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Chapter 6

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Psychology 2220A/B
Scott Mac Dougall- Shackleton

Chapter 6: The Visual System LIGHT ENTERS THE EYE AND REACHES THE RETINA • Some animals have special adaptations that allow them to see under very dim illumination, but no animal can see in compete darkness • If there is no light, there is no vision • Light can be though of as discrete particles of energy traveling through space, or as waves of energy • Light is sometimes defined as waves of electromagnetic energy that are between 380 and 760 nanometers in length • Some animals can see wavelengths that we cannot • Wavelength and intensity are two properties of light that are of particular interest - wavelength because it plays an important role in the perception of colour, and intensity because it plays an important role n the perception of brightness • The Pupil and the Lens • The amount of light reaching the retinas is regulated by the donut-shaped bands of contractile tissue, the irises, which give our eyes their characteristic colour • Light enters the eye through the pupil, the hole in the iris • The adjustment of pupil size in response to changes in illumination represents a compromise between sensitivity and acuity • When the level of illumination is high, the visual system takes advantage of the situation by constricting the pupils • When the pupils are constricted, the images falling on each retina is sharper and there is a greater depth of focus • However, when the level of illumination is too low to adequately activate the receptors, the pupils dilate to let in more light, thereby sacrificing acuity and depth of focus • Behind each pupil is a lens, which focuses incoming light on the retina • The tension on the ligaments holding each lens in place is adjusted by the ciliary muscles, and he lens assumes its natural cylindrical shape • This increases the ability of the lens to refract (bend) light and thus brings close objects into sharp focus • The process of adjusting the configuration of the lenses to bring images into focus on the retina is called accommodation • Eye Position and Binocular Disparity • One reason vertebrates have two eyes is that vertebrates have two sides: left and right • By having one eye on each side, vertebrates can see in almost every direction without moving their heads • Humans, have their eyes mounted side by side for our visual system’s ability to create 3D perceptions (to see depth) from 2D retinal images • The movements of your eyes are coordinated so that each point in your visual world is projected to corresponding points on your two retinas • To accomplish this, your eyes must converge (turn slightly inward); convergence is greatest when you are inspecting things that are close • Binocular disparity - the difference in the position of the same image on the two retinas - is greater or close objects than for distant objects Chapter 6: The Visual System • Your visual system can use the degree of binocular disparity to construct one 3D perception from two 2D retinal images THE RETINAAND TRANSLATION OF LIGHT INTO NEURAL SIGNALS • The retina converts light to neural signals, conducts them toward the CNS, and participates in the processing of the signals • The retina is composed of five layers of different types of neurons: receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells • Each of these five types of retinal neurons comes in a variety of subtype • Amacrine cells and horizontal cells are specialized for lateral communication • Retinal neurons communicate both chemically via synapses and electrically via gap junctions • Light reaches the receptor layer only after passing through the other four layers • Then, once the receptors have been activated, the neural images is transmitted back out through the retinal layers to the retinal ganglion cells, whose axons project across the inside of the retina before gathering together in a bundle and exiting the eye ball • This inside-out arrangement creates two visual problems: one is that the incoming light is distorted by the retinal tissue through which it must pas before reaching the receptors • The other is that for the bundle of retinal ganglion axons to leave the eye, there must be a gap in the receptor layer; this gap is called the blind spot • The first of these two problems is minimized by the fovea • The fovea is an indentation, about .33 cm in diameter, at the center of the retina; it is the area of the retina that is specialized for high-acuity vision • The thinning of the retinal ganglion cell layer at the fovea reduces the distortion of incoming light • The blind spot requires a more creative solution • The visual system uses information provided by the receptors around the blind spot to fill in the gaps in your retinal images • Completion is a fundamental visual system function • When you look at n object, your visual system extracts key information about the object and conducts that information to the cortex, where a perception of the entire object is created from that partial information • Surface Interpolation - the process by which we perceive surfaces; the visual system extracts information about edges and from it infers the appearance of large surfaces • The central role of surface interpolation in vision is an extremely important but counterintuitive concept • Cone and Rod Vision • There are two different types of receptors in the human retina: cone-shaped receptors called cones, and rod-shaped receptor called rods • Species active only in the day tend to have cone-only retinas and species active only at night tend to