Sensation & Perception PSYC420-01 Thomas E. Van Cantfort, Ph.D The Eye The Eye The function of the eyeball is to protect the photoreceptors The role of the eye is to capture an image of objects that we see And to translate that light energy into electrical energy The eye also performs some computation on the image. Morphology of the eye Cornea is a clear front part of the eye, which functions like the first lens in a camera Ú Most of the optical powers of the eye is contained in the curved front surface of the cornea. Ú The cornea bends the light rays that comes from distant objects. The iris is a smooth ring of muscle with a central opening the size of which depends on the state of contraction of the iris. The central opening is the pupil Ú The area of the pupil varies as a function of the amount of light impinging on the eye. Ú As well as, being influenced by emotional states. < Belladonna literally means beautiful woman Just behind the iris lies the crystalline lens. Ú It purpose is to assist the cornea in producing a focused image of the visual world onto the back of the eye. Ú The lens of the eye is convex and is normally a clear tissue with no blood vessels. It is this lens that the eye muscle can bend and flatten to focus the light onto the back of the eye Major landmarks at the back of the eyes are: Ú Retina Ú Fovea Ú And optic disc
ight Enters the Eye ight Enters the Eye (continued) ight is bent and the image is inverted. The light falls on the retina and stimulates the photoreceptors. Morphology of the Retina Photoreceptors Ú Rods respond to low intensity light < There are approximately twenty times as many rods as there are cones. < There are approximately 120-140 million rods in each eye. < Rods are concentrated towards the periphery. Ú Cones respond to high intensity light and wavelength < There are approximately 6-7 million cones in each eye. < Cones are concentrated near the center area of the retina (called fovea centralis) Morphology of the Retina (continued) Bipolar cells Horizontal cells Amacrine cells Ganglionic cells Morphology of the Retina (continued) The Fovea The fovea is compose of only photoreceptors The lens focuses the light on the fovea. The fovea is entirely composed of cones As we move away from the fovea the number of rods increase until finally there are only rods in the periphery of the retina.
The Fovea (continued) Receptors in the Retina It has long been assumed that some kind of cell in the retina had to be specifically sensitive to light. But it was not fully established that two different types of cells (rods and cones) had this function until the development of modern techniques for studying the retina in microscopic detail. Ú Yet, even if these techniques were not available to us now Ú There would be considerable behavioral data that would strongly implicated a two-receptor theory of the retina Ú Often referred to as the duplicity theory. One Evidence for Two Receptors Duplicity Theory The first clue originated from experiments involving dark-adaptation. Ú In studies of this type, the observer is suddenly in a dark environment after viewing a very bright light for 2 minutes. Ú A faint light is then introduced and increased in intensity, so that a detection threshold can be determined. Ú As one may expect, as the time in darkness lengthens, the threshold declines. Duplicity Theory (continued) Notice that the curve falls in two stages. Ú With a plateau reached after 10 minutes in the dark Ú Followed by a further fall until a minimal point is reached. Ú In approximately 30 minutes. If only one type of cell were gaining sensitivity in the dark, It would seem strange that the dark- adaptation would appear as a two-stage decline. George Wald (1954) From the pioneering work of George Wald (1954) the dark-adaptation curve is a result of the changes that occur in the light-sensitive pigments in the rod and cone cells. Ú If the light is very bright, a substantial amount of pigments in both cells is bleached. < That is depleted Ú As a result, the cells temporarily lose much of their sensitivity to light. Ú When in the dark, the pigments begins to be resynthesized in a process that is completed in approximately 30 minutes.
Duplicity Theory (continued) Cone pigment (called iodopsin) resynthesizes first; Ú And as the resynthesis continue, the sensitivity of cones rises. Ú As a result the resynthesis process becomes complete. Ú We reach a temporary limit (the plateau midway in the dark-adaptation process) Resynthesis of the rod pigment (called rhodopsin) begins at this point. Ú Resulting in a second rise in sensitivity until a final limit is reached. Dark Adaptation If only one type of cell were gaining sensitivity in the dark, it would seem strange that the darkadaptation would appear as a two-stage decline. Purkinje Shift Another set of behavioral observations strongly implicating a functional distinction between rods and cones comes from the study of relative brightness. Johannes Purkinje, a Czech biologist, first observed this phenomena in 1825. Ú At dusk, the red flowers in his garden quickly darkened Ú While his blue-violet flowers remained bright < We now know that this observation is a result of a shift. < Often referred to as the Purkinje Shift Purkinje Shift (continued) < A shift in luminosity function, as a conesprocessing system which is dominate during the day < Is replaced by a rod-processing system which is dominant during the night. < Under bright illumination conditions, the peak in luminosity is reached at 555 nm wavelengths < Under dim illumination condition, the peak is reached at 505 nm wavelength. Purkinje Shift (continued) Ú In the transition from cone system during the late afternoon hours, the brightness of wavelengths around 400-450 nm (the blueviolet flowers) would be shifted upward Ú While the brightness of wavelengths around 600-650 nm (the red flowers) would be shifted downward. We would see an equivalent (but opposite) effect in the early morning hours. When the shift in the retinal processing system would take place in the opposite direction. in Rods and Cones As mentioned earlier, the first step in the processing of light by the nervous system Is taken when the light is absorbed by the pigments: Ú Rhodospsin in the rods Ú Iodopsin in the cones A description of this critical process can best be made in terms of rhodopsin.
in Rods When a sufficient amount of light (as little as a single photon) strikes a particular rod A portion of the 100 million or so rhodopsin molecules are broken into two components: Ú Retinal Ú And a protein called rod opsin. Ú The process of rhodopsin breakdown is completed very quickly, about 2 msec. Continued present of light results in the subsequent change of retinal to a related molecule, retinol (another name for Vitamin A) in Rods (continued) Only when the light is removed will the retinol begin to revert back to the retinal form. Setting the stage for resynthesis of rhodopsin. In the Cones The process of iodopsin breakdown, resulting from light absorption in cone cells, follows the same general course. Ú Iodopsin splits into retinal and a protein called cone opsin, until resynthesis can occur during darkness. Ú While the chemical process of breakdown in rods and cones may be similar. Ú An important difference lies in the fact that there is only a single opsin in the rod cell Ú While there three distinct opsins in cone cells In Cones Ú Each cone has one of the three possible opsins. Ú Brown and Wald (1964) found the maximal sensitivities of the three populations of cone opsins in human retina < 455 nm < 535 nm < 570 nm Ú It is interesting that these wavelength peaks correspond roughly to blue, green, and red hue. Neural Activity In Photoreceptors ight causes the rod or cone to produce a hyperpolarization By acting on a molecule similar to cyclic AMP (camp) called guanosine monophosphate (cgmp). Normally cgmp binds to the channel of the membrane in these cells and controls the permeability to sodium ions. Neural Activity In Photoreceptors ight causes the levels of cgmp to drop and causes a closing of the channels. When the sodium permeability is blocked, the membrane potential becomes hyperpolarized. While hyperpolarization ordinarily would serve as an inhibitory influence. The overall impact of light upon the rods and cones is actually excitatory. The neurotransmitter released by these cells is glutamic acid, an inhibitory neurotransmitter. Thus, light produces an inhibition of an inhibitory process.(