Michael Cleary-Gaffney: The Hidden Eye
The eye is one of the most complex organs in the mammalian species. It provides two functions, the first being the most obvious of allowing us make conscious perception of objects in our visual field. The second is that it is involved in non-image functions such as keeping our circadian clock in sync and pupillary reflex. As most who did leaving cert biology will know, the eye contains rods and cones (photorecptors) which allow for us to perceive an object. However, it was also thought that these photoreceptors were the only photoreceptors which passed light information to our internal body clock located in the brain. This was shown not to be the case with blind people demonstrating an ability to respond to light despite having all of their rods and cones destroyed. This was also repeated in animals who had their rods and cones deleted and they too demonstrated an ability to respond to light and carry out non-image functions.
This idea that there were other photoreceptors located within the eye changed our view of the eye and as result a significant amount of research was conducted to find where these other photorepceptors were located. It was found that a group of retinal ganglion cells located within the eye expressed a unique photopigmant called melanopsin after light hit the eye and this sent signals to area of the brain involved in coordinating our circadian time-keeping. Specifically, these cells are called intrinsically photosensitive retinal ganglion cells (ipRGCs). To provide sufficient evidence that this was in fact a key photoreceptor involved in non-image functioning animals who had a deletion to their rods and cones were employed. They expressed a circadian response to light. Animals who had a deletion to all photoreceptors (rods, cones and ipRGCs) expressed no response to light indicating that each of these photoreptors were involved in non-image functioning. In order to ascertain whether deletion of ipRGCs would result in complete loss of photoentrainment animals who had deletion to just their ipRGCs expressed a response to light however, this response was weakened. This indicated that the rods and cones pass light information to the ipRGCs and also that the ipRGCs works independently to receive light information.
This realisation of a third class of phtoreceptors has had a significant impact on vision biology. This work has impact for those who are blind. It indicates that those who are blind are still able able to carry out non-image functions despite not being able to consciously perceive an image. It also indicates that in cases where the eye is not at risk (e.g not rotting), the eye should then not be removed as non-image functions will also be extinguished resulting in our clock not being in synchronisation with the external environment. For those who have been reading my blogs up until now you will hopefully appreciate the deleterious consequences this has for our physiology and behaviour.
For those interested in more detailed explanation please see:
Hankins, M. W., Peirson, S. N., Foster, R. G. (2007). Melanopsin: an exciting photopigment. Trends in Neuroscience, 31, 27-36.
Schmidt, T. M., Chen, S. K., Hattar, S. (2011). Intrinsically photosensitive retinal ganglion cells: many subtypes, diverse functions. Trends in Neuroscience, 34, 572-580.