Vision Flashcards
% of the cerebral cortex plays a direct role in processing visual information
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energy detectable to human eye
electromagnetic
3 layers of retina
- photoreceptors (rods and cones), at back of retina
- bipolar cells (transparent)
- ganglion cells (transparent), surface of retina, sends axons out of eye
photoreceptors: rods
- more numerous
- more in periphery
- sensitive to small changes in light intensity/shading
- mediate vision in dim illumination
- more outlines and contours than details
photoreceptors: cones
- fewer
- concentrated in central area (fovea)
- sensitive to color
- higher resolution visual discrimination, fine details
fovea
central area of retina that contains only cones and has greatest visual acuity
optic disk
where ganglion cell axon exit eye in optic nerve; contains no photoreceptors and creates a blind spot
main pathway of visual stimuli
eye > optic nerve > optic chiasm > optic track > lateral geniculate nuclei in thalamus, sends optic radiations to > primary visual cortex / striate cortex > association visual cortex
second pathway of visual stimuli
eye > optic nerve > optic chiasm > superior colliculus > thalamus > secondary visual cortex (V5) - for orientation of head and eyes toward new stimuli in periphery
third pathway of visual stimuli
eye > optic nerve > optic chiasm > hypothalamus - for control of circadian rhythms
receptive field
the area in the visual field a neuron responds to
photoreceptors-to-ganglion cell ratio
- nearly one-to-one correspondence in fovea
- many receptors connect with single neuron in periphery
optic chiasm
- neurons carrying information from medial half of visual field (inner / nasal) cross to contralateral hemisphere
- ipsilateral visual info from each side crosses and goes to other hemisphere
- contralateral visual info from each side stays in same hemisphere
left visual field
- left half of visual field in each eye
- seen by right half of retina
- processed by right hemisphere
right visual field
- right half of visual field in each eye
- seen by left half of retina
- processed by left hemisphere
lateral geniculate nucleus (LGN)
- in thalamus
- 6 layers
- receptive field organization is reflected in LGN organization
- segregates visual information
- optic radiations project from LGN to primary visual cortex
- segregation of information transmitted is preserved in the cortex
optic radiations
info projected from LGN to primary visual cortex
map of primary visual cortex
visual field is represented in a topographic map on the cortex
- Contralaterally represented
- Upside down
- Upper visual field processed in inferior primary cortex
- Lower visual field processed in superior visual cortex - Central – peripheral
- Central visual field processed in posterior primary visual cortex
- Peripheral visual process in anterior primary visual cortex
% of primary visual cortex process info from fovea
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primary visual cortex / striate cortex / V1
color, form, and movement are processed separately
Composed of 1000s of modules, which consist of:
- blobs: color
- interblobs: orientation, movement
Some neurons are sensitive to orientation and only respond to certain line angles
cortical blindness
aka blind sight
conscious blindness is in the part of the visual field that damaged part of the cortex corresponds to
visually mediated bx still possible
V2
V1 projects to V2, an area of extrastriate cortex (region of visual association cortex that surrounds striate cortex) Information segregation (color, form, movement) happens in V1 then projects to V2 V2 maintains segregation of info (important)
association visual cortex
Fragmented component processing of primary visual cortex and V2 is transmitted to visual association areas where it is integrated into perception
Two visual pathways after primary visual cortex (V1 and V2)
- dorsal – WHERE – parietal lobe, perception of spatial location and self-movement
- ventral – WHAT – temporal lobe, perception of objects and color, identification of things
V4
- visual association cortex in occipital lobe
- part of ventral stream of visual info (V2 to V4)
- processes color and form
- neurons respond to specific color
- some to color in a specific orientation, indicating this area mediates form and color perception
- damage to V4 in humans results in impaired color perception and color constancy
V8 (or TEO)
- area in temporal lobe anterior to V4 - also involved in color discrimination
- Damage to V8 in humans results in loss of color vision but not form perception
- neuron receptive fields are larger than V4 and smaller than TE
- primary input from V4
- primary output to TE
- damage impairs monkeys’ ability to make even simple visual discriminations of form, size, orientation, color or brightness
location of integration for 3-D, object, and background perception
TEO and TE
inferior temporal lobe
TE
usually referred to as inferior temporal lobe
- 2nd level of visual assoc cortex, involved in processing most complex visual information
- recognition of objects is based on patterns
- neurons respond best to 3-D objects or photos of them, poorly to spots, lines, etc. and continue to respond if object is moved or changed in size, shape, brightness, etc.
