Optokinetic response

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Horizontal optokinetic nystagmus. Optokinetic nystagmus.gif
Horizontal optokinetic nystagmus.

The optokinetic reflex (OKR), also referred to as the optokinetic response, or optokinetic nystagmus (OKN), is a compensatory reflex that supports visual image stabilization. [1] The purpose of OKR is to prevent image blur on the retina that would otherwise occur when an animal moves its head or navigates through its environment. This is achieved by the reflexive movement of the eyes in the same direction as image motion, so as to minimize the relative motion of the visual scene on the eye. OKR is best evoked by slow, rotational motion, and operates in coordination with several complementary reflexes that also support image stabilization, including the vestibulo-ocular reflex (VOR).

Contents

Characteristics of OKR

Eliciting OKR

OKR is typically evoked by presenting full field visual motion to a subject. The optokinetic drum is a common clinic tool used for this purpose. The drum most commonly contains sinusoidal or square-wave stripes that move across the subject's field of view to elicit strong optokinetic eye movements. However, nearly any moving texture evokes OKR in mammals. Outside of laboratory settings, OKR is strongly evoked by natural image motion, including when looking out the side window of a moving vehicle.

Eye movement patterns

When viewing constant, unidirectional motion, OKR consists of a stereotyped "sawtooth" waveform that represents two types of eye movements. During slow nystagmus, the eyes smoothly follow the direction of the stimulus. Though slow nystagmus closely resembles smooth pursuit eye movements, it is distinct; several species that do not exhibit smooth pursuit nonetheless have slow nystagmus during OKR (though in humans, it is possible to substitute slow nystagmus for smooth pursuit during a version of OKR referred to as "look nystagmus", in which subjects are specifically instructed to track the moving stimuli [2] ). Fast nystagmus is the second constituent eye movement in OKR. It consists of a rapid, resetting saccade in the opposite direction of the slow nystagmus (i.e., opposite to the stimulus motion). The purpose of the fast nystagmus is to keep the eye centered in the orbit, while the purpose of the slow nystagmus is to stabilize the moving visual scene on the retina.

Comparative biology

OKR is one of the best preserved behaviors in the animal kingdom. It has been identified in insects, invertebrates, reptiles, amphibians, birds, fish, and all mammals. [3] There are subtle differences in how OKR plays out across species. For instance, in fruit flies, individual segments of the compound eye move in response to image motion, [4] whereas in mammals and several other species the entire eye moves together. In addition, OKR patterns vary across species according to whether stimuli are presented monocularly or binocularly: in most species monocular presentation of stimuli results in asymmetric responses, with stimuli moving in the nasal-to-temporal direction resulting in larger responses than stimuli moving in the temporal-to-nasal direction. In humans, this asymmetry is seen only in infants, and monocular OKR becomes symmetric by six months of age because of cortical development. [3] In several species, OKR is also more reliably evoked by upward motion than by downward motion. [5] [6] [7] Both vertical and horizontal asymmetries are often attributed to functional adaptations that reflect common natural scene statistics associated with forward terrestrial locomotion.

Neural mechanisms

OKR is driven by a dedicated visual pathway called the accessory optic system (AOS). [8]

Retina

The AOS begins in the retina with a specialized class of retinal ganglion cell known as ON direction selective retinal ganglion cells (oDSGCs). These cells respond selectively to motion in one of three cardinal directions (upward, downward, or nasal motion), [9] [10] and inherit their direction selectivity at least partially from asymmetric inhibition from starburst amacrine cells. [11] Glycinergic inhibition produces a speed tuning preference for slow stimulus motion in oDSGCs, [12] [13] which has been used to explain the analogous slow tuning of OKR. [14] In some species, oDSGCs constitute the displaced ganglion cells, whose cell bodies reside in the inner nuclear layer of the retina. oDSGCs that respond to different directions of motion have slightly different response properties that are also reflected in OKR behavior, and it is thought that a linear subtraction of oDSGC spikes may predict the magnitude of the OKR slow phase. [7]

Midbrain

oDSGC axons do not target common visual structures. Instead, they are likely the only retinal ganglion cell type to innervate the three midbrain nuclei of the AOS: [8] the nucleus of the optic tract (NOT), the lateral terminal nucleus (LTN), and the medial terminal nucleus (MTN). These nuclei are targeted by oDSGCs that prefer nasal, downward, and upward image motion, respectively. Recurrent inhibitory connections exist between these AOS nuclei, further suggesting a subtraction of signals between different oDSGC types. There are only modest connections between these nuclei and the cortex. The activity of neurons in the AOS nuclei are well-correlated with the velocity of the OKR slow phase.

