Chapter 2perception.pdf

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Matthias Niemeier

Chapter 2 – The First Steps in Vision: From Light to Neural Signals A Little Light Physics • Two ways to conceptualize light: o Wave ▪ An oscillation that travels through a medium by transferring ener- gy from one particle or point to another without causing any per- manent displacement of the medium ▪ Visible light waves have wavelengths between (violet) 400 and (red) 700 nanometres (nm; nm=10 metre) o A stream of photons ▪ Tiny particles that each consist of one quantum of energy • In empty space the electromagnetic radiation from a star travels in a straight line at the speed of light (about 186,000 miles per second) o Once it reaches the atmosphere: ▪ Some light will be absorbed or scattered (by encounters with dust) ▪ Most will make it through the atmosphere and will eventually hit the surface of an object • If it hits something light-surfaced, most of the light is re- flected • If it hits something dark-surfaced, most of the light is ab- sorbed • If light is neither reflected or absorbed, it will be bent or refracted Eyes That See Light • Cornea o The first tissue that light encounters when it reaches the eye o Has a rich supply of transparent sensory nerve endings ▪ There to force the eyes to close and tears if the cornea is scratched • Aqueous Humor o A fluid derived from blood o Fills the space immediately behind the cornea o Supplies oxygen and nutrients to and removes waste from the cornea and the crystalline lens • Lens o Completely transparent o Shape of the lens is controlled by the ciliary muscle • Pupil o A hole in a muscular structure called the iris o Controls the amount of light that reaches the retina (via the pupillary light reflex) • Iris o Gives the eye its distinctive colour o Iris automatically expands and contracts to allow more less light into the eye o “Photic sneeze reflex” • Vitreous humor o Light is refracted for the fourth and final time o Comprises 80% of the internal volume of the eye • Retina o Image is brought into focus here o Only some of the light reaches the retina ▪ Good deal of light is lost in the eyeball o Detect light and “tell the brain about aspects of light that are related to objects in the world” Focusing Light onto the Retina • Cornea forms the most refractive surface in the eye o Aqueous and vitreous humors also help refract light o Lens can alter the refractive power by changing its shape ▪ Accommodation • Accomplished through contraction of the ciliary muscle ▪ Attached to the ciliary muscles through “zonules of Zinn” (tiny fi- bres) • When the ciliary muscle is relaxed o Zonules are stretched o Lens is relatively flat o Eye will be focused o Eye will be focused on very distant objects • When the ciliary muscle contracts o Reduced tension on zonules o Enables the lens to bulge o Allows us to focus on something closer • Accommodation enables the power of the lens to vary by as much as 15 diopters o Ability declines with age ▪ Starting from about 8 years old, we lose about 1 diopter of ac- commodation every five years up to the age of 30 ▪ Presbyopia – loss of vision because of insufficient accommodation • Lens becomes sclerotic (harder) and the capsule that encir- cles the lens loses its elasticity • Cataracts o Opacities of the lens (composed of crystallins – class of proteins – packed very densely) ▪ Caused by irregularity of the crystallins o Congenital cataracts (present at birth) are relatively rare o Absorb and scatter more light than the normal lens does • Emmetropia o When the refractive power of the four optic components of the eye are perfectly matched to the length of the eyeball • Myopia (nearsightedness) o Occurs if the eyeball is too long o The image will be focused in front of the retina o Can be corrected with negative lenses • Hyperopia (farsightedness) o Occurs if the eyeball is too short o Image is focused behind the retina o Can be corrected with positive lenses • Average adult human eye is 24 mm long • Astigmatism o Occurs when the cornea is not spherical o Vertical lines must be focused slightly in front of the retina, while hori- zontal lines are focused slightly behind it (or vice versa) The Retina • The optics involved are similar to those in most cameras o “stop” is analogous to the iris in human eyes HOWEVER • The purpose of a camera is just to record an image, the purpose of the human visual system is to interpret the image • Light is transduced into neural energy that can be interpreted in the brain • Eye doctors use an ophthalmoscope to look at the fundus (back surface of their patient’s eye) ▯ • Optic disc o Where arteries and veins that feed the retina enter the eye o Where axons of ganglion cells leave the eye via the optic nerve o Contains no photoreceptors ▪ Blind spot, but our visual system fills it in with information from the surrounding area • Retina is a layered sheet of clear neurons with another layer of darker ce -lls ly ing beneath the final layer o Pigment epithelium • When photoreceptors sense light they can stimulate neurons in the intermedi- ate layers o Bipolar cells, horizontal cells, and amacrine cells ▪ Connect with the front-most layer of the retina, made up of gan- glion cells whose axons pass through the optic nerve to the brain Retinal Information Processing Light Transduction Processing • Two types of photoreceptors (duplex retinas): o Rods ▪ Humans have more rods (about 90 million) ▪ Completely absent from the centre of the fovea • “rod-free” area subtends a visual angle of about 1 degree • Directly behind the centre of the pupil ▪ Density increases to a peak at about 20 degrees and then declines again ▪ Contains the pigment rhodopsin • Concentrated mainly in the stack of membranous discs in the outer segment ▪ Function relatively well under conditions of dim (scotopic) illumi- nation ▪ Because they all have one kind of pigment, they cannot signal dif- ferences in colour o Cones ▪ Each cone has one of three pigments • Respond to long, medium, and short ▪ Less cones (about 4-5 million) ▪ Most concentrated in the centre of the fovea • Density drops off dramatically with retinal eccentricity o Distance between the retinal image and the fovea ▪ Require brighter (photopic) illumination ▪ Short wavelength-sensitive cones (S-cones) • 5-10% of cone population • Essentially missing from the centre of the fovea o Fovea is dichromatic (only 2 colour-sensitive cones) ▪ Long wavelength-sensitive cones (L-cones) ▪ Medium wavelength-sensitive cones (M-cones) • On average about twice as many L-cones as M-cones, but varies greatly ▪ Because cones have three photopigments they are better at sig- naling differences in colour • Both types of photoreceptors consist of an/a: o Outer segment ▪ To the pigment epithelium ▪ Visual pigments are stored here o Inner segment ▪ Visual pigments are made here • Consist of a protein (opsin) which determines which wave- lengths of light they absorb • Consist of a chromophore which captures light photons o Synaptic terminal • Evidence suggests a third type of photoreceptor that “lives” among the gan- glion cells o Involved in adjusting our biological rhythms to match the day and night of our external world o Contain the photopigment melanopsin o Sensitive to the ambient light level o Send their signals to the suprachiasmatic nucleus (SCN) ▪ Home of the brain’s circadian clock • Photoactivation (activation by light) o Initiates the closing of channels in the cell membrane that normally al- low ions to flow into the rod outer segment ▪ Makes the cell more negatively charged • Hyperpolarization o Closes calcium channels at the synaptic terminal ▪ Reduces the concentration of neurotransm - it
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