Topics and Speakers Michael E. Goldberg, MD
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Synopsis
In this lecture, Dr Michael E. Goldberg explains the control of eye movements and various eye reflexes. This is an example-heavy lecture, full of illustrative case videos and slides that present both normal and abnormal eye movements.
Why do we make eye movements? As Dr Goldberg describes, we make eye movements so that we can keep an object on the fovea, the most high-resolution part of the retina. If the object is still, then you want to keep your eyes still or fixated on that object. The medical term for this is fixation. If the object moves, you want to follow the object by moving your head but keeping your eyes still in space. In order to keep your eyes still while your head moves, you need two reflexes: the vestibulo-ocular reflex and the optokinetic reflex, the latter compensating for some problems in the former.
The vestibulo-ocular reflex (VOR) is a reflex eye movement that stabilizes images on the fovea during head movement by producing an eye movement that is in the direction opposite to the head movement. For example, if the head moves to the left, the eyes move to the right. However, the VOR is imperfect. If you are sat on a chair and rotated very quickly, the VOR will work in the first five seconds but after that, the optic nerve loses the signal that relates head velocity. The result is vestibular nystagmus: the eyes quickly jerk from side to side to reset, so to speak. The optokinetic reflex compensates for the motion of the visual field by using the relative velocity of the image on the retina to induce eye movements in the same direction and at the same velocity as the external world to preserve stable vision.
Other eye movements are then explained, in particular smooth pursuit, saccades, and vergence. In contrast to the VOR, smooth pursuit matches target velocity to eye velocity instead of matching target velocity to head velocity. Smooth pursuit only happens when you have an object to pursue; otherwise you end up making saccades, which is another eye movement Dr Goldberg focuses on. Saccades are quick, simultaneous movements of the eyes in the same direction. For instance, if you were to look at a painting, your eyes would dart all over it in order to create a mental map of the image. Saccades are necessary because only the fovea, the central part of the retina, contain cones, which are color-sensitive. The rest of the retina contains rods, which are not color-sensitive. Finally, vergence is the opposite of the saccade: the eyes move simultaneously in different directions (e.g., staring at your nose).
Dr Goldberg moves on to the anatomy of single eye movements. He briefly reviews the anatomy of the six ocular muscles (lateral rectus, medial rectus, inferior rectus, superior oblique, superior rectus, and inferior oblique) and then moves on to the two kinds of single eye movements: abduction and adduction. Abduction is a movement away from the nose, and adduction is a movement toward the nose. Vertical movements describe the pupil moving up and depression movements describe the pupil moving down. An extreme example of a single eye movement is torsion, where the sphere of the eye rotates within the socket. For instance, an intorsion moves one eye toward the nose; an extorsion moves an eye toward the ear.
Torsion however is problematic. When the sphere of the eye rotates in the socket, vertical lines still remain vertical. How is this possible? This was solved by Listing's law, which states that torsion must be constrained in some way or else vertical lines would not remain vertical. The way it is constrained is through an axis of rotation, a notion that implies that the eye has a starting position and it moves to any other position in a single plane, called the Listing's plane. When that happens, the angle by which the axis changes is equal to half the angle that the eye itself moves.
Dr Goldberg concludes the lecture with a description of oculomotor neurons. Oculomotor neurons contain two signals: the step (in charge of eye position) and the pulse (in charge of eye velocity). In smooth pursuit, the major signal is the pulse signal, which comes from the contralateral medial vestibular nucleus; whereas for horizontal saccades, the major signal is the step signal. Saccades are driven by attention. For instance, if you ask someone to look at a photograph, their eyes dart to the "interesting things" such as the eyes and mouths instead of hair and less important features.
Lesions in areas such as the parietal lobe can cause an anti-saccade, resulting in slightly hypometric saccades with longer reaction times. For instance, if you ask a lesioned patient not to look at your hand moving, they will immediately look at the stimulus instead of looking away. A person without a lesion would be able to resist the urge to look at the stimulus and could look away. Dr Goldberg expands on this topic by briefly touching upon supranuclear control of pursuit.
