The Second Face of Blindness: Processing Speed Deficits in the Intact Visual Field after Pre- and Post-Chiasmatic Lesions

Abstract

PurposeDamage 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.MethodsPatients 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.ResultsPatients 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.ConclusionsProcessing 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.

“Sightblind”: Perceptual Deficits in the “Intact” Visual Field

“…At the same time, persistent visual field defect might lead to widespread changes in the brain, e.g., disturbance of synchronization, oscillations, or functional connectivity (e.g., Dai et al, 2012), causing non-specific slowing secondary to the loss of visual input. Further, because intact field deficits were found in both post-chiasmatic and pre-chiasmatic patients (Bola et al, 2013), this suggests that not only cortical lesions but also pre-chiasmatic lesions might cause intact field deficits as well, although we do not know if the mechanisms of this impairment is different. These hypotheses need to be tested in future studies.…”

“…Indeed, in patients with visual field loss the temporal processing speed in the intact field is related to its distance to the scotoma – the closer to the scotoma the stimulus is presented the longer the RT (Bola et al, 2013). We interpret this as a sign of a local, spatially constrained (retinotopic) influence of the scotoma.…”

Remaining Visual Field and Preserved Subjective Visual Functioning Prevent Mental Distress in Patients with Visual Field Defects

“…Instead of using averaged results of monocular visual field tests, binocular visual fields are possibly more strongly related to subjective vision. Also temporal processing deficits in the intact visual field may turn out to be a relevant variable (Bola et al, 2013). Psychological variables, like individual coping strategies or social resources, could be considered as possible confounders (Boerner and Wang, 2012).…”

Disturbed temporal dynamics of brain synchronization in vision loss

“…Specifically, the areas considered to be "intact" based on the perimetry results actually exhibit subtle perceptual deficit in the processing speed (Bola et al, 2013b;Poggel et al, 2011;Cavezian et al, 2010) and contour integration (Paramei and Sabel, 2008;Schadow et al, 2009). These subtle deficits in the "intact" field are "sightblind" (review: Bola et al, 2013a), as they are the flip-side of "blindisight" where subtle residual vision exists deep in the "blind" field. The "sightblind" effect was found in patients with post-chiasmatic damage (e.g.…”