4. The Visual Brain Flashcards
crossover point
- optic nerves from from retinal ganglion cells (RGCs) meet at the optic chiasm
e.g., object in the left visual field( not the left eye but the left half of your FOV):
- image passes through lens and projects on the right part of the retina in both eyes
- at optic chiasm, optic nerves from right side of both eyes meet
- optic tracts (optic nerve after the optic chiasm) project to the LGN in the thalamus
- then to the right half of the visual cortex in the occipital lobe
Lateral geniculate nucleus LGN
layers of lateral geniculate nucleus (LGN) of the thalamus (sensory processing centre)
parvocellular
RCG input: p-cells
small cell body
many of these
slow conduction speed
sustained response type
small receptive field
high spatial detail
colour
magnocellular
RCG input: m-cells
large cell body
few in number
rapid conduction speed
transient (responding to change) response type
large receptive field size
motion sensitive
black and white
there are 2 visual pathways
- geniculostriate
- tectopulvinar
geniculostriate visual pathway
- P-cells, some M-cells (P and M pathways) → LGN
- LGN has 6 layers; each has a reinopic map: adjacent neurons correspond to spatially related points on the retina
(LGN also receives substantial feedback connections from the cortex)
- then, optic radiations project to cortex in occipital lobe:
- primary visual cortex (V1, “striate cortex”)
- and secondary visual cortex (V2)
tectopulvinar visual pathway
- remaining M-cells → superior colliculi of the tectum: part of brain stem; guide visual attention
- then projects to thalamus: pulvinar and lateral posterior nuclei, then to V2 and beyond
- controls eye movements/fixations; detection/orientation to visual stimuli, motion and location
this process bypasses the primary visual cortex
The Striate Cortex
- has 6 layers (magno → 4Cα, parvo → 4Cβ)
- layers 2 and 3 contain: blobs and interblobs
blobs
sensitive to wavelength, but not orientation (mostly receive input from parvo)
interblobs
areas between blobs; sensitive to orientation, but not wavelength (receive input from parvo only)
Cells in the Primary visual cortex/V1
simple cell,
complex cell,
hypercomplex/end-stopped cell
David Hubel & Torsten Wiesel (1962, 1968):
- shared 1981 Nobel Prize for research on information processing by cortical cells in the visual system
simple cell
(layer 4) responds best to:
▸ a bar, line, or edge of light
▸ in a particular location on the retina
▸ having a specific orientation
complex cell
(layers 2/3 and 6) responds best to:
▸ a bar, line or edge of light
▸ in a particular location on the retina
▸ having a specific orientation
▸ and moving in a certain direction
cells respond to stimuli that have the same ______
when inserting electrode perpendicular to surface of cortex, cells responded to stimuli having the same PROPERTY
this includes: location columns, ocular dominance columns, orientation columns, and hyper columns
hypercomplex/end-stopped cell
(beyond V1) responds best to:
▸ a bar, corner, or angle having a certain length and/or width
▸ in a particular location on the retina
▸ having a specific orientation
▸ moving in a certain direction
location column
cells respond to stimuli from same retinal location
ocular dominance column
cells respond to stimuli presented to one eye only
orientation column
cells respond to line stimuli having the same orientation; adjacent orientation columns differ in orientation selectivity by 10°
hyper column
region containing a single location column, which contains left and right ocular dominance columns, which contain the set of orientation columns from 0° to 180°
the occipital lobe is made of areas…
V3 and V4
V3
cells sensitive to moving edges of a certain orientation
- believed to handle perception of forms and local motion (local motion is one thing moving, global is your whole FOV moving)
- projects to temporal lobe
V4
cells respond to perceived colour of a surface (not wavelength)-the difference in color can be perceived but not identified
- projects to temporal lobe
Achromatopsia
Verrey (1888):
▸ 61-year-old female stroke patient with cerebral Achromatopsia : unable to perceive colour in the right half of her visual field
▸ could perceive but not identify different colours
e.g., purple and orange look different, but colours could not be identified
the temporal lobe is comprised of:
the inferior and the medial temporal cortex