When asked about vision (Lee and Harris, 1999) PD patients report problems 1) judging distance and motion in the street, 2) reaching for objects, and 3) moving through narrow spaces within their homes. ''It's not as simple as it looks ... I am still going, but I have run out of space to move in ...my space, our space, is nothing like your space: our space gets bigger and smaller...'' (from a parkinsonian patient, in Sacks, 1990).
Primary visual processes in PD have been summarized in recent reviews (Bodis-Wollner, 2003). Dopaminergic neurons control the preganglionic organization of the receptive field of ganglion cells (Fig. 1). The PD specific preganglionic retinopathy, shown in MPTP monkeys, is quantified with the ERG which can be also performed in patients. The consequence of dopaminergic deficiency of the human retina is apparently retinal nerve fiber (axons of the retinal ganglion cells) thinning. This can be clinically seen and quantified in PD patients. Retinal dopaminergic deficiency leads to specific loss of spatial contrast sen-
Fig. 1. Left: The network of the mammalian retina consists of five different classes of neurons arranged in different layers: photoreceptors (rods R and cones C), horizontal cells (H), bipolar cells of different classes (invaginating midget bipolars 1MB, flat midget bipolars FMB, invaginating diffuse bipolars DB, and rod bipolars RB), amacrine cells (A), and ganglion cells (falling into two main classes, midget ganglion cells MG, and parasol ganglion cells P). An orderly subset of amacrine cells are dopaminergic. The relative proportion of DA amacrine cells is higher in the fovea. There are two sizes of DA amacrines: large and small (not shown). Right: Simplified schema of the D1-D2 interaction in the retina. The D1 dopamine pathway locally enhances the "surround" signal for individual ganglion cells, while the D2 pathway enhances the "center" signal by coupling photoreceptors. Experimental results suggest that these two dopamine pathways are not independent from each other. Consistent with the results we postulate that D2 pre-synaptic receptors are located in the D1 pathway. Thus D2 ligands postsynaptically amplify the "center" response while presynaptically attenuate the "surround". The effect is functionally synergistic. Conversely postsynaptic D2 blockade attenuates the center response and pre-synaptic blockade enhances the surround. Thus ganglion cells with large surrounds are particularly affected by D2 blockade while neurons with small centers are easily amplified by D2 agonists. Conversely D1 agonists have a larger effect on large ganglion cells' and negligible effect on small ganglion cells' receptive fields (after Bodis-
Wollner, 1990; Bodis-Wollner and Tzelepi, 2002)
sitivity (CS). CS, a measure of contrast vision apart from visual acuity and color vision worsens with progressive motor impairment.
Many tests of visual-spatial deficits show sensitivity to visual and other cognitive processes. Given the hierarchical nature of the visual system, degraded visual information may cause cognitive visual dysfunction. Difficulty seeing and discriminating visual stimuli could be due to impaired CS and could occur as the result of dopaminergic deficiency of the retina. For goal directed movements PD patients have an increased dependence on visual information compared to someone without the disease, suggesting the behavioural consequence and importance of retinal visual impairment. Yet, contrast sensitivity losses in other diseases which affect only the retina do not cause the kind of visuo-cognitive deficits observed in PD.
We summarize below those select cognitive visual defects in PD which are unlikely to be caused only by primary visual impairment.
Visuospatial deficits should lead to difficulty navigating oneself around one's environment. Space exploration is comprised of tasks such as the ability to scan the environment and to move the body to interact with this environment. Scanning the world is impaired in PD but how scanning problems interact with visuospatial deficits has not been established.
Spatial cognition is the mental manipulation of spatial information, and this requires visual memory and egocentric representation. Oliver Sacks (1990) describes parkinsonian patients who walk on a tilt but perceive their own body and the environment to be perfectly upright. It was shown some thirty years ago by Bowen and colleagues that PD patients show deficits in judging the visual vertical and horizontal, and indeed, advanced PD patients do demonstrate very exaggerated bent postures. However, it has not been established whether the problem is due to visual impairment, to a distorted body centered coordinate system, to a visual working memory deficit, or all of the above. In a simple experimental paradigm of visuo-spatial ability, patients are asked to judge the orientation of lines (Benton). This test is very sensitive to PD; however it may depend on primary visual dysfunction of orientation selective visual neurons.
Dopaminergic mechanisms in the neostria-tum play an important role in categorization. Visual categorization can be quantified with event-related potentials (ERPs) obtained when subjects have to decide whether a briefly presented image contains, for instance, animals or non-animals. In normals, the mean ampli tudes of N1 (150-250 ms) and N2 components (400-600 ms) are more negative for non-animal scenes as compared with stimuli containing animals, whereas P2 (250-350 ms) is more positive for animal scenes. In PD, N1 and N2 components were similar for both animal and non-animal stimuli, and P2 is reduced. Apparently, both perceptual (N1) and semantic (N2) processes related to the categorization of natural scenes are specifically impaired in PD. In the temporal domain however the slowed semantic processing is preceded by a relatively normally paced perceptual analysis. These electro-physiological results are consistent those hypothesis which emphasize the importance of striatal dopaminergic mechanisms in classification functions.
PD patients have difficulty with the mental rotation of objects. This deficit, in the absence of concurrent visual input, is unlikely to depend on primary visual dysfunction. Brown and Marsden (1998) hypothesized that the link between voluntary effort, sensory input and sequencing of motor movements or thought suffers in PD due to a deficient role of the basal ganglia in neuronal binding. Gamma-frequency rhythms of discharge activity from thalamic and cortical neurons are facilitated by cholinergic arousal and resonate in thalamocortical networks. Experimental evidence shows the visual perceptual role of synchronized gamma range brain activity when illusory contours are perceived and when blindfolded subjects execute voluntary saccades. It is probable that gamma recorded over the visual cortices is modulated by pre-frontal attentional mechanisms. The role of gamma oscillatory deficits in many aspects of PD has not been widely researched.
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