- neurons are arranged in columns with neurons in adjacent regions responding to slightly different versions of the same object (tiger perception separated out into more basic shapes/color)
- largest receptive fields, often an entire contralateral half of visual field
visual agnosia
- Visual acuity intact
- No dementia
- Cannot identify objects by sight
- Can identify objects by other sensory modalities
apperceptive visual agnosia
failure in higher level perception
a. perceptual impairment and visual agnosia (can’t copy figures or recognize objects)
b. bilateral lateral occipital damage
associative visual agnosia
disconnect btwn perception and verbal systems
a. perception intact (can copy figures) with visual agnosia (can’t recognize objects)
b. damage in the anterior ventral/temporal stream
lateral occipital cortex
large region of ventral visual association area that responds to a large variety of objects and shapes
fMRI study found only three regions that showed greater activation to specific categories
faces
bodies
scenes
fusiform face area (FFA)
- visual association area on inferior surface of temporal (and occipital) lobes in R hemisphere
- part of ventral stream
- Separate specialized neural circuits that mediates face recognition
- FFA is influenced by experience; activated in people who are experts in viewing something: bird experts, car experts, dog judges
- People are better able to recognize faces of their own race
prosopagnosia
impaired ability to recognize familiar faces and learn new faces
- Developmental prosopagnosia
extrastriate body area (EBO)
- posterior to FFA
- most activated to bodies and parts of bodies and not other objects
- intermediate activation to faces
parahippocampal place area
activated by scenes and backgrounds
V5 (MT)
- V5 (or MT for medial temporal) mediates perception of movement
- receives input from primary visual cortex, other areas of visual association cortex, superior colliculus
- neurons are more heavily myelinated and input arrives faster to V5 than to V4
- V5 neurons responded best to movement and many are sensitive to movement in a particular direction
- bilateral damage in humans results in impaired perception of movement; disruption of V5 with TMS impairs perception of movement without affecting perception of form
- Patterns of movement can also give rise to perception of 3-D form
- these abilities can be dissociated in brain injury in humans, with V5 damage can have:
- impaired perception of movement without impairment of form perception based on movement
- or accurate perception of movement but impaired perception of form based on movement
V5a (MST)
V5a or MST (medial superior temporal) adjacent to V5 receives input from V5 and is sensitive to complex movements i.e. circular, radial, spiral
- MST also important in analysis of optic flow
analysis of optic flow
- V5a or MST important
- analysis of the relative movements of visual elements in the environment; e.g. when walking, some objects move to the R, L, up or down in your visual field
- analysis of optic flow allows you to determine what direction you are going, how fast you are approaching objects and if they will pass on the R, L, above or below
dorsal visual stream
- posterior parietal
- V5 projects to the posterior parietal area (intraparietal sulcus), area mediates visually guided movements, about where things are located
a. extensive connections with frontal eye fields and areas controlling hand and arm movements
b. bilateral damage impairs person’s ability to assume the proper hand position when reaching for an object - posterior parietal also involved in mental construction of 3-D images, along with visual association cortex (extrastriate) and dorsal prefrontal (planning movements)
blobs
in V1 modules, process color
interblobs
in V1 modules, process orientation, form, movement
processing pathway, starting with V1
V1, primary visual cortex, blobs and interblobs
> to V2, extrastriate cortex, segregation is maintained
then to V4 or V5
> V4, visual association cortex, color and form, beginning of perception, then ventral stream (what):
- to V8/TEO, integration of color/form for 3D, object, and background perception
- to TE, second level of visual assoc cortex, involved in processing most complex visual info, 3D objects
> V5, detects/distinguishes movement, then dorsal stream (where) to V5a/MST and to posterior parietal, interparietal sulcus, mediating visually guided movements
