Visual evoked potentials in parkinsonism and dopamine blockade reveal a stimulus-dependent dopamine function in humans

Effect of dopamine and acetylcholine on the visual evoked potential

Visual evoked potentials were measured on patients with Parkinson's disease and Alzheimer's disease and normal controls to assess the function of dopamine and acetylcholine in the visual system. Dopamine is a neurotransmitter known to be present in the retina of primates and is found to be severely depleted in the substantia nigra of patients with Parkinson's disease. Acetylcholine is also known to be present in the retina, visual cortex, and superior colliculus and is found to be grossly reduced in patients with Alzheimer's disease. Stimuli were designed to preferentially activate functionally separate pathways in the visual system described as magnocellular and parvocellular. The four stimuli were a diffuse flash; an achromatic, 73' check counterphasing at 6 Hz at a contrast of 30%; an achromatic 10' check counterphasing at 2 Hz at a contrast of 85%; and an isoluminant red/green grating of 4 cpd presented using an on and off cosine ramp of 200 ms. The results indicate that an acetylcholine deficit produces a delay to the flash P2 component of the visual evoked potential. No change was detected when other stimuli were used.

Color VEPs in Parkinson's disease

Evoked potential studies have confirmed visual pathway impairment in Parkinson's disease. Dopamine is also known to be involved in retinal color vision mechanisms. In this study, pattern evoked potentials were recorded in 20 parkinsonian patients in "on" and "off" conditions to compare the sensitivity of black-and-white and color pattern stimuli. Evoked responses to colored patterns proved more sensitive to L-DOPA therapy. This finding supports the proposed dopamine modulation of the retinal color system and suggests that color pattern evoked potential studies might be used in monitoring dopamine therapy in parkinsonian patients.

Event-related potentials associated with covert orientation of visual attention in Parkinson's disease

Event-related potentials (ERPs) were measured in a group of patients with idiopathic Parkinson's disease and compared with a matched control group during a task involving covert orientation of visual attention. Central warning cues directed attention to the probable location of a lateralized signal that required a button-press response. Parkinson patients had enhanced post-cue P1 (at Oz) and P2 (at Cz) amplitudes, delayed N1 (at Oz) latencies, and diminished CNV amplitudes. Post-target amplitudes were similar to controls, except for an enhanced P1 to invalidly cued targets, and delayed N1 and P3 target latencies. These results indicate that Parkinson patients, in addition to their motor deficits, process spatial cues more effortfully and slowly, have impaired response preparation, and process imperative stimuli more slowly.

Dopaminergic modulation of visual sensitivity in man

A large body of experimental evidence supports the hypothesis that dopamine is a functional neuromodulator at many levels of the visual system. Intrinsic dopaminergic neurons were characterized in most mammalian retina, including man. These neurons give rise to a dendritic plexus covering the retina. Thus, dopamine seems to be involved in the organization of the ganglion cell and the bipolar cell receptive fields and modulates physiological activity of photoreceptors, both processes which underlie sensitivity and spatial selectivity of visual processing in the early stage of the visual system. Moreover, few data are now available concerning the functional significance of dopaminergic modulation of visual sensitivity in man. Parkinson's disease is a specific disorder of central dopaminergic systems. Abnormalities in the pattern-evoked potentials and electroretinogram have been found in parkinsonian patients. Contrast sensitivity, a useful tool for measuring visual spatio-temporal sensitivity in man, has also been shown to be modified due to this affection. Dynamic contrast sensitivity is primarily decreased in these patients, distinguishing them from the normal aging process. Because these modifications in shape of the contrast sensitivity function are reversed by L-Dopa, and that neuroleptic administration could reproduce them in schizophrenia patients, it was suggested that dopamine might tune the contrast sensitivity function in man. We have recently shown that subcutaneous apomorphine induces changes in contrast sensitivity in healthy volunteers, which preferentially affect motion sensitivity. These dopaminergic sensitive modifications in the shape of the contrast sensitivity function might reflect a change in the range of sensitivity of the visual system, both in dynamic and spatial properties. This could be explained by a modification in the spatial and dynamic properties of the ganglion cell responses in the retina. Moreover, we suggest both from our results and from the review of the literature that human psychophysical data confirm the hypothesis that dopamine may be involved in light retinal adaptation, as light-induced and dopamine-induced modifications in the shape in the contrast sensitivity function are quite similar.

