Visual System II: Processing and Perception Flashcards
Review of Ocular Anatomy
The eye itself is a hollow sphere composed of three layers of tissue. The outermost layer is the fibrous tunic, which includes the white sclera and clear cornea. The sclera accounts for five sixths of the surface of the eye, most of which is not visible.
The transparent cornea covers the anterior tip of the eye and allows light to enter the eye. The middle layer of the eye is the vascular tunic, which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by suspensory ligaments, or zonule fibres. These two structures bend the lens, allowing it to focus light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris—the coloured part of the eye. The iris is a smooth muscle that opens or closes the pupil, which is the hole at the centre of the eye that allows light to enter. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light. The innermost layer of the eye is the neural tunic, or retina, which contains the nervous tissue responsible for photoreception.
The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. It is filled with a watery fluid called the aqueous humour. The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more viscous fluid called the vitreous humour.
The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer. There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye as the optic nerve. Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nerve begins. This creates a “blind spot” in the retina, and a corresponding blind spot in our visual field.
Detection of Light
Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment. The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane. There are three cone photopigments, called opsins, which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its colour. The pigments in human eyes are specialized in perceiving three different primary colours: red, green, and blue.
The opsins are sensitive to limited wavelengths of light. Rhodopsin, the photopigment in rods, is most sensitive to light at a wavelength of 498 nm. The three colour opsins have peak sensitivities of 564 nm, 534 nm, and 420 nm corresponding roughly to the primary colours of red, green, and blue (Figure 14.18). The absorbance of rhodopsin in the rods is much more sensitive than in the cone opsins; specifically, rods are sensitive to vision in low light conditions, and cones are sensitive to brighter conditions. In normal sunlight, rhodopsin will be constantly bleached while the cones are active. In a darkened room, there is not enough light to activate cone opsins, and vision is entirely dependent on rods. Rods are so sensitive to light that a single photon can result in an action potential from a rod’s corresponding RGC.
The three types of cone opsins, being sensitive to different wavelengths of light, provide us with colour vision. By comparing the activity of the three different cones, the brain can extract colour information from visual stimuli. For example, a bright blue light that has a wavelength of approximately 450 nm would activate the “red” cones minimally, the “green” cones marginally, and the “blue” cones predominantly. The relative activation of the three different cones is calculated by the brain, which perceives the colour as blue. However, cones cannot react to low-intensity light, and rods do not sense the colour of light. Therefore, our low-light vision is—in essence—in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colours in the dark, it is most likely because your brain knows what colour something is and is relying on that memory.
Primary Visual Pathway
The connections of the optic nerve are more complicated than those of other cranial nerves. Instead of the connections being between each eye and the brain, visual information is segregated between the left and right sides of the visual field. In addition, some of the information from one side of the visual field projects to the opposite side of the brain. Within each eye, the axons projecting from the medial side of the retina decussate at the optic chiasm. For example, the axons from the medial retina of the left eye cross over to the right side of the brain at the optic chiasm. However, within each eye, the axons projecting from the lateral side of the retina do not decussate. For example, the axons from the lateral retina of the right eye project back to the right side of the brain. Therefore the left field of view of each eye is processed on the right side of the brain, whereas the right field of view of each eye is processed on the left side of the brain.
Extending from the optic chiasm, the axons of the visual system are referred to as theoptic tractinstead of the optic nerve. The optic tract has three major targets, two in the diencephalon and one in the midbrain. The connection between the eyes and diencephalon is demonstrated during development, in which the neural tissue of the retina differentiates from that of the diencephalon by the growth of the secondary vesicles. The connections of the retina into the CNS are a holdover from this developmental association. The majority of the connections of the optic tract are to the thalamus—specifically, thelateral geniculate nucleus. Axons from this nucleus then project to the visual cortex of the cerebrum, located in the occipital lobe. Another target of the optic tract is the superior colliculus.
In addition, a very small number of RGC axons project from the optic chiasm to thesuprachiasmatic nucleusof the hypothalamus. These RGCs are photosensitive, in that they respond to the presence or absence of light. Unlike the photoreceptors, however, these photosensitive RGCs cannot be used to perceive images. By simply responding to the absence or presence of light, these RGCs can send information about day length. The perceived proportion of sunlight to darkness establishes thecircadian rhythmof our bodies, allowing certain physiological events to occur at approximately the same time every day.
Visual Cortex
There are two main regions that surround the primary cortex that are usually referred to as areas V2 and V3 (the primary visual cortex is area V1). These surrounding areas are the visual association cortex. The visual association regions develop more complex visual perceptions by adding colour and motion information. The information processed in these areas is then sent to regions of the temporal and parietal lobes.
Visual processing has two separate streams of processing: one into the temporal lobe and one into the parietal lobe.
These are the ventral and dorsal streams, respectively. The ventral stream identifies visual stimuli and their significance. Because the ventral stream uses temporal lobe structures, it begins to interact with the non-visual cortex and may be important in visual stimuli becoming part of memories. The dorsal stream locates objects in space and helps in guiding movements of the body in response to visual inputs. The dorsal stream enters the parietal lobe, where it interacts with somatosensory cortical areas that are important for our perception of the body and its movements. The dorsal stream can then influence frontal lobe activity where motor functions originate.
Eye Movement
• Visual motion information is used for perception and to direct self-motion
• Area MT outputs to various cortical and subcortical structures that control eye movement
• For high acuity vision, the light from an object of interest must fall on the fovea
• When the object is moving, or during self-motion, the eyes rotate to track the object
• Fixation: actively suppresses eye movement
• Saccades: very rapid, stereotyped eye movements
• Smooth Pursuit: slower, for tracking moving objects
• Vergence: for focussing on near or far objects
Two reflex pathways stabilise gaze during self-motion:
• the optokinetic reflex (slow-phase and fast-phase)
• the vestibulo-ocular reflex (fast)
Saccades
- Very rapid (20-200 ms) stereotyped eye movements: no visual perception (prevent image blurring)
- Alternating saccades and fixations used for scanning images or reading
- Without saccades, static visual images disappear
Smooth Pursuit
- Used for tracking moving objects
- Maximum velocity —30 degrees/sec. Faster movement and error correction requires catch-up saccades
- Initiation under voluntary control, execution automatic
Vergence and Depth Perception
- Simultaneos movements of both eyes in opposite directions to maintain binocular vision
- A subset of neurons in area MT use binocular disparity for depth perception
- Also used to initiate convergent/divergent eye movements to bring. objects into focus
The Superior Colliculus
Layered structure in the midbrain, has both sensory and motor functions
• Superficial layers receive input from the retina and visual cortex
• Deeper layers receive input from auditory and somatosensory brain regions
The Nucleus of the Optic Tract
- Neurons have large receptive fields that are direction-selective and speed-selective
- Respond strongly to slow, large-scale movements of the visual image (i.e., during self-motion)
- Involved in the optokinetic reflex and vestibulo-ocular reflex
- Also known as the accessory optic system; may contribute to blindsight
