E N D
1. Physiological Psychology Vision I
2. Sensation and perception Sensation – The process in which the sense organs’ receptor cells are stimulated and relay information to the brain.
Perception – The process in which an organism selects and interprets sensory input so that it acquires meaning
Sensation – Detect Stimuli
Perception - Comprehension
3. Vision Conversion of light energy to graphical information that the brain can “see”
visual receptors are specialized to absorb light and transduce it into an electrochemical pattern in the brain
not a duplicate, picture-like pattern of the object
5. Cornea – the transparent tissue covering the front of the eye
Sclera – the tough outer layer of the eye; the “white” of the eye
Iris – The pigmented muscle of the eye that controls the size of the pupil
Lens – The transparent organ situated behind the iris of the eye; helps focus an image on the retina
Accommodation - The change in the shape of the lens to adjust for distance
6. Vitreous Humor – the clear fluid in the eye
Retina – the tissue at the back inside surface of the eye that contains the photoreceptors
Macula - 3mm X 5mm center of retina with greatest ability to resolve detail
Fovea – Small pit near the center of the retinal containing many receptors responsible for the most acute and detailed vision
8. The Retina Light passes through ganglion and bipolar cells, without distortion, to visual receptors
bipolar cells receive input from visual receptors
ganglion cells receive input from bipolar cells
amacrine cells exchange information with bipolar cells and send information to ganglion and other amacrine cells
provides many options for complex processing of information
Optic nerve is made up of axons of ganglion cells
the point where optic nerve leaves the eye does not have receptors and is our blind spot
11. Many bird species have two foveas per eye
one pointing ahead and one pointing to the side
Visual receptors in some predators and prey are designed to facilitate survival
hawks have greater density on top half (looking down) than on the bottom half (looking up)
rats have greater density on the bottom half (looking up)
12. Photoreceptors
Two types: Rods and Cones
Rods – Very sensitive to light but cannot detect changes in color
respond best to low light conditions
bleached by bright light
Cones – responsible for acute daytime vision and color vision
Distribution different: Cones almost entirely in fovea, rods spread across
13. 100 million cones and 6 million rods
But rods send 10 times more responses to brain than cones
Each cone has direct line to brain while many rods share same line
Both rods and cones contain photopigments, chemicals that release energy when struck by light
light is absorbed and 11-cis-retinal is converted to all-trans-retinal
14. Color Vision Rods specialize in picking up very dim light while cones specialize in fine detailed vision and color vision.
Colors of light correspond to different wavelengths of electromagnetic energy
Something has color because of the wavelength of light that it reflects
White reflects all wavelengths
Black absorbs all wavelengths
How does the visual system convert these wavelengths into our perception of color?
15. Psychological dimensions of color Hue – Color: the psychological property of light referred to as color
Based on the wavelength of the light
Brightness – The lightness or darkness of reflected light
Based on the intensity of the light
Saturation – Purity: the depth and richness of the hue
Based on the mixture of several wavelengths
17. Theories of Color Vision Trichromatic theory- or the Young Helmholtz theory
Cones respond to three primary colors
Blue
Green
Red
Separate type of cone for each primary color
Color vision depends on the relative rate of response by 3 types of cones.
18. 3 types of cones
19. Opponent Process Theory There are 6 basic colors to which people respond, but 3 types of receptors
Red-green
Blue-yellow
Black-white
We perceive color in terms of “paired opposites” red-green, black-white and yellow-blue
explains why we can’t see reddish green or bluish yellow
explains negative color afterimages
20. Support for opponent-process Negative afterimages- Image seen after a portion of the retina is exposed to an intense visual stimulus
A negative afterimage consists of colors complementary to those of the physical stimulus
When the cones have been fatigued, the opposite color comes through.
21. Color afterimage
25. Opponent-processes for Detection of Movement? http://psylux.psych.tu-dresden.de/i1/kaw/diverses%20Material/www.illusionworks.com/html/motion_aftereffect.html
26. Opponent processes is not a complete explanation since afterimages depend not only on the retina but also on the area of the brain that produces it.
