Lecture 5
Vision 4
Visual Cortex: (Area 17, V1)
Topics:
1. Cell types/response types
2. Cortical organization
3. Population coding & perception
I. Response features that V1 cells can have:
elongated receptive fields + ORIENTATION SELECTIVITY
center-surround (simple, etc.)
no center-surround (complex)
direction selectivity (magnocellular layers)
“linear” vs. “non-linear” (simple vs. complex)
color tuning (in cells with round receptive fields & no orientation tuning)
monocular input
binocular input & disparity tuning
Simple Cells
like RGCs, LGN cells
Center-surround
but not round - elongated!
respond best to a bar of light (or dark) in center of RF
oriented along the axis of RF
inhibited by light (or dark) in surround, also oriented
= Orientation selective
responses = linear, sum of responses to component spots
Complex cells
also orientation selective
larger RF's
but no longer center-surround
respond to bar anywhere in RF
can’t predict responses from testing with smaller stimuli –
non-linear
cells in visual cortex have elongated receptive fields
respond best to bar shaped stimuli
simple cells:
on center - off surround or off center - on surround (simple cells)
probably due to converging input from a series of (LGN cells
with suitably located receptive fields
complex cells:
no center surround organization
instead, respond to onset & offset
(How does this relate to X-Y or M-P divisions? These cell types can get input
from either sources).
II. Organization of cortex:
1. Laminar
2. retinotopic
cortical magnification
3. "columnar" orientation
ocular dominance
1. Layers
6 layers
Distinct input/output roles
e.g. layer 4 receives input from thalamus
2. Retinal topography
A. Map of retina across surface (parallel to layers)
B. Cortical magnification:
relative area of cortex representing a particular area of visual field
is not proportional to size of that area of the visual field:
Fovea = small
Foveal cortical region = huge
3. Columnar organization
A. Orientation columns
cells with common orientation preference = clustered together =>
orientation columns.
Systematic progression through orientation space
B. Ocular dominance columns
columns of cortex receive input from predominantly one eye
or other
established during development
sorting of inputs based on experience
Hubel & Wiesel -raise kittens with one eye sutured shot
columns = all other eye
but relative experience
suture both eyes shut
normal ocular dominance column pattern
critical period
it matters when you do the monocular deprivation
Note, though, that unlike LGN many cells do receive
input from both eyes.
4. other: Color also shows local clustering – more on that later.