Can colour blindness be reversed?

A pioneering approach to restoring elements of colour vision in people with colour blindness was discussed by Professor John Barbur, Director of the Henry Wellcome Laboratories for Vision Science at City St George’s, University of London, during a recent research seminar.

The talk, hosted by the Centre for Applied Vision Research in the School of Health and Medical Sciences, examined whether normal colour vision can be restored in people with congenital colour blindness, specifically those who struggle to see red/green, using targeted stimulation of the individual light-sensitive photoreceptors in the eye, which are known as cones.

Colour perception

In the retina, which is the light-sensitive tissue lining the back of the eye, colour vision depends on three cones, specifically ‘blue’, ‘green’ and ‘red’ ones, and there are over 110 million photoreceptors in total. These are also often referred to as S, M and L cones, reflecting their sensitivity to short, middle and long wavelengths of light, respectively. Each cone responds to a broad range of wavelengths, and this has been taken as proof that colour vision cannot be achieved using single cone stimulation. As a result “a single cone is often described as ‘colour-blind’ with the inevitable conclusion that the human eye does not perceive colour when only one class of cones is stimulated with light," said Professor Barbur.

Professor Barbur then went on to examine whether the eye can perceive colour even when individual cones are ‘colour-blind’ and the mechanisms involved in colour vision when only one cone type is stimulated.

In the seminar, Professor Barbur spoke about how recent experiments have in fact shown that people with normal colour vision, known as trichromats, do perceive colour when light stimulates only one class of cone. Professor Barbur said they report “saturated ‘red’ in response to extreme long wavelength light, ‘blue’ for extreme short wavelengths, and ‘saturated green/teal’ when only M-cones are stimulated with light”. The isolation of single M-cone photoreceptors has only recently been possible using advanced retinal imaging equipment which makes possible the delivery of microdoses of light only to M-cones. This novel process, which is also sometimes referred to as the “Wizard of Oz technique”, can directly control the human eye’s individual photoreceptor excitation via cell-by-cell light delivery.

The evidence examined demonstrates beyond doubt that the normal human eye perceives ‘blue’, ‘green’ or ‘red’ colours in response to single cone-class stimulation, according to Professor Barbur.

The experience is very different for people with congenital colour blindness, who are often missing a particular cone and are thus known as dichromats. The question of interest that follows is whether dichromats who lack either the red/green or the yellow/blue colour mechanism continue to perceive colour in response to single cone stimulation. Protanopes have normal M-cone pigment (but no functioning L-cones) and deuteranopes have normal L-cone pigment (but no functioning M-cones), however, unlike normal trichromats, no red/green dichromat can see either green or red in response to single cone stimulation. This is simply because both deuteranopes and protanopes lack the normal post-receptor neural mechanisms for red/green colour processing, according to Professor Barbur.

As a result, while dichromats do not experience colour from single-cone stimulation, normal trichromats can perceive colour even with single-cone stimulation. According to Professor Barbur, this suggests that colour perception does not arise within the cones themselves, but from later stages of visual processing that receive inputs from different cone classes. This includes a yellow/blue post-receptor mechanism that receives signals from all three cone classes, and a red/green chromatic mechanism that receives signals from only M and L cones. Ultimately, it’s the combined post-receptor yellow/blue and red/green signals that largely determine what we see, and in those who lack certain cones, either one or the other of these mechanisms fails to develop and its absence explains why dichromats fail to see either ‘red’ or ‘green’  in response to single cone class stimulation.

Understanding anomalous trichromats

The situation is also slightly different again for the most common form of colour vision deficiency, known as anomalous trichromacy. Those who fall into this group have three, spectrally distinct cones (like in normal vision), but the sensitivity of one of the cones is shifted towards the other, making it harder to tell certain colours apart, especially reds and greens, reducing considerably the magnitude of red/green colour signals.

Further analysis presented during the talk suggested an even more striking possibility. If selective stimulation of each cone class could be achieved using the Oz technique, as the post-receptor chromatic mechanisms remain fully functioning in anomalous trichromats, “normal colour vision could be achieved with a single monochromatic wavelength (~488 nm) in both colour-anomalous and normal trichromats,” explained Professor Barbur.

Concluding the talk, Professor Barbur said: “The seminar showed that post-receptor mechanisms for processing colour remain intact in anomalous trichromats, and that in these subjects, appropriate, selective stimulation of each cone photoreceptor class using the Oz technique can restore ‘normal’ colour vision.”

This means that, although cone signals are different in anomalous trichromats when compared to normal vision, this difference can be corrected for by controlling how much light each cone class receives. When this is done using the Oz technique, normal colour vision is restored in anomalous trichromats.

The findings presented by Professor Barbur illustrate how the normal functioning of post-receptor chromatic mechanisms ensure that the eye perceives colours in response to single cone-class stimulation, even when single cone responses are ‘colour blind’. These post-receptor chromatic mechanisms continue to function normally in anomalous trichromats, but are absent in dichromats. The results suggest that an enhanced version of the Oz technique for controlled stimulation of single cones could be used to restore normal colour vision in anomalous trichromats.