2. Light: The Stimulus for Vision
- We see light or electromagnetic energy whichcomes to us:
- Directly from a light producing object (a light bulb)
- Indirectly from a light source reflecting off an object
- Light waves come in different lengths
- The small range of visible lightis due to properties of visual receptors, not properties of the light itself.
- Inter-species differences in light range detection
- The eye is sensitive to a narrow band of wavelength
- The human visual system is sensitive to
- wavelengths from 400 to 700 nanometers
4. The Electromagnetic Spectrum 5. The Eye Cornea : Transparent tissue covering the front of the eye. Does not have any blood vessels; does have nerves. Iris:Circular band of muscles that controls the size of the pupil. The pigmentation of the iris gives "color" to the eye. Lens:Transparent tissue that bends light passing through the eye. To focus light, the lens can change shape by bending. Pupil:Hole in the center of the eye where light passes through. Sclera :Protect coating around the posterior five-sixths of the eyeball Retina:Layer of tissue on the back portion of the eye that containscells responsive to light (photoreceptors) 6. The Eye
- The lensprojects an inverted image onto the retina and the brain adjusts this inversion so we see the world in its correct orientation.
- The cornea and lens bend or refract light rays as they enter the eye, in order to focus images on the retina.
- The eye can change theextent to which rays are bentand thus can focus images of objects by varying curvature of the lens.
- Through the ciliary muscle
- This ciliary muscle is smooth or involuntary muscle.
- Light passes through the cornea, pupil, and lens on its way to theretinaPHOTORECEPTORS,which absorb and then convert it into electrical potentials that carry information to thebrain.
7. 8. 9. 10.
- The retina performs dual function involving
- the two different receptors :
The Photoreceptors 11. The Photoreceptors
- A sensory receptor is a specialized neuron that
- detects a specific physical stimulus.
- *Note:Do not confuse a sensory receptor with
- the protein molecules that function as
- neurotransmitter receptors in the membranes
- Most sensory receptors do not have axons.Their
- cell bodies synapse on dendrites or cell bodies of
12. The Photoreceptors
- The photoreceptors process the light energy.
- There are two types of photoreceptors: "rods" and "cones."
- Rodsare sensitive enough to respond to a single photon, but functioning together they are optimized for seeing inpoor light.
- Conesare optimized for responding to fine detail and color; they need a lot more light and work best inbroad daylight.
- Inside the human eye, there are eighteen times more rods than cones. These are arranged in such a way as to produce the best possible combination ofnight and day vision.
13. The Photoreceptors:located in the back of the retina Whenlight hits the retinait first processed in the photoreceptors located in the back of the retina. PHOTORECEPTORSBIPOLAR CELLS GANGION CELLS The ganglion cells axons join together to form theoptic nerve,which exits through the back of the eye at theoptic disk. 14. The Photoreceptors: RODS
- 120 million rods distributed over most of the retina except near the fovea
- Rods are connected in groups; there are far fewer optic nerves going to the brain than rods
- Rod vision detects edges and motion very well
- Rod pigment is bleached by light and is less effective in bright light; rods take about 20 - 30 minutes of dark adaptation before they are most efficient
- Absent in fovea centralis
- Increase number towards periphery
- Night vision (scotopic vision)
- The photochemical substance is rhodopsin
16. The Photoreceptors: CONES
- There is a concentration in fovea, region about 1.5 mm in diameter. Most acute vision limited to foveola, covering ~0.4 mm
- No rods at all in central part of fovea
- Color vision is provided by 3 types of cone with different colored light absorptions:red, green, and blue cones .
- Daylight vision(photopic vision)
- Concerned with color vision
- The photochemical substance isiodopsin
18. 19. The Photoreceptors:PHOTOTRANSDUCTION
- Rods and cones containphotopigments-chemicals that release energy when struck by light.
- Photopigments consist of11- cis- retinal(derivative of Vitamin A) which is bound toopsin(a protein).
- Light converts 11- cis- retinal to all- trans -retinal, which ultimately activates 2 ndmessenger systems that work tocloseNA+ channels, hyperpolarizing the receptor.
- More light =hyperpolarization .
20. Photoreceptors input to retinal bipolar cells, which input to retinal ganglion cells. Photoreceptors and bipolar cells do not produceaction potentials .They producegraded potentials. Retinal ganglion cells produce action potentials. light The Photoreceptors:Phototransduction 21. Glutamate hyperpolarizes some bipolar cells, and depolarizes other bipolar cells. At rest, (i.e. in the dark) photoreceptors continuously releaseneurotransmitter (glutamate). in the dark Some bipolar cells provide hyperpolarizing input to ganglion cells, and some bipolar cells provide depolarizing input. The Photoreceptors:Phototransduction . . . . . . . . . 22. 23. Dark and Light Adaptation
- Periphery of the retina sensitive to dam light
- Dark adaptation: 60% accomplished in the first 5 minutes (12%/min) and completed ->20minutes (2.66%/min).
- Light adaptation:loss of sensitive attainedthrough dark adaptation occurs upon re-exposure of the eyes to the light.
- Less time to loss dark adaptationthan to acquired it
- Change Taking Place During Dark Adaptation
- Change in sensitive to different wave length
- Increase sensitive to the blue-green
- Change over from cone vision to rod vision
25. Change involved in light adaptation
- Constriction of the pupil
- Purkinje shift to the yellow side
- Change over from rod vision to cone vision
26. close your right eye. With your left eye, look at the red circle. Slowly move your head closer to the image. At a certain distance, the blue line will not look broken!!THE OPTIC DISK: YOUR BLIND SPOT 27. The Receptive Field
- The receptive fieldof a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron and itusually have a "hot spot"
- Receptive fields have been identified for neurons of the auditory system, the somatosensory system, andthe visual system .
28. Processing in the Retinal Ganglion Cell
- Receptive fieldis the area of visual space to which a given cell responds
- Ganglion cellreceptive fields are circular.
- Ganglion cell receptive fields have a concentric antagonistic surround.
- on-centercells mostly excited by light falling on the center of the receptive field
- off-centercellmostly excited by light in the surround.
Each ganglion cell ONLY responds to the presence or absence of light in its receptive field OR 29. 30. Receptive Fields The receptive field of a retinal ganglion cell is roughly circular with 2 concentric regions. center periphery 31. Receptive Fields
- excitatory burst when light
- shines on the center of their
light time Action potentials light 32. Receptive Fields
- ON-cells are inhibited when light
- shines on the periphery of their
Action potentials light light time 33. Receptive Fields
- ON-cells respond with fairly
- constant activity when light
light time Action potentials light 34. Receptive Fields