Chapter 6- Vision Flashcards
How do we perceive objects?
By the energy they transmit. Each of our sense has specialized receptors to a particular kind of energy. After information reaches your nervous system, you encode it.
Receptors for vision are sensitive to light.
How do we store the information in terms of responses by neurons?
Which neurons response, their amount of response, and timing of response .
Receptors transduce energy into electrochemical patterns so the brain can percieve
What is the Law of Specific Nerve Energies?
Done by Mueller, whatever excites a particular nerve establishes a special kind of energy unique to that nerve. Impulses in one energy indicate light, while another indicated sound
What does sensory coding depend on?
The frequency of firing
True of False, the cornea of our eyes are adjustable while the lens is not?
False. The cornea is fixed, the lens is adjustable. light is focused by the lens and the cornea onto the rear surface of the eye ( retina)
What is the pupil?
Center of the Iris, light enters the eye through an opening in the center of the iris
What is the retina?
Rear surface of the eye, lined with visual receptors. Light from the left side of the world strikes the right side of the retina and vice versa ( contralateral)
What is the route within the retina?
visual receptors send messages to neurons called bipolar cells located closer to the center of the eye, which then send messages to the Ganglion cells (even closer to the center of the eye)
The ganglion axons then join together and travel to the back of the brain which form the optic nerve
What are Amacrine cells?
Additonal cells that get information from bipolar cells and send it to other bipolar cells, other amacrine cells and ganglion cells
What is the blind spot?
part of the eye that has no receptors.
We do not notice it because our eyes fills in the gap and anything in the blond spot of one eyes if visible to the other eye
What is the Fovea?
Tiny area specialized for acute, detailed vision. Vision is dominated by what we see in the fovea.
Each receptor( cone) in fovea connects to a single bipolar cells, which in turn connects to a single ganglion cell, which has an axon to the brain. This allows the registering of exact location of input. Packed tight with receptors, no ganglion axons or blood vessels.
What are midget ganglion cells?
Ganglion cells in fovea of humans and primates
What happens toward the periphery of the retina?
More and more receptors converge onto bipolar and ganglion cells. ( peripheral vision) Detailed vision is decreased here but allows for the greater detection of much fainter light
What about the eyes in birds?
Arrangement of visual receptors in the high are highly adaptive, birds have a greater density of receptors on top of the eye while rats have a greater density on the bottom of the eye
What are rods?
abundant in periphery of retina respond to faint light, not useful in bright light. 120 million per retina
What are cones?
abundant in and near fovea, less active in dim light, more useful in bright light, and essential for color vision. 6 millions per retina
What is the ratio of rods to cones?
20/1 . Cones are about 90% of brain’s input
What is the average number of axons in the optic nerve?
1 million. Allows heightened visual responses.
What are photo-pigments?
Chemicals in both rods and cones that release energy when struck by light . Consists of 11- CIS- retinal bound to proteins called opsins.
light energy converts 11-cis- retinal quickly into all-trans- retinal
light is absorbed and energy is released that activates 2nd messengers inside the cell
Visible wavelengths are dependent on…
Specific receptors.
Color discrimination depends on combination of responses by different neurons
What is the shortest wavelength that humans can perceive?
400 nanometers ( violet) longest is 700 ( red)
What is the Trichromatic theory?
Recognized that color perception required a biological explanation, said that we perceive color through the relative rates of response by three kinds of cones, each maximally sensitive to a different set of wavelengths.
Short, medium and long wavelengths.
More intense light increases the brightness of the color but does not change the ratio. The nervous system determines the color and brightness of light by comparing the responses of different types of cones
Why is the Trichromatic theory incomplete?
It doesn’t explain negative color afterimages
What is the Opponent-Process Theory?
Proposed by Hering, we percieve color in terms of opposites, the brain has a mechanism that perceives color on a continuum. Red to Green. Yellow to Blue. White to Black.
the possible hypothetical mechanism is the excitation and inhibition of bipolar cells.
What is wrong with the Opponent- Process Theory?
Don’t explain color constancy .
Afterimages depend on the whole context, not just the light on individual receptors- most likely the cerebral cortex is responsible, not the the bipolar cells or ganglion cells
What is the Retinex Theory?
The cortex compares information from various parts of the retina to determine the brightness and color for each area. This better explains color and brightness constancy
What is Color Vision Deficiency ( Color Blindness)
Impairment of perceiving color differences. Some people lack one or two types of the cones, or one is abnormal. The gene is located on the X chromosome
What is the most common form of color vision deficiency?
Distinguishing between red and green. It results from long and medium wavelength cones having the same photo pigment
Rods and Cones make synapses with _____ cells and _____cells
Horizontal and bipolar
Horizontal cells make inhibitory contact onto…
bipolar cells
Bipolar cells make synapses onto ____cells and ____ cells
amacrine and ganglion
What is the optic chiasm?
where the optic nerve from the two eyes meet.
In humans, half of the axons from each eyes cross over to the opposite side of the brain. Information form nasal half of each eye crosses over to contralateral hemisphere.
Information from temporal half of each eye foes to ipsilateral hemisphere
Where doe most ganglion cells axons go?
To the Lateral Geniculate Nuclues : part of the thalamus
It is specialized for visual perception
The thalamus and the cerebral cortex are constantly sending information back and forth
What is Lateral Inhibition?
The reduction of activity in one neuron by activity in neighboring neurons. It’s main function is to sharpen contrasts to emphasize the borders of objects
The response of cells in the visual system depends on?
the net result of excitatory or inhibitory of the nearest bipolar cells
What is the Receptive field?
part of visual field that excites or inhibits it- each cell in the visual system of the brain has a receptive field.
