Presbyopia monovision correction and cortical adaptation

The article ‘Immediate cortical adaption in visual and non‐visual areas functions induced by monovision’ in this issue of The Journal of Physiology approaches adaptation to monovision with contact lenses, one way of compensating the loss of near vision that affects all humans with ageing (Zeri et al. 2018).

In this study, the authors investigated which cortical changes take place when a group of presbyopes have one eye corrected for distance vision while the other eye, the non‐dominant eye, has near vision correction. By using visual evoked potentials (VEPs), the authors found that there is a reduction in the early components of the electrocortical activity triggered by the visual task. This means that there is lower activity in the primary visual cortex, which is expected as a result of the blurring induced by monovision, as monovision creates an unbalanced correction across the two eyes.

Interestingly, monovision also led to an increase in the amplitude of VEP components arising from prefrontal regions. These extrastriate areas seem to be activated as a way of compensating the degraded optics created by monovision. This attentional compensatory activity would allow the extraction of more useful information from the less sharp image that reaches the primary visual cortex. Indeed, it has been shown that cortical regions associated with the attentional network are less activated after learning a visual task, and that the decrease in brain activation is highly correlated with the magnitude of expertise acquisition and performance improvement (Mukai et al. 2007). Because the group of presbyopes in this study were fitted with monovision contact lenses for the first time, it is likely that they were adapting to the visual disparity and therefore requiring feedback activity in such prefrontal areas. Consequently, the hypo‐functioning of the primary visual cortex did not negatively affect the visual perception due to the contribution of other visual and non‐visual areas, which increased their activity to compensate a reduced sensorial input, still ensuring effective vision (Zeri et al. 2018).

A similar conclusion was reached by authors using functional magnetic resonance imaging to study presbyopic patients who had surgical implantation of multifocal intra‐ocular lenses during cataract surgery (Rosa et al. 2017). This type of lens also creates a sort of rivalry between distance and near images. However, with multifocal lenses both eyes are simultaneously corrected for distance and near vision, but the near image is blurred when a distant object is focused on and vice versa. Patients with recently implanted multifocal lenses also displayed a reduced signal in the primary visual cortex, especially under glare, and significant activation of the attention network (frontal, middle frontal, parietal–frontal areas) (Rosa et al. 2017).

Both studies provide evidence of visual plasticity/adaptation, a possible target for improving contact lenses or surgical results. As a broader implication, they also suggest the adaptive or neuroplastic nature of the adult human brain, contrasting with the classical notion of immutability of the visual cortex after childhood (Bavelier et al. 2010).

In addition, these electrophysiological findings have the potential to be used in the future as markers for favourable adaptation, for example as a pre‐operative testing procedure. It is likely that favourable adaptation is associated with an effective activation of extrastriate visual and non‐visual areas associated with attention, learning, cognitive control, task planning and solving.

In conclusion, this is an innovative paper that will certainly inspire further studies on the cortical aspects of presbyopia correction and highlights the important role of the brain in our perceptual construction of vision.

Additional information

Competing interests

None declared.