Visual reflexes

The pupil light reflex controls the quantity of light entering the eye by varying the pupil size. This ranges between 1.5 to 8 mm in diameter, being maximal in absolute darkness. However this permits only a 30-fold change in light entry (that is small as compared to the range of light intensity the visual system can experience) the reflex is helpful as it operates more than the light levels usually encountered during the daylight. The light shone in one eye produces pupil constriction of the same eye (i.e., the direct reflex) and of the contralateral eye (i.e., the consensual reflex) since of reciprocal crossed connections in the midbrain. The reflex pathway is as shown in figure below. The Optic nerve axons synapse in the pretectum that sends output to the preganglionic parasympathetic fibers in Edinger–Westphal (accessory) oculomotor nucleus. Such autonomic fibers travel in the oculomotor nerve to the ciliary ganglion that lies in the orbit. The Postganglionic fibers from there go to the pupillary sphincter. The light stimulation of optic nerve fibers excites the parasympathetic terminals to discharge acetylcholine that contracts the sphincter. Lesions of the oculomotor and optic nerves or of the midbrain can be diagnosed by examining the defects in pupil light reflex which is shown in the figure below.

Figure: Altered pupil reflexes damage as follows to either optic (II) or oculomotor (III) nerves on the left side: (a) reflex pathway; (b) optic nerve damage, left eye stimulated; (c) optic nerve damage, right eye stimulated; (d) oculomotor nerve damage, left or right eye stimulated.

For close objects, the light rays are diverging as they enter the eye and therefore greater refraction is required to bring them to focus at the fovea. This is attained by the accommodation reflex. The contraction of ciliary muscles pulls the ciliary body forwards and inwards, easing the tension in the suspensory ligament and lens capsule, permitting the lens to become more spherical and decreasing its focal length. The stimulus for the accommodation reflex is blurring of the retinal image. This is monitored by the visual cortex that projects to the pretectum through the corticobulbar pathway. Through connections between the pretectum and the Edinger–Westphal nucleus, the parasympathetic fibers are activated that contract the ciliary muscles. The accommodation takes place in both eyes equally.

Observing a close object also causes convergence to the visual axes of both eyes, the vergence reflex. This allows both eyes to fix their gaze on an object. Additionally, the degree of convergence gives a cue for stereopsis, as the closer an object is, the greater the convergence should be. The Vergence can be triggered by a blurred retinal image or by consciously modifying gaze to a point at a dissimilar distance. The circuitry is from the visual cortex to the frontal eye fields in the frontal cortex concerned with the planning and execution of eye movements.