slide 1 vision is remarkable! extremely complicated and costly process as a reward over the course...
DESCRIPTION
Slide 3 Properties of Light Optics Light rays travel in straight lines until they interact with atoms and molecules in the medium Reflection =Bouncing of light rays off a surface =Most of what we see is reflected light Absorption =Transfer of light energy to a particle or surface =Basis for color perception - reflected light off a surface is absorbed in the retina Refraction =Bending of light rays from one medium to another - toward a line that is perpendicular to the surface =Due to the speed (of light travel) difference - the greater the difference, the greater the angle of refraction =Basis for image forming on the retinaTRANSCRIPT
Slide 1
Vision is Remarkable!
•Extremely complicated and costly process•As a reward over the course of evolution, vision provided
•New ways of communication•Ability to predict the trajectory of objects and events in time and space•New froms of mental imagery and abstraction•World of visual art
•Eyes like a camera •can adjust to differences in illumination and focus itself on objects of interest•Additional functions such as the ability to track moving objects and self-cleaning system
•Retina like film (but much more than that) •Photoreceptors: Converts light energy into neural activity•Output does not faithfully reproduce the intensity of the light falling on it
•Detects differences in intensity of light falling on different parts of it•Image processing on the retina
Slide 2
Properties of LightLight Electromagnetic radiation is all around us!
• radio, wireless phones, x-ray machines, sun Light is the electromagnetic radiation that is visible to our eyes! (400-700 nm)
Energy content is proportional to frequency • Hot colors: Orange, red : lower energy
• Cool colors: blue, violet: higher energy
• Colors are themselves “colored” by the brain
Slide 3
Properties of LightOptics Light rays travel in straight lines until they interact with atoms and molecules in the medium• Reflection
Bouncing of light rays off a surface Most of what we see is reflected light
• Absorption Transfer of light energy to a particle or surface
Basis for color perception - reflected light off a surface is absorbed in the retina
• Refraction Bending of light rays from one medium to another - toward a line that is perpendicular to the surface
Due to the speed (of light travel) difference - the greater the difference, the greater the angle of refraction
Basis for image forming on the retina
Slide 4
The Structure of the EyeGross Anatomy of the Eye Pupil: Opening where light enters the eye
Iris: Control the amount of light coming into eye (aperture)• The sphincter muscle lies around the very edge of the pupil. In bright light, the sphincter contracts, causing the pupil to constrict. The dilator muscle runs radially through the iris, like spokes on a wheel. This muscle dilates the eye in dim lighting.
• Of which pigmentation determines the eye color
Sclera: White of the eye• provides tough wall of eyeball• Extraocular muscles (3pairs) are embedded and control the movement of eyeball
Slide 5
The Structure of the EyeGross Anatomy of the Eye
Eye’s orbit: bony socket of skull
Conjunctiva: membrane connecting sclera with eyelids
Cornea: • Glassy transparent external surface of the eye (lens-like regractive power)
• Innervated by unmyelinated nerve endings : very sensitive to pressure (touch)
Optic nerve: Bundle of axons from the retina
Slide 6
The Structure of the EyeOphthalmoscopic Appearance of the Eye Blood vessels on the surface of Retina Optic disk :
• A pale circular region• Gate for entering blood vessels andExiting optic nerve fibers • Blind spot
No photoreceptors present Brain is deceiving you!
Macula (spot):• Central vision• Relative absence of blood vessels - improves the quality of central vision
Fovea (pit):• A thinner center of macula• The center of retina - serves as a anatomical reference point Nasal vs temporal Superior vs inferior
Slide 7
The Structure of the EyeOphthalmoscopic Appearance of the Eye Blood vessels on the surface of Retina Optic disk :
• A pale circular region• Gate for entering blood vessels andExiting optic nerve fibers • Blind spot
No photoreceptors present Brain is deceiving you!
