Chapter 4: The Visual Cortex and Beyond Flashcards
how did the study of the visual cortex begin?
with cases of people who had brain damage
Optic chiasm
an x-shaped bundle of fibres on the underside of the brain
LGN
part of the thalamus that regulates neural information as it flows from the retina to the cortex and information flow
Visual receiving area/ V1/ Striate cortex
the place where signals from the retina and the LGN first reach the cortex
the retina to the cortex steps
- Visual signals from both eyes leave the back of the eye through the optic nerve and meet at the optic chiasm
- At the optic chiasm, some of the fibres cross to the opposite side of the brain from the eye they came from, resulting in each hemisphere corresponding to the contralateral visual field
- 90% of the signals from the retina proceed to the LGN, located in the thalamus of each hemisphere. The other 10% travels to the superior colliculus
- From the LGN, the visual signal travels to V1
how are neurons in the LGN organized
have centre-surround organization & receptive fields
where does the LGN receive most of its signals from
the cortex (feedback mechanism) but also receives signals from the retina
where is V1 located
in the occipital lobe
why is V1 called the striate cortex
it has a striped appearance when viewed in cross-section
how is a neuron’s receptive field determined
by presenting a stimulus to different places on the retina to determine which areas result in an exhibitory response, inhibitory response, or no response
Hubel & Wiesel’s receptive field method
projected stimuli on a screen and anesthetized an animal and focused its eyes with glasses. its eyes remained stationary and each point on the screen corresponded to a point on the cat’s retina, allowing them to determine receptive fields
where is the receptive field ALWAYS located
on the receptor surface
where is the receptive field for vision
the retina
who discovered simple, complex, and end-stopped cortical cells?
Hubel & Wiesel
simple cortical cells
Cells in the striate cortex that have inhibitory and excitatory areas arranged side by side. They respond best to bars of a particular orientation
oriented tuning curve
the relationship between orientation and firing, which is determined by measuring the responses of simple cortical cells to bars with different orientations
complex cortical cells
cells in the striate cortex that only respond when a correctly oriented bar of light moves across the entire receptive field
end-stopped cortical cells
cells in the striate cortex that respond to corners, angles, or bars of a particular length moving in a particular direction
feature detectors
neurons that respond selectively to a specific feature of the stimulus such as the orientation or direction of the motion
selective adaptation
A procedure in which a person or animal is selectively exposed to one stimulus and then the effect of this exposure is assessed by testing with a wide range of stimuli. Typically, sensitivity to the exposed stimulus is decreased
what are the 2 main effects of selective adaptation
Selective adaptation causes the neuron’s during rate to decrease and the neuron to fire less when that stimulus is immediately presented again
what neurons are affected by selective adaptation
those that respond to the stimulus
contrast threshold
the minimum intensity difference between the light and dark bars until the bars can just barely be seen
steps for measuring the effect of selective adaptation
- Measure a person’s contrast threshold to gratings with several different orientations
- Adapt the person to one orientation by having the person view a high-contrast adapting stimulus for a minute or two
- Remeasure the contrast threshold of all the test stimuli presented in step 1
psychophysical curve
a function of the increase in contrast threshold to the orientation of the stimulus
orientation tuning curve
a function of the firing rate of a neuron to the orientation of the stimulus
psychophysical vs. orientation tuning curve for simple cortical cells
The psychophysical & orientation tuning curves are very similar for a simple cortical neuron. Demonstrates that feature detectors play a role in the perception
selective rearing
A process by which neurons adapt to the specific environment within which they live. This is achieved when neurons change their response properties so that they respond best to stimuli that have been repeatedly experienced in the environment
Blakemore & Cooper’s kitten orientation study
exposed kittens to only vertical or horizontal orientations. When they were removed from the tubes after 5 months, they seemed blind to orientations they hadn’t seen in the tube
selective rearing might help explain ____
the oblique effect
neural plasticity/experience-dependent plasticity
the capacity of the nervous system to change in response to experience
duration of selective rearing vs. adaptation
selective rearing: long-term
selective adaptation: short-term
locations in the cortex vs. retina
Locations on the cortex correspond to locations on the retina
retinotopic map
the electronic map of the retina on the cortex
spatial representation on the cortex
is distorted, with more space being allotted to locations near the fovea than to locations in the periphery
cortical magnification
the apportioning of a large area on the cortex to the small fovea
Cortical magnification factor
the size of the cortical magnification
measuring the cortical magnification of your finger
- Hold your index finger up with your left hand at arm’s length
- While you look at it, hold your right hand at arm’s length about a foot to the right of your finger, positioned so the back of your hand is facing you
- Your left index finger activates an area of the cortex as large as the area activated by your whole right hand
who discovered columns in the cortex
Hubel & Wiesel
how is the striate cortex organized
in location & orientation columns
location columns
columns that are perpendicular to the surface of the cortex so that all of the neurons within a location column have their receptive field at the same location on the retina and prefer stimuli with the same orientation
orientation columns
columns that contain cells that respond best to a particular orientation
where are orientations & location columns located relative to each other?
