LECTURE 5 Flashcards
VISION
Construction of the world based on incoming information provided by light interacting with
photoreceptors on the retina.
* Most important/informative sense for humans
* Greatest amount of sensory cortex is devoted to vision
* Conscious experience dominated by visual information
EARLY NOTIFICATION VS RICH DETAIL
Tradeoff between accuracy and speed
LIGHT
Comprised of photons – particle and wave
* Color (and visibility) of light related to its
wavelength
* Brightness of light related to its amplitude
RETINA
Light stimulates photoreceptors at far end of retina
* Neural signals converge onto ganglion cells through bipolar cells
* Ganglion cell axons group into the optic nerve, which projects into brain
RODS
Low light vision (scotopic)
* Sensitive to fast motion
* Low resolution; poor edge detection (high convergence)
* Concentrated at peripheries of eye
* ~120 million per eye
CONES
Requires bright light (photopic)
* Three types – short, medium, and long – correspond to wavelength most sensitive to
* High resolution; good edge detection (low convergence)
* Concentrated in the fovea (middle of retina; focus point)
* ~6 million per eye
CONES VS RODS
Although the rods greatly outnumber the cones, each cone provides a much larger relative input to visual cortex due to neural convergence
* ~90% of brain’s input originates from the cones
DOES RIGHT EYE GO TO LEFT BRAIN AND VICE VERSA?
No. Left visual field projects to the right hemisphere of
visual cortex and right visual field projects to the left hemisphere of visual cortex
THROUGH THE BRAIN
Information from the optic nerve splits to both hemispheres in the optic chiasm
* Routed through the lateral geniculate nucleus (LGN) in the thalamus
* Projected to primary visual cortex, area V1 in the Occipital lobe
LGN NEURON (THALAMUS) COMPRISED OF
Parvo and Magno cells
PARVO CELLS
- Connections from foveal cones
- Color sensitive
- Detail and edge detection
- Good spatial resolution
MAGNO CELLS
- Connections from peripheral rods and cones
- Sensitive to motion over wide areas
- Bad spatial resolution
- “Fast track”
VISUAL CORTEX
- Area V1 (primary visual area)
- Back of occipital lobe
- Individual neurons responsive to edges of specific orientations
- Retinotopic organization
- Cortical magnification
PARIETAL LOBE PATHWAY
Dorsal pathway (spatial location and action)
TEMPORAL LOBE PATHWAY
Ventral pathway (characteristics of objects)
COLOR - PHYSICAL AND PERCEPTION DEFINITION
- Physics definition
- Wavelength of light
- Can be emitted from a light source or reflected off a surface
- Perceptual definition
- Experience of “color”
- Based on wavelength – but only partially
- Perceptual experience of color changes based on
brightness and hue of light source and lightness of object
COLOR MIXING
Additive color mixing
* Mixing light
* Every new color “adds” emitted color
* Colors can be defined as a combination of red, green,
and blue (RGB)
* Photoshop RGB color picker
CONES AND COLOR
- Three types of cones, corresponds to size of
wavelength most sensitive to: Short (Blue), Medium (Green) & Long (Red) - Combination of information from three cones is
unique for all visible wavelengths - S-cones are less common, and absent in fovea
- M-cones are a more recent development (cf. dog
vision)
OPPONENT-PROCESS THEORY
Color “opposites”
* Red-Green, Blue-Yellow, Black-White
* Color adaptation effect
* Neurons “fatigue” when responding to same stimulus
* When stimulus is removed, “opposite” is perceived
COLOR WHEEL
Colors opposite from each other are opposing
Opposing colors “pop” when combined = Color contrast
BRIGHTNESS
Intensity of light; luminance
* Depends on lighting, transient
LIGHTNESS
Degree to which a surface reflects light;
reflectance
* Quality of an object itself, stable
LIGHTNESS CONSTANCY
Interplay of lighting and lightness determines brightness (lightness constancy)
* Both a white sheet of paper in a dark room (a) or a grey sheet of paper in a bright room (b) have the same brightness
* Therefore - a must be a lighter surface than b since brightness is the same, but a is viewed with less overall light
COLOR CONSTANCY
This same process occurs when evaluating the color of objects in different lighting contexts
* Colors reflect different spectrums under different colored lights
* Like lightness constancy, need to figure out the color of the object itself, so need to “subtract” ambient lighting
