Retinocortical Gain in the Foveal Pathway: The Effect of Spatial Frequency and Stimulus Size

The amplitude and the phase of the simultaneously recorded steady-state pattern electroretinogram (PERG) and visual evoked potential (VEP) were evaluated in humans as a function of the vertical diameter (D) of unidimensional Gabor stimuli. In the other dimension, parallel to the horizontal gratings, the patterns all had constant diameter (see Methods and Materials). Spatial frequencies (SFs) of 1 cycle per degree (cpd) and 5.3 cpd were counterphase modulated at a rate of 6.8 Hz. After off-line artifact rejection, the response was subject to Fast Fourier Transformation (FFT). Amplitude and phase of the first and second harmonics of both ERG and VEP were displayed for each SF and stimulus diameter.

Both ERG and VEP amplitude were found to increase as a function of D. Using trend analysis we found that ERG amplitude increased linearly as a function of D. VEP amplitude was found to be both linear and cubic, as a function of D. We calculated the ratio of VEP amplitude and ERG amplitude at each D and termed it retinocortical gain (G). G normalized to stimulus area was high for small D and decreased with D independently of SF. Unity gain occurred at stimulus sizes of 6° to 7°.

ERG phase was found to be more negative at 5.3 cpd than at 1 cpd. Although no significant difference was found between VEP phases at the two SFs tested, neither ERG nor VEP phase changed as a function of D.

The results suggest that retinocortical gain is highest for the foveally centered low spatial frequency small patch Gabors. The results provide support for the notion of the “foveal window” in human vision.

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.

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.

Role of the calcarine cortex (V1) in perception of visual cues for saccades

OBJECTIVE

To determine the initial level at which the pathways for cue perception, saccades and antisaccades diverge.

METHODS

Two procedures: single pulse transcranial magnetic stimulation (sTMS) over posterior occiput and backward masking were used. A visual cue directed saccades to the left or right, either a pro-saccade (to the side of the cue but beyond it) or an antisaccade, i.e., contraversive saccade. No visual target was presented.

RESULTS

Latencies of the two types of saccades did not differ. Focal sTMS applied unilaterally over V1 suppressed both perception of a cue flashed 80-90ms earlier contralaterally (but not ipsilaterally) and the appropriate saccade. Masking at a delay of 100ms abolished the appropriate saccade and cue perception.

CONCLUSIONS

V1 is essential for the perception of a flashed cue and for executing appropriate pro- and contraversive saccades. Masking may occur beyond V1, where the pathways for perception and for saccades at least to the next visual processing level start separating.

SIGNIFICANCE

VI is needed for rapid, accurate perceptual and motor responses to the crudest (left versus right) cues. It is unlikely that the "where" system can have a major direct input bypassing V1.

Getting around and communicating with the environment: visual cognition and language in Parkinson's disease

Vision in PD. In PD an impairment of dopaminergic neurons of the preganglionic retina and a defect of the retinal nerve fibers (axons of the retinal ganglion cells) has been demonstrated and a correlation of loss of spatial contrast sensitivity, with the progression of motor impairment in PD has been described. These low level visual deficits contribute but do not directly explain behavioural visual deficits in PD involving spatial cognition, internal representation, space navigation and visual categorization. Language deficits in non-demented PD patients can include impairments in comprehension, verbal fluency, and naming. Comprehension deficits become evident when patients are required to process sentences with non-canonical, irregular grammatical structures. Semantic memory deficits may result in the impairments in category fluency and confrontational naming. Selective language deficits may be due to impaired dynamics of the "phonological loop" connecting the pre-frontal cortex and the basal ganglia. A more encompassing linguistic and functional model of PD specific language impairments would be useful for evaluating language deficits in the context of motor dysfunction.

Nicotinic receptors and cognition in Parkinson's Disease: the importance of neuronal synchrony

Parkinson's Disease (PD) is associated with cognitive deficits. The earliest impairment is evident for executive abilities, visuospatial orientation and memory. Dopamine deficiency is unlikely to be singly responsible for all cognitive changes in PD. Acetyl-choline has an essential role in cognition, thus cholinergic transmission may have an important role in non-dopaminergic cognitive changes. If so, some cognitive defects could possibly be treated with choline-esterase inhibitors. A concern is the potential negative motor effect of cholinergic medication in PD. Surprisingly, these are reported only in a few patients studied. Establishing the relationship between select cognitive deficits and nicotinic neurotransmission may lay the foundation for rational pharmacotherapy of cognitive dysfunction in PD. We summarize anatomical, physiological and pharmacological aspects of nicotinic receptor function. The focus is on those nicotine receptor dependent cognitive dysfunctions which are likely to contribute to motor impairment. Lastly, we discuss hypotheses concerning cholinergic involvement in neuronal synchrony and sensorimotor integration in PD.

