Supra-nuclear palsy is quite a devastating condition that affects the
visual system. It occurs when damage is sustained to the
superior colliculus, the part of the
optic nerve primarily responsible for controlling
eye movements. Among its functions is the mediation of the automatic response to fixating an object of immediate interest in our
visual field – things that might hurt us, such as an animal or flying object.
When looking at an object, we make continuous quick, sharp eye movements, known as “saccades”, across it. Our eyes flick all over the object, fixating different parts of it rather than just fixating one pertinent area. To discern any finer detail of an object we need to fixate it in the centre of our visual field, as it is in the centre of the retina where the highest concentration of cone cells are located (this area is known as the fovea and it is very small). This high concentration of cells elicits a greater spatial acuity, and thus finer detail is discerned.
We have no real control over the saccadic eye movements that occur when we look at an object as it is an autonomic process mediated by the superior colliculus visual pathway. However, you can notice yourself performing these continuous eye movements if you look at an object. So, to look at an object, we take many different “samples” across it, and store this information in visual memory to produce a representation in our minds.
When the superior colliculus is damaged or disrupted, a person loses the ability to make autonomic eye saccades. This produces a situation where the sufferer has to consciously control all their eye movements. So, they lose the ability to automatically focus their attention on important stimuli. They also lose the ability to make saccadic eye movements over an object. This produces great problems in that they can only focus their highly sensitive area of visual field on one small part of an object, and therefore they only able to distinguish finer detail on a very small scale.
This is a problem in itself, but a greater problem can arise from the physiological nature of the photoreceptor cells on the retina. Photoreceptor cells contain pigment chemicals that react to light falling on them. This is how they react and send impulses when light is incident on them. However, the pigment chemicals can become “bleached” if the same amount and type (e.g., wavelength) of light is incident on them for a certain length of time. When the pigment chemicals become bleached, the cell no longer responds to light stimuli. This is why the eye moves continuously - doing so changes the amount and type of light incident on the photoreceptor cells due to the shifting around of the visual field.
In supra-nuclear palsy sufferers, the ability to make these automatic movements is lost and as a consequence, the sufferer’s photoreceptor cells become bleached and unresponsive if they look at the same object for too long. This results in an eventual loss of all sight when looking at an object.
Bleaching of the photoreceptor cells is only temporary and is recoverable, so the condition can be controlled by the sufferer continuously making an conscious effort to move their eyes. This must be a very hard thing to do. In many ways, I imagine that this is like having to remember to conduct some other automatic process, like breathing.