Macula (spot):• Central vision• Relative absence of blood vessels - improves the quality of central vision
Fovea (pit):• A thinner center of macula• The center of retina - serves as a anatomical reference point
Nasal vs temporal Superior vs inferior
Slide 8
The Structure of the EyeCross-Sectional Anatomy of the Eye Aqueous Humor
• fluid filling space between corneaand lens• supply nourishment
Ciliary muscles• Ligaments (zonule fibers) that suspend lens are attached• Connect to sclera
Lens: Change shape to adjust focus• Aqueous humor in anterior chamber• Jelly-like vitreous humor in posterior chamber
its pressure serves to keep the spherical shape of eyeball
Slide 9
Image Formation by the EyeEye collects light, focuses on retina, forms images Refraction of light by the cornea Parallel lights from far distance must be bent by refraction
Air to aqueous humor changemakes refraction on the surfaceof cornea Focal distance depends onthe curvature of cornea Refractive power - reverse ofFocal distance (diopter)
• Cornea has 42 diopters• Depends on the slowing of light (Blurry vision underwater)
Slide 10
Image Formation by the EyeAccommodation by the Lens Changing shape of lens allows for extra focusing power (~12 diopters)
Important for focusing images of objects within 9m ranges - requires greater refraction for diverging (not parallel) rays
Accommodations by ciliary muscle contraction
• tension of zonule fubers Decrease• lens becomes rounder • increased curvature• lose this function with age (presbyopia)
Slide 11
Image Formation by the Eye
The Pupillary Light Reflex Connections between retina and brain stem neurons that control muscle around pupil
Continuously adjusting to different ambient light levels
Consensual : bilateral reflex Pupil similar to the aperture of a camera • increase the depth of focus by the constriction of pupil
• same as increasing the f-stop
Slide 12
Image Formation by the Eye
The Visual Field Amount of space viewed by the retina when the eye is fixated straight ahead
Visual Acuity Ability to distinguish two nearby points : depends on several factors including the spacing of photoreceptors in the retina and the precision of eye’s refraction
Visual Angle: Distances across the retina described in degrees
Slide 13
Microscopic Anatomy of the Retina
Photoreceptors: Cells that convert light energy into neural activity
Direct (vertical) pathway:
Horizontal cells, Amacrine cells : modify the responses of bipolar cells and ganglion cells via lateral connections
Ganglion cells Output from the retina
Photoreceptors bipolar cells
ganglion cells
Slide 14
Microscopic Anatomy of the RetinaThe Laminar
Organization of the Retina Cells organized in layers
Inside-out :• Upper cells are relatively transparent
• Pigmented epithelium is critical to maintain photoreceptors and photopigments
Slide 15
Microscopic Anatomy of the RetinaPhotoreceptor Structure
Electromagnetic radiation to neural signals
Four main regions• Outer segment
Stack of membraneous disks that contain photopigments
Lights are absorbed by photopigments and lead to changes in membrane potential
• Inner segment• Cell body• Synaptic terminal
Types of photoreceptors• Rods
have more disks and higher photopigments concentration - 1000 times more sensitive to light than cones
Scotopic retina• Cones detect colors
Photopic retina
Slide 16
Microscopic Anatomy of the RetinaRegional Differences in Retinal Structure Varies from fovea to retinal periphery
Peripheral retina• Higher ratio of rods to cones
• Higher ratio of photoreceptors to ganglion cells - lower acuity
• More sensitive to light (rods are specialized for low light)
Rods outnumber cones in the human retina (20 to 1)
Slide 18
Microscopic Anatomy of the RetinaRegional Differences in Retinal Structure Cross-section of fovea:
• Pit in retina due to lateral displacement of the cells above the photoreceptors
• Maximizes visual acuity by allowing light to strike photoreceptors directly (no scattering)
Central fovea: All cones (no rods)• 1:1 ratio with ganglion cells• Area of high visual acuity
Slide 19
Phototransduction Phototransduction in Rods Depolarization in the dark: “Dark current” -30 mV• Due to steady influx of Na+ through cGMP gated sodium channel
• Constant production of cGMP by guanylyl cyclase
Hyperpolarization in the light • Light reduces cGMP to close Na+ channel - hyperpolarize
transducin
Slide 20
Phototransduction Phototransduction in Rods Depolarization in the dark: “Dark current” to -30 mV• Due to steady influx of Na+ through cGMP gated sodium channel
• Constant production of cGMP by guanylyl cyclase
Hyperpolarization in the light • Light reduces cGMP to close Na+ channel - hyperpolarize
PDE
Slide 21
Rhodopsin•Photopigment that absorb electromagnetic radiation•Receptor protein that is embedded in the membrane of the stacked disks in the rod outer segments•Receptor protein with a prebound chemical agonist [Opsin (GPCR) + retinal (vitamin A derivative)]•Bleaching : change in conformation of retinal by light absorption leading to the activation of opsin (by dissociation)
