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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)
- the wavelength of light determines the color we see
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
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)
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
- 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.
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
light- sensitive chemicals called?
rod photopigment is called?
is more sensitive to light than is cone photopigment
the cone photopigment
- 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
- 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
Color refers to a person’s experience rather than the true nature of an object
- Young and Helmholtz proposed that only 3 types of color receptors (red, green, blue) detect every visible color
- Television operates on this principle
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)
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.
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),
- There is a retinotopic map in the visual cortex
- Form vision is the detection of an object’s boundaries and features
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
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.
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.