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CHAPTER _9.docx

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John Mitterer

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INTRODUCTION  Light is electromagnetic energy that is emitted in the form of waves.  The visual system begins with the eyes. At the back of the eye is the retina which contains photoreceptors specialized to covert light energy into neural activity.  The output of the retina is not a reproduction of the intensity of the light falling on it but is specialized to detect differences in the intensity of light falling on different parts of it.  The first synaptic relay in the pathway that serves visual perception occurs in a cell group of the dorsal thalamus called the lateral geniculate nucleus (LGN). From the LGN, visual information ascends to the cerebral cortex where it is interpreted and remembered. PROPERTIES OF LIGHT Light Electromagnetic radiation:  Wavelength: the distance between successive peaks or troughs  Frequency: the number of waves per second  Amplitude: the difference between wave trough and peak The energy content of electromagnetic radiation is proportional to its frequency [i.e. high frequency (short wavelengths) has the highest energy content].  Only a small part of the electromagnetic spectrum is detectable by our visual system. This visible light consists of wavelengths of 400-700 nm. (see Fig. 9.2)  “Hot” colours (e.g. red or orange) have long wavelength light and less energy than “cool” colours (e.g. blue or violet). Optics In a vacuum, a wave of electromagnetic radiation will travel in a straight line. This is a ray.  Reflection: the bouncing of light rays off a surface. This depends on the angle at which it strikes the surface. A ray striking a mirror perpendicularly is reflected 180˚ back upon itself and a ray striking a mirror at 45˚ angle is reflected 90˚.  Absorption: the transfer of light energy to a particle or surface. Black surfaces absorb the energy of all visible wavelengths. Some absorb light energy only in a limited range of wavelengths and reflect the remaining wavelengths. For example, a blue pigment absorbs long wavelengths but reflects a range of short wavelengths centered on 430 nm that are perceived as blue.  Refraction: the bending of light rays that occur when they travel from one transparent medium to another. It occurs because the speed of light differs in two media (e.g. light passes through air more rapidly than through water). The greater the difference between the speeds of lights in the two media, the greater the angle of refraction. (see Fig. 9.3). THE STRUCTURE OF THE EYE Gross Anatomy of the Eye  Pupil: the opening that allows light to enter the eye and reach the retina. It appears dark due to the light-absorbing pigments in the retina.  Iris: surrounds the iris and provides the eye’s colour. It contains two muscles: one to make the pupils smaller when it contracts and the other to make the pupils larger.  Cornea: a glassy transparent external surface of the eye that covers the pupil and iris.  Sclera: the “white of the eye” which is continuous with the cornea. It forms the tough wall of the eyeball. The eyeball sits in the eye’s orbit. Into the sclera are three pairs of extraocular muscles lying behind the conjunctiva and move the eyeball in the orbit.  Optic nerve: carries axons from the retina to the base of the brain near the pituitary. Ophthalamoscopic Appearance of the Eye  Optic disk: a pale circular region that gives rise to the retinal vessels and also where the optic nerve fibers exit the retina. Sensation of light cannot occur here because there are NO photoreceptors. Also sensation of light cannot occur where the large blood vessels exist because the vessels cast shadows on the retina.  Macula: located at the middle of the retina. It is responsible for central vision and lacks large blood vessels which improve the quality of central vision.  Fovea: a dark spot about 2mm in diameter. Cross-Sectional Anatomy of the Eye  Aqueous Humor: it is the fluid behind the cornea which nourishes it  Lens: located behind the iris and is suspended by zonule fibers attached to the ciliary muscles. Changes in the shape of the lens allow the eyes to adjust their focus to different viewing distances. o The ciliary muscles are attached to the sclera and form a ring inside the eye. They also divides the interior of the eye into two compartments containing slightly different fluids:  The aqueous humor which lies between the cornea and lens, and  The more viscous, jellylike vitreous humor which lies between the lens and the retina and serve to keep the eyeball spherical. IMAGE FORMATION BY THE EYE Cornea, rather than the lens, is the site of most of the refractive power of the eyes Refraction by the Cornea Light strikes the cornea and passes from the air into the aqueous humor. Then light rays bend and converge on the back of the eye (see Fig. 9.7)  Focal distance: the distance from the refractive surface to the point where parallel light rays converge and depends on the curvature of the cornea (i.e. tight the curve, the shorter the focal distance).  Diopter: reciprocal of the focal distance in meters. The cornea has a refractive power of 42 diopters which means that parallel light rays striking the corneal surface will be focused 0.024 m (2.4 cm). Refractive power depends on the slowing of light at the air-cornea interface. If we replace air with a medium that passes light at about the same speed as the eye, the refractive power will eliminated. This is why one’s vision is blurry when you open your eyes underwater. Accommodation by the Lens The lens contributes another dozen or so diopters to the formation of a sharp image at a distance. It is, however, more importantly involved in forming crisp images of objects located closer than about 9m from the eye.  