have rod only retinas • Duplexity theory - the theory that cones and rods mediate different kinds of vision • Photopic vision predominates in good lighting and provides high-acuity coloured perceptions of the world Chapter 6: The Visual System • In dim illumination, there is not enough light to reliably excite the cones, and the more sensitive scotopic vision predominates • The sensitivity of scoptic vision is not achieved without cost: Scoptic vision lacks both the detail and the colour of photopic vision • The differences between photopic and scotopic vision result in part from a difference in the way the two systems are wired • In the scoptic system, the output of several hundred rods converge on a single retinal ganglion cell, whereas in the photopic system only a few cones converge on each retinal ganglion cell to receive input from only a few cones • The effects of dim light simultaneously stimulating many rods can summate to influence the firing of the retinal ganglion cell onto which the output of the stimulated rods converges, whereas the effects of the same dim ight applied to a sheet of cones cannot summate to the same degree, and the retinal ganglion cells may not respond at all to the light • The convergent scoptopic system pays for its high degree of sensitivity with a low level of acuity • Cones and rods differ in their distribution on the retina • There are no rods at all in the fovea, only cones • At the boundaries of the foveal indentation, the proportion of cones declines markedly, and there is an increase in the number of rods • There are more rods in the nasal hemiretina than in the temporal hemiretina • Spectral Sensitivity • More intense lights appear brighter • However, wavelength also has a substantial effect on the perception of brightness • Lights of the same intensity but of different wavelengths can differ markedly in brightness • A graph of the relative brightness of lights of the same intensity presented at different wavelengths is called a spectral sensitivity curve • The most important thing to remember about spectral sensitivity curves is that humans and other animals with both cones and rods have two of them: a photopic spectral sensitivity curve and a scotopic spectral sensitivity curve • Under photopic and scotopic conditions, the visual system is maximally sensitive to wavelengths of about 560 nm; thus, under photopic conditions, a light at 500 nm would have to be much more intense than one at 560 nm to be seen as equally bright • Under scotopic conditions, the visual system is maximally sensitive to wavelengths of about 500 nm; thus, under scotopic conditions, a light of 560 nm would have to be much more intense than one at 500 nm to be seen as equally bright • Eye Movement • What we see s determined bot just by what is projected on the retina at that instant • Although we are not aware of it, the eyes continually scan the visual field, and our visual perception at any instant is a summation of recent visual information • It is because of this temporal integration that the world does not vanish momentarily each time we blink • These involuntary fixational eye movements are of three kinds: tremor, drifts, and saccades Chapter 6: The Visual System • Although, we are normally aware of fixational eye movements, they have a critical visual function • We must fix our gaze to perceive the minute details of our world, but, ironically, if we were to fixate perfectly, our world would fade and disappear • Visual Transduction: The Conversion of Light to Neural Signals • Transduction is the conversion of one form of energy to another • Visual transduction is the conversion o light to neural signals by the visual receptors • When rhodopsin was exposed to continuous intense light, it was bleached and it lost its ability to absorb light; but when it was returned to the dark, it regained both its redness and its light-absorbing capacity • It is now clear that rhodopsins absorption of light is the first step in rod-mediated vision • The degree to which rhodopsin’s absorbs light in various situations predicts how humans see under the very same conditions • In dim light, our sensitivity to various wavelengths is a direct consequence of rhodopsins ability to absorb them • Rhodopsin is a G-protein-coupled receptor that responds to light rather than to neurotransmitter molecules • Rhodopsins initiate a cascade of intracellular chemical events when they are activated • When rods are in darkness, their sodium channels are partially open, thus keeping the rods slightly depolarized and allowing a steady flow of excitatory glutamate neurotransmitter molecules to emanate form them • When rhodopsin receptors are bleached by light, the resulting cascade of intracellular chemical events closes the sodium channels, hyperpolarized the rods, and reduces the release of glutamate • Signals are often transmitted through neural systems by inhibition FROM RETINA TO PRIMARY VISUAL CORTEX • Retina-geniculate-striate pathways, which conduct signals from each retina to the primary visual cortex, or striate cortex via the lateral geniculate nuclei of the thalamus • All signals form the left visual field reach the right primary visual cortex, either ipsilaterally form the temporal hemiretina of the right eye or contralaterally form the nasal hemiretina of the left eye - an that the opposite is true of all signals for the right visual field • Each lateral geniculate nucleus has six layers of each nucleus receives input from all parts of the contralateral visual field of one eye • Most of the lateral geniculate neurons that project to the primary visual cortex terminate in the lower part of cortical later