Physiological roles of dopamine and neuropeptides in the retina

The retina is a highly complex nervous tissue that converts light into patterns of electrical action potentials in order to process visual information. To carry out its function as a transducer and processor of visual information, the retina must be structurally and biochemically organized to send a coherent signal to the visual areas of the brain. In recent years, a number of biologically active substances have been demonstrated to be located within neurons in the retina. Most of them are thought to be involved in the modulation of the signal and its transmission to the brain through the optic nerve. The present paper attempts to summarize the immunocytochemical distribution and physiology of some neuronally localized substances in the mammalian retina, namely dopamine and neuropeptides.

Acutely administered haloperidol has little effect on steady-state visual evoked potentials from pattern-reversal stimulations in treated schizophrenics

The amplitude changes in steady-state visual evoked potentials (VEPs) from pattern-reversal stimulations at 30 min after an intramuscular injection of haloperidol were examined in 10 treated schizophrenics. The VEP amplitudes to hemi-field or full-field pattern-reversal stimulations with a standard check size were almost unchanged after the haloperidol injection as compared with those before the injection. However, while the VEP amplitudes to the full-field stimulations were significantly higher in the midline occipital portion than in the right occipital portion before the injection, no significant difference was observed after the injection. Further, the VEPs similarly responded to the change in the check sizes used in the full-field stimulations before and after the injection, both showing significantly lower amplitudes only at the large check size of 4.1 degrees in a visual angle than that of 1.0 degrees. These results indicate that acutely administered haloperidol has little effect on steady-state VEPs from pattern-reversal stimulations in treated schizophrenics.

Low contrast stimuli enhance PERG sensitivity to the visual dysfunction in Parkinson's disease

The pattern electroretinogram (PERG) was recorded at different contrast levels (96%, 71%, 47%) in 10 Parkinson's disease patients before and during dopaminergic monotherapy. The data were compared to a control group of 8 normal subjects recorded with the same procedure. PERG P50 latency progressively increased as contrast was decreased both in normal subjects and patients; however, this trend was much more pronounced in PD patients without therapy; consequently in this group the difference between P50 latency obtained with 96% and 47% contrast was statistically significant (P = 0.01, analysis of variance corrected by post-hoc Tukey test). By contrast this was not seen in the control group. Statistical analysis (Bonferroni's t test) showed at the 47% contrast level a significant P50 latency increase (P less than 0.01) in PD patients without therapy if compared with the control group. Dopaminergic monotherapy induced a P50 latency recovery in PD patients. We conclude that low contrast stimuli enhance PERG sensitivity to the visual dysfunction of PD patients. Moreover, the effects observed after therapy confirm that abnormal contrast response functions in PD patients are linked to dopaminergic deficiency.

Visual evoked cortical responses and electroretinograms following implantation of human fetal mesencephalon to the right caudate nucleus in Parkinson's disease

Pattern electroretinograms and visual evoked cortical responses following flash and checkerboard pattern reversal stimulation were performed in 7 patients with advanced Parkinson's disease before and after implantation of human fetal ventral mesencephalon to the head of the right caudate nucleus. Six to nine months following surgery there was bilateral enhancement of flash-evoked response amplitudes. Electroretinogram measurements were unchanged. Individual increases in amplitude and decreases in latency were seen following pattern stimulation which were not statistically significant. A relationship between change in dyskinesia, and change in checkerboard pattern stimulation amplitude at 2.4 cycles per degree suggests that amplitude effects at this stimulus frequency are mediated via altered dopaminergic receptor status in the grafted hemisphere. This may be relevant to the mechanisms leading to generalised clinical improvements following this grafting technique.

