The WHO/WHAT (Ventral Stream/Parvocellular) Pathway Flashcards
Color
Spectra of “visible light” include wavelengths ranging from ~350nm to ~700nm
rainbow
short wavelengths =
med?
med-long?
long =?
- prism refracts different wavelengths
- ROYGBV
- short wavelengths = violet and blue
- Med = green
- med-long = yellow
- long wavelengths = orange and red
Trichromatic Color Vision
- Young-Helmholtz Theory
- 3 Cone types (Short, Medium, Long); Each w/diff type of Opsin that determine which wavelengths that Cone responds to
- Each responds best to Short, Medium or Long wavelengths and less well to adjacent wavelengths.
So Note: response ranges overlap!
- e.g. Orange (~630nm) = high response from Long wavelength Cones, less from Medium wavelength Cones, and none from Short wavelength Cones
Across Fiber Coding
color is, in part, coded by the RATIO of activity across multiple receptor types
Color Opponency
- Trichromatic System is then recoded into a Red/Green & Blue/Yellow (& Black/White) Opponent System
- e.g. If adapt to red, see green afterimage & if adapt to blue, see yellow afterimage (& vice/versa)
- e.g. If colorblind to red, will be colorblind to green, but not to blue or yellow (or if to B&Y not to R&G)
- arises via Lateral Inhibition
Blobs
In V1, each Hypercolumn has one set of Red/Green & another of Blue/Yellow “Blobs” for color processing
Lateral Inhibition
opponent cells?
(e.g. via Horizontal Cells in Retina & inter-neurons in LGN & V1)
- e.g. Some Ganglion & LGN cells are “Opponent Cells” = have Receptive Fields that are R+G-, G+R-, B+Y- or Y+B-
- i.e. R+G- = Red light on RF increases Ganglion’s response, while green light on same RF dec’s that cell’s response
- These converge in complex ways, to cancel out/ enhance one another, or form more complex “detectors” (e.g. for ripe fruit)
Color Constancy
- Humans can recognize colors even under changing light conditions (where actual s change)
- e.g. If whole scene tinted green, can still ID white, red, blue etc. as long as can compare across scene (not if only see one object)
- Retinex Theory
Retinex Theory
(Retina + Cortex) = Visual cortex (probably V4) compares color & light levels across scene, and somehow (? probably via feedback circuits) “subtracts out” shared tint, enables comparative analysis/identification of colors
Shape
To determine shape and texture, Visual Cortex begins by analyzing scene per Orientations & Spatial Frequencies
Simple Cell
- in V1
- gives its best response to lines of particular Orientation, less well to similar orientations
Complex Cells
- in V2
- give best response to Moving lines of particular Orientation
- often respond best to Sine Wave Gradients - gradually changing bands of dark & light - at diff Spatial Frequencies
Spatial Frequency
dark-light changes / degree of visual angle: High SFs for detail, Low SFs for Gross outline
Visual Scene coded as simultaneous activity…
…along multiple, (semi)-independent Spatial Frequency Channels for diff Orientations
- Combinations of these, at multiple orientations = image, for identifying objects, faces, etc.
- Spatial Freq changes across a whole scene also involved in depth perception (processed along other pathway - see below)
