Midterm 1 Flashcards
How do pink and red differ?
Saturation but not hue
Why have color?
Object detection Perceptual organization Segmentation Fruit theory Object recognition and identification
Additive color mixing
Adding light
More photos = whiter light
Subtractive color mixing
Adding pigments
In pigment mixing we only see the colors that do net get absorbed
Simultaneous color contrast
Two colors simultaneously present
Artists aware of phenomenon before scientists
Successive color contrast
Color aftereffects
See complimentary color on same shape after
What kind of light level are rods sensitive to
Scotopic: dim light levels at or below the level of moonlight
Can rods discriminate wavelength
No because they are sensitive to all wavelengths
What photopigmemtation molecule are all rods sensitive to?
Rhodopsin
Can we perceive many different hues at nighttime
No because we have to really on rod system
S-cones
Cones that are preferentially sensitive to short wavelengths (“blue” cones)
M-cones
Cones that are preferentially sensitive to middle wavelengths (“green” cones)
L-cones
Cones that are preferentially sensitive to long wavelengths (“red” cones)
Why not just have a single receptor type?
Ambiguity in responses of each cone type
The higher the number of absorbed photos
The higher the activity
Problem of univariance
An infinite set of different wavelength intensity combinations can elicit exactly the same response from a single type of photoreceptor – Thus, one type of photoreceptor, by itself, cannot serve as the basis for color discriminations
Ill-posed problem
Problem which lacks the necessary amount of problem to solve
Wavelength mapping
One-to-one matching of wavelength to cone type but no the reverse
Metameters
Two physically different stimuli that are perceptually identical
Example of metameter in vision
Wavelength 580 + 620 = same color as 580
Spectral colors
Colors produced by wavelengths in spectrum
Nonspectral hues
Colors that can only result from light mixtures e.g. purple magenta
How does brain derive reflectance curve of an object no matter the light?
The brain assumes that light follows a normal distribution and the reflectance follows a normal distribution
Related colors
Colors that only hold their value by comparison to other hues (gold, silver, brown)
Nonrelative colors
Always a particular hue e.g. Red, blue, yellow
Color constancy
The principle that certain colors do not change hue based on their environment
Chromