Test 4 Chapter 6 Vision We receive information about the environment from sensory receptors specialized neutrons that detect a variety of physical events Stimuli impinge on the receptors and alter their membrane potentials This process is known as sensory transduction because sensory events are transduced into changes in the cells membrane potential These electrical changes called receptor potentials affect the release of neurotransmitters and hence modify the pattern of firing in neurons with which sensory receptors form synapses Approximately 20 of the cerebral cortex plays a direct role in the analysis of visual information Stimulus Light is a narrow band of the spectrum of electromagnetic radiation Electromagnetic radiation with a wavelength of between 380 to 760 nm is visible Honeybees can detect differences in ultraviolet radiation reflected by flowers that appear white to us The range of wavelengths we call light is not qualitatively different from the rest of the electromagnetic spectrum it is simply part of the continuum that we humans can see The perceived color of light is determined by three dimensions hue saturation and brightness Light travels at a constant speed of 300 000 kilometers per second Wavelength determines the first of the three perceptual dimensions of light hue the dominant wavelength Brightness light can vary in intensity Saturation refers to the relative purity of the light that is being perceived If all the radiation is of one wavelength the perceived color is pure or fully saturated Conversely if the radiation contains all visible wavelengths it produces no sensation of hue it appears white Anatomy of the Visual System For an individual to see an image must be focused on the retina the inner lining of eye The retina is actually part of the brain it and the optic nerve are in the CNS The Eyes The eyes are suspended in orbits bony pockets in front of the skull Held in place and moved by 6 extraocular muscles attached to the tough white outer coat of the eye called the sclera We cannot look behind our eyeballs and see those muscles because their attachments to the eyes are hidden by the conjunctiva These mucous membranes line the eyelid and fold back to attach to the eye preventing a contact lens from falling behind the eye Our eyes make jerky saccadic movements the rapid jerky movement of the eyes used in scanning a visual system slowly on the fovea entry of light When you read a book your eyes stop several times and move quickly between each stop you cannot consciously control the speed of movement between stops Only by performing a pursuit movement can you make your eyes move more o The movement that the eyes make to maintain an image of a moving object The white outer layer of most of the eye the sclera is opaque and does not permit The cornea the outer layer at the front of the eye is transparent The amount of light that enters is regulated by the size of the pupil which is an opening in the iris the pigmented ring of muscles situated behind cornea The lens situated immediately behind the iris consists of a series of transparent onion like layers Its shape can be altered by contraction of the ciliary muscles a set of muscle fibers attached to the outer edge of the lens These changes in shape permit the eye to focus images of near or distant objects on the retina a process called accommodation After passing through the lens light traverses the main part of the eye which is filled with vitreous humor glassy liquid a clear gelatinous substance Light then falls on the retina the interior lining of the back of the eye In the retina are located the receptor cells the rods and cones collectively known as photoreceptors The human retina contains approximately 120 million rods and 6 million cones Cones provide us with most of the visual information about our environment o They are responsible for our day time vision o They provide us with information about small features in the environment and thus are the source of vision of the highest sharpness of acuity The fovea or central region of the retina which mediates our most acute vision contains only cones Cones are also responsible for color vision our ability to discriminate light of different wavelengths Rods are more sensitive to light in very dim light we are color blind and lack foveal vision Another feature of the retina is the optic disk where the axons conveying visual information gather together and leave the eye through the optic nerve The optic disk produces a blind spot because no receptors are located there The primate retina which is divided into three main layers the photoreceptive layer the bipolar cell layer and the ganglion cell later Photoreceptors are at the back of the retina light must pass through the overlying layers The photoreceptors form synapses with bipolar cells neurons who two arms connect the shallowest and deepest layer of the retina Bipolar cells form synapses with the ganglion cells neurons whose axons travel through the optic nerve the 2nd cranial nerve and carry visual information into the rest of the brain The retina contains horizontal cells and amacrine cells both of which transmit information in a direction parallel to the surface of the retina and thus combine messages from adjacent photoreceptors Photoreceptors Rods and cones consist of an outer segment connected by a cilium to an inner segment which contains the nucleus The outer segment contains several hundred lamellae or thin plates of membrane The first step in transduction of visual information involves a special chemical called a photopigment Photopigments are special molecules embedded in the membrane of the lamellae a single human rod contains approximately 10 million of them The molecules consist of two parts an opsin a protein and retinal a lipid There are several forms of opsin the photopigment of human rods Rhodopsin consist of rod opsin retinal Retinal is synthesized from vitamin A When a molecule of Rhodopsin is exposed to light it breaks into its two The opsin changes from its rosy color to a pale yellow hence we say that the light constituents rod opsin retinal bleaches the photopigment The splitting of the photopigment produces the receptor potential a change in the membrane potential of the photoreceptor The receptor potential affects the release of neurotransmitter by the photoreceptor which alters the firing rate of the bipolar cells with which the photoreceptors communicate This information