Shaping the visual system: cortical and subcortical plasticity in the intact and the lesioned brain

Visual system is endowed with an incredibly complex organization composed of multiple visual pathway affording both hierarchical and parallel processing. Even if most of the visual information is conveyed by the retina to the lateral geniculate nucleus of the thalamus and then to primary visual cortex, a wealth of alternative subcortical pathways is present. This complex organization is experience dependent and retains plastic properties throughout the lifespan enabling the system with a continuous update of its functions in response to variable external needs. Changes can be induced by several factors including learning and experience but can also be promoted by the use non-invasive brain stimulation techniques. Furthermore, besides the astonishing ability of our visual system to spontaneously reorganize after injuries, we now know that the exposure to specific rehabilitative training can produce not only important functional modifications but also long-lasting changes within cortical and subcortical structures. The present review aims to update and address the current state of the art on these topics gathering studies that reported relevant modifications of visual functioning together with plastic changes within cortical and subcortical structures both in the healthy and in the lesioned visual system.

Vision restoration after brain and retina damage: the "residual vision activation theory"

Vision loss after retinal or cerebral visual injury (CVI) was long considered to be irreversible. However, there is considerable potential for vision restoration and recovery even in adulthood. Here, we propose the "residual vision activation theory" of how visual functions can be reactivated and restored. CVI is usually not complete, but some structures are typically spared by the damage. They include (i) areas of partial damage at the visual field border, (ii) "islands" of surviving tissue inside the blind field, (iii) extrastriate pathways unaffected by the damage, and (iv) downstream, higher-level neuronal networks. However, residual structures have a triple handicap to be fully functional: (i) fewer neurons, (ii) lack of sufficient attentional resources because of the dominant intact hemisphere caused by excitation/inhibition dysbalance, and (iii) disturbance in their temporal processing. Because of this resulting activation loss, residual structures are unable to contribute much to everyday vision, and their "non-use" further impairs synaptic strength. However, residual structures can be reactivated by engaging them in repetitive stimulation by different means: (i) visual experience, (ii) visual training, or (iii) noninvasive electrical brain current stimulation. These methods lead to strengthening of synaptic transmission and synchronization of partially damaged structures (within-systems plasticity) and downstream neuronal networks (network plasticity). Just as in normal perceptual learning, synaptic plasticity can improve vision and lead to vision restoration. This can be induced at any time after the lesion, at all ages and in all types of visual field impairments after retinal or brain damage (stroke, neurotrauma, glaucoma, amblyopia, age-related macular degeneration). If and to what extent vision restoration can be achieved is a function of the amount of residual tissue and its activation state. However, sustained improvements require repetitive stimulation which, depending on the method, may take days (noninvasive brain stimulation) or months (behavioral training). By becoming again engaged in everyday vision, (re)activation of areas of residual vision outlasts the stimulation period, thus contributing to lasting vision restoration and improvements in quality of life.

Perimetry while moving the eyes: implications for the variability of visual field defects

BACKGROUND

In standard perimetry, subjects fixate so that saccades are reduced and testing precision is increased. However, because vision in daily life requires eye movements, it is appropriate to assess visual fields during eye movement.

METHODS

Perimetry was carried out in 8 healthy subjects and in 16 patients with visual field defects under conditions of a stable and moving fixation spot. Eye movements were simultaneously recorded with an eye tracker. Outcome measures included stimulus detection, variability of visual field border, and saccade amplitudes.

RESULTS

Perimetric performance during stable fixation was comparable to that during eye movement. All subjects showed 92%-96% correct detections of the fixation controls and a stable and comparable blind spot position in the stable and moving fixation spot conditions. The eye tracker revealed that 97% of the time the eyes were positioned within +/-1 from fixation.

CONCLUSIONS

Visual fields obtained by perimetry while moving the eyes is comparable to standard perimetry in which a stable fixation spot minimizes eye movements.

