Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,Chapter Six,Vision,Visual Coding and Retinal Receptors,Reception-absorption of physical energy by receptors,Transduction-the conversion of physical energy to an electrochemical pattern in the neurons,Coding-one-to-one correspondence between some aspect of the physical stimulus and some aspect of the nervous system activity,Visual Coding and Retinal Receptors,From Neuronal Activity to Perception,coding of visual information in the brain does not duplicate the stimulus being viewed,General Principles of Sensory Coding,Muller and the law of specific energies-any activity by a particular nerve always conveys the same kind of information to the brain,Qualifications of the Law of Specific Energies,the rate of firing or pattern of firing may signal independent stimuli,timing of action potentials may signal important information indicating such things as movement,the meaning of one neuron depends on what other neurons are active at the same time,Visual Coding and Retinal Receptors,The Eye and Its Connections to the Brain,Pupil-opening in the center of the eye that allows light to pass through,Lens-focuses the light on the retina,Retina-back surface of the eye that contains the photoreceptors,The Fovea-point of central focus on the retina,The Route Within the Retina,photoreceptors-rods and cones,bipolar cells-receive input from rods and cones,ganglion cells-receive input from bipolar cells,optic nerve-made up of axons of ganglion cells,blind spot-the point where the optic nerve leaves the eye,Figure 6.2Cross section of the vertebrate eye,Note how an object in the visual field produces an inverted image on the retina.,Figure 6.4Visual path within the eyeball,The receptors send their messages to bipolar and horizontal cells,which in turn send messages to the amacrine and ganglion cells.The axons of the ganglion cells loop together to exit the eye at the blind spot.They form the optic nerve,which continues to the brain.,Animation,Figure 6.6Two demonstrations of the blind spot of the retina,Close your left eye and focus your right eye on the o in the top part.Move the page toward you and away,noticing what happens to the x.At a distance of about 25 cm(10 inches),the x disappears.Now repeat this procedure with the bottom part.At that same distance what do you see?,Visual Receptors:Rods and Cones,Rods,abundant in the periphery of the retina,best for low light conditions,see black/white and shades of gray,Cones,abundant around fovea,best for bright light conditions,see color,Transduction,Both Rods and Cones contain photopigments(chemicals that release energy when struck by light),11-cis-retinal is transformed into all-trans-retinal in light conditions,this results in hyperpolarization of the photoreceptor,the normal message from the photoreceptor is inhibitory,Light inhibits the inhibitory photoreceptors and results in depolarization of bipolar and ganglion cells,Color Vision,The Trichromatic(Young-Helmholtz)Theory,we perceive color through the relative rates of response by three kinds of cones,each kind maximally sensitive to a different set of wavelengths,The Opponent-Process Theory,we perceive color in terms of paired opposites,The Retinex Theory,When information from various parts of the retina reaches the cortex,the cortex compares each of the inputs to determine the brightness and color perception for each area,Figure 6.12Possible wiring for one bipolar cell,Short-wavelength light(which we see as blue)excites the bipolar cell and(by way of the intermediate horizontal cell)also inhibits it.However,the excitation predominates,so blue light produces net excitation.Red,green,or yellow light inhibit this bipolar cell because they produce inhibition(through the horizontal cell)without any excitation.The strongest inhibition is from yellow light,which stimulates both the long-and medium-wavelength cones.Therefore we can describe this bipolar cell as excited by blue and inhibited by yellow.White light produces as much inhibition as excitation and therefore no net effect.(Actually,receptors excite by decreasing their usual inhibitory messages.Here we translate that double negative into excitation for simplicity.),Color Vision Deficiency,Color Vision Deficiency-inability to perceive color differences,Generally results from people lacking different subsets of cones,Neural Basis of Visual Perception,An Overview of the Mammalian Visual System,Rods and Cones synapse to amacrine cells and bipolar cells,Bipolar cells synapse to horizontal cells and ganglion cells,Axons of the ganglion cells leave the back of the eye,The inside half of the axons of each eye cross over in the optic chiasm,Pass through the lateral geniculate nucleus,Transferred to visual areas of cerebral cortex,Processing Visual Stimuli,Mechanisms of Processing in the Visual System,Receptive Field-the part of the visual field to which any one neuron re