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Chapter 10: Central Visual System (Extrastriate visual cortical areas (MST…
Chapter 10: Central Visual System
Central visual pathways
axons of retinal ganglion cells leave retina in the optic nerve
axons in nasal hemiretina cross to opposite side of the brain at optic chiasm
temporal hemiretina axons DO NOT cross
Physiological optics
binocular visual field: visual field viewed by both eyes
left side of the brain views right visual hemifield
right side of brain views left visual hemifield
Central projections of retinal ganglion cells
thalamus
lateral geniculate nucleus
LGN contains retinotopic map of contralateral visual hemifield
parallel pathways: segregation of inputs in LGN
M/P/K
left eye/right eye
ON/OFF center RFs
retino-geniculo-cortical projection: course of optic radiations to striate cortex
optic nerve leads from eye to optic chiasm
nasal hemiretinal fibers cross at optic chiasm
goes to optic tract to LGN
optic radiation leads from LGN to primary visual cortex
hypothalamus
suprachiasmatic nucleus
midbrain
superior colliculus, pretectum
Retinotopic organization of primary visual cortex (V1)
more space devoted to central visual field
stria of genari: "striate cortex"
heavily myelinated, many more neurons here receive input from central retina than from peripheral retina
Visual field deficits
anopsias: large visual field deficits
scotomas: small visual field deficits
lesions
optic nerve
ipsilateral eye blindness
optic chiasm
peripheral blindness
optic tract
blindness in ipsilateral visual fields of each eye
Vertical organization of primary visual cortex
LGN K cells --> BLOBS in II/III
LGN M cells --> 4C(alpha)
LGN P cells --> 4C(beta)
Layer 4C is primary recipient zone for LGN afferents
M,P pathways remain segregated
cells are still center-surround, ON and OFF pathways remain segregated
cells are monocular
Receptive field properties emerging inV1
Ocular dominance
convergence of left and right eye inputs
important for depth perception
ocular dominance columns
share same eye dominance
extend from Pia to white matter
Layer 4C monocular
binocular cells outside of layer 4C (but driven better by one eye or the other)
Disparity tuning
retinal disparity (difference in images from left and right eye), brain uses as a binocular cue (stereopsis)
determines depth/distance of an object
Orientation Tuning
convergence of inputs from ON center and OFF center RFs
important for form perception
visual system uses info from local orientation and contrast to construct contours and surfaces of objects
Simple cell receptive field
convergence of inputs from rows of ON and OFF center cells
RF is elongated along particular axis
elongated ON subregions excited by light on
elongated OFF subregions excited by light off
good stimulus is bar of light at particular orientation bad stimulus: inhibition cancels excitation
orientation selectivity
Complex RF
can't make any predictions to luminance pattern, complex cells are still orientation selective though
convergence of several like-oriented simple cells
Orientation columns extends from Pia to white matter
same orientation preference
Direction Selectivity
convergence of inputs from cells with different spatiotemporal RF properties (preferred direction of motion)
important for motion perception
Optically imaging intrinsic brain signals
voltage sensitive dyes
intrinsic differences in light scattering of active and inactive cells
orientation pinwheel (functional column)
cells with progressively different orientation preference
two-photon calcium imaging
CA concentrations can be measured by introducing Ca sensitive fluorescent dye into neuron
amount of light emitted from neuron correlates with neuron firing rate
Cytochrome oxidase blobs in primary visual cortex
cells in blobs have higher firing rates than inter blobs (cells outside of blobs)
blobs centered on an ocular dominance stripe
Extrastriate visual cortical areas
MT- motion perception "M" (dorsal) pathway
some MT neurons activated by structure from motion and biological motion
lesions of MT --> abolish ability to perceive structure from motion
some MT neurons activated by illusory motion
what motion we perceive, even though there's no motion
V4 - color perception "P" (ventral) pathway
Color constancy: ability to determine color of an object despite changing illumination
perception of color of an object is based on light reflected from it AND light reflected from surrounding objects
lesions of V4 lead to chromatopsia: inability to distinguish colors
dorsal and ventral processing stream
dorsal: to parietal lobe
"M" Pathway
"where" "action"
location and grasping
V4/IT
ventral: to temporal lobe
"P"/"K" pathway
"what" "who"
recognizing object
MST/MT
MST respond to optic flow stimuli
optic flow provides info about environment
near objects move faster than more distant objects
RFs cover large part of visual field
neurons respond to motion and difference in speed between center and periphery
lesions of MT and MST can lead to movement agnosia: inability to see movement
face selective cells in IT
require nearly all essential features of face
requires more convergence of info to single cells
lesions of IT
prosopagnosia
object agnosia