It is the point in space from which light strikes the cells.
What type of cells converge to from receptive field?
Ganglion Cells . The receptive field of a ganglion cells is circular center with antagonistic ( donut shaped) surround
What are the three categories of ganglion cells in primates?
1) Parvocellular neurons- have small cell bodies and small receptive fields ( located mostly in or near fovea) . Well suited to detect visual details and to rspond to color
2) Magnocellular neurons- have larger cells bodies and receptive fields and are distributed evenly throughout the retina. Sensitive to overall patterns and moving stimuli , not to details.
3) Koniocellular neurons - have small cell bodies, but occur throughout the retina. Several functions, axons terminate in different places.
What is the Primary Visual Cortex?
located in the occiptal cortex, AKA Area V1.
People with damage to this area show blindsight
What is the first stage of visual processing?
Lateral Geniculate Nucleus
What is blindsight?
an ability in people with damage to area V1, they respond to visual information that they report not seeing
Conscious Visual Perception requires activity where?
Area V1
Name the three types of neurons ( cells) in the visual cortex
Simple Cell- has receptive field with fixed excitatory and inhibitory zones. These cells are only found in the primary visual cortex and have bar shaped or edge shaped receptive fields.
Bar shaped receptive fields with vertical and horizontal orientation outnumber diagonal ones
Complex Cells- located in areas V1 and V2- does not respond to exact location of stimulus. Responds to pattern of light in a particular orientation anywhere within its large receptive field. responds to moving stimulus
End- Stopped ( hypercomplex) cells- resembles a complex cell, but has a strong inhibitory area at one end of its bar shaped receptive field. Responds to a bar-shaped pattern of light provided the bar does not extend beyond a certain point.
True or False. Cells within a given column respond to either the left eye, the right eye or both eyes equally?
True. They process similar information and do not fire at the same time.
Neurons in area V1 respond strongly to what type of patterns?
Bar or edge shaped. Therefore, activity of these cells may be necessary for perception of a bar, line, or edge.
What is a feature detector?
a neuron that indicates the presence of a particular feature/ stimuli
True or False. Prolonged exposure to a given feature decreases sensitivity to that feature?
True.
Most researchers believe that neurons in area V1 detect _______ _________, rather than bars or edges?
Spatial frequencies
Early lack of stimulation of one eyes leads to synapses in the visual cortex becoming…
Unresponsive to input from that eye
When does the critical period for the development of the visual system end in newborns?
With the onset of chemicals that inhibit axonal sprouting
What happens if both eyes of a kitten are kept shut for the first few weeks?
For the first 3 weeks cortex remains responsive to both eyes cortical responses become sluggish and lose well-defined receptive fields
What is a sensitive period?
when experiences have particularly strong and enduring influences
What is retinal disparity?
Discrepancy between what the left and right eye sees
What does comparison of slightly different inputs from the two eyes allow us to achieve?
Stereoscopic depth perception
Ability of cortical neurons to adjust their connections to detect retinal disparity is shaped through…
experience
What is Strabismus?
lazy eye. If two eyes carry unrelated inputs, cortical cell strengthens its synapses with axons from only one eye. Focusing.
What happens if early exposure is limited to only a certain pattern?
leads to nearly all of the visual cortex cells becoming responsive to only that pattern
What is astigmatism?
blurring of vision for lines in one direction, caused by an asymmetric curvature of the eyes
What are cataracts?
cloudy spots on one or both eyes. Difficulty in recognizing objects, unable to tell that components are part of a whole.
In infants impairment is more extreme if cataracts remain until alter life.
What is the Secondary Visual Cortex( Area V2)?
the area that receives information from primary visual cortex, processes it further, and transmits it to additional areas
What is the dorsal stream?
Visual path in parietal cortex. the “ where” pathway that helps motor system locate objects
Individuals with damage to the dorsal stream..
know what things are but not where they are
What is the ventral stream?
collection of visual paths in temporal cortex. the “ what” pathway. Specialized for identifying and recognizing objects
Individuals with damage to the ventral stream…
cannot describe what they see
What is Visual Agnosia?
Inability to recognize objects despite otherwise satisfactory vision. Usually results from damage in temporal cortex
What is the Inferior Temporal Cortex?
it contains cells that respond selectively to complex shapes but are insensitive to distinctions that are critical to other cells. The cells respond to what viewer perceives, but not what the stimulus is physically
What is the Fusiform Gyrus of the Inferior Temporal Cortex?
Area ( especially in the right hemisphere) that responds strongly to faces, much more than anything else
What is Prosopagnosia?
Inability to recognize faces. Occurs after damage to fusiform gyrus of the inferior temporal cortex
Color Perception depends on both…
the light reflected on an object and how it compares with objects around it
What brain area is likely responsible for color constancy?
Area V4
What brain area is especially activated by motion?
Area MT ( V5) and Area MST ( medial superior temporal cortex)
True or False. The middle temporal cortex ( MT) responds to a stimulus moving in a particular direction?
True
True or False. MST ( medial temporal cortex) neurons enable you to distinguish between result of eye movements and result of object movements?
True. Most neurons in MST are silent during eye movements but respond vigorously if something moves relative to the background
What is Motion Blindness?
Inability to determine the direction or speed of objects moving or if they are moving at all. Results from damage to the MT and MST
What is a Saccade?
a decrease in the activity of the visual cortex during quick eye movements
Neuron activity and blood flow decrease _____ ____ before and during eye movements
75 ms
