Chapter 3: Introduction to Vision

Chapter 3: Introduction to Vision

Light is the Stimulus for Vision • Electromagnetic spectrum • Energy is described by wavelength. • Spectrum ranges from short wavelength gamma rays to long wavelength radio waves. • Visible spectrum for humans ranges from 400 to 700 nanometers. • Most perceived light is reflected light

Figure 3.1 The electromagnetic spectrum, showing the wide range of energy in the environment and the small range within this spectrum, called visible light, that we can see.

Focusing Images on the Retina • The cornea, which is fixed, accounts for about 80% of focusing. • The lens, which adjusts shape for object distance, accounts for the other 20%. • Accommodation results when ciliary muscles are tightened which causes the lens to thicken. • Light rays pass through the lens more sharply and focus near objects on retina.

Figure 3.2 An image of the cup is focused on the retina, which lines the back of the eye. The close-up of the retina on the right shows the receptors and other neurons that make up the retina.

Figure 3.3 Focusing of light rays by the eye. (a) Rays of light coming from a small light source that is more further than 20 feet away are approximately parallel. The focus point for parallel light is at A on retina. (b) Moving an object closer to the relaxed eye pushes the focus point back. Here the focus point is at B, but light is stopped by the back of the eye. (c) Accommodation of the eye (indicated by the fatter lens) increases the focusing power of the lens and brings the focus point for a near object back to A on the retina.

Focusing Images on Retina - continued • Myopia or nearsightedness - Inability to see distant objects clearly • Image is focused in front of retina • Caused by • Refractive myopia - cornea or lens bends too much light • Axial myopia - eyeball is too long

Focusing Images on Retina - continued • Solutions for myopia • Move stimulus closer until light is focused on the retina • Corrective lenses can also be used. • LASIK surgery can also be successful.

Figure 3.5 Focusing of light by the myopic (nearsighted) eye. (a) Parallel rays from a distant spot of light are brought to a focus in front of the retina, so distant objects appear blurred. (b) As the spot of light is moved closer to the eye, the focus point is pushed back until, at the far point, the rays are focused on the retina, and vision becomes clear. (c) A corrective lens, which bends light so that it enters the eye at the same angle as the light coming from the far point, brings light to a focus on the retina. Angle A is the same in (b) and (c).

Focusing Images on Retina - continued • Hyperopia or farsightedness - inability to see nearby objects clearly • Focus point is behind the retina. • Usually caused by an eyeball that is too short • Constant accommodation for nearby objects can lead to eyestrain and headaches.

Retinal Processing - Rods and Cones • Differences between rods and cones • Shape • Rods - large and cylindrical • Cones - small and tapered • Distribution on retina • Fovea consists solely of cones. • Peripheral retina has both rods and cones. • More rods than cones in periphery.

Figure 3.12 The distribution of rods and cones in the retina. The eye on the left indicates locations in degrees relative to the fovea. These locations are repeated along the bottom of the chart on the right. The vertical brown bar near 20 degrees indicates the place on the retina where there are no receptors because this is where the ganglion cells leave the eye to form the optic nerve. (Adapted from Lindsay & Norman, 1977.)

Retinal Processing - Rods and Cones - continued • Number - about 120 million rods and 5 million cones • Blind spot - place where optic nerve leaves the eye • We don’t see it because: • one eye covers the blind spot of the other. • it is located at edge of the visual field. • the brain “fills in” the spot.

Figure 3.14 There are no receptors at the place where the optic nerve leaves the eye. This enables the receptor’s ganglion cell fibers to flow into the optic nerve. The absence of receptors in this area creates the blind spot.

Diseases that Affect the Retina • Macular degeneration • Fovea and small surrounding area are destroyed • Creates a “blind spot” on retina • Most common in older individuals • Retinitis pigmentosa • Genetic disease • Rods are destroyed first • Foveal cones can also be attacked • Severe cases result in complete blindness

Measuring Dark Adaptation • Three separate experiments are used. • Method used in all three experiments: • Observer is light adapted • Light is turned off • Once the observer is dark adapted, she adjusts the intensity of a test light until she can just see it.

Figure 3.18 Viewing conditions for a dark adaptation experiment. The image of the fixation point falls on the fovea, and the image of the test light falls in the peripheral retina.

Measuring Dark Adaptation - continued • Experiment for rods and cones: • Observer looks at fixation point but pays attention to a test light to the side. • Results show a dark adaptation curve: • Sensitivity increases in two stages. • Stage one takes place for three to four minutes. • Then sensitivity levels off for seven to ten minutes - the rod-cone break. • Stage two shows increased sensitivity for another 20 to 30 minutes.

Figure 3.19 Three dark adaptation curves. The red line is the two-stage dark adaptation curve, with an initial cone branch and a later rod branch. The green line is the cone adaptation curve. The black curve is the rod adaptation curve. Note that the downward movement of these curves represents an increase in sensitivity. The curves actually begin at the points indicating “light-adapted sensitivity,” but there is a slight delay between the time the lights are turned off and when measurement of the curves begins. (Partial data [purple curve] from Ruston, 1961.)

Measuring Dark Adaptation - continued • Experiment for cone adaptation • Test light only stimulates cones. • Results show that sensitivity increases for three to four minutes and then levels off. • Experiment for rod adaptation • Must use a rod monochromat • Results show that sensitivity increases for about 25 minutes and then levels off.

