Saccades Study Documents

Saccades         In primates (e.g. humans), saccades aim to line up the fovea to maintain a clear image of the object. Animals that don’t have fovea also have saccadic movements, but these are associated with head movements. Classifications: o Volition: elective saccades. o Reflexive: to a novel stimulus. o Quick phase: associated with nystagmus or resetting following drift. o Spontaneous. The rate of saccadic movement has been tested. o Ask subject to focus on a middle point. o Quickly change the point to either temporal (lateral) or nasal (medial) areas. o Measure time taken for fovea to travel to new point of focus. The further away a new focus point is, the faster the fovea will move. Often times, the fovea will not focus entirely on the new point (dysmetria). o The normal occurrence would be to undershoot (hypometria) the target. o Within about 100ms after reaching this point, the fovea will carry out an express saccade to make up for the remaining distance. Saccades occur because of higher centres in the brain that determine target position. o Signal is transformed by neurons in the brainstem into velocity and position instructions for the motor neurons. The decision of when and where to make a saccade is made in the cerebral cortex. o Cerebral cortex triggers saccades via descending pathways; controls the saccadic system through the superior colliculus. o Basal ganglia has inhibitory connections to the substantia nigra and to superior colliculus – inhibit extraneous saccades during fixation and facilitate volitional saccades. o Cerebellum calibrates saccadic amplitude and saccadic pulsestep match.  Horizontal component organised in the paramedian pontine reticular formation (PPRF) and nucleus prepositus hypoglossi. o The two important nuclei are premotor nuclei. o Conjugate eye movements require opposing muscle groups (medial rectus and lateral rectus) to be excited in different eyes (left and right eye). o The PPRF and nucleus prepositus hypoglossi receive information from the eyes to process information and elicit a response. o Their first synapse is to the abducen nucleus, which excites the lateral rectus on the ipsilateral side of the nuclei. o The abducen nucleus also projects to the oculomotor nuclei of the contralateral side via the medial longitudinal fasciculus, hence exciting the medial rectus there.   Vertical component organised in the mesencephalic reticular formation; rostral interstitial nucleus of the MLF, interstitial nucleus of Cajal. o The two nuclei project into the oculomotor and nucleus nuclei, which innervate the superior and inferior rectus, and the superior and inferior oblique. Measuring the firing rate of neurons during eye movement produces an activity pattern; the pulse-step or burst-tonic innervation: o An initial increase in activity (pulse/burst) to the extra-ocular muscles in order to move against inertia. o Then, there is a period of constancy, as the eye moves to focus on to the object. o The drag of inertia has some sort of elasticity, which normally will bring the eye back to its resting position. To prevent going out of focus, the neurons to the extra-ocular muscles maintain a constant firing rate, enough to maintain the fovea’s position on the image.    Tonic cells: maintain the static position of the eyeball. Burst cells. o Medium lead: begin discharging ~12ms before saccade. o Long lead: begin discharging ~40ms before saccade. o Inhibitory burst neurons: suppress contralateral neurons. Omnipause cells: discharge continuously except immediately prior to and during saccades.