Chapter 6: Vision Flashcards
Pupil
opening through centre of eye where light enters
Lens
focuses light (adjustable)
Cornea
Focuses light (not adjustable)
Retina
- where light is projected onto
- rear surface of eye
- lined with visual receptors
Visual path to brain
-image is coded by different types of neuronal activity
back of eye -> bipolar cells -> ganglion cells-> ganglion axons that join together and travel back to brain
Amackine cells
refine and input to ganglion cells and enable them to respond to shapes, movement, and other visual features
Fovea
- acute detailed vision
- each receptor connects to single ganglion cell that has axon to the brain
Midget Ganglion Cells
- small
- responds to a single cone
- each cone in fovea is connected to the brain with direct route that registers exact location of input
- 70% of input provided by midget ganglion cells therefore vision is dominated by what we see in the fovea
process by which three types of cones, and the neurons they connect with, can produce a rich spectrum of perceived color
3 types of cones
1) short wavelength
2) medium wavelength
3) long wavelength
Trichromatic or Young Helmholtz Theory
- perceive colour through relative rates of response by the 3 kinds of cones
- compare the response to cone with response of other cones
- discriminate wavelengths by ratio of activity across 3 types of cones
ex) light @550nm excites medium and long receptors equally but not short receptors at all. This ratio determines perception of yellow-green - mice only have 1 type of cone and are colourblind
Opponent Process Theory
- perceive colour in terms of opposites
- brain has a mechanism that perceives colour on a continum
- cerebral cortex must be responsible for after images not individual receptors
trade-off between acuity for detail and sensitivity to dim light.
cones=colour vision, abundant in fovea, useful in bright light, essential for colour vision
rods=good in dim light, abundant in periphery, respond to faint light, not useful in day light because bright light bleaches them
-20:1 more rods than cones
-each cone has own line to brain due to midget ganglion cells
-periphery rod receptors share a line with 10,00-100,00 other rods which is why acuity is not good
Receptive Field
- area in visual space that excites or inhibits it
- point in space from which light strikes the cell
- receptive field grows as info becomes more processed (receptive fields converge)
- cone/rod -> bipolar cell -> ganglion cell
Parvocellular Neuron
- small cell body
- small receptive field
- mostly near fovea
Magnocellular neuron
- large cell body
- large receptive field
- evenly throughout retina
Koniocellular neuron
- small cell body
- throughout retina
- axons from ganglion cells form optic nerve that goes to optic chiasm, where 1/2 axons cross to opposite hemisphere
- most go into lateral geniculate nucleus of thalamus
- when info reaches cortex it becomes more complex
- info from lateral geniculate nucleus of thalamus goes to primary visual cortex in occipital lobe aka Area V1
- primary visual cortex processes conscious visual perception
Simple Cells
- receptive field with fixed excitatory and inhibitory zones
- more light that shines on excitatory=more cells respond
- more light that shine in inhibitory zone=less cells respond
Complex Cells
- do not respond to exact location of stimulus
- respond to pattern of light in a particular orientation anywhere in its receptive field
- responds strongly to moving stimulus
End stopped or hypercomplex cells
-similar to complex but has strong inhibitory area at 1 end of bar shaped receptive field
Area V1 neurons respond strongly to…
-bar or edge shaped patterns meaning activity of this cell is perception of bar, line, or edge and spatial frequency
Feature Detector
- neuron whose responses indicate presence of particular feature
- 80 brain areas (at least process info)
Ventral Stream
-what pathway
-specializes in identifying and recognizing objects
ventral and dorsal stream communicate and help each other
Dorsal Stream
-where pathway
-help motor system locate objects
ventral and dorsal stream communicate and help each other
Area V1
Primary visual cortex
- sends info to Area V2 (secondary visual cortex)
- simple and complex cells
- connections are reciprocal
- V1 sends info to V2, V2 returns info to V1
Area V2
- processes info and transmits it to additional areas
- connections are reciprocal
- V1 sends info to V2, V2 returns info to V1
Inferior Temporal Cortex
- responds to identifiable objects
- cells respond to what the viewer perceives not what the stimulus actually is
- physically can recognize different views of the same object
Area V4
- colour perception
- colour of an object in V4 corresponds to apparent or perceived colour depending on context
Motion Perception
- Area MT (middle temporal cortex) aka area V5 and Area MST (middle superior temporal cortex)
- receive most info from magnocellular path
- MT=colour insensitive
Area MT
- middle temporal cortex
- responds selectively
- detects acceleration and absolute speed
- responds to motion in all dimensions
- responds to photos of movement
Area MST
- middle superior temporal cortex
- responds best to expansion, contraction, or rotation of large visual scene (dorsal part)
- message converges to temporal part of MST where cells respond object that moves in relation to its background
- MST neurons enable you to distinguish between results of eye movement and result of object movements
Early visual deprivation: one eye
- deprived experience in one eye: synapses in cortex become unresponsive to input from deprived eye
- need to fix eye problems in sensitive period or eyes will not adapt or heal as quickly after this period is over
Early visual deprivation: two eys
-deprived experience in both eyes: when both eyes have no input there is no competition
for at least 3 weeks cortex remains responsive to visual input
-over 3 weeks response starts to become sluggish and lose well defined receptive fields
-eventually visual cortex starts responding to auditory or touch stimuli
-cortical plasticity is greatest in early life but never ends
-need to fix eye problems in sensitive period or eyes will not adapt or heal as quickly after this period is over
Strabismus
- eyes do not point in same direction
- children attend to 1 eye but not the other
- impacts developing depth perception
- need to fix eye problems in sensitive period or eyes will not adapt or heal as quickly after this period is over
Astigmatism
- blurring of vision for lines in 1 direction caused by asymmetric curvature of eyes
- need to fix eye problems in sensitive period or eyes will not adapt or heal as quickly after this period is over