Phototransduction
11-cis-retinal all-trans-retinal
Slide 22
Light - retinal - opsin - transducin - PDE - cGMP - cGMP gated Na+ channelSignal amplification : very low number of photons can be detected
Phototransduction
Slide 23
Phototransduction Phototransduction in Cones Similar to rod phototransduction
• Rods are hyperpolarized constantly - saturated
• Daytime vision depends on cones, whose photopigments require more energy to become bleached
Different opsins : major difference• Red, green, blue cones
Color detection• Contributions of blue, green, and red cones to retinal signal
• Young-Helmholtz trichromacy theory of color vision Brain assigns colors based on a comparison of the readout of the three color types
Color blindness - significant spectral abnormality (beyond normal variation), mostly due to genetic errors
Abnormal red-green vision is the most common abnormality that is more frequently found in men
Peak sensitivity of rods is to a wavelength of 500 nm (blue-green)
Slide 24
Phototransduction
Dark and Light Adaptation
Dark adaptation—Increasing the sensitivity to light (106 fold)• Dilation of pupils - 2-8 mm diameter; 16 fold • Regeneration of unbleached rhodopsin• Adjustment of functional circuitry - signals from more rods are available to each ganglion cells
All-cone daytime vision All-rod nighttime vision20–25 minutes
Slide 25
Phototransduction
Dark and Light Adaptation Calcium’s Role in Light Adaptation
• Blinding sensation reflects saturation of both rods and cones (hyperpolarization)
• Cones gradually adapt their membrane potential to - 35 mV
• cGMP-gated Na+ channel also allow Ca++ that inhibit guanylyl cyclase (balancing GC in the dark)
• Under the light, low Ca++ concentration in the hyperpolarized cones gradually activates GC, recovering cGMP - open the Na+ channel again
• Ca++ also affect photopigments and PDE to reset their responses to light
• What we see is the relative difference in light level, not the absolute level!
Slide 26
Retinal Processing
Only ganglion cells fire APs! All other cells respond with graded changes in membrane potential : difficult to detect!
Transformations in the Outer Plexiform Layer Photoreceptors form synapses with bipolar cells and horizontal cells
Output of photoreceptors is generated by dark rather than light
Dark is the preferred stimulus• When a shadow passes across a photoreceptor, it responds by depolarizing and releasing neurotransmitter glutamate
Slide 27
Retinal Processing
Bipolar Cell Receptive Fields Two classes of bipolar cells
• OFF bipolar cells : depolarized through glutamate gated Na+ channel - active when photoreceptors are depolarized (dark)
• ON bipolar cells : GPCR respond to glutamate to hyperpolarize - active (depolarized) when there’s less glutamate (light)
Receptive Field• Area of retina that changes the cell’s mem-brane potential upon Light stimulation• Consists of centerand surround• Antagonistic center-surround receptive fields; complex interaction ofhorizontal cells, photoreceptors, and bipolar cells
Slide 28
Light on surround
On center becomes off
Slide 29
Retinal OutputGanglion Cell Receptive Fields On-Center and Off-Center cells
(corresponding connections with bipolar cells) Center-surround cancellation
Responsive to differences in illumination within their receptive fields
Slide 30
Retinal Output
Ganglion Cell Receptive Fields The center-surround organization of the receptive fields leads to a neural response that emphasizes the contrast at light-dark edges
Illusion of the perception of light level can be explained by the level of inhibition by surround
Slide 31
Retinal Output
Types of Ganglion Cells Two types of ganglion cells in monkey and human retina• M-type (Magno, 5%), P-type (Parvo, 90%), nonM-nonP (5%)
Slide 32
Retinal OutputColor-Opponent Ganglion Cells Some P cells and nonM-nonP cells Response to one wavelength in the receptive field center is canceled by another wavelength in the receptive field surround
Red versus green and blue versus yellow
• White light will equally activate both center and surround to cancel each other
Ganglion cells output A stream of information concerning three different comparisons : Light vs dark, red vs green, blue vs yellow
Slide 33
Retinal Output
Parallel Processing - independent but simultaneous information processing Simultaneous input from two eyes
• Information from two streams is compared in the central visual system to give the perception of ‘Depth’
Information about light and dark• ON-center and OFF-center ganglion cells provide independent
streams of information Different receptive fields and response properties
of retinal ganglion cells• M cells : sensitive to subtle contrasts over large receptive
field and contribute to low-res vision• P- cells : small receptive field contribute to high-res
vision (detail)• nonM-nonP cells : color opponency