eye movements Flashcards
**What is saccades?
Saccades are rapid, ballistic movements of the eyes that abruptly change the direction of fixation. Saccades can be elicited voluntarily, but they occur reflexively whenever the eyes are open, even when they are fixated on a target,
**what is Smooth pursuit movements
Smooth pursuit movements are much slower tracking movements of the eyes designed to keep a moving stimulus on the fovea once foveation is achieved. Such movements are under voluntary control in the sense that the observer can choose whether or not to track a moving stimulus.
**Vergence movements
Vergence movements align the fovea of each eye with targets located at different distances from the observer. Although vergence movements are required to track a visual target that may be moving closer or farther away, they are more commonly employed when abruptly shifting the direction of gaze, for example, from a near object to one that is more distant. Unlike other types of eye movements, in which the two eyes move in the same direction (conjugate eye movements), vergence movements are disconjugate (or disjunctive); they involve either a convergence or divergence of the lines of sight of each eye to foveate an object that is nearer or farther away.
Describe the near reflex triad and its components
Convergence is one of the three reflexive visual responses elicited together to shift gaze from a distant to a near object. The components of the near reflex triad are convergence, accommodation of the lens, and pupillary constriction. Convergence involves disconjugate movements of the eyes to foveate an object that is close to the person. Lens accommodation is a process in which the curvature of the lens is
4. There are 3 types of eye movements that serve to shift the direction of gaze. Define each and
describe their characteristics (e.g. speed, amplitude, voluntary/involuntary, conjugate/disconjugate,
etc.).
increased to bring the close object into focus. Pupillary constriction is a process in which spherical aberration is reduced in order to increase the depth of field and sharpens the image on the retina.
2 types of eye movements that serve to stabilize gaze. Describe
a)Vestibulo-ocular movements
When a person fixates their gaze on an object and moves their head from side to side, the eyes automatically compensate for the head movement by moving the same distance and at the same velocity but in the opposite direction, thus keeping the image of the object at more or less the same place on the retina. The vestibular system detects brief, transient changes in head position and produces rapid, corrective eye movements. Sensory information from the semicircular canals directs the eyes to move in a direction opposite to the head movement.
b) Optokinetic eye movements
Optokinetic eye movements rely not on vestibular information, but on visual cues indicating motion of the visual field. When a visual image slowly “slips” across the retina (slow movements (below 1 Hz) over large areas of the visual field), and the optokinetic system responds by slowly inducing compensatory movements of the eyes at the same speed and in the opposite direction.
How is the amplitude of a saccadic eye movement controlled?
The amplitude of a saccadic eye movement is encoded by the duration of neuronal activity in the lower motor neurons of the oculomotor nuclei.
What and where is the paramedian pontine reticular formation?
the paramedian pontine reticular formation (PPRF), also called the horizontal gaze center, is a collection of local circuit neurons near the midline in the pons. These neurons are responsible for generating horizontal eye movements.
(An example of how the PPRF works with the abducens and oculomotor nuclei to generate a horizontal saccade to the right is shown in the figure below. Neurons in the PPRF innervate cells in the abducens
nucleus on the same side of the brain. The abducens nucleus contains two types of neurons. One type comprises the lower motor neurons that innervate the lateral rectus muscle on the same side. The other type, called internuclear neurons, sends their axons across the midline. These axons ascend in a fiber tract called the medial longitudinal fasciculus and terminate in the portion of the oculomotor nucleus that contains lower motor neurons that innervate the medial rectus muscle. As a result of this arrangement, activation of PPRF neurons in the right side of the brainstem causes horizontal movements of both eyes to the right; likewise, activation of PPRF neurons in the left half of the brainstem induces horizontal movements of both eyes to the left.)
