The most striking feature of PSP patients is the fixity of gaze that, resulting from supranuclear ophthalmoplegia, gives the disease its name. However, far from being a purely oculomotor problem, PSP patients' ocular movement difficulties are accompanied by a severe deficit in orienting attention. Rafal (114) has outlined very well the difficulties that PSP patients encounter in everyday life and how these difficulties interact with other components of the disease (gait disorder, dysphagia, and nuchal dystonia). When conversing with others, reaching for objects, eating, or dressing, PSP patients typically tend not to look at what they are doing. This failure to orient spontaneously the lower visual field contributes to gait disequilibrium (the source of falls) and to the ingestion of boluses that are too large to swallow (the source of ab ingestis). Loss of spontaneous social orienting is also a striking feature of PSP and it is kind of unique to this disease. Relatives tend to note this as an early sign and often attribute it to a change of mood or to carelessness.
A further characteristic trait of visuomotor impairment in some PSP patients is, conversely, difficulty in inhibiting orienting responses in situations in which orienting is disadvantageous. One need only think of patients who, as they walk, tend to orient toward the door or a wall mirror or TV set they have just passed, as if they were magnetically attracted to and fixed upon that object in the environment. This visual behavior, also called "visual grasping" by Ghika et al. (115), is associated with repeated backward head movements while walking and is, consequently, a further cause of backward falls (114).
As early as 1981, Kimura et al. (116) demonstrated that PSP patients are impaired in visual search and scanning tasks. Subsequently, Fisk et al. (117) performed systematic observations in an attempt to relate the performances of PSP patients on visual search and scanning tasks to the pattern of their oculomotor deficits. In one of these tasks, subjects were required to scan lines composed of a variable number of dots and dashes and to report either the number of dashes or the number of dots. Scanning in each of the four directions was measured: left to right, right to left, top to bottom, and bottom to top. PSP patients were less accurate than controls only when scanning along the vertical plane. This confirmed that bradykinesia and psychomotor retardation could not account for impaired performance and that some specific defect of visual search and scanning along the vertical plane had to be assumed.
Impaired performance on a visual search task, as well as on the Benton's Judgement of Line Orientation Test (8), was also demonstrated by Soliveri and coworkers, in comparison with both normal controls (118) and PD patients (119). The same authors could not demonstrate any difference on these tasks between PSP and CBD patients (119).
In a detailed study of 25 patients, Esmonde et al. (120) confirmed a severe visuo-perceptual deficit in PSP. They found marked differences between patients and controls in two subtests of the Visual Object and Space Perception Battery (121), (the Fragmented Letter and Cube analysis tests) which were explicitly chosen to minimize the effect of oculomotor scanning on performance.
Subsequently, in a series of experiments, Rafal, Posner, and colleagues (45,122,123) demonstrated that PSP patients are not only slow in scanning and searching when allowed to move their eyes, but also in covertly shifting their visual attention, especially in the vertical plane. However, unlike PD patients, who demonstrate defective voluntary control of covert attention, PSP patients are especially impaired in reflexive orienting.
The authors came to this conclusion after comparing attentional movements from both endogenous and exogenous cues. In both cases the subjects' task was to press a button upon detecting a target. In these tasks, targets are preceded by cues that may orient the attention to the target location (valid cue) or to another location (invalid cue), or may have only an alerting value and provide no spatial information (neutral cue). For testing voluntary control of attention, the (endogenous) cue is typically an arrow in the center of the display, instructing the subject where to expect the forthcoming target. Reflexive orienting is tested by an exogenous cue, e.g., lighting up of the box where the target might appear. In this case, unlike the case of the central cue paradigm, the cue has no predictive value and the target is equally likely to appear in an uncued as in the cued location. Results demonstrated that PSP patients show no "validity effect" on reflexive orienting of attention along the vertical plane: i.e., they are no faster in responding to a target when it is preceded by a valid exogenous cue. A similar trend for smaller orienting effects in the vertical plane is also seen in endogenous orienting but it is not as dramatic as in reflexive orienting. In other words PSP patients have problems in "shifting" attention, especially in the vertical plane and especially in response to exogenous signals.
Kertzman et al. (124) also provided evidence consistent with the conclusion that PSP has little or no effect on endogenous orienting of attention. In their experiment the cue was lighting up of a peripheral box that predicted the location of the forthcoming target. Vertical and horizontal attention movements were tested in separate blocks, thus giving the subjects maximum opportunity to use the cue to shift attention. Under these conditions the PSP patients did not show a smaller effect of cue validity in the vertical plane. This demonstrated that PSP patients' attentional deficit can be contrasted with that of inferior parietal lobe patients. These latter patients are typically impaired in the presence of endogenous cues, in the horizontal plane, and in the invalid cue condition, i.e., when they have to "disengage" attention that was cued by a central signal to the side opposite the target.
Rafal et al. (114) demonstrated quite convincingly that PSP attentional deficit is likely to be a result of the degenerative damage to the superior colliculus and the adjacent tectal nuclei. This part of the midbrain constitutes the phylogenetically older retinotectal pathway, which retains important functions for regulating visual attention and visually guided behavior. The frontal eye fields are also important in controlling saccadic eye movements (125), show dramatic hypometabolism in PSP patients (126), and might also be responsible for visual attention deficits. Nonetheless, patients with lesions restricted to the dorsolateral prefrontal cortex (including the frontal eye fields) perform normally in covert orienting of attention (127), even if they show increased latencies for endogenous saccades to targets contralateral to the lesion (128). Furthermore, converging evidence from normal subjects (129) and from hemianopic patients (130) demonstrates that the midbrain retinotectal pathway is important in reflexive orienting to exogenous signals and corroborate the view that it is decisive in determining the visuomotor and attentional deficit typically found in PSP patients.
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