The visuo-cognitive and motor effect of amantadine in non-Caucasian patients with Parkinson's disease. A clinical and electrophysiological study

It has been reported that non-Caucasian populations often suffer from an atypical type of Parkinson's disease (PD) characterized by poor levodopa response, early cognitive impairment and autonomic dysfunction. We tested the effect of a well known antiparkinsonian compound, amantadine, in 23 Afro-American patients with PD in a time-limited (six months), open-label, clinical and electrophysiological (simultaneously recorded primary and cognitive visual evoked potentials) trial. Patients were given amantadine either as monotherapy (first group) or added to levodopa treatment (second group). Amantadine produced a significant (p < 0.05) shortening of the latency of the event related potential (P300) obtained in a visual discrimination paradigm, while the timing of primary visual evoked potentials was little or not at all affected. Amantadine also showed significant beneficial effects (p < 0.01) on the motor score of both groups as assessed by the Rated Parkinson's Disease Neurological Exam, including items related to autonomic dysfunction. These findings suggest that amantadine alone and as adjuvant to levodopa can significantly improve both the speed of visual cognitive processing and the clinical score in non caucasian patients with PD. For these populations amantadine can be thus considered a helpful therapeutical option.

Wavelet transform of the EEG reveals differences in low and high gamma responses to elementary visual stimuli

Multiunit electrophysiological studies indicate that oscillatory activity is common in the awake mammalian central nervous system. Synchronous 20-80 Hz oscillations, so called gamma rhythms, have been proposed as a possible fundamental physiological mechanism of binding neuronal activity underlying object recognition. The purpose of this study was to determine whether or not gamma band oscillatory activity in the human brain is modulated by attributes of elementary visual stimulation. The experiment was performed on 7 normal subjects. Sinusoidal gratings were presented over a range of spatial frequencies. Evoked potentials were recorded over 5 surface electrodes placed in a horizontal occipital chain across the back of the head. Discrete wavelet transform was performed on the first 200 msec following stimulus onset on the average data of 256 sweeps. Power was analyzed with ANOVA across conditions. In our previous studies we have separated a "low" (14-28 Hz) and "high" (28-55 Hz) gamma band. The current results indicate that both gamma bands to full-field stimulation have the highest power at the midline (inion) electrode to a spatial frequency of 5.5 cpd, which is the peak spatial frequency from foveal psychophysical data. However, the spatial frequency bandwidth is considerably narrower in the HG than in the LG band. Occipital spatial frequency tuning of the massed high gamma response is narrower than the tuning of individual cortical neurons. The bandwidth difference between low and high gamma band suggests that different frequency gamma range oscillations may represent not only different functional properties of visual processing, but may also reflect underlying differences in excitatory and postsynaptic inhibitory circuits shaping the contrast sensitivity of the human observer. Our study emphasizes the importance of elementary visual filter properties for gamma responses and the need to subdivide gamma frequency ranges according to functional properties.

Parkinson's disease changes the balance of onset and offset visual responses: an evoked potential study

OBJECTIVES

We investigated whether the transient pattern onset and offset visual evoked potential (VEP) can distinguish between patients with Parkinson's disease (PD) and normal subjects.

METHODS

Two horizontal sinusoidal gratings differing in spatial frequency, i.e. 1 and 4 cycles per degree, were presented to 17 patients with PD and 16 age-matched control subjects. We analyzed the responses in the time-domain and measured the latencies and amplitudes of N1 and P1 to the onset and the offset of the stimulus; we also derived the measures of offset N1 and P1 amplitude responses 'normalized' to onset N1 and P1 amplitude values, respectively (amplitude ratios).

RESULTS

Absolute and normalized offset P1 amplitude is a distinguishing feature of PD patients from controls. Offset P1 amplitude was significantly larger in PD patients than in controls, particularly to the lower spatial frequency stimulus (P<0.01 for absolute and P<0.001 for normalized values, respectively).

CONCLUSIONS

We conclude that the pattern onset/offset VEP amplitude provides a simple measure to evaluate visual processing deficits in PD and could contribute to an understanding of the pathophysiology of these changes.

The paracentral visual field in multiple sclerosis: evidence for a deficit in interneuronal spatial summation?

