Neuro-ophthalmology has two divisions Afferent - vision Efferent - eye movements Efferent neuro-ophthalmology has 6 supranuclear control systems There are 6 systems of supranuclear control 1. Saccade 2. Pursuit 3. Vergence 4. Fixation - microsaccade, microtremor and microdrifts 5. Vestibulo-ocular reflex (talking only about angular VOR here) 6. Optokinetic nystagmus (OKN) [slow OKN = slow phase and fast OKN = fast phase] Why do we need 6? For that we have to go back to evolution. The earliest animals had only vision WITHOUT a fovea which is the point with the highest visual acuity in the retina --> this was around 520-500 million years ago So they needed to keep only the visual field in line with the head since there was no fovea! So they developed the angular VOR and the OKN which had two different functions VOR --> head movement WITHOUT movement of visual field OKN --> only visual field moves WITHOUT head movement But these usually operated together since most situations had simultaneous movement of the head and visual field Around 450-400 millions ago, the fovea began to emerge --> this allowed fixation and much better visual acuity Fixation --> keeping the object of interest on the fovea for maximum vision Fixation involves miniscule movements called micro saccades, microtremors and microdrifts which help keep the object on the fovea --> these miniscule movements allow phototransduction rather than maintain fixation (that is a different issue altogether) But how to maintain fixation on an object when the head of the animal is constantly moving? Pursuit --> this moves the eye along an x and y axis Vergence --> this maintains fixation along a z axis Now what to do if we want to break fixation and focus on a new target? We have saccades! Now what about the older supranuclear systems of VOR and OKN? They do well with fixation but they have evolved for at least 100 million years where they did not have fixation! So they can do WITH and WITHOUT fixation. In conclusion VOR and OKN --> maintains visual field with and without fixation Pursuit and vergence --> essential to maintain fixation Saccades --> essential to break up fixation and acquire new fixation This is a heavily simplified account of the evolutionary basis of eye movements! #MedTwitter #NeuroTwitter #Neuroophthalmology #EyeMovements