IV, producing a characteristic stripe, or striation, when viewed in cross section - hence the name striate cortex • Retinotopic Organization • The retina-geniculate-striate system in retinotopic each level of the system is organized like a map of the retina • This means that two stimuli presented to adjacent areas of the retina excite adjacent neurons are all levels of the system Chapter 6: The Visual System • The retinotopic layout of the primary visual cortex has a disproportionate representation of the fovea; although the fovea is only a small part of the retina, a relatively large proportion of the primary visual cortex is dedicated to the analysis of this input • The M and P Channels • At least two parallel channels of communication flow through each lateral geniculate nucleus • One channel run through the top layers • These layers are called the parvocellular layers because the are composed of neurons with small cell bodies • The other channels runs through the bottom two layers, which are called the magnocellular layers because they are composed of neurons with large cell bodies • The parvocellular neurons are particularly responsive to colour, to fine patter details, and to stationary or slowly moving objects • The magnocellular neurons are particularly responsive to movement • Cones provide the majority of the input to the P layers, whereas rods provide the majority of the input to the M layers SEEING EDGES • Edges are the most informative features of any visual display because they define the extent and position of the various objects in it • Visual systems of many species are particularly good at edge perception • A visual edge is merely the place where two different areas of a visual image meet • The perception of an edge is really the perception of a contrast between two adjacent areas of the visual field • Lateral Inhibition and Contrast Enhancement • The nonexistent stripes of brightness and darkness running adjacent to the edges are called Mach bands; they enhance the contrast at each edge and make the edge easier to see • Contrast enhancement is not something that occurs just in books • Every edge we look at is highlighted for us by the contrast-enhancing mechanisms of our nervous systems • Large receptors, called ommatidia • The axons of the ommatidia are interconnected by a lateral neural network • If a single ommatidium is illuminated, it fires at a rate that is proportional to the intensity of the light striking it • When a receptor fires it inhibits its neighbors via the lateral neural network; this inhibition is called lateral inhibition because it spreads laterally across the array of receptors • The receptor adjacent to the edge on the more intense side fires more than the other intensely illuminated receptors while the receptor adjacent to the edge on the less well-illuminated side fires less than the other receptors on that side • Lateral inhibition accounts for these differences • Receptive Fields of Visual Neurons • The receptive field of a visual neuron is the area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron Chapter 6: The Visual System • Receptive Fields: Neurons of the Retina-Geniculate-Striate System • Hubel and Wiesel began their studies of visual system neurons by recording from the three levels of the retina-geniculate system: first from retinal ganglion cells, then from lateral geniculate neurons and finally from the striate neurons of lower layer IV, the terminus of the system • When Hubel and Wiesel compared the receptive fields recorded form retinal ganglion cells, lateral geniculate nuclei, and lower layer IV neurons, four commonalities were readily apparent: • At each level, the receptive fields in the foveal area of the retina were smaller than those at the periphery, this is consistent with the fact that the fovea mediates fine-grained vision • All the neurons had receptive fields that were circular • All the neurons were monocular, that is, each neuron had a receptive field in one eye but not the other • Many neurons at each of the three levels of the retina-geniculate-striate system had receptive fields that comprised an excitatory area and an inhibitory area separated by a circular boundary • On-center cells respond to lights shone in the central region of their receptive fields with “on” firing and to lights shone in the periphery of their receptive fields with inhibition, followed by “off” firing when the light is turned off • Off center cells display the opposite patterns • In effect, on-center and off-center cells respond best to contrast • The most effective way to influence the firing rate of an on-center or off-center cell is to maximize the contrast between the center and the periphery of its receptive field by illuminating either the entire center or the entire surround, while leaving the other region completely dark • Diffusely illuminating the entire receptive field has little effect on firing • Hubel and Wiesel thus concluded that one function of many of the neurons in the retina-geniculate-striate system is to respond to the degree of brightness contrast between the two areas of their receptive fields • Visual system neurons: most are continually active, even when there is no visual input • Receptive Fields: Simple Cortical Cells • The receptive fields of most primary visual cortex neurons fall into one of two classes: simple or complex • Simple cells, like lower layer IV neurons, have receptive fields that can be divided into antagonistic “on” and “off” regions and are thus unresponsive to diffuse light • They are monocular • The main difference is that the borders b
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