only by the velocity of horizontal saccades. All remaining metrics were similar between both patient groups. EP patients present extensive oculomotor disturbances probably due to manganese-induced damage to the basal ganglia, reflecting their role in oculomotor system.