Sensation and Perception (Vision) Flashcards
Layering of the retina
Photoreceptors, then ganglion cells, then axons of the ganglion cells form the optic nerve
Photoreceptors
- Rods and cones
- contain pigments that absorb light energy and turn it to electrochemical energy used in the nervous system
Parallel processing (visual system)
The visual system simultaneously processes different aspects of the visual scene at the same time
Rods
Rods contain the pigment rhodopsin: sensitive to small amounts of light
Rods are located in the periphery of the retina
Many rods feed into ganglion cells
Cones
- 3 types of cones: sensitive to different wavelengths- red, blue, green
- Cones are most densely packed in the fovea
- Only a few cones feed into ganglia
Ganglion Cells
- Have axons that extend to the brain to form the optic nerve
- Many rods feed into a single ganglion cells, very few cones feed into one ganglion
- Two main types of ganglia: M cells (large) and P cells (small)
- the two types of ganglia send input to different parts of the brain
M cells (Magnocellular cells)
The larger ganglion cells
- Responsive to coarse pattern and rapid motion
- goes down tectopulvinar pathway
P cells (Parvocellular cells)
Preserve colour information
The smaller ganglion cells
Receptive Fields
- Specific cells that respond to specific regions in the visual space
- Light strikes different parts of the eye depending on where the eye is facing
- the brain knows where light has struck based on which ganglion cells are excited
Centre-surround receptive fields
Receptive fields have a centre-surround structure
- light in a particular spot will excite the ganglion cell, light in the donut-shaped area surrounding that spot will be inhibited/ weakened
- this enhances contrast in vision
Pathway from the eye to the cortex
- Right visual field goes to the left of each eye and vice versa
- All right visual field information goes to the left of the brain to V1, and vice versa
- Information travels from the eye, through the optic nerve, to the optic chiasma (where information crosses over), reaching the Lateral Geniculate Nucleus (LGN), and finally, the Primary Visual Cortex
The Tectopulvinar Pathway
- Receives the majority of input from M ganglion cells
- Fast acting, sensitive to both motion and the appearance of novel objects on the vision periphery
-Sends information directly to the superior colliculus
»> The pathway then extends to the pulvinar nucleus in the thalamus and to cortical areas that process information about visual motion.
»> Superior colliculus also sends projections to motor regions that control eye & head movements
Superior Colliculus
Part of the tectum (in the midbrain), alongside the inferior colliculus
SC is visual, IC is auditory
SC sends projections to motor regions that control eye and head movements
The Geniculostriate Pathway
90% of optic fibres project to this
- Axons terminate in the LGN of the thalamus
— information continues to the straite (V1)
— enables colour and detail perception
— information on the right sides of both retinas is sent to the right side of the brain + vice versa
- Once optic nerve fibres cross (at the optic chiasm), they are referred to as the optic tract instead of the optic nerve
Lateral Geniculate Nucleus (LGN)
6 main layers, stacked on each other, then folded into a knee shape
- additional minilayers between each layer: konicellular layers
- Each layer receives input from only one eye, all layers receive information from the contralateral visual field
- Has 2 Magnocellular layers and 4 Parvocellular layers
— Koniocellular layers recieve input from bistratified ganglion calls + superior colliculus
Each layer contains a retinotopic map of the visual world, which helps organize information
Retinotopic mapping
Made to look like the visual world
In the V1, it is flipped and reversed compared to reality
Primary Visual Cortex/ V1/ Striate Cortex
Projections from LGN to V1 maintain spacial organization
Has varying cortical magnification factor- fovea is higher than periphery, for example
Has distinct layers that make it appear “striped”
Cortical Magnification Factor
The millimetres of cortical surface dedicated to one degree of an angle in the visual world
Cells of the Striate Cortex
Simple cells- respond to bars of differing orientations
Complex cells- best respond to certain orientations, not picky about where the line is, and show preference for direction of movement
Hyper-complex cells- prefer lines of specific lengths
Columns in the Striate Cortex
Cells that prefer a given line orientation group together to form orientation columns
- ocular dominance columns are segregated depending on which eye sent the input
- hypercolumn contains cells tuned to respond to stimulation at a specific spatial location
Binocular Integration in the Striate Cortex
Information from both eyes is integrated
- binocular disparity — the difference between the image on each retina, used to determine depth
- there are specific striate cells tuned to certain amounts of binocular disparity
Area V4 and colour
Patients with posterior ventral cortex damage (cerebral achromatopsia) see the world in shades of grey.
-colour-sensitive cells have been found ion the V4 area of the macaque monkey
— other studies found responsiveness to line orientation, depth, and motion
-Human studies imply an association between ventral extrastriate subregions and color processing, but the exact association is still subject to debate.
Cortical blindness
Blindness due to cortical issues instead of a problem in the eye or optic nerve
Demonstrates the importance of V1 for conscious awareness of the visual world
Blindsight
People have no conscious experience of seeing despite being able to make rudimentary visual discrimination
Likely because of intact tectopulvinar pathway + damaged geniculostriate pathway AND/OR a small number of LGN pathways bypassing V1 and going to extrastriate regions instead
What and where pathways in vision
What = identifying objects, ventral stream
Where = representing spatial locations, dorsal stream
Two main routes that leave the striate cortex