Mapped distribution of pattern reversal VEPs to central field and lateral half-field stimuli of different spatial frequencies

Visual evoked potentials (VEPs) to pattern reversal vertical bar stimuli of 3 different sizes (1, 2, 4 c/deg) were recorded from 19 scalp derivations in 50 controls. The stimuli were presented on a full-field (FF) screen of 24 degrees visual angle, and on left and right half-fields (HF) of 12 degrees radius. In 15 controls partial HF stimuli were presented on the central 3 and 6 degrees and as hemiannular stimuli of 12 degrees with occlusion of the central 3 and 6 degrees. An antero-posterior polarity reversal of the N1-P1-N2 sequence was observed for FF VEPs. A tangential polarity reversal was observed for HF VEPs. Also with central or hemiannular stimuli polarity reversals of all VEP components were observed within the scalp. Variants of VEP distribution, absence or prominence of some of the ipsi- or contralateral VEP components were observed in 8-40% of controls. The FF and HF VEP distribution, and the variant VEP asymmetries were partly dependent on the pattern spatial frequency.

Visual dysfunction in treated schizophrenia suggested by visual evoked potentials from pattern-reversal stimulation

Steady-state visual evoked potentials (steady-state VEPs) from pattern-reversal stimulations were compared in treated schizophrenic patients and normal subjects matched for sex and age. The VEP amplitudes were more variable in the patients than in the controls. Furthermore, the VEP amplitudes of the patients mostly showed little or no change when the check size was varied, in contrast to the controls who showed a marked check size effect. These results suggest that schizophrenics receiving drugs have dysfunction of the visual system, especially an inability to respond adequately to changes of visual information.

Attenuation of the early anterior negativity of median nerve somatosensory evoked potential in the MPTP-treated monkey

Median nerve somatosensory evoked potentials (SEP) were recorded in 7 Cynomolgus monkeys, before and after the administration of N-Methyl 1,4 Phenyl 1,2,3,6 tetrahydropiridine (MPTP), a neurotoxin which induces a parkinsonian syndrome in primates. Following MPTP administration, the amplitude of the negative component recorded at 15 ms over the frontal derivations (N15) decreased by 70% or more. This amplitude reduction was not modified by administration of dopamine precursors. These findings shed light on recent findings in human parkinsonian patients.

Effects of chlorpromazine on pattern and flash ERGs and VEPs compared to oxazepam and to placebo in normal subjects

Antidopaminergic drugs delay the pattern-reversal VEP (P-VEP) and the flash VEP (F-VEP) and, in separate studies, reductions in the amplitude and increases in the latencies of scotopic ERGs have been reported. This study investigated the effects of chlorpromazine (CPZ) on the pattern ERG (P-ERG), P-VEP, flash ERGs and VEPs and oscillatory potentials (OPs). Normal volunteers (N = 15) were administered a placebo, or a single dose of CPZ 100 mg or oxazepam (OZP) 15 mg at weekly intervals, in a double-blind crossover design. A gold foil-ipsilateral ear derivation and an Oz'-Fz derivation were used for the ERG and VEP recordings, respectively. The latencies of 'mixed' and cone ERGs were significantly prolonged after CPZ compared to both placebo and to OZP. Amplitudes of rod- and cone-dominated ERGs were reduced following CPZ administration. All components of the OPs were significantly delayed after CPZ administration. No significant intertreatment differences were found in the F-VEP results. The P-ERG P50 peak and the P-VEP N70 and P100 peaks were significantly delayed after CPZ in the case of 28' checks but not 55' checks. Retinocortical times and P-ERG and P-VEP amplitudes were not significantly affected. In contrast to CPZ, the administration of OZP had virtually no significant effects compared to placebo. These findings suggest that the antidopaminergic CPZ has a primary effect on retinal electrophysiology. Similar findings have been reported in Parkinson's disease and in animal models.

