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The Visible Spectrum
- the adequate stimulus of visible light makes up 1/70th of the electromagnetic spectrum light is a form of energy that travels in oscillating waves, like sound. the unit of measurement is the nanometer (billionth of a meter)
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wavelength of light determines the color we see
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Light and the Visual Apparatus
the eye contains 126 million light receptors, plus a complex network of cells between the receptors and the optic nerves
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The Eye and its Receptors
- rods and cones are found in the retina- the photoreceptors of light:
- rods are in periphery (>20 degrees from center) of retina, detect variations in light but not color, work in low level light and are very sensitive to movement
- cones are most concentrated in the fovea, the area of greatest visual acuity, require bright, and differentiate among different wavelengths of light (red, green, blue)
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Pathways to the Brain
- where the optic nerve exits the eye there are no receptors; this is known as the blind spot
- The optic nerves from each eye join at the optic chiasm then separate; neurons from the nasal side of the retina cross to the contralateral side, resulting in information from each side of the visual field going to the opposite hemisphere; information transmitted to lateral geniculate nucleus of the thalamus, then the occipital lobe
- retinal disparity is the discrepancy in the location of an image on the two retinas (due to the 6cm separation of the eyes), providing one of the depth cues
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retinal disparity
- is the discrepancy in the location of an image on the two retinas (due to the 6cm separation of the eyes), providing one of the depth cues
- The visual field is the part of the environment that is being registered on the retina.
- –Information from the right half of each eye will be transmitted to the right hemisphere.
- –An image in the right visual field will similarly be projected to the left hemisphere.
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retina
the light- sensitive structure at the rear of the eye, is made up of two main types of light- sensitive receptor cells, called rods and cones, and the neural cells that are connected to them
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light- sensitive chemicals called?
photopigments
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rod photopigment is called?
rhodopsin
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Rhodopsin
is more sensitive to light than is cone photopigment
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Iodopsin
the cone photopigment
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Rod System
- function: best in dim light, detail vision is poor, does not distinguish with colors
- location: mostly in periphery of retina
- receptive field: large, due to convergence on ganglion cells, contributes to light sensitivity
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Cone System
- function: best in bright light, detail vision is good, distinguishes among colors
- location: mostly in fovea and surrounding area
- receptive field: small, with one or a few cones converging on a single ganglion cell; contributes to details vision
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Color Vision
Color refers to a person’s experience rather than the true nature of an object
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Trichromatic Theory
- Young and Helmholtz proposed that only 3 types of color receptors (red, green, blue) detect every visible color
- Television operates on this principle
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Opponent Process Theory
- Ewald Hering recognized that yellow is also one of the primary colors, but proposed the eye has only 2 color receptors (one for red and green, one for blue and yellow)
- Red and green are complements, as are blue and yellow. Complementary colors combine to a neutral gray or white; overstimulation of the eye with one color makes it quite sensitive to its complement (negative color aftereffect)
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A Combined Theory
- Hurvich and Jameson proposed 3 types of color receptors (red, green, and blue) connected in an opponent-process fashion to produce yellow
- This theory requires three color processes at the receptors and four at the ganglion cells
- Red cones excite R/G and Y/B ganglia, green cones inhibit R/G and excite Y/B. Blue cones inhibit both ganglia.
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Color Blindness (color vision deficiency)
- ·Usually dichromatic, due to a defect in one of the cone systems. Two major types:
- Red-green color blind, deficient in either the red (protanopia) or green (deuteranopia) cone or photochemical
- Blue color blind (tritanopia, rarest)
- ·Diagnostic tests include Ishihara (1917),
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Form Vision
- There is a retinotopic map in the visual cortex
- Form vision is the detection of an object’s boundaries and features
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The Perception of Objects,Color, and Movement: Processing
- Modular Processing: Each component of processing is segregated into separate locations
- Hierarchial Processing:
- information is processed by ascending through increasingly complex levels of
- the nervous system
- Visual functions are distributed across a wide area of the brain
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The Perception of Objects, Color, and Movement: The Two Pathways of Visual Analysis
- 1.The ventral stream handles the “what” of processing
- ·Flows from the visual cortex to the temporal lobes and is dominated by the parvocellular system
- ·People with damage in the ventral stream have trouble identifying objects visually but walk around them or reach for them
- 2.The dorsal stream handles the “where” of processing
- ·Flows from the visual cortex to the parietal lobes and is dominated by the magnocellular system
- ·Patients with damage to this area can identify objects visually but have trouble orienting toward them, reaching for them, and shaping the hand to grasp them.
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The Perception of Objects, Color, and Movement: Disorders of Visual Perception
- 1.Object and Face Agnosia
- ·Damage to the inferior temporal cortex (ventral stream)
- ·Object Agnosia is the impaired ability to recognize objects
- ·Prosopagnosia is the impaired ability to recognize familiar faces
- 2. Face Blindness and Blindsight
- ·Damage to V1 causes blindness, but information from superior colliculus to striate areas allows people with blindsight to react to unseen objects unconsciously.
- ·The visual word form area (VWFA) of the inferior temporal cortex responds to whole written words.
- 3. Color Agnosia is the inability to perceive color due to brain damage
- 4. Movement Agnosia is the inability to perceive movement
- 5.Neglect and the Role of Attention in Vision
- ·Neglect refers to the patient ignoring visual, touch, and auditory stimulation on the side contralateral to the injury.
- ·Due to a deficit in attention resulting from injury to the posterior parietal cortex, not a lack of sensation or visual processing.
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