The quest to restore vision long after brain damage in adulthood

It has long been thought that blindness after brain lesions is permanent. Patients who suffer visual field defects as in hemianopia following visual cortex lesions or the more diffuse visual field loss found after optic neuropathy have had no real options to be treated. Some clinicians trained patients to better explore the missing areas of their visual field by compensatory (scanning) eye movements into the blind field.¹ Though this approach results in enlargements of search fields and reduced reaction times to locate targets on the side of the visual field defect, actual improvements of perceptual dysfunction could not be achieved by this approach.

Areas of residual vision

It is now becoming clear that ARV are functional representations of regions of the visual pathway, which are only partially injured and this might explain why perception in ARV is variable (i.e., more prone to sensitivity changes during the course of the day, such as attention loss or fatigue, and other influences that alter their functional activation state). Vision is not a static affair, determined by the optical apparatus only. It also engages many brain mechanisms of perception and attention, which seem less 'physiological' and more 'psychological'.

For instance, light detection performance within ARV may increase with higher levels of alertness and attention and decrease with fatigue.⁸ In fact, patients feel that their performance varies during the day and assessment in the clinic or the laboratory typically shows some variable performance that is typically seen in the size or defect depth of the relative defect, which can be measured with threshold perimetry.

Stimulation approaches

Both approaches, behavioural training and non-invasive electrical stimulation, are similar in that repetitive stimulation may lead to functional visual improvements. However, visual field training uses small visual stimuli to which the patient responds by pressing a key (just like in perimetry) in daily training sessions for 6 months. The recently developed non-invasive transorbital alternating current stimulation (tACS) achieves repetitive stimulation by electric current with comparable effects after only 10 days.

The stimulation protocols of visual field training uses rather unspecific stimuli. For example, in vision restoration training small white dots are used on black background. The tACS approach, in contrast, uses no visual stimuli at all but trains of electrical current in particular frequencies that stimulate brain plasticity. It is interesting to note that the number of stimulations to achieve clinical effects in both procedures are roughly in a similar range: Whereas training requires about 1000 visual stimulus presentation per day for 3–6 months (i.e., a total of approximately 90000–180000), the tACS protocol requires the delivery of about 10000 pulses per day, which can be done in about 30 min, for a total of 10 days (i.e., 100000 stimulations). Though both lead to similar clinical improvements, visual training is a rather laborious affair to the patient, a hurdle in scaling up its use. In contrast, current stimulation, which is not focused only on the ARV but stimulates the entire visual system, is a more accelerated method. It cuts down therapy time from 6 months to about 10 days. This means less effort is required of both the patient and medical personnel, facilitating adoption in clinical practice.^9–11