As objects approach, the light rays originating at a point can no longer be consider to be parallel. These rays diverge and greater refractive power is required to bring them into focus on the retina. This focusing power, called accommodation, is provided by changing the shape of the lens (see Fig. 9.8).  During accommodation, the ciliary muscles contract and swells in. The lens becomes rounder and thicker, increasing the curvature of the lens surfaces and increases the refractive power.  The ability to accommodate changes with age. The Pupillary Light Reflex The pupil continuously adjusts for different ambient light levels.  Pupillary light reflex involves connections between the retina and neurons in the brain stem that control the muscles that constrict the pupils. o This reflex is consensual. Shining a light into only one eye cause the constriction of the pupils of both eyes. Constriction of the pupil increases the depth of focus. The Visual Field It is the points where one can no longer see an object while staring straight ahead (see Fig. 9.9). The left visual field is imaged on the right side of the retina and the right visual field is imaged on the left side of the retina. Visual Acuity It is the ability of the eye to distinguish two nearby points. This depends on several factors including the spacing of photoreceptors in the retina and the precision of the eye’s refraction. The distance across the retina can be described in terms of degrees of visual angel (see Fig. 9.10). MICROSCOPIC ANATOMY OF THE RETINA The most direct pathway for visual information to exit the eye is: photoreceptors  bipolar cells  ganglion cells. In response to light, ganglion cells fire action potentials which propagate down the optic nerve to the rest of the brain. Retinal process is influenced by two additional cell types:  Horizontal cells: receives input from the photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors.  Amacrine cells: receive inputs from bipolar cells and project laterally to influence surrounding ganglion cell, bipolar cells and other amacrine cells.  There are two important points to remember: o Only light-sensitive cells in the retina are the photoreceptors. All other cells are influence by light only via direct and indirect synaptic interactions with the photoreceptors. o The ganglion cells are the only source of output from the retina The Laminar Organization of the Retina Laminar organization means that the cells are organized in layers (see Fig. 9.12). Light must pass from the vitreous humor through the ganglion cells and bipolar cells before it reaches the photoreceptors. The pigmented epithelium lies below the photoreceptors and plays a critical role in the maintenance of the photoreceptors and photopigments. It also absorbs any light that passes entirely through the retina and minimizes the reflection of light within the eye.  Ganglion cell layer: innermost layer and contains the cell bodies of the ganglion cells. o Inner plexiform layer: between the ganglion cell layer and inner nuclear layer and contains the synaptic contacts between the bipolar, amacrine cells and ganglion cells.  Inner nuclear layer: below the ganglion cell layer and contains the cell bodies of the bipolar cells, the horizontal and amacrine cells. o Outer plexiform layer: between the outer and inner nuclear layers where the photoreceptors make synaptic contact with the bipolar and horizontal cells.  Outer nuclear layer: contains cell bodies of the photoreceptors  Layer of photoreceptor outer segments: contains light-sensitive elements of the retina and is embedded in the pigmented epithelium. Photoreceptor Structure Every photoreceptor has four regions: an outer segment, an inner segment, a cell body and synaptic terminal. The outer segment contains a stack of membranous disks which contain light- sensitive photopigments that absorb light and triggers changes in the photoreceptor membrane potential.  There are two types of photoreceptors (the retina is duplex): o Rod photoreceptors: have a long, cylindrical outer segment, containing many disks. It has a greater number of disks and a higher photopigment concentration. Thus they are over 1000 times more sensitive to light than cones. Under night-time lighting (i.e. scotopic conditions) only rods contribute to vision. All rods contain the same photopigment. o Cone photoreceptors: have shorter, tapering outer segment with fewer membranous disks. Under day-time lighting (i.e. photopic conditions), cones mostly contribute to vision. There are three types of cones, each containing a different pigment and make the cones sensitive to different wavelengths of light. Regional Differences in Retinal Structure  The peripheral retina has a higher ratio of rods to cones and also a higher ratio of photoreceptors to ganglion cells. Thus the peripheral retina is more sensitive to light: o Rods are specialized for low light, and o There are more photoreceptors feeding information to each ganglion cells.  Day-time vision requires cones and good visual acuity requires a low ratio of photoreceptors to ganglion cells. o The fovea is highly specialized for
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