Perception-Chapter 3

Absorption spectrum

*Is a plot of the amount of light absorbed by a substance vs. the wavelength of the light.

Lateral inhibition

*Inhibition that is transmitted across the retina.

Rods and cones

*2 kinds of visual receptors that contain light-sensitive chemicals called visual pigments. *Visual pigments react to light and trigger electrical signals

Macular degenerations

*A condition in which most common in older people, destroys the cone-rich fovea and a small area that surrounds it. *This creates a "blind spot" in central vision, so when a person looks at somethings he or she loses sight of it.

Dark adaptation curve

*A plot of how visual sensitivity changes in the dark, beginning with when the lights are extinguished.

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

Spectral sensitivity

*An observer's sensitivity to light at each wavelength across the visible spectrum.

Retinitis pigmentosa

*Another condition which is a degeneration of the retina that is passed from one generation to the next. *This condition first attacks the peripheral rod receptors and results in poor vision in the peripheral visual field. -eventually in severe cases, the foveal cone receptors are also attacked, resulting in complete blindness.

Dark adaptation

*Causes the eye to increase its sensitivity in the dark.

Optic Nerve

*Conducts signals toward the brain.

Physiological Reaction after Isomerization

*Current research in physiology and chemistry shows that isomerization triggers an enzyme cascade. -Enzymes facilitate chemical reactions. -A cascade means that a single reaction leads to increasing numbers of chemical reactions. -This is how isomerizing one pigment leads to the activation of a rod receptor.

Visual acuity

*Detail vision *Visual acuity is highest at the fovea. *Cones have better visual acuity because they have less convergence.

Rod and cone absorption spectra

*Difference in spectral sensitivity is due to absorption spectra of visual pigments *Rod pigment absorbs best at 500 nm. *Cone pigments absorb best at (S) 419nm, (M) 531nm, and (L) 558nm -Absorption of all cones equals the peak of 560nm in the spectral sensitivity curve

Retinal Processing - Rods and Cones

*Differences between rods and cones -Shape 1) Rods - large and cylindrical 2) Cones - small and tapered -Distribution on retina 1) Fovea consists solely of cones. 2) Peripheral retina has both rods and cones. 3) More rods than cones in periphery.

Limulus

*Horseshoe crab

The Eye

*Is where vision begins *Light reflected from objects in the environment enters the eye through the pupil and is focused by the cornea and lens to form sharp images of the objects on the retina, which contains receptors for vision

Photons

*Light can also be described as this *Small packets of energy with one photon being the smallest possible packet of light energy.

Monochromatic light

*Light that contain only a single wavelength

Experiment for cone adaptation

*Make sure the test light only stimulates cones. -Look directly at test light so image will fall on the all cone fovea. -Results show that sensitivity increases for three to four minutes and then levels off.

Light adapted sensitivity

*Measured while the eyes are adapted to the light.

Visual pigment regeneration

*Molecules that have been split apart are undergoing this process in which the retinal and opsin become rejoined *Process needed for transduction: -Retinal molecule changes shape -Opsin molecule separates -The retina shows pigment bleaching. -Retinal and opsin must recombine to respond to light. -Cone pigment regenerates in 6 minutes. -Rod pigment takes over 30 minutes to regenerate. *This process occurs more rapidly in cones than in rods.

Solutions for myopia

*Move stimulus closer until light is focused on the retina -Distance when light becomes focused is called the far point. *Corrective lenses can also be used. *LASIK surgery can also be successful

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.

Measuring Dark Adaptation -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.

Simultaneous contrast

*Occurs when our perception of the brightness or color of one area is affected by the presence of an adjacent or surrounding area.

Blind spot

*One area in the retina where there are no receptors because the this is where the ganglion cells leave the eye to form the optic nerve.

Rod monochromat

*People who have no cones due to a rare genetic defect.

Psychophysical Experiment by Hecht et al.