Ex. McCollough effect
Research has found that trichromatic theory and opponent-process theory are both wrong and right
Cones are probably like Trichromatic theory
Bipolar cells may be opponent processes
28. The retinex theory
color perception requires some reasoning
the cortex compares information from various areas of the retina to determine the brightness and color perception for each area
color constancy: we see the right colors despite lighting changes, e.g., we subtract green tint to see white house and red rose but we only see green house if viewed in isolation
brightness requires a comparison with other objects
29. Visual perception depends on both bottom-up and top-down processing.
Bottom-up processing
Data-driven- raw sensory information
We see something and interpret what we see.
Top-down processing
Conceptually driven
Uses past experiences to interpret information
We see something and interpret it based on our previous knowledge and expectations
“From the brain”
Causes illusions
32. Abnormal Color Vision: Color Blindness
34. Red-green color blindness – are missing either a red or green cone.
inability to distinguish red from green is most common deficiency
recessive gene on X chromosome
8% in men and 1% in women
Lack of blue cones also exists, but is extremely rare.
Others may lack cones altogether and thus do not see color.
Acquired achromotopsia – damage to parts of the occipital cortex. Lose ability to see aspects of color.
35. Visual Pathways Within the eyeball
rods and cones synapse to horizontal cells and bipolar cells
horizontal cells make inhibitory synapse onto bipolar cells
bipolar cells synapse to amacrine and ganglion cells
axons of the ganglion cells leave the back of the eye
38. The inside half of the axons of each eye cross over in the optic chiasm
most visual information goes through the lateral geniculate nucleus of the thalamus
some goes to the superior colliculus
LGN inputs to other parts of thalamus and to visual areas of cerebral cortex, which sends back axons to modify input
39. Processing in the Retina Receptive field: the point in space from which incoming light strikes a receptor
receptors have both excitatory and inhibitory regions since receptive field is normally an array of light patterns
Ex: light in center of ganglion cell might be excitatory, with the surround inhibitory
Lateral inhibition
each active receptor and it’s visual path tends to inhibit the visual path of neighboring receptors
Increases contrasts in the image
40. An active receptor excites both a bipolar and horizontal cell; in turn, horizontal cell inhibits bipolar cell, but net potential is excitatory on bipolar
But, horizontal cell does inhibit neighboring bipolar cells on border of visual field
Effect is to heighten contrast: receptors inside visual field are excited and those on border tend to be inhibited
42. Retina and Lateral Geniculate Pathways Parvocellular: smaller ganglion cell bodies and small receptive fields, located near fovea
detect visual detail and color
all axons go to lateral geniculate nucleus
Magnocellular: larger ganglion cell bodies and receptive fields, distributed fairly evenly throughout retina
respond to moving stimuli and patterns
not color sensitive
most axons go to lateral geniculate nucleus
43. Koniocellular: small ganglion cell bodies that occur throughout the retina
many functions
axons go to lateral geniculate nucleus, thalamus and superior colliculus
Many different types of ganglion cells implies analysis of information from the beginning
44. Most visual information from lateral geniculate nucleus goes to primary visual cortex (V1)
first stage of visual processing
Output of V1 goes to secondary visual cortex (V2)
second stage of visual processing which transmits visual information to additional areas
feedback loop to V1
V1 and V2 also exchange information with other cortical areas and thalamus
30-40 visual areas reported in brain of macaque monkey
45. Magnocellular and parvocellular paths split into three paths
Magnocellular path
ventral branch to temporal cortex is sensitive to movement
dorsal branch to parietal cortex integrates vision with action
Parvocellular path to temporal cortex is sensitive to details of shape
Mixed parvo/magnocellular path to temporal cortex is sensitive to brightness and color
47. Visual paths in temporal cortex form the ventral stream
the “what” path, specialized for identifying and recognizing objects
if damaged, we can find and pick up objects but cannot describe them
Visual path in parietal cortex is the dorsal stream
the “where” or “how” path, helps motor system find objects, move toward them and pick them up
if damaged, we can describe object but can’t find and pick up object