Visual evoked potentials in Alzheimer's disease: correlations with age and severity

Flash and pattern reversal visually evoked potentials were recorded in 25 patients fulfilling NINCDS-ADRDA criteria for 'probable' Alzheimer's disease (AD). Results were compared with those obtained from groups of normal elderly subjects and patients with mild memory impairment not amounting to dementia. The P2 component of the flash VEP was significantly delayed in AD patients of 74 years or less and correlated with severity whereas the P100 component of the pattern reversal VEP remained relatively unchanged. The latency difference between the P2 and the P100 proved to be a valid discriminator between demented and non-demented subjects only in the younger age group. This limits its usefulness as a diagnostic test of dementia in patients over the age of 75.

Visual deficits related to dopamine deficiency in experimental animals and Parkinson's disease patients

In patients affected by Parkinson's disease, and in the monkey model of this disease, visual defects have been shown using psychophysical and electrophysiological measures of spatial and temporal contrast sensitivity. These studies imply an essential role for dopamine in primate vision. There is electrophysiological and neurochemical evidence to suggest that at least part of the problem is impaired retinal processing caused by systemic dopaminergic deficiency. Some of the deficits that have been demonstrated, consistent with physiological studies, suggest that center-surround interaction of neurons may suffer as a consequence of dopaminergic deficiency. The role of the regulation of retinal dopamine (D1 and D2) receptors in primate vision and of the balance of these receptors in presynaptic dopaminergic deficiency is not yet determined. Using sinusoidal grating stimuli in cognitively loaded tasks may increase understanding of the behavioral consequences of visual deficits seen in dopamine deficiency syndromes.

Visuospatial orientation in Parkinson's disease

Visuospatial functioning in patients with Parkinson's disease was investigated using neuropsychological measures of basic visual perception, complex perceptual discrimination, and spatial orientation. Three subgroups of patients were described: (a) those with broadly impaired visuospatial abilities, (b) those with generally intact abilities, and (c) those whose performance on a task of spatial orientation was lower than their performance on a task of complex perceptual discrimination. These subgroup differences were also concordant with three other variables: age, duration of disease, and degree of dementia. It is suggested that decreases in spatial orientation functioning in Parkinson's disease may reflect the speed of progression of this disease.

Simultaneous VEP and PERG investigations in early Parkinson's disease

To evaluate whether visual evoked potential (VEP) and pattern electroretinogram (PERG) abnormalities may be detectable early in the course of Parkinson's disease (PD) and to assess a possible interdependence between retinal and cortical potentials, simultaneous VEP and PERG were carried out in nine patients with early PD. The mean (SD) duration of the disease was 10 (6) months. None of the patients had been previously treated with levodopa or other anti-parkinsonian drugs. The results show VEP and PERG abnormalities that are spatial stimulus dependent, with higher frequencies being more involved and also indicate that VEP changes are not entirely dependent on alterations at the retinal level.

Dopaminergic pharmacological manipulations in normal humans confirm the specificity of the visual (PERG-VEP) and cognitive (P300) electrophysiological alterations in Parkinson's disease

Retinal and occipital visual evoked potentials and event-related potentials (P300) have been recorded in normal human subjects before and after the administration of the dopaminergic receptor antagonist, haloperidol, and/or the dopaminergic precursor L-DOPA. The data show that either retinal or occipital visual potentials and P300 are delayed by haloperidol. These findings are consistent with the hypothesis that haloperidol in healthy subjects mimicks the electrophysiological abnormalities observed in Parkinson's disease. On the other hand, L-DOPA does not generally modify these latencies in normals, while it is known to decrease the same parameters in parkinsonian patients. This is in accord with the involvement of a specific mechanism in the recovery observed in parkinsonian patients after L-DOPA therapy. Our data confirm that the alterations of visual and cognitive potentials observed in Parkinson's disease are closely related to the impairment of dopaminergic transmission.

