Bio-Psych: Vision (the Fifth Sense).doc

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Biomedical Physio & Kines
BPK 142
Neil Watson

Biological Psychology 7edition Website-sourced Study Questions Chapter 10: Vision: From Eye to Brain The Visual System Extends from the Eye to the Brain • The vertebrate eye acts in some ways like a camera • Visual processing begins in the retina • Photoreceptors transduce light into chemical reactions • Different mechanisms enable the eyes to work over a wide range of light intensities • Acuity is best in foveal vision • Brightness is created by the visual system Study questions: 1) The visual field is the entire area that you can see without moving your head or eyes, but the sharpness of the visual image, known as visual acuity, is far greater toward the center than at the periphery. It is for this reason that we constantly shift our gaze between different objects in the environment. 2) The basic dimensions of our perception of light include hue, which varies through blue, green, yellow, orange, red, and intermediates; brightness, which varies from light to dark; and saturation, which varies from fullness to paleness. People with color blindness are impaired in the perception of hue: about 8% of males and 0.5% of females exhibit some degree of color blindness. Complete color blindness is therefore extremely rare. 3) The apparent brightness of visual objects is subject to modification by the visual system; for example, interactions between neighboring retinal cells can enhance contrast at the boundaries of patches. The process that acts to enhance boundaries is called lateral inhibition. 4) The visual system responds to electromagnetic radiation. In humans, the eyes detect quanta of light energy, called photons, within a very narrow range of wavelengths, ranging from 400 to 700 nanometers (nm). 5) The blind spot is located laterally from the point of fixation for each eye and corresponds to the location at which the optic nerve exits the eye (called the optic disc). In normal vision, the brain fill(s) in this perceptual gap, so the blind spot is not noticeable. 6) Light travels in straight lines until it encounters the cornea, which causes the light to bend in a process known as refraction. As this light enters the eye, it is further modified by the lens, which is able to change its shape due to the ciliary muscles. This process is called accommodation and results in the projection of a sharply focused image on the retina, where the photoreceptors are located. The amount of light entering the eye is governed by the size of the pupil, which may be increased by drugs (such as belladonna, or “beautiful lady”) that block cholinergic transmission in the parasympathetic nervous system. 7) Fill in the missing information, identifying the four types of intermediate cells found in the retina, their connections, and the potentials they produce. a) Bipolars connect to ganglion cells and produce graded potentials. b) Horizontal cells connect to receptor cells and produce graded potentials. 1 c) Amacrine cells connect to bipolar cells and ganglion cells and produce action potentials. d) Ganglion cells connect to brain cells and produce action potentials. 8) The optic nerve is made up of the axons of ganglion cells; there are roughly 100 more ganglion cells than receptor cells, so the degree of convergence is about 100 to 1. 9) The rods are especially involved in the scotopic visual system, which works in dim light and does not give rise to color perception. The cones are especially involved in the photopic visual system, which requires more light but gives highly detailed vision, including color information in many species. 10)Fill in the missing information about the sequence of events that occurs from the point at which photons strike a receptor to the release of transmitter by the receptor. a) Photons strike rhodopsin and cause it to split into RETINAL and opsin. b) Activated opsin combines with a G protein called transducin, which activates the enzyme phosphodiesterase. c) Phosphodiesterase transforms cGMP to 5′-GMP, which causes Na+ channels (which had been held open by the cGMP) to close. + d) Blockade of Na ions produces a hyperpolarizing generator potential, which regulates the rate of release of the transmitter glutamate and signals light information 11)Bipolar cells release the neurotransmitter glutamate, which always depolarizes ganglion cells onto which they synapse. 12)The strongest light that you can detect is about 10 billion times as intense as the weakest light you can detect. 13)Adaptation of the sensitivity of photoreceptors is accomplished in three main ways: a) Ca2+ availability is modulated. b) Rate is limited by slow recombination of opsin and RETINAL into rhodopsin. c) Retinal chemicals become less available at high intensities, so larger numbers of photons are required for activation. 14)One reason that acuity is heightened in the center of the visual field is the increased density of receptors there, especially cones in the central part of the retina, which is called the fovea. The diameter of photoreceptors is lesser in this location than in the periphery. Some species, such as hawks and eagles, have a much greater density of cones than humans do, and their receptors have a smaller diameter. 15)You more likely to be able to see a dim star at night if you look 20° away from it than if you look directly at it because rods tend to be found in the periphery, ringing the fovea. Because rods are the basis of the scotopic system and are more sensitive to dim light, you are more likely to detect a very dim object in peripheral vision than in central vision. 2 Table 10.1 Properties of the Human Photopic and Scotopic Visual Systems Property Photopic System Scotopic System Receptors Cones Rods Approximate # receptors/eye 4 million 100 million Photopigments Three classes of cone opsins; Rhodopsin the basis of color vision Sensitivity Low; needs relatively strong High; can be stimulated by stimulation; used for weak light intensity; day vision used for night vision Location in Retina Concentrated in and near Outside fovea fovea; present less densely throughout retina Receptive field size & visual Small in fovea, so acuity is Larger, so acuity is lower acuity high; larger outside fovea Temporal Responses Relatively rapid Slow Neural Signals Travel from the Retina to Several Brain Regions • The retina projects to the brain in a topographic fashion • Know Figure 10.10: Visual Pathways in the Human Brain • View Animation 10.2 for the same topic Study questions: 16)Primary visual cortex is known as area V1. This area contains a “retinotopic” map in which the area corresponding to the fovea is greatly enlarged. In primates, including humans, about 50% of this cortical area is devoted just to the fovea, which contributes to heightened acuity in central vision. 17)A scotoma is a blind spot in the visual field caused by a lesion somewhere in the visual pathway. Because the visual pathway is orderly (retinotopic), there is a direct correspondence between the location of the lesion and the location of the scotoma in the visual field. Although there is no conscious visual perception within the scotoma, objects may sometimes be discriminated in this region due to a phenomenon known as blindsight. 18)Complete the sentences below about the five evolutionary steps that seem to have occurred in the development of eyes across the various phyla. a) Localized groups of light-sensitive cells developed. b) Light receptors clustered at the bottom of cuplike depressions, allowing for some directional sensitivity. c) The rim of the cup narrowed to produce a small aperture like a pinhole camera, allowing the eye to focus. d) The opening of the cup was closed with transparent skin, and/or the cup was filled with transparent material to keep out foreign material. e) The skin or some other component developed into a lens; this aids in focusing, but also allows the aperture to be larger than the “pinhole” version, allowing more light to enter and conferring low-light sensitivity 3 19)Although the evolution of eyes is often cited as an example of convergent evolution, it appears that the eyes of all seeing species develop using the Pax gene; this shared feature suggests a common origin. 20)Fill in the missing details describing the major parts of the visual pathways. a) The axons of the ganglion cells of the retina form the optic nerve. b) In humans, fibers from the nasal retina cross over to the opposite side of the brain at the optic chiasm. c) Once they enter the brain, the axons of the retinal ganglion cell are called the optic tract. d) The axons carrying visual information from the eyes terminate chiefly at the lateral geniculate nucleus of the thalamus. e) Axons from the LGN form a fiber tract called the optic radiations, which terminates at the visual cortex. Neurons at Different Levels of the Visual System Have Very Different Receptive Fields • Photoreceptors excite some retinal neurons and inhibit others • Neurons in the retina and the LGN have concentric receptive fields • Neurons in the visual cortex have varied and complicated receptive fields • Most cells in the primary visual cortex are tuned to particular spatial frequencies • Area V1 is involved in the formation of mental images •
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