A visual complaint such as blurred or "washed-out vision" can be one of the early signs of multiple sclerosis (MS). Although visual deficits are commonly attributed to optic nerve demyelination even with preserved visual acuity, the results of a considerable number of visual studies are inconsistent with this interpretation [Camisa, Mylin, & Bodis-Wollner, Annals of Neurology 10 (1981) 532-539; Regan & Neima, British Journal of Ophthalmology 68 (1984) 310-315]. However, a retinal axonal (nerve fiber layer) defect can be detected in some eyes, this is not the rule. Routine visual field (VF) tests, with a low sampling rate may also be non-informative in MS and optic neuritis, possibly because the VF abnormalities may be small and spotty or they can be found between tested points. The present study combined the advantages of VF and contrast sensitivity (CS) testing by applying contrast perimetry (CP), to the central 16 degrees of the VF. Four paracentral VF quadrants were tested in clinically affected and unaffected eyes of 31 MS patients and 26 controls. The stimuli were vertical Gaussian apertured sinusoidal gratings (Gabors) of 1 cpd. CS was obtained as a function of the diameter of the Gábor ranging from 1 to 7.4 degrees. The CP data of controls and definite and probable MS groups were significantly different for each pattern size, but the largest difference was found at diameters 2.5-3.7 degrees. Our study adds to previous evidence showing that optic nerve pathology does not explain "subclinical" and manifest visual dysfunction in MS. Given previous studies revealing orientation dependent monocular visual deficits and our study results, parsimony suggests that MS affects a network relying on myelinated lateral axonal branches of the visual cortex, binding monocular columns of neurons with like-with-like specificity.

L-alpha-glycerylphosphorylcholine enhances the amplitude of the pattern electroretinogram in rhesus monkeys. A pilot study

The effects of L-alpha-glyceryl-phosphorylcholine (LGPC), a putative acetylcholine precursor, were studied on the pattern electroretinogram (PERG) in two rhesus monkeys. In the time interval 10-30 min after LGPC administration (85 mg/kg i.m.), the amplitude of the PERG increased significantly. In both monkeys the amplitude enhancement was more pronounced for low spatial frequency stimuli (1 cycle/degree) than for medium spatial frequency gratings (5 cycles/degree). This result suggests that besides the general facilitatory properties, LGPC may have specific spatial frequency tuning effects at the level of retinal information processing.

Visuo-cognitive dysfunctions in Parkinson's disease

Loss of will, decreased activity, and poverty of behavior are among the common symptoms observed in Parkinson's disease (PD). In line with these clinical observations, PD patients display prominent deficit in neuropsychological tests, requiring self-generated and effort-demanding operations. However, recent evidence suggests that this impairment is not generalized: visuo-spatial working memory and attentional set-shifting seem to be selectively impaired in the early stages of the disease. Electrophysiological studies also demonstrate the dysfunction of higher-level visual information processing. In this article, we discuss some current results to show the connection between clinical symptoms and neuropsychological deficits. We also consider dysfunction in underlying neural mechanisms, with particular emphasis on the dysregulation of fronto-striatal circuits. However, it is conceivable that visuo-cognitive impairment in PD reflects dysfunction of neural assemblies, involving basal ganglia, dorsal visual stream, and frontal-prefrontal circuits.

Psychophysical examination of paracentral defects in glaucoma

More than 50% of ganglion cells may be damaged before visual field loss is measurable by conventional methods in primary open angle glaucoma. There is general agreement on the need to improve early diagnosis of visual field loss in primary open angle glaucoma. In this article, new techniques that enlist measurement of paracentral regions are discussed, and the ability of each method to detect visual field loss prior to perimetric loss is described.

Functional properties of sub-bands of oscillatory brain waves to pattern visual stimulation in man

The scalp recorded transient visual evoked potential (VEP) represents the massed activity of a large number of neurons of the human visual cortex. Animal studies show that intracerebrally-recorded high frequency electrical activity represents binding between neurons participating in a cooperative response. We evaluated the relationship between scalp recorded high frequency activity and transient VEPs elicited by a repetitive (grating) pattern. Stimuli were 1 and 4 cycles/degree sinusoidal gratings, presented in an on/off mode. Following conventional averaging, the discrete wavelet transform (DWT) was applied. Multi-resolution decomposition was used to divide the responses into 6 orthogonal frequency bands. The results show that high frequency oscillatory activity in the beta and gamma frequency range is closely related in time to the N70 peak of the simultaneous VEP. Power in both bands is modulated by spatial frequency. Beta range response to hemifield stimulation recorded over a chain of electrodes over the occipital area lateralizes in the same manner as N70, while gamma range activity is insensitive to lateralization and is more closely linked to foveal stimulation. This dissociation between beta and gamma range activity suggests that different bands of high frequency oscillatory activity in humans, linked to visual stimulation, may represent different aspects of visual processing.

