Remodeling of the fovea in Parkinson disease.

To quantify the thickness of the inner retinal layers in the foveal pit where the nerve fiber layer (NFL) is absent, and quantify changes in the ganglion cells and inner plexiform layer. Pixel-by-pixel volumetric measurements were obtained via Spectral-Domain optical coherence tomography (SD-OCT) from 50 eyes of Parkinson disease (PD) (n = 30) and 50 eyes of healthy control subjects (n = 27). Receiver operating characteristics (ROC) were used to classify individual subjects with respect to sensitivity and specificity calculations at each perifoveolar distance. Three-dimensional topographic maps of the healthy and PD foveal pit were created. The foveal pit is thinner and broader in PD. The difference becomes evident in an annular zone between 0.5 and 2 mm from the foveola and the optimal (ROC-defined) zone is from 0.75 to 1.5 mm. This zone is nearly devoid of NFL and partially overlaps the foveal avascular zone. About 78 % of PD eyes can be discriminated from HC eyes based on this zone. ROC applied to OCT pixel-by-pixel analysis helps to discriminate PD from HC retinae. Remodeling of the foveal architecture is significant because it may provide a visible and quantifiable signature of PD. The specific location of remodeling in the fovea raises a novel concept for exploring the mechanism of oxidative stress on retinal neurons in PD. OCT is a promising quantitative tool in PD research. However, larger scale studies are needed before the method can be applied to clinical follow-ups.

Cortical control of saccades in Parkinson disease and essential tremor.

A number of studies suggest that some features of essential tremor (ET) and Parkinson disease (PD) overlap. Besides tremor, also some cognitive features have been implicated in ET and PD. There is recent evidence that a common genetic mutation occurs in ET and PD. Saccadic eye movements could provide an easily quantifiable procedure to help in the differential diagnosis in early PD and ET. Being able to distinguish early on the two diseases may help in tailoring therapy. Cortical control of saccades and antisaccades as they pertain to the potential discrimination of PD and ET is reviewed. Imaging and electrophysiological studies are highlighted; however, there are still few studies. Hopefully this review will stimulate further research, in particular in the direction of differences and similarities in the neural circuits involved in PD and ET.

Cortical control of voluntary saccades in Parkinson's disease and pre-emptive perception.

Gamma range EEG has been associated with cognition. Bodis-Wollner et al. [Ann NY Acad Sci 2002;956:464-7] and Forgacs et al. [Perception 2008;37:419-32] described posterior perisaccadic gamma (35-45 Hz) modulation associated with voluntary saccades. Voluntary impairment is a hallmark of Parkinson's disease (PD). We have done correlational analysis of frontally and posteriorly (posterior-parietal) recorded intrasaccadic gamma (ISG) powers, to understand cortical control of voluntary saccades in PD and healthy controls. Fifteen PD patients (55-71 years, 4 females) and 17 healthy controls (54-72 years, 9 females) participated in the study. The EEG was recorded over frontal and posterior-parietal scalp sites. Saccades were recorded with electro-oculogram and infra-red ISCAN camera. Subjects executed horizontal voluntary saccades to a mark; 15 degree distance rightwards or leftwards (centrifugal CF) from the central fixation, then back to the center (centripetal CP) and so on, for 2 minutes. Perisaccadic EEG segments were wavelet transformed followed by Hilbert transform to obtain ISG (35-45 Hz) powers. ISG power was trial-averaged, separately for the 4 possible saccade types; CP and CF, rightwards and leftwards. The perisaccadic EEG revealed disorganization in the intrasaccadic period. The correlations between frontal and posterior ISG power are high in PD (correlation coefficient >0.6) while low in controls (correlation coefficient <0.02). We interpret these results as lack of modulatory coupling between frontal and posterior intrasaccadic mechanisms in PD. Impaired volition in PD may be due to impaired circuitry of preemptive perception (PEP). Interareal phase coupling analysis will help in investigating the cortical voluntary saccade control with greater temporal precision.

Perisaccadic gamma modulation in Parkinson disease patients and healthy subjects.

We quantified the anterior-posterior distribution of the gamma modulation index (GMI), an index of perisaccadic phasic modulation of the gamma (35-45 Hz) range electroencephalogram (EEG), in healthy human subjects and Parkinson disease (PD) patients. The EEG was recorded over the frontal, parietal, temporal and occipital sites in 11 idiopathic PD patients (age 50-70 years, four females), 4 age matched healthy volunteers (1 female) and 17 young healthy controls (age 21-30 years, four females) Eye movements were recorded with EOG and ISCAN camera. Subjects executed saccades to a mark at right and back to fixation point and vice versa. The saccades directed away from center/fixation (centrifugal CF) were analyzed. Two minutes of EEG were obtained from each subject for the two possible saccade types (centrifugal rightwards and leftwards at 15 degrees). Each perisaccadic EEG segment was analyzed using continuous wavelet transform for quantifying the power and time course of gamma EEG ranges for each saccade type. A three way ANOVA was used for statistical analysis. Perisaccadic GMI (peak intrasaccadic power divided by mean power) in healthy subjects was higher over the contralateral hemisphere to the saccade direction, for both centrifugal saccades at anterior, posterior and occipital recording sites. Contrary to the healthy subject GMI remained near one in PD, i.e., there was no evidence of intrasaccadic gamma power increase in PD patients.