Rehabilitation of reading and visual exploration in visual field disorders: transfer or specificity?

Reading and visual exploration impairments in unilateral homonymous visual field disorders are frequent and disabling consequences of acquired brain injury. Compensatory therapies have been developed, which allow patients to regain sufficient reading and visual exploration performance through systematic oculomotor training. However, it is still unclear whether the reading and visual exploration impairments require specific compensatory training for their improvement. We present the first cross-over rehabilitation study to determine whether the training-related performance improvements are task-specific, or whether there is a transfer of training-related improvements between reading and visual exploration. We compared the therapeutic effects of compensatory oculomotor reading and visual exploration training in 36 patients with unilateral homonymous visual field loss in a cross-over design. In addition, we explored whether the training sequence determines the overall treatment outcome. Our findings demonstrate that the training-related improvements in reading and visual exploration are highly specific and task-dependent, and there was no effect of training sequence.

Line bisection in unilateral homonymous visual field defects.

The contralesional line bisection error in unilateral homonymous hemianopia is a frequent but neglected clinical phenomenon. Our knowledge about this bisection error is based on small samples of hemianopic patients. Moreover, horizontal line bisection has never been investigated in other unilateral visual field defects. The present study is the first to examine line bisection in a large, representative sample of patients with unilateral homonymous visual field defects. We investigated horizontal line bisection in 129 patients with left- or right-sided homonymous hemianopia (60.5%), upper and lower quadranopia (24.8%), and paracentral scotoma (14.7%), and determined the magnitude and direction of line bisection error. The contralesional horizontal line bisection error was present not only in patients with hemianopia but also in those with upper or lower quadranopia or paracentral scotoma. Neither the type nor the severity of the visual field defect was found to determine the bisection error. Only the side of the field defect seemed to determine the horizontal direction of the bisection error (left-/rightward). The contralesional bisection error is not a specifically "hemianopic" phenomenon. It is frequently associated with any unilateral homonymous visual field defect, i.e., hemianopia, upper/lower quadranopia, paracentral scotoma. Moreover, our results further support the recent finding that the contralesional bisection error is not a direct consequence of the visual field defect. Yet, they also suggest that, although the visual field defect does not seem to be the primary cause of the contralesional bisection error, it may nevertheless contribute to it.

What and where in mirror reading.

In a combined voxel-based morphometry and functional magnetic resonance imaging study on the practice of mirror reading, we recently found a shift of activation from right superior parietal to right dorsal occipital cortex and a corresponding increase of gray matter. We interpreted this shift of activation and the corresponding structural changes as a shift from effortful visuospatial transformation to a more direct processing of mirrored words (Ilg et al., 2008). To test this hypothesis, we now analyzed brain activation patterns associated with different aspects of mirror reading. Activation at the dorsal occipital cortex and bilateral parietal cortex (dorsal visual stream) was related to inverse text processing, whereas activation of areas at the inferior and ventral occipitotemporal cortex (ventral visual stream) was associated with decoding of mirrored words. This indicates that the dichotomy of content-related ("what") and process-related ("where") higher visual functions also applies to mirror reading.

On the origin of line bisection error in hemianopia.

The contralesional line bisection error in hemianopia is a well-known clinical phenomenon. Its origin, however, is still unclear. We therefore investigated the causes of the hemianopic bisection error in 84 patients with unilateral homonymous hemianopia without visuospatial neglect. Our results suggest that the contralesional bisection error is neither a consequence of the visual field defect itself nor a manifestation of strategic adaptation of attention and eye movements into contralesional hemispace. Additional extrastriate brain injury, presumably to occipito-temporal areas including the occipital white matter, seems to be critical for the emergence of the contralesional bisection error that is frequently associated with but separable from homonymous hemianopia.

Gray matter increase induced by practice correlates with task-specific activation: a combined functional and morphometric magnetic resonance imaging study.

The neurophysiological basis of practice-induced gray matter increase is unclear. To study the relationship of practice-induced gray matter changes and neural activation, we conducted a combined longitudinal functional and morphometric (voxel-based morphometry) magnetic resonance imaging (MRI) study on mirror reading. Compared with normal reading, mirror reading resulted in an activation of the dorsolateral occipital cortex, medial occipital cortex, superior parietal cortex, medial and dorsolateral prefrontal cortex, as well as anterior insula and cerebellum. Daily practice of 15 min for 2 weeks resulted in an increased performance of mirror reading. After correction for pure performance effects, we found a practice-related decrease of activation at the right superior parietal cortex and increase of activation at the right dorsal occipital cortex. The longitudinal voxel-based morphometry analysis yielded an increase of gray matter in the right dorsolateral occipital cortex that corresponded to the peak of mirror-reading-specific activation. This confirms that short-term gray matter signal increase corresponds to task-specific processing. We speculate that practice-related gray matter signal changes in MRI are primarily related to synaptic remodeling within specific processing areas.