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 motion 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).
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.
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.
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.
OKR is driven by a dedicated visual pathway called the accessory optic system (AOS). [8]
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]
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.
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.
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.
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]
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.
In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived from optic stalks during the seventh week of development and is composed of retinal ganglion cell axons and glial cells; it extends from the optic disc to the optic chiasma and continues as the optic tract to the lateral geniculate nucleus, pretectal nuclei, and superior colliculus.
The visual system is the physiological basis of visual perception. The system detects, transduces and interprets information concerning light within the visible range to construct an image and build a mental model of the surrounding environment. The visual system is associated with the eye and functionally divided into the optical system and the neural system.
In neuroanatomy, the lateral geniculate nucleus is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, ventral projection of the thalamus where the thalamus connects with the optic nerve. There are two LGNs, one on the left and another on the right side of the thalamus. In humans, both LGNs have six layers of neurons alternating with optic fibers.
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.
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, it is also known as the Cervico-ocular reflex. 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.
Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an animal's ability to recognize small details with precision. Visual acuity depends on optical and neural factors. Optical factors of the eye influence the sharpness of an image on its retina. Neural factors include the health and functioning of the retina, of the neural pathways to the brain, and of the interpretative faculty of the brain.
The fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina.
A retinal ganglion cell (RGC) is a type of neuron located near the inner surface of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina amacrine cells. Retina amacrine cells, particularly narrow field cells, are important for creating functional subunits within the ganglion cell layer and making it so that ganglion cells can observe a small dot moving a small distance. Retinal ganglion cells collectively transmit image-forming and non-image forming visual information from the retina in the form of action potential to several regions in the thalamus, hypothalamus, and mesencephalon, or midbrain.
Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene Opn4. In the mammalian retina, there are two additional categories of opsins, both involved in the formation of visual images: rhodopsin and photopsin in the rod and cone photoreceptor cells, respectively.
In neuroanatomy, the pretectal area, or pretectum, is a midbrain structure composed of seven nuclei and comprises part of the subcortical visual system. Through reciprocal bilateral projections from the retina, it is involved primarily in mediating behavioral responses to acute changes in ambient light such as the pupillary light reflex, the optokinetic reflex, and temporary changes to the circadian rhythm. In addition to the pretectum's role in the visual system, the anterior pretectal nucleus has been found to mediate somatosensory and nociceptive information.
Motion perception is the process of inferring the speed and direction of elements in a scene based on visual, vestibular and proprioceptive inputs. Although this process appears straightforward to most observers, it has proven to be a difficult problem from a computational perspective, and difficult to explain in terms of neural processing.
Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms to fixate, inspect and track visual objects of interests. A special type of eye movement, rapid eye movement, occurs during REM sleep.
In the anatomy of the eye, amacrine cells are interneurons in the retina. They are named from Greek a– 'non' makr– 'long' and in– 'fiber', because of their short neuronal processes. Amacrine cells are inhibitory neurons, and they project their dendritic arbors onto the inner plexiform layer (IPL), they interact with retinal ganglion cells, and bipolar cells or both of these.
Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), are a type of neuron in the retina of the mammalian eye. The presence of an additional photoreceptor was first suspected in 1927 when mice lacking rods and cones still responded to changing light levels through pupil constriction; this suggested that rods and cones are not the only light-sensitive tissue. However, it was unclear whether this light sensitivity arose from an additional retinal photoreceptor or elsewhere in the body. Recent research has shown that these retinal ganglion cells, unlike other retinal ganglion cells, are intrinsically photosensitive due to the presence of melanopsin, a light-sensitive protein. Therefore, they constitute a third class of photoreceptors, in addition to rod and cone cells.
Retinotopy is the mapping of visual input from the retina to neurons, particularly those neurons within the visual stream. For clarity, 'retinotopy' can be replaced with 'retinal mapping', and 'retinotopic' with 'retinally mapped'.
Nystagmus is a condition of involuntary eye movement. People can be born with it but more commonly acquire it in infancy or later in life. In many cases it may result in reduced or limited vision.
Feature detection is a process by which the nervous system sorts or filters complex natural stimuli in order to extract behaviorally relevant cues that have a high probability of being associated with important objects or organisms in their environment, as opposed to irrelevant background or noise.
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.