Parkinson's disease: a conceptualization of neuropsychological deficits within an information-processing framework

Parkinson's disease patients are frequently impaired in a variety of neuropsychological tasks involving memory, perceptual motor performance and cognitive flexibility. Although various theories have been forwarded to explain specific impairments, few attempts have been made to account for all the deficits within a single theoretical framework. Furthermore, the frequent occurrence of dementia, the adverse side effects of medication and the motor symptoms of the disease tend to interfere with neuropsychological performance. This has hindered the clear delineation of the neuropsychological profile of Parkinsonian patients. The present paper examines the evidence for intellectual deficits in non-demented Parkinson's disease patients. A number of problems inherent in cognitive research on Parkinson's disease are discussed. The contribution of dementia, motor symptom severity and medication in the expression of these intellectual impairments is examined. It is suggested that many of the neuropsychological deficits described in Parkinson's disease may result from a common underlying deficit in some aspect of information processing. Although it is not possible to pinpoint the precise mechanism(s) involved on the basis of the available evidence, several possibilities are suggested by cognitive and electrophysiological data.

The effect of intraocular 6-hydroxydopamine on retinal processing of primates

In previous studies, we showed that in the monkey, systemically administered N-methyl, 4-phenyl, 1-2-3-6 tetrahydropyridine (MPTP) produces a chronic parkinsonian syndrome accompanied by spatial frequency-dependent abnormalities in both the pattern electroretinogram and visual evoked potential. We describe the effect of intravitreally administered 6-hydroxydopamine (6-OH-DA) on the pattern electroretinogram and pattern visual evoked potential of 3 aphakic monkeys. Because of the aphake condition, several complexities of intravitreal injection of 6-OH-DA could be avoided. Nevertheless, following 6-OH-DA treatment, both the phase and the amplitude of pattern electroretinogram and pattern visual evoked potential became abnormal. This abnormality was most pronounced for the higher spatial frequencies (2.5 and 3.5 cycles per degree), whereas lower spatial frequencies (0.5 and 1.2 cycles per degree) were less impaired. The effects of systemically administered MPTP on pattern electroretinogram and pattern visual evoked potential are similar to the effects of intravitreal injections of 6-OH-DA, suggesting that a retinal catecholaminergic system plays an important role in pattern vision of primates.

Short-latency somatosensory and brainstem auditory evoked potentials in patients with Parkinson's disease

Short-latency somatosensory evoked potentials (SSEPs) and brainstem auditory evoked potentials (BAEPs) were recorded in 44 patients with Parkinson's disease (mean age 67.3 years) and 23 normal subjects (mean age 69.3 years). Patients with Parkinson's disease and normal subjects did not show any significant difference with regard to the interpeak latencies between N13 and N20 central conduction time (CCTs). Likewise, there were no significant differences in CCTs between patients with and without dementia. The interpeak latencies between waves I and V (I-V IPLs) in patients with Parkinson's disease were significantly longer than those of the normal subjects (p less than 0.05). In particular, patients with dementia revealed significant prolongation of I-V IPLs compared to patients without dementia and normal subjects (p less than 0.01, p less than 0.001) although no significant differences were observed between patients without dementia and normal subjects. These results show that auditory brainstem pathways are involved in Parkinson's disease patients with dementia.

Associated central and peripheral demyelination: an electrophysiological study

A case is reported in which retrobulbar neuritis preceded Guillain-Barré syndrome by 4 weeks. The visual evoked potential latencies were prolonged. After peripheral nervous system signs had cleared, median and peroneal somatosensory evoked potentials showed prolonged cervical N13, scalp N20 and L3-scalp conduction times.

Systemic 1-methyl,4-phenyl,1-2-3-6-tetrahydropyridine (MPTP) administration decreases retinal dopamine content in primates

Following MPTP administration, 4 Cynomolgus monkeys developed a parkinsonian syndrome, accompanied by specific changes of both pattern visual evoked potential and electroretinogram. Retinal dopamine and dihydroxyphenylacetic acid contents were measured in the 4 MPTP-treated monkeys and in 3 normal monkeys. Dopamine and dihydroxyphenylacetic acid levels were significantly lower in the retinas of the MPTP-treated animals (p less than 0.001), suggesting that dopamine has a specific function in the visual system of primates.