visual part 3+4 Flashcards
Describe the neural sequence from retina to primary visual cortex
Right and left visual fields project to opposite lateral geniculate body and primary visual
cortex
Partial decussation of ganglion cell axons in the optic chiasm
What is partial decussation?
(crossing) of fibres from the temporal visual fields (the nasal hemi-retina) of both eyes
What is the difference between homonymous and heteronymous hemianopsia?
Hemianopsia: loss of half visual field
Heteronymous: loss of opposite visual fields due to lesion of
central optic chiasm (pituitary tumors)
Homonymous: loss of same visual fields due to lesion of
optic system between optic chiasm and visual cortex
Describe the differences between ON and OFF retinal cells
Light stimulates rod and cone receptors which activate ON and OFF bipolar & ganglion cells
ON bipolar cells have glutamate receptors that respond to decreased glutamate with depolarization and excitation of the ganglion cell, activated in the presence of light in center of the RF–> OFF cells are inhibited
OFF: With darkness in center of the RF, photoreceptors release more glutamate
OFF bipolar cells have glutamate receptors that respond to increased glutamate with depolarization that excites the OFF ganglion cell
What are center-surround receptive fields in the retin
Circular areas of retina are organized into concentric rings
where inner receptors activate ganglion cells one way, the
surrounding region activates it in the opposite way.
On-center retinal ganglion cells respond–>light spots
surrounded by dark backgrounds
Off-center ganglion cells–>dark spots, light backgrounds
how do center-surround receptive fields affect perception of visual boundaries?
Center-surround fields enable ganglion cells to encode contrast. emphasize EDGES
Rate of activity of ganglion cells indicates amount of contrast between and center part of its
receptive field.
SOOOO
Because ganglion cells enhance contrast, a moderately bright region may appear brighter or darker depending on the background.
What is the difference between the parvocellular and magnocellular pathways from retina to LGN and visual cortex?
different types of ganglion cells–>parallel processing conveys different types of visual information along each route
Parvocellular (midget): color and shape information for object perception and identification.
90% of axons in optic nerve,receive input from 1 or a few
photoreceptor (esp cones in fovea),high acuity with smaller center-surround receptive fields
Magnocellular (parasol) : movement for perception of motion and direction to enable visual attention, alerting,
grasping.
5% of axons in optic nerve, receive input from larger group of photoreceptors (esp rods in periphery) ,low acuity with large center-surround receptive fields
dark or night vision
ventral and dorsal streams cortical and processing of visual information
ventral=parvocellular stream: information from the
fovea (cones), acuity and color, through association
cortices including the inferotemporal gyrus. high levels
of visual perception and cognition, ‘What’ (not where).
dorsal=magnocellular stream:peripheral retina (rods) into
posterior parietal cortex. This computes vision for ‘action’ ‘How’ and ‘Where’.
streams converge into prefrontal cortex to form working
memory
Give a brief description of simple and complex cells in the primary cortex.
Primary visual cortex develops perception of form, color,direction of movement and binocular vision through activity of simple and complex cells organized in columns
Simple:input from several geniculate cells that individually respond to contiguous points along the bar of light w/ specific orientation
Complex:respond to a properly oriented EDGE or bar of light, but anywhere within the receptive field. take info from SIMPLE cells
What characterizes visual cortical columns?
vertical zones of cortex that use combinations of simple
and complex cells to sort information into various properties.
What impact does cortical feedback have on incoming visual input?
Cortical columns generate complex activity that is
used to provide feedback onto sensory pathways. –> focus on particular aspects of the environment
Cortical feedback to lateral geniculate nucleus ->sensory input according to the ongoing behavior of the organism
Enhances the acuity of their responses to bars of light by
altering lateral-inhibition-like mechanisms among LGN neurons
What impact does neuromodulation by the reticular formation have on incoming visual information?
LGN cells are modulated by brainstem pathways using norepinephrine, serotonin (5HT), ACh, NO (reticular formation).
Cortical cells that respond optimally to a particular RF (orientation, movement, ) will sharpen the responses of any LGN cell that responds to those particular features–> amplification
What are the visual association cortices?
V1: projects different aspects of visual cognition into dorsal and ventral streams of association cortices, integrating the information
V2 cells are tuned to complex properties: angles between lines, orientation of illusory contours and whether the stimulus is part of the figure or the ground
V4:tuned to color and more complex shape complex outline-shapes
Inferotemporal cortex: scale and position, objects such as faces
MT (middle temporal) is part of the dorsal stream
for detecting complex, global motion
How do the inferotemporal and fusiform cortices differ from the other visual association cortices?
inferotemporal responds to both halves of visual field, information transmitted by interhemispheric axons via the corpus callosum, EMOTION. focus on something
and you notice it more, short and long term memory
Fusiform: Face recognition, esp. with emotional valence (connection with amygdala) . Synesthesia
Describe visual neglect and its impact on perception of one’s visual field.
Loss of function in the right visual association cortices
left side of picture missing