1 mm location columns contain within them orientation columns that cover all possible orientations
hypercolumn
a location column with all of its orientation columns
how is an image represented in the cortex?
by the firing of neurons sensitive to a specific orientation in many separate columns in the cortex
tiling
the process by which columns work together to cover the entire visual field
where does the visual signal go after v1?
proceeds to other areas in the occipital lobe & beyond, known as V2, V3, V4, V5
extrastriate cortex
collective term for the visual areas in the occipital lobe and beyond known as V2, V3, V4, V5
size of the receptive fields in the extrastriate cortex
gradually increases as we move to higher-level extrastriate areas because we are adding more aspects of the visual scene
ablation
the destruction or removal of tissue in the nervous system
object discrimination problem (Ungerleider & Mishkin experiment)
the animal was shown one object and presented with a two-choice task which included the target object and another stimulus. If the subject was able to discriminate between the two objects, it received a food reward
landmark discrimination problem (Ungerleider & Mishkin experiment)
the animal’s task was to remove the cover of the food well that was closest to the landmark
ventral pathway
what pathway; takes information from the cortex to the temporal lobe
ventral part of the brain
the lower part where the temporal lobe is located
dorsal pathway
where the pathway; takes information from the cortex to the parietal lobe
dorsal part of the brain
the upper surface, where the parietal lobe is located
feedback
the backward flow of information
where do the ventral & dorsal pathways begin
the retina
are the dorsal & ventral pathways connected?
yes
what direction does information go in the dorsal & ventral pathways
bidirectional
Milner & Goodale claimed that ____
the dorsal stream provides information for how to direct action to a stimulus & should thus be called the how or action pathways
Milner & Goodale’s double dissociation experiment
Demonstrated double dissociation experiments with DF, a woman who suffered damage to her ventral pathway. She performed poorly in the static orientation-matching task but did well as soon as action was involved
double dissociation
in brain damage when function A is present and function B is absent in one person, and function A is absent and function B is present in another. The presence of a double dissociation means that the two functions involve different mechanisms and operate independently of each other
length estimation task
people were better at estimating lines if they grasped them (dorsal pathway) as opposed to perceiving them (ventral pathway)
infereotemporal (IT) cortex
an area of the brain outside V1 involved in object perception and facial recognition
where do neurons have the largest receptive fields
Infereotemporal cortex (IT)
Gross et al’s IT cortex experiment in monkeys
recorded single neurons in anesthetized monkeys’ IT cortexes while presenting them with a variety of stimuli. They found a neuron that refused to respond to any of the standard stimuli and would only respond to a handlike shape with fingers pointing up, demonstrating that neurons in the IT cortex respond to complex stimuli.
hippocampus
an area associated with forming and storing memories located in the medial temporal lobe (MTL)
patient HM
Patient HM had his hippocampus removed in an attempt to eliminate epileptic seizures. The procedure eliminated his ability to store experiences in his memory
Gelbard-Savig et al’s MTL neurons & videos experiment
Participants viewed video clips and some neurons responded better to particular clips. When they thought about the clip, the neurons also responded, demonstrating that MTL neurons that respond to perceiving specific stimuli might also be involved in remembering those stimuli
do receptive fields change?
yes, they are flexible in response to our environment
Kapadia et al’s neural flexibility experiment
recorded neurons from a monkey’s visual cortex and found that when the “outside the receptive field” bars are presented along with the “inside the field” bar, there is a large increase in firing, demonstrating neurons’ flexibility
contextual modulation
the effect of stimulating outside the receptive field
perceptual organization
the process by which small elements become perceptually grouped into larger objects
how do feature detectors demonstrate the stimulus-physiology relationship
they show how the stimulus leads to the activation of cells
how do feature detectors demonstrate the physiology-behaviour relationship
they show how the activation of cells makes you perceive something
the neural map in V1 is determined by ___
stimulating various places on the retina and noticing where neurons fire in the cortex
nasal retina
the part of the retina closest to the nose
temporal retina
the part of the retina closest to the temples
connections from the temporal retina are ___
ipsilateral
connections from the nasal retina are ____
contralateral
how are receptive fields in the retina organized?
in circular receptive fields
what is each location and orientation column responsible for?
detecting its own orientation
where are neurons’ receptive fields located in the retina vs. the cortex
every neurons has its receptive field at about the same location on the retina
cortical representation of a stimulus
has to contain information that represents the stimulus
damage to the ventral pathways results in ___
difficulties with object discrimination
damage to the dorsal pathway results in ___
spatial deficit
Mortimer Mishkin
discovered the what and where pathways by doing experiments on monkeys
experiments with patient DF demonstrate
that the dorsal stream is associated with action
size of receptive fields throughout the visual system
The increase in receptive field size continues through the what stream so that neurons at the apex of this stream in the IT cortex have the largest receptive fields
