Abstract
Visual field loss after brain lesions is commonly determined using perimetric tests of light detection (perimetry). Many patients with visual field defects complain about perceptual difficulties in areas that are perimetrically normal. To look at a potential cause for such difficulties, we topographically determined temporal characteristics of visual information processing in those patients and compared them to those of healthy subjects. In nine patients with visual field loss we measured thresholds of double-pulse resolution (DPR), i.e., the minimum perceivable duration of a temporal gap between two light pulses, at eccentricities up to 20°. Furthermore, high-resolution maps of visual reaction times (RT) were obtained in a computer-based campimetric test. Performance was compared to healthy controls from a cross-sectional study of temporal perception across the life span (Toelz Temporal Topography Study). Compared to healthy subjects, DPR thresholds and RTs in patients are elevated in the entire visual field, including areas that are perimetrically intact. Performance on temporal variables depends on the degree of intactness of the respective visual field position. DPR thresholds correlate considerably with RTs, and both parameters increase with eccentricity. However, whereas DPR thresholds are increased around blind regions relative to the intact field, this is not the case for RTs. Temporal processing in patients with cerebral vision loss is impaired to a certain extent independently from perimetric light detection performance. This may partly explain reported subjective perceptual problems. The findings may have important implications for visual rehabilitation, i.e., the potential generalization of light detection training to temporal processing performance.
Highlights
Conventional perimetry of light detection is the method of choice for evaluating the intactness of the visual field (Harrington, 1981)
Many patients with visual field defects complain about difficulties of visual perception that cannot be identified with standard perimetry or other common clinical measures of visual function, e.g. in glaucoma (Horn et al, 1999; Tyler, 1981) and after cortical damage (Castelo-Branco et al, 2006; Poggel, 2002)
The variability of perimetric maps of different psychophysical measures in healthy individuals is not surprising given the high degree of functional specialization within the visual pathway, even at the earliest stages (e.g. between magno- and parvo-cellular systems (Callaway, 2005; DeYoe and Van Essen, 1988) and ventral vs. dorsal pathways beyond the primary visual cortex (Goodale et al, 2005; Mishkin and Ungerleider, 1982))
Summary
Conventional perimetry of light detection is the method of choice for evaluating the intactness of the visual field (Harrington, 1981). Topographical deviations between different visual functions may explain why some forms of visual impairment remain undetected in standard clinical testing (Bachmann and Fahle, 2000; Castelo-Branco et al, 2006). The situation becomes even more complex in patients with damage to the visual system because it is possible that lesions affect only parts of the visual pathway while others remain intact, again resulting in different topographic features of visual field maps (Bachmann and Fahle, 2000; Castelo-Branco et al, 2006)
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