Cortical activation patterns during voluntary blinks and voluntary saccades

OBJECTIVE

To investigate the activation of frontal, parietal, and occipital areas in normal volunteers during voluntary blinks and during voluntary saccades using functional MRI (fMRI).

BACKGROUND

A previous fMRI study revealed the activation of the precentral and posterior middle frontal gyrus ("frontal eye field" [FEF]), the medial part of the superior frontal gyrus ("supplementary eye field" [SEF]), and the visual cortex. The parietal cortex was not included in this study. Frontal and occipital cortical areas involved in voluntary blinking have not been shown previously using fMRI.

METHODS

A 1.5-T standard clinical scanner was used for both anatomic and functional studies in 12 observers. To conduct data analyses the authors used voxel-by-voxel cross-correlation.

RESULTS

Voluntary blinks led to the activation (p < 0.05) of the FEF, the SEF, the posterior parietal cortex ("parietal eye field" [PEF]), and the visual cortex. Voluntary blinking produced activity in the same cerebral structures as voluntary saccades. However, the number of activated voxels was smaller during voluntary blinking than during voluntary saccades in the visual cortex and in the FEF (p < 0.01). In contrast, the extent of activation was significantly higher (p < 0.003) in the SEF and in the PEF during voluntary blinking.

CONCLUSIONS

Voluntary blinks and saccades are associated with similar loci of activation patterns; however, the quantitative distribution of activation suggests that the middle part of the frontal gyrus and posterior parietal cortex are of special significance for voluntary blinks. The results argue for the importance of considering quantitative distributional properties of parallel cortical activities associated with saccades and blinks.

A mixed D1 and D2 antagonist does not replay pattern electroretinogram alterations observed with a selective D2 antagonist in normal humans: relationship with Parkinson's disease pattern electroretinogram alterations

The human retina produces a tuned response to stimuli of increasing spatial frequency reversed at a steady state. The peak amplitude response, at medium spatial frequencies, is decreased in Parkinson's disease and in normal subjects (n = 18) treated with a D2 dopaminergic antagonist (l-sulpiride). Here, we report that a mixed D1-D2 receptor antagonist (haloperidol) in normal subjects (n = 18) does not produce an amplitude decrease of medium spatial frequencies (SFs) responses but it decreases low-frequency response. It could argued that the increased dopamine release produced by the presynaptic D2 antagonistic action of haloperidol is subsequently counteracted at postsynaptic level by its D1 antagonistic effect, producing a net counterbalance at medium SFs. These data suggest that the two dopamine receptors may play different roles in the retinal function and in the origin of visual alterations in Parkinson's disease.

The push-pull action of dopamine on spatial tuning of the monkey retina: the effects of dopaminergic deficiency and selective D1 and D2 receptor ligands on the pattern electroretinogram

Retinal dopamine depletion in monkeys using either systemic MPTP or 6-OHDA results in attenuated electroretinographic (ERG) responses to peak spatial frequency stimuli. Diverse dopamine receptors have been identified in the primate retina. ERG studies performed using Haloperidol (a mixed antagonist), L-Sulpiride (D2 antagonist) and CY 208-243 (a D1 agonist) cause spatial frequency dependent diverse effects. 'Tuning' of the normal spatial contrast response PERG, was quantified by dividing the amplitude of the response at the peak spatial frequency with the amplitude to the low spatial frequency response yielding a number greater than one. Tuning for the pharmacological experiments was defined by dividing the actual amplitude obtained at the normal peak response with the actual amplitude at the low spatial frequency response. The PERG spatial contrast response function is discussed as the envelope output of retinal ganglion cells or the average or 'equivalent' retinal ganglion cell. However, we postulate the existence of two dopamine sensitive pathways with different weights for two classes of ganglion cells. It is inferred that D1 receptors are primarily affecting the 'surround' organization of ganglion cells with large centers, while D2 post-synaptic receptors contribute to 'center' response amplification of ganglion cells with smaller centers. These inferences are consistent with some lower vertebrate data. It is also inferred that low affinity D2 autoreceptors may be involved in the D1 'surround' pathway. An understanding of the logic performed by retinal D1 and D2 receptors may be useful to discern the functional role of diverse dopamine receptors in DA circuits elsewhere in the CNS.