Kompensatorische und restitutive Methoden des Gesichtsfeldtrainings

Gesichtsfelddefekte sind eine häufige Folge von Hirnschäden. Es existieren zwei Behandlungsansätze: 1. Kompensation durch Spiegelbrillen, Prismen, sakkadische Augenbewegungen, exzentrisches Lesetraining und 2. Gesichtsfeldvergrößerung durch ein spezielles visuelles Restitutionstraining. Spiegelbrillen und Prismen wurden vielfach kritisiert, tauchen aber auch in aktuellen Studien immer wieder auf. Kompensation durch ein Training sakkadischer Augenbewegungen in den blinden Bereich führt rasch zu Alltagsverbesserungen und ist als Methode weitgehend unumstritten. Für ein exzentrisches Lesetraining wurde aktuell ein computergestütztes System entwickelt. Erste Forschungsergebnisse des Restitutionstrainings wurden positiv bewertet, seit der Jahrtausendwende tauchte jedoch zunehmende Kritik auf – insbesondere bezüglich mangelnder Fixationskontrolle. Der Artikel setzt sich mit diesen Kritikpunkten auseinander und gibt einen Überblick über aktuelle Forschungsergebnisse, die zeigen, dass eine Gesichtsfelderweiterung zwar klein, aber durchaus möglich ist. Alternativerklärungen wie z. B. sakkadische Augenbewegungen und Veränderungen der Aufmerksamkeit konnten in neuen Studien weitgehend ausgeräumt werden. Hinsichtlich der Fahreignung können offenbar viele Patienten den Defekt gut kompensieren; Studien hierzu sind aber widersprüchlich. Empfohlen wird die bessere Verquickung von restitutiven und kompensatorischen Behandlungsstrategien, um Patienten optimale Hilfe zu geben.

Computer-based training of stimulus detection improves color and simple pattern recognition in the defective field of hemianopic subjects

In a previously conducted randomized placebo-controlled trial, we were able to demonstrate significant visual field enlargement induced by restitution therapy in patients with cerebral lesions [Kasten, E., Wuest, S., Behrens-Bamann, W., & Sabel, B. A. (1998c). Computer-based training for the treatment of partial blindness. Nature Medicine, 4, 1083-1087.]. Visual field training was performed on a computer monitor for 1 hr per day over a period of 6 months. Since the procedure included only stimulation with white light, in the present study we investigated if this simple detection training had a transfer effect on color or form recognition in the trained area (i.e., in the absence of modality specific training). Answering this question would be crucial for planning optimal restitution therapy: In case there is no transfer of training effects to other visual modalities, a specific treatment of each visual function must be performed in order to achieve maximum benefit. Therefore, we analyzed the data from 32 patients with visual field defects who had participated in the original trial and whose form and color recognition had been investigated. The experimental group (n = 19, restitution training) experienced not only an increase of 12.8% correctly detected stimuli (PeriMa program, p <.05), but also an improvement of 5.6% in pattern recognition (PeriForm) and of 6.1% in color perception (PeriColor), respectively. In contrast, the placebo group (n = 13, fixation training) showed no significant changes from baseline to final outcome in any of the visual modalities (PeriMa: 0.3%; PeriForm: -0.3%; PeriColor: 0.4%). Conventional perimetry yielded an increase of 7.8% detected stimuli in the experimental group, but only of 1.2% in the placebo group (p <.05). For form recognition and color perception, the differences between the results of the experimental and the placebo groups narrowly missed significance. However, correlations of diagnostic results showed that mainly those patients who had achieved visual field enlargement also improved in color and form perception: r =.67 (p <.05) between PeriMa and PeriForm and r =.32 between PeriMa and PeriColor. We conclude that visual restitution training using a simple white light stimulus has at least some influence on improving other visual functions such as color and pattern recognition. This result supports the "bottleneck theory" of visual restitution, i.e., training effects can be explained as a process of perceptual learning and increased processing of information by residual structures surviving lesions of the primary visual pathways.