The second face of blindness: processing speed deficits in the intact visual field after pre- and post-chiasmatic lesions

Abstract

Purpose: Damage along the visual pathway results in a visual field defect (scotoma), which retinotopically corresponds to the damaged neural tissue. Other parts of the visual field, processed by the uninjured tissue, are considered to be intact. However, perceptual deficits have been observed in the "intact" visual field, but these functional impairments are poorly understood. We now studied temporal processing deficits in the intact visual field of patients with either pre- or post-chiasmatic lesions to better understand the functional consequences of partial blindness.

Methods: Patients with pre- (n = 53) or post-chiasmatic lesions (n = 98) were tested with high resolution perimetry--a method used to map visual fields with supra-threshold light stimuli. Reaction time of detections in the intact visual field was then analyzed as an indicator of processing speed and correlated with features of the visual field defect.

Results: Patients from both groups exhibited processing speed deficits in their presumably "intact" field as indicated by comparison to a normative sample. Further, in both groups processing speed was found to be a function of two factors. Firstly, a spatially restricted (retinotopic) influence of the scotoma was seen in longer reaction times when stimuli were presented in intact field sectors close to the defect. Secondly, patients with larger scotomata had on average longer reaction times in their intact field indicating a more general (non-retinotopic) influence of the scotoma.

Conclusions: Processing speed deficits in the "intact" visual field of patients with visual system damage demonstrate that visual system lesions have more widespread consequences on perception than previously thought. Because dysfunctions of the seeing field are expected to contribute to subjective vision, including visual tests of the presumed "intact" field may help to better understand vision loss and to improve methods of vision restoration and rehabilitation.

Figure 1. Visual field charts of patients with pre-chiasmatic and post-chiasmatic visual system damage.

Detection accuracy of stimuli presented in high resolution perimetry (HRP), an indicator of functional state (defect depth), is denoted by color: black represents absolute defect (0% detection), grey represents areas of residual vision (relative defect; detection >0% and <100%) and white represents intact field (100% detection). Reaction time (RT) is color-coded in RT charts. RT histograms show the distribution of RT in the HRP sectors defined as intact or relative defect.

Figure 2. Reaction time is related to the functional state of the visual field area.

In both groups, pre-chiasmatic and post-chiasmatic, RT depends on the functional state of the visual field sector, as defined by HRP detection accuracy. Processing speed upon detected stimuli was higher in the relative defect regions than in the intact field.

Figure 3. Intact field reaction time is related to the functional state of the immediate surround.

Group analysis (A) and single cases as representative examples (B) are shown. Functional status of the immediate neighborhood, defined as the percent of defective sectors in the 5-degree surround, partially explains within-subject variability of RT in patients with pre-chiasmatic (case 1 and 3) and post-chiasmatic lesions (case 2 and 4). However, between-subject RT variability is still substantial.

Figure 4. Intact field reaction time is related to the size of the scotoma.

In both groups, pre-chiasmatic and post-chiasmatic, a significant correlation was found between the size of the scotoma and the intact field RT. This indicates that subjects with a larger defect field are more impaired in the intact field as well.

Figure 5. Intact field reaction time in quadrantanopia.

RT was calculated for each of the three quadrants with respect to the position of each quadrant in relation to the scotoma: RT in the adjacent quadrant of the same hemisphere (I) was slower than in the adjacent (mirror-symmetric) quadrant of the contralateral hemisphere (II) and the diagonal quadrant (III). The inset is a simplified sketch of a visual field test showing an example of a lower right quadrantanopia to illustrate the position of the three “intact” quadrants. Processing speed in all three intact quadrants was impaired in comparison to healthy controls. Additionally, the intact quadrant located in the same hemispace as the scotoma (I) was found to be more impaired than the two quadrants processed by the uninjured hemisphere (II and III). There was no evidence of symmetric transcallosal connectivity influences, since the quadrant mirror-symmetric to the scotoma (II) did not show more pronounced deficits than the diagonal quadrant (III).