D1 agonist CY208-243 attenuates the pattern electroretinogram to low spatial frequency stimuli in the monkey

We investigated whether or not the D1 agonist, CY 208-243, affects the spatial tuning function of pattern electroretinogram (PERG). Two lightly anaesthetised monkeys were studied before and after CY 208-243 or placebo administration. The results show that the PERG response to 0.5 cycles/degree (c/d; coarse), but not to 2.3 c/d (medium) spatial frequency stimuli disappears following systemic administration of this drug. Since previous results show that D2 blockers attenuate the PERG only above 2.3 c/d, foremost the peak of the normal spatial frequency response function, the current results suggest that dopamine itself, via D1 receptors, may be responsible for the low spatial frequency decline of normal spatial PERG tuning function. We infer that the synergistic activation of D1 and D2 receptors is needed to shape the spatially tuned primate ERG.

Electrophysiological evidence for visuocognitive dysfunction in younger non Caucasian patients with Parkinson's disease

A study of "primary" (VEPs) and "cognitive" (ERPs) visual evoked potentials was carried out in a group of non-demented Afro-American Parkinson's disease (PD) patients. Current studies suggest that differences exist in the clinical manifestations of PD in Caucasian and non-Caucasian populations. Two horizontal sinusoidal gratings differing in spatial frequency, i.e., 1 and 4 cycles per degree (cpd), were presented in an "odd-ball" paradigm to 17 patients with PD and 17 age-matched control subjects. While the 1 cpd stimulus, is not expected to reveal retinal dopaminergic deficency, but only visuocognitive deficits, the 4cpd may give direct information of both "retinal" and "cognitive" visual deficits. We measured the latencies and amplitudes of N70, P100 and P300 components, and derived the "normalized" measures of P300-N70 latency difference (Central Processing Time-CPT70), the P300-P100 latency difference (CPT100) and the P300 amplitude responses normalized to either N70 and P100 amplitude (Amplitude Ratios AR70 and AR100). Our results do show that cognitive electrophysiological deficits in younger PD patients exist in non-Caucasians, perhaps to an even greater degree than in Caucasians, and confirm that absolute and normalized ERP amplitude and latency abnormalities are a distinguishing feature of younger PD patients from controls. In particular P300 measures are abnormal for 1 cpd pattern. A negative correlation exists between P300 amplitude and the motor score. By comparing the results for 1 and 4cpd stimuli it can be concluded that "primary" and "cognitive" visual abnormalities are independently affected in PD, implying that visuo-cognitive abnormalities are not passively determined by retinal dopaminergic deficiency.

Hidden visual loss in optic neuropathy is revealed using Gabor patch contrast perimetry

Visual sensitivity was evaluated in the central 16 degrees of the visual field in normal subjects, in patients with glaucomatous optic neuropathy, in glaucoma suspects, and in eyes of patients with multiple sclerosis without evidence of active optic neuritis. A novel method was used to assess sensitivity called contrast perimetry (CP). CP essentially samples every point in the central 16 degrees field, and the normal and deviant responses are relatable to spatial summation of contrast signals. In each visual field quadrant 1 cycles/degree sinusoidal grating stimuli limited in area by a gaussian circular aperture (called a Gabor stimulus) were presented. Contrast sensitivity was measured as a function of stimulus size. The normal curve is nearly S-shaped: For small Gabors contrast sensitivity increases slowly, then accelerates and then flattens again. Patients' results fell into two broad categories: a loss more or only evident for small Gabors and another type of loss for both small and large Gabor size. Glaucoma suspect and most glaucoma eyes showed predominant losses to small Gabors. There were more eyes and more VF quadrants identified by contrast perimetry as abnormal compared to the diagnostic yield of the Humphrey 30-2 (central) visual field even though a Humphrey VF defect was defined liberally. Apparently, contrast perimetry may yield diagnostically useful information of paracentral visual sensitivity. Furthermore, the results suggest that selective losses in POAG and some glaucoma suspects occur to spatially broad-band retinal mechanisms, presumably ganglion cells.

Dopamine D2 receptor blockade alters the primary and cognitive components of visual evoked potentials in the monkey, Macaca fascicularis

The effects of sulpiride, a dopamine D2 receptor antagonist, were studied on visual event-related potentials in a monkey performing in a visual oddball task in order to investigate receptor specific mechanisms in visuo-cognitive processes. Following the injection of 0.35 mg/kg sulpiride i.m., the amplitude and latency of the primary (P100) and cognitive (P300) components did not change significantly. When 1.05 mg/kg sulpiride was administered, the latency of the primary and cognitive components increased. The amplitude of the P100 component decreased, while that of the P300 component increased. These data suggest that D2 receptors play an important role in visuo-cognitive processes in both physiological and pathological conditions such as Parkinson's disease and schizophrenia.

