[Dissertation] Effects of homonymous visual field defects on visuo-spatial perception and performance – Full Text PDF

General Abstract

Homonymous visual field defects are typically associated with three distinct types of visual disorders beyond the field cut: (i) a visual exploration or scanning deficit, (ii) reading disturbances (“hemianopic alexia”), and (iii) a contralesional visuospatial bias towards the blind field in localizing the midline position (“hemianopic line bisection error”, HLBE). While the exploration and reading disorder are well explored and their causes often analysed, the origin of the HLBE – although already known for more than 100 years – have remained largely unclear and are still a matter of debate. The present Ph.D. thesis addresses several unresolved issues of the HLBE in three subsequent, already published studies.

First, it was investigated, whether and to what extent patients with homonymous quadrantranopia display a contralesional visuospatial error when indicating the visual midline. Interestingly, in earlier studies the HLBE was almost exclusively found in horizontal (left or right) or vertical (altitudinal) hemianopia. All 15 tested patients with quadranopia showed distinct and large shifts towards their blind quadrant when estimating their visual subjective straight ahead in a bowl perimeter. Moreover, patients with dorsal lesions respectively lower quadrantanopia showed the largest errors.

Second, the matter of eccentric fixation as a possible cause of the HLBE was analysed in this study and in the subsequent study with patients showing horizontal hemianopia by using the technique of perimetric blind spot mapping. The results revealed in both studies that static fixation as measured by the position of the blind spot(s) was completely normal in nearly all subjects and was neither (cor)related to shifts of the visual straight ahead nor the HLBE. In addition, it was found that the capacity to scan the blind field with saccadic eye movements (“saccadic search field”) was not related to the HLBE, thus ruling out visual scanning deficits as a possible cause of the HLBE.

The last issue that was analysed in this thesis was the question of attention in relation to the HLBE. Deficits in line bisection are a frequent finding in patients with visual neglect. Many studies in this context have shown that manipulations of visuospatial General Abstract IV attention, i.e. via attentional cueing to one side of space, significantly modulate the ipsilesional spatial error in patients with visual neglect.

In a similar logic, we evaluated in the second and third study of this thesis, whether attentional cueing to the left or right side of the horizontal line that had to be bisected modulated the HLBE in hemianopic patients. Surprisingly, cueing had virtually no effect on the HLBE in hemianopic subjects while the very same manipulation clearly modulated bisection in a small group of patients with left visual neglect, thus showing the principal efficacy of the attentional manipulation.

In summary, this thesis reports novel evidence of an oblique contralesional spatial error in homonymous quadrantanopia akin to the HLBE in horizontal hemianopia. Moreover, the present results in chronic patients with hemianopia do neither support the notion of eccentric fixation nor of hypo-/hyperattention to one side of space as possible determinants of the HLBE. Furthermore, gross visual exploration deficits do not seem to contribute to the HLBE either. Finally, possible limitations of the present studies are mentioned and alternative theoretical accounts shortly discussed.

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