*Prior physical evidence showed that it takes one photon to isomerize a pigment molecule. *Purpose of experiment - to determine how many pigment molecules need to be isomerized for a person to see *Method of constant stimuli used to determine absolute threshold.

Transduction of Light into Nerve Impulses

*Receptors have outer segments, which contain: -Visual pigment molecules, which have two components: 1) Opsin - a large protein 2) Retinal - a light sensitive molecule *Visual transduction occurs when the retinal absorbs one photon. -Retinal changes it shape, called isomerization.

Experiment by Hecht et al. - continued

*Results showed: -a person can see a light if seven rod receptors are activated simultaneously. -a rod receptor can be activated by the isomerization of just one visual pigment molecule. *Good example of using psychophysic/psychophysical approach to draw conclusions about the physiology of the visual system.

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

Spectral Sensitivity of Rods and Cones

*Sensitivity of rods and cones to different parts of the visual spectrum -Use monochromatic light to determine threshold at different wavelengths -Threshold for light is lowest in the middle of the spectrum -1/threshold = sensitivity, which produces the spectral sensitivity curve

Lens

*Supplies the remaining 20% of the eye's focusing power, can change its shape to adjust the eye's focus for stimuli located at different distances.

Spectral sensitivity curve

*The ability to see wavelengths across the spectrum is often plotted in terms of sensitivity vs. wavelength. *We can convert threshold to sensitivity with the rolling equation:1/threshold = sensitivity.

Accommodation

*The ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that it gets thicker. *The increased curvature bends the light rays passing through the lens to pull the focus point back to A to create a sharp image on the retina. *Accommodation brings focus point forward. *Accommodation enables you to bring both near and far objects into focus, although objects at different distances are not in focus at the same time

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

Far point

*The distance at which the spot of light becomes focused on the retina *When the spot of light is at the far point, a myope can see it clearly.

Near point

*The distance at which your lens can no longer adjust to bring close objects into focus

Wavelength

*The distance between the peaks of the electromagnetic waves.

Presbyopia

*The distance of the near point increases as a person gets older

Visible light

*The energy within the electromagnetic spectrum that humans can perceive, has wavelengths ranging from about 400 to 700 nanometers.

Focusing Images on Retina - continued

*The near point occurs when the lens can no longer adjust for close objects. *Presbyopia - "old eye" -Distance of near point increases -Due to hardening of lens and weakening of ciliary muscles -Corrective lenses are needed for close activities, such as reading *At about age 45 the ability to accommodate begins to decrease rapidly, and the near point moves beyond a comfortable reading distance.

Rod-Cone break

*The place where the rods begin to determine the dark adaptation curve

Dark adapted sensitivity

*The sensitivity at the end of dark adaptation which is about 10,000 times greater than the light-adapted sensitivity measured before dark adaptation began.

Enzyme cascade

*The sequence of reactions triggered by the activated visual pigment molecule. *EX: lighting one match to a fuse can trigger fireworks consisting of thousands of points of light.

Purkinje shift

*The shift from cone vision to rod vision that causes this enhanced perception of short wavelengths during dark adaptation.

Lateral Inhibition and Lightness Perception

*Three lightness perception phenomena explained by lateral inhibition 1) The Hermann Grid: Seeing spots at an intersection 2) Mach Bands: Seeing borders more sharply 3) Simultaneous Contrast: Seeing areas of different brightness due to adjacent areas

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.

Cornea

*Transparent covering of the front of the eye, accounts for 80% of the eye's focusing power. *It is fixed in place so it cannot adjust it's focus.

Detached retina

*Traumatic injuries *When part of the retina becomes detached, it has become separated from a layer that it rests on, the pigment epithelium, which contains enzymes that are necessary for pigment regeneration. *The results are that when visual pigments are bleached, so the retinal and opsin are separated, they can no longer be recombined in the detached part of the retina, and the person becomes blind in the area of the visual field served by this area of the retina.