Functional MRI mapping of occipital and frontal cortical activity during voluntary and imagined saccades

We investigated the activation of frontal and occipital cortical areas in 14 normal volunteers during voluntary saccades in light or dark and during imagined saccades using functional magnetic resonance imaging (FMRI) with electro-oculogram monitoring. Voluntary saccades in light or dark and imagined saccades led to a significant activation (p < 0.005) of the precentral and posterior medial frontal gyrus (frontal eye field). The medial part of the superior frontal gyrus (supplementary eye field) also showed significant activity during voluntary saccades in all subjects, but only in four subjects during imagined saccades. In addition to frontal activity we found an activated primary visual cortex during voluntary saccades, both in light and in dark. In contrast to executed saccades, imagined eye movements revealed to occipital response under either condition. Our FMRI study supports the concept of frontal eye fields during voluntary saccades and demonstrates that occipital areas are associated with the generation of voluntary eye movements. However, the primary visual cortex is not active when eye movement is only imagined.

Visual electrophysiology in Parkinson's disease: PERG, VEP and visual P300

A retinal dopaminergic deficiency underlies some visual changes in Parkinson's disease (PD), in particular those elicited by stimuli near the peak of the human and monkey spatial contrast sensitivity. The correspondence of retinal changes and VEP alterations is not perfect: they do not seem to rely on identical mechanisms. It seems that additional pathology beyond the retina affects visual responses, including VEPs. The relevance of "distal" primary VEP changes to higher cognitive visual abnormalities in PD is not established at present.

Longitudinal study of a heteroplasmic 3460 Leber hereditary optic neuropathy family by multiplexed primer-extension analysis and nucleotide sequencing

Nucleotide-sequencing and multiplexed primer-extension assays have been used to quantitate the mutant-allele frequency in 14 maternal relatives, spanning three generations, from a family that is heteroplasmic for the primary Leber hereditary optic neuropathy (LHON) mutation at nucleotide 3460 of the mitochondrial genome. There was excellent agreement between the values that were obtained with the two different methods. The longitudinal study shows that the mutant-allele frequency was constant within individual family members over a sampling period of 3.5 years. Second, although there was an overall increase in the mutant-allele frequency in successive generations, segregation in the direction of the mutant allele was not invariant, and there was one instance in which there was a significant decrease in the frequency from parent to offspring. From these two sets of results, and from previous studies of heteroplasmic LHON families, we conclude that there is no evidence for a marked selective pressure that determines the replication, segregation, or transmission of primary LHON mutations to white blood cells and platelets. Instead, the mtDNA molecules are most likely to replicate and segregate under conditions of random drift at the cellular level. Finally, the pattern of transmission in this maternal lineage is compatible with a developmental bottleneck model in which the number of mitochondrial units of segregation in the female germ line is relatively small in relation to the number of mtDNA molecules within a cell. However, this is not an invariant pattern for humans, and simple models of mitochondrial gene transmission are inappropriate at the present time.

Simultaneously evoked primary and cognitive visual evoked potentials distinguish younger and older patients with Parkinson's disease

While it is known that both primary visual processes and visuocognitive responses are affected in Parkinson's Disease (PD), their relationship is not known. It is known that both of these measures can be affected by age per se. Our aims were to determine if in non-demented PD patients visual cognitive event-related potential (ERP) changes simply reflect abnormal primary visual processing and to determine the effects of age and disease on their relationship. In order to do so, we introduce a new normalizing procedure for visual ERPs. In addition to the latencies and amplitudes of P100, N140, P200, N200 and P300 components, the P300-P100 latency difference (termed "central processing time"-CPT) were measured. In order to avoid confounding factors of absolute amplitude differences due to say, generally low voltage recordings or poor primary visual responses, P300 responses normalized to P100 responses were also evaluated (P300/P100 amplitude ratio). Visual ERPs were obtained in an "oddball" paradigm in 20 nondemented patients with PD and 20 normal age-matched subjects. The stimuli were horizontal sinusoidal gratings differing only in spatial frequency (0.5 and 1 cycle/degree). While simple ERP latency criteria did not distinguish non-demented PD patients as a group from controls, when younger PD patients were compared to older PD patients and controls CPT acceleratedly increased in younger PD patients. The amplitudes of both N200 and P300 provided significant distinction between patient and control groups. The surprising result emerging from this study is that an individually normalized P300 amplitude provides significant distinction of younger PD patients from age matched normals.