Oculomotor plant

The projection neurons of the NOT, LTN, and MTN converge on the oculomotor plant in the brainstem, where their activity is integrated to drive the eye movements. This occurs through Cranial Nerves III, IV, and VI, and their associated brainstem nuclei.

Plasticity

Potentiation of the OKR slow phase is known to occur after long periods of continuous stimulation. These mechanisms are cerebellar-dependent, and may be associated with corresponding changes to the VOR.

Scientific and medical interest

The reflexive nature of OKR has made it a popular method for objectively measuring vision in many contexts. OKR-based tests have been developed to objectively assess visual acuity, color vision, stereopsis and more. [15] [16] [17] Changes to the stereotypical OKR waveform can also be a biomarker of disease, including stroke, concussion, drug or alcohol intoxication, and parkinsonism. [18] OKR is also commonly used in basic science as an objective measure of acuity in animal disease models.

In neurobiology, the isolation of the AOS from other visual pathways, its clear connection to a behavioral readout in the form of OKR, and its conservation across species make it an attractive model system to study. The AOS has been used to understand molecular mechanisms of synapse formation, feature tuning and direction selectivity in the retina, neural circuit development, axon targeting, plasticity mechanisms, and computational strategies for integrating primary sensory information. [19] [20] [21] [22]

See also

Related Research Articles

<span class="mw-page-title-main">Retina</span> Part of the eye

The retina is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then processes that image within the retina and sends nerve impulses along the optic nerve to the visual cortex to create visual perception. The retina serves a function which is in many ways analogous to that of the film or image sensor in a camera.

<span class="mw-page-title-main">Optic nerve</span> Second cranial nerve, which connects the eyes to the brain

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<span class="mw-page-title-main">Visual system</span> Body parts responsible for vision

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<span class="mw-page-title-main">Lateral geniculate nucleus</span> Component of the visual system in the brains thalamus

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<span class="mw-page-title-main">Vestibulo–ocular reflex</span> Reflex where rotation of the head causes eye movement to stabilize vision

The vestibulo-ocular reflex (VOR) is a reflex that acts to stabilize gaze during head movement, with eye movement due to activation of the vestibular system. The reflex acts to stabilize images on the retinas of the eye during head movement. Gaze is held steadily on a location by producing eye movements in the direction opposite that of head movement. For example, when the head moves to the right, the eyes move to the left, meaning the image a person sees stays the same even though the head has turned. Since slight head movement is present all the time, VOR is necessary for stabilizing vision: people with an impaired reflex find it difficult to read using print, because the eyes do not stabilise during small head tremors, and also because damage to reflex can cause nystagmus.

<span class="mw-page-title-main">Pupillary light reflex</span> Eye reflex which alters the pupils size in response to light intensity

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<span class="mw-page-title-main">Pretectal area</span> Structure in the midbrain which mediates responses to ambient light

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<span class="mw-page-title-main">Eye movement</span> Movement of the eyes

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<span class="mw-page-title-main">Amacrine cell</span> Interneuron cells in the retina of the eye

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<span class="mw-page-title-main">Parasol cell</span>

A parasol cell, sometimes called an M cell or M ganglion cell, is one type of retinal ganglion cell (RGC) located in the ganglion cell layer of the retina. These cells project to magnocellular cells in the lateral geniculate nucleus (LGN) as part of the magnocellular pathway in the visual system. They have large cell bodies as well as extensive branching dendrite networks and as such have large receptive fields. Relative to other RGCs, they have fast conduction velocities. While they do show clear center-surround antagonism, they receive no information about color. Parasol ganglion cells contribute information about the motion and depth of objects to the visual system.

References

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