Visual pigment bleaching

*When separation causes the retina to become lighter in color

Belongingness

*Which states that an area's appearance is influenced by the part of the surroundings to which the area appears to belong. -Effect probably occurs in cortex rather than retina. -Exact physiological mechanism is unknown.

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.

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.

Isomerization

*when a photon of light hits the retinal, it changes shape, so it is sticking out from the opsin. *It is this step that triggers the transformation of the light entering the eye into electricity in the receptors. *Isomerization of one visual pigment molecule activates about a million other molecules.

Myopia or nearsightedness

- Inability to see distant objects clearly - Image is focused in front of retina so that the image reaching the retina is blurred. *Caused by - Refractive myopia - cornea or lens bends too much light - Axial myopia - eyeball is too long

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 -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.

Experiment for rod adaptation

-Must use a rod monochromat -Results show that sensitivity increases for about 25 minutes and then levels off.

Lateral inhibition and 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.

Perception is Indirect

-Stimuli in environment impinge on receptors. -Transduction takes place causing electro-chemical impulse in neurons. -Provides accurate information about world -Feels direct but that is an illusion -This is true for all senses

Convergence in the Retina - continued

1) 126 million rods and cones converge to 1 million ganglion cells. 2) 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

Results of three dark adaptation experiments:

1) As soon as the light is extinguished, the sensitivity of both the cones and rods begin increasing. 2) Cones are the most sensitive receptor system, which means they are more sensitive at the beginning of dark adaptation and therefore determine the early part of the dark adaptation curve. 3) After about 3-5 minutes the cones are finished adapting, so their curve levels off. 4) The rods' sensitivity continues to increase, until by about 7 minutes of dark adaptation the rods have caught up to the cones and then become more sensitive than the cones. 5) Once the rods become more sensitive they then being controlling the person's vision, and the course of rod dark adaptation becomes visible.

Light is the Stimulus for Vision

1) Electromagnetic spectrum -Energy in this spectrum 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

Lateral Inhibition of Neurons

1) Experiments with eye of Limulus -Ommatidia allow recordings from a single receptor. -Light shown into a single receptor leads to rapid firing rate of nerve fiber. -Adding light into neighboring receptors leads to reduced firing rate of initial nerve fiber.

Diseases that Affect the Retina

1) Macular degeneration -Fovea and small surrounding area are destroyed -Creates a "blind spot" on retina -Most common in older individuals 2) Retinitis pigmentosa -Genetic disease -Rods are destroyed first -Foveal cones can also be attacked -Severe cases result in complete blindness

William Rushton's findings

1) Our sensitivity to light depends on the concentration of a chemical-the visual pigment 2) The speed at which our sensitivity is adjusted in the dark depends on a chemical reaction-the regeneration of the visual pigment. *Also found that when he compared the course of pigment regeneration to the of psychophysical dark adaptation rate, the rate of cone/rod pigment regeneration and the rate of cone/rod adaptation matched.

Lateral inhibition and simultaneous contrast

1) 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.

Mach Bands

1) 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.

Hermann Grid

1) People see an illusion of gray images in intersections of white areas. 2) 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.

Convergence in the Retina

1) Rods and cones send signals vertically through -bipolar cells. -ganglion cells. -ganglion axons. 2) Signals are sent horizontally -between receptors by horizontal cells. -between bipolar and between ganglion cells by amacrine cells.

Measuring spectral sensitivity

1) Rods are more sensitive to short wavelength light than are cones. 2) This difference in the sensitivity of the cones and the rods to different wavelengths means that as vision shifts from the cones to the rods during dark adaptation, we become relatively more sensitive to short wavelength light-that is, light nearer the blue and green end of the spectrum.

Greater convergence of the rods compared to cones translates into 2 differences in perception:

1) Rods result in better sensitivity 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 2) Cones result in better detail vision than rods.

The difference between the rod and cone spectral sensitivity curves is caused by:

Differences in the absorption spectra of the rod and cone visual pigments.