The pattern electroretinogram in Parkinson's disease reveals lack of retinal spatial tuning

Spatio-temporal visual abnormalities, involving processing of medium coarse stimuli, are known to occur in Parkinson's disease (PD). While these deficits have been related to retinal dopaminergic deficiency, previous ERG studies in PD patients have provided conflicting results, probably due to differences in stimulus conditions. The influence of pattern element size (spatial frequency, SF) on the pattern electroretinogram (PERG) in PD has not been systematically studied. We recorded steady-state PERG to sinusoidal gratings of 50% contrast, counterphase modulated at 7.5 Hz with a series of SFs ranging from 0.5 to 6.9 c/deg in 20 PD patients and 20 healthy volunteers, subdivided in 10 "young" and 10 "age-matched" (AM) subjects. The PERG was analyzed by means of Fast Fourier Transform and the amplitude and the phase of the second harmonic response (15 Hz) were taken into account. We evaluated the medium-to-low SF amplitude ratio and termed it "PERG tuning ratio" (TR). The results indicate that aging affects all the studied SF, but the pattern of age-related loss differs from that observed in PD. Compared to AM subjects, PD patients show a specific deficit at medium SF, with a distorted PERG SF response function. Consequently, all PD patients show an attenuated PERG TR and 17 of them (85%) have an inverted TR. A significant TR decrease is correlated with the clinical stage of PD. There is a marked TR difference between patients receiving and not receiving L-DOPA. We conclude that stimulus SF is a crucial variable of the PERG in PD. PERG measurements and the derived PERG TR may provide a simple tool to evaluate retinal dopaminergic mechanisms and could contribute to the clinical assessment and monitoring of dopaminergic therapy in PD.

Modality dependent changes in event-related potentials correlate with specific cognitive functions in nondemented patients with Parkinson's disease

The relationship between event-related potentials (ERPs) and cognitive functioning was studied in patients with Parkinson's Disease (PD) but without dementia. Auditory and visual stimuli were used; 30 subjects participated in the auditory study and 20 in the visual study. Patient groups did not differ with respect to gender, age, education, illness duration, and level of cognitive functioning. Visual stimuli were 2.3 cpd sinusoidal grating patterns randomly presented in an oddball paradigm (oblique vs. vertical spatial orientation). Auditory stimuli were tones presented at 70 dB SPL at a rate of 1.1/second, also using the oddball paradigm (1.5K vs. 1K tones). All patients were given neuropsychological tests to measure verbal fluency, memory, visual spatial perception, and abstract reasoning. P300 and N200 abnormalities correlated with a number of these measures, such that longer ERP latencies were related to lower scores on tests of cognitive functioning. Patterns of results suggest that auditory and visual ERPs correlate with different subsets of neuropsychological functions in nondemented PD patients and that N200 may provide a new metric for clinical use.

Spatial frequency tuning of the monkey pattern ERG depends on D2 receptor-linked action of dopamine

The pattern electroretinogram (PERG) has been previously shown to be sensitive to dopaminergic manipulations in the monkey's retina. In order to study the role of retinal D2 receptors were recorded the PERG before and during the acute administration of l-sulpiride, a selective D2 blocker, in three monkeys. The stimuli were sinusoidal vertical gratings, with a contrast of 70% counterphase modulated at 7.5 Hz. The response to four different spatial frequencies (0.5, 1.1, 2.3 and 4.6 c/deg) was explored. PERGs were recorded before and after 20 min of i.m. administration of l-sulpiride. Two different doses (0.07 and 0.35 mg/kg) were administered in different sessions for each spatial frequency (SF). Baseline (before sulpiride) responses showed high intersession reproducibility, with a clear SF tuning. Both doses of the drug affected the PERG to the peak SF of the stimulus, but the higher one was more consistently effective in all of the three monkeys. Our results confirm previous studies which suggested that DA is involved in retinal processing in the primate and reveal the new information that D2 receptors are necessary for spatio-temporal tuning of pattern vision.