Spectral Sensitivity of Rods and Cones - continued • Rod spectral sensitivity shows: • more sensitive to short-wavelength light. • most sensitivity at 500 nm. • Cone spectral sensitivity shows: • most sensitivity at 560 nm. • Purkinje shift - enhanced sensitivity to short wavelengths during dark adaptation when the shift from cone to rod vision occurs

Convergence in the Retina • Rods and cones send signals vertically through • bipolar cells. • ganglion cells. • ganglion axons. • Signals are sent horizontally • between receptors by horizontal cells. • between bipolar and between ganglion cells by amacrine cells.

Convergence in the Retina - continued • 126 million rods and cones converge to 1 million ganglion cells. • Higher convergence of rods than cones • Average of 120 rods to one ganglion cell • Average of six cones to one ganglion cell • Cones in fovea have one to one relation to ganglion cells

Convergence and Sensitivity • Rods are more sensitive to light than cones. • Rods take less light to respond • Rods have greater convergence which results in summation of the inputs of many rods into ganglion cells increasing the likelihood of response. • Trade-off is that rods cannot distinguish detail

Figure 3.26 The wiring of the rods (left) and the cones (right). The dot and arrow above each receptor represents a “spot” of light that stimulates the receptor. The numbers represent the number of response units generated by the rods and the cones in response to a spot of intensity of 2.0.

Convergence and Detail • All-cone foveal vision results in high visual acuity • One-to-one wiring leads to ability to discriminate details • Trade-off is that cones need more light to respond than rods

Figure 3.28 Neural circuits for the rods (left) and the cones (right). The receptors are being stimulated by two spots of light.

Figure 3.29 Circuit with convergence and inhibition form Figure 2.16. Lateral inhibition arrives at neuron B from A and from C.

Lateral Inhibition and Lightness Perception • Three lightness perception phenomena explained by lateral inhibition • The Hermann Grid: Seeing spots at an intersection • Mach Bands: Seeing borders more sharply • Simultaneous Contrast: Seeing areas of different brightness due to adjacent areas

Figure 3.32 The Hermann grid Notice the gray “ghost images” at the intersections of the white areas, which decrease or vanish when you look directly at the intersection.

Hermann Grid • People see an illusion of gray images in intersections of white areas. • Signals from bipolar cells cause effect • Receptors responding to white corridors send inhibiting signals to receptor at the intersection • The lateral inhibition causes a reduced response which leads to the perception of gray.

Mach Bands • People see an illusion of enhanced lightness and darkness at borders of light and dark areas. • Actual physical intensities indicate that this is not in the stimulus itself. • Receptors responding to low intensity (dark) area have smallest output. • Receptors responding to high intensity (light) area have largest output.

Figure 3.35 Mach bands at a contour between light and dark. (a) Just to the left of the contour, near B, a faint light band can be perceived, and just to the right at C, a faint dark band can be perceived. (b) The physical intensity distribution of the light, as measured with a light meter. (c) A plot showing the perceptual effect described in (a). The bump in the curve at B indicates the light Mach band, and the dip in the curve at C indicates the dark Mach band. The bumps that represent our perception of the bands are not present in the physical intensity distribution.

Mach Bands - continued • All receptors are receiving lateral inhibition from neighbors • In low and high intensity areas amount of inhibition is equal for all receptors • Receptors on the border receive differential inhibition

Mach Bands - continued • On the low intensity side, there is additional inhibition resulting in an enhanced dark band. • On the high intensity side, there is less inhibition resulting in an enhanced light band. • The resulting perception gives a boost for detecting contours of objects.

Figure 3.37 Circuit to explain the Mach band effect based on lateral inhibition. The circuit works like the one for the Hermann grid in Figure 3.34, with each bipolar cell sending inhibition to its neighbors. If we know the initial output of each receptor and the amount of lateral inhibition, we can calculate the final output of the receptors. (See text for a description of the calculation.)

Figure 3.38 A plot showing the final receptor output calculated for the circuit of figure 3.37. The bump at B and the dip at C correspond to the light and dark Mach bands, respectively.

Figure 3.39 Simultaneous contrast. The two center squares reflect the same amount of light into your eyes but look different because of simultaneous contrast.

Simultaneous Contrast • People see an illusion of changed brightness or color due to effect of adjacent area • An area that is of the same physical intensity appears: • lighter when surrounded by a dark area. • darker when surrounded by a light area.

Simultaneous Contrast - continued • Receptors stimulated by bright surrounding area send a large amount of inhibition to cells in center. • Resulting perception is of a darker area than when this stimulus is viewed alone. • Receptors stimulated by dark surrounding area send a small amount of inhibition to cells in center. • Resulting perception is of a lighter area than when this stimulus viewed alone.

Figure 3.40 How lateral inhibition has been used to explain the simultaneous contrast effect. The size of the arrows indicate the amount of lateral inhibition. Because the square on the left receives more inhibition, it appears darker.

Illusions not Explained by Lateral Inhibition • White’s Illusion • People see light and dark rectangles even though lateral inhibition would result in the opposite effect.

Figure 3.41 White’s illusion. The rectangles at A and B appear different, even though they are printed from the same ink and reflect the same amount of light. (From White, 1981.)

Figure 3.42 When you mask off part of the White’s illusion display, as shown here, you can see that rectangles A and B are actually the same. (Try it!)

Figure 3.43 The arrows indicate the amount of lateral inhibition received by parts of rectangles A and B. Because the part of rectangle B is surrounded by more white, it receives more lateral inhibition. This would predict that B should appear darker than A (as in the simultaneous contrast display in Figure 3.39), but the opposite happens. This means that lateral inhibition cannot explain our perception of White’s illusion.

Explanation of White’s Illusion • Belongingness • An area’s appearance is affected by where we perceive it belongs. • Effect probably occurs in cortex rather than retina. • Exact physiological mechanism is unknown.

Chapter 3: Introduction to Vision