Chapter 4. The Visual Cortex and Beyond

optic chiasm

an x-shaped bundle of fibers on the underside of the brain

• some fibers cross to the opposite side of the brain from the eye

• each hemisphere responds to the contralateral (opposite) side of the visual field

lateral geniculate nucleus (LGN)

• in the thalamus (relay station before cerebral cortex)

• 90% of retina signals proceed here after optic chiasm

• regulates neural information as it flows from the retina to the cortex

• receives more signals from cortex than retina (feedback)

superior colliculus

• 10% of retina signals proceed here after optic chiasm

• structure involved in controlling eye movements

occipital lobe/ striate cortex

• visual receiving area

• where retina and LGN signals first reach the cortex

• area V1 = striped appearance in cross section

Hubel and Wiesel (1965)

study of receptive fields

• anesthetized animals looked at a screen with projected stimuli

• recorded from single cells/fibers at various points along the visual pathway

• determined optimum stimulus for each cell

simple cortical cells

cells with side-by-side receptive fields (excitatory and inhibitory areas)

• best respond to a vertical bar of light, line, or edge of a particular orientation

orientation tuning curve

indicates the relationship between orientation and firing; determined by measuring the responses of a simple cortical cell to bars with different orientations

complex cortical cells

respond only when a correctly oriented bar of light moves across the entire receptive field

• respond best to a particular direction of movement

end-stopped cells

fire to moving lines of a specific length or to moving corners or angles

• respond best to bars of a certain length moving in a particular direction

• feature detectors

ganglion cell

• center-surround receptive field

• responds best to small spots, but will also respond to other stimuli

stimulus-physiology relationship

measured when neurons respond to oriented lines

physiology-behavior relationship

measured to demonstrate a link between physiology and perception

• use selective adaptation

selective adaptation

firing in response to a particular stimulus causes neurons to eventually become fatigued or adapt; adaptation causes:

• neuron firing rate decreases

• neuron fires less when the stimulus is immediately presented again

only neurons that were firing adapt

contrast threshold

the minimum intensity difference between two adjacent bars that can be detected

• change intensity difference between light and dark bars in a grating

selective rearing

if an animal is reared in an environment that contains only certain types of stimuli, then neurons that respond to these stimuli will become more prevalent

• “use it or lose it”

neural plasticity/ experience-dependent plasticity

the response properties of neurons can be shaped by perceptual experience

• ex: rearing an animal in an environment that contains only vertical lines should result in the animal’s visual cortex having simple cells that respond predominantly to verticals

retinotopic map

electronic map of the retina on the cortex

• organized spatial map shows 2 points that are close together on an object and on the retina will activate neurons that are close together in the brain

cortical magnification

apportioning of a large area on the cortex to the small fovea

location column

the striate cortex organized into columns perpendicular to the surface of the cortex

• all neurons in one have receptive fields at the same location on the retina

orientation column

the striate cortex is organized with each column containing cells that respond best to a particular orientation

hypercolumn

a location column with all of its orientation columns

• receives info about all possible orientations that fall within a small area of the retina

• well suited to process info from a small area in the visual field

extrastriate cortex

are beyond the striate cortex (V1) where the visual signal proceeds

ablation

destruction or removal of tissue in the nervous system

object discrimination problem

a monkey is shown one object, then presented a 2-choice task including that object and another. If the monkey can discriminate between the 2 objects by pushing away the target object, it received the food reward under the object

landmark discrimination problem

a monkey is shown one object. The monkey’s task is to remove the cover of the food well closest to the “landmark” (the object)

ventral (what) pathway

the pathway leading from the striate cortex to the temporal lobe

• related to an object’s identity

dorsal (where + how/action) pathway

the pathway leading from the striate cortex to the parietal lobe

• related to an object’s location

• determines how a person carries out an action

double dissociations

we can understand the effects of brain damage through 2 people

• Person 1 has brain damage in an area causing function A to be absent while function B is present

• Person 2 has brain damage in an area causing function B to be absent while function A is present

inferotemporal (IT) cortex

neurons in the apex of this have the largest receptive fields

• encompass whole objects in one’s visual field

• neurons respond to more complex objects that occupy a larger portion of the visual field

hippocampus

an area of the brain associated with forming and storing memories