Visual discrimination and P300 are affected in parallel by cholinergic agents in the behaving monkey

We studied the acute effect of cholinergic agents, scopolamine alone and scopolamine pretreatment and acetyl-L-carnitine (ALC) on visual event-related potentials (ERPs) in a monkey performing in a go/no-go visual oddball discrimination task. The same monkey was studied repeatedly for a period extending over 1.5 years. Scopolamine is known to cause cognitive impairment in primates. In the concentrations used, primary evoked potentials did not change significantly whereas P300 latency increased maximally 20-40 min following scopolamine administration. Acetyl-L-carnitine on its own increased P300 amplitude and decreased its latency. When ALC administration was preceded by scopolamine, both P300 latency and amplitude maximally increased in 25-45 min. Apparently ALC's effect on the latency and amplitude of visual P300 of the monkey is largely via muscarinic mechanisms. Our results suggest that scopolamine may provide a valuable tool to separate cognitive from primary visual processes.

Visual and visual perceptual disorders in neurodegenerative diseases

Optic neuropathy is reported to occur in 10-30% of patients, while retinopathy is characteristic of type III OPCA. The use of visual and visual electrophysiological tests in early diagnosis and classification is promising. The relationship between visual impairment and other manifestations of these diseases has not been explored.

Retinal periphlebitis in multiple sclerosis

BACKGROUND

The ocular and visual manifestations of multiple sclerosis are varied. The most uncommon is sheathing of the veins, periphlebitis.

METHODS

A 39-year-old white female presented with primary visual complaints of contrast problems on a computer terminal. The patient was lost to follow up for 4 years, however, upon return a clinical diagnosis of multiple sclerosis was confirmed. Six years after the initial presentation, ophthalmoscopic exam of both eyes revealed marked focal sheathing of the retinal veins in the periphery, and associated hemorrhages in the right eye.

RESULTS

During follow-up, the left eye became further involved developing hemorrhages and a retinal hole, which was treated. The visual consequence of the multiple sclerosis as a result of further exacerbations were bilateral centrocecal scotomas and midly reduced visual acuity.

CONCLUSIONS

Multiple sclerosis should be considered in the differential diagnosis of patients with retinal periphlebitis.

The effect of levo-acetyl-carnitine on visual cognitive evoked potentials in the behaving monkey

We studied acute and chronic effects of levo-acetyl-carnitine (LAC) on event-related potentials (ERPs) in 3 monkeys trained in a "go"/"no-go" visual "oddball" discrimination task. The stimuli were 2.5 cpd sinusoidal gratings differing in their respective orientation only (0 degrees or 45 degrees). Each monkey was trained to release a lever during a prespecified time window. Target stimulus presentation probabilities were between 0.25 and 0.5. ERPs had comparable mean latencies and amplitudes in all monkeys. Primary evoked potentials recorded to either the target or non-target stimulus did not change significantly as a result of LAC treatment. On the other hand, P300 latency decreased following LAC administration, with a maximum occurring in 15-20 min. The major effects of LAC were consistent within each animal and for all three of them.

Sensory evoked potentials: PERG, VEP, and SEP

The review summarizes current developments in pattern electroretinography practice and theory: generator sources and clinical applications. This is particularly relevant to neuronal disease of the central nervous system (CNS). Visual evoked potentials (VEPs) are discussed in reference to the evolution and follow up of multiple sclerosis (MS) and in conjunction with magnetic resonance imaging (MRI) demonstrated abnormalities. These reveal that the techniques are complementary. Sensory evoked potentials (SEPs) in conjunction with MRI studies are highlighted as quantitatively comparable in spinal cord disorders.

A short latency cortical component of the foveal VEP is revealed by hemifield stimulation

Transient evoked potentials were recorded simultaneously over 5 electrodes placed in a horizontal row across the occiput. A range of spatial frequencies were presented as either full-field or hemifield stimuli. Subjects were 11 normal observers and 5 patients with lesions causing a homonymous hemianopic field defect. The shortest latency peak response was at approximately 70 msec, a negative potential (N70). For all spatial frequencies, full-field stimuli evoked a lower amplitude N70 at the midline than the sum of N70 amplitudes to two hemifield stimuli, suggesting partial cancellation. The latency and amplitude of N70 increased as spatial frequency increased. N70 and P100 differed in respect to their response to spatial frequency and field size, further suggesting that they may not be subsets of a unitary response. For hemifield stimulation, N70 had an ipsilateral maximum and attenuated or completely reversed in polarity across the midline. Consistent with the data of normals using hemifield stimuli, in 5 patients a full-field stimulus elicited an N70 lateralized contralaterally to the homonymous hemianopia, i.e., the ipsilateral N70 was absent. The absolute amplitude difference between the left and right electrodes was significant for hemifield stimulation in normals and full-field stimulation in the patients, but not for full-field stimulation in normals. Our results imply that the evaluation of N70 hemispheric distribution is useful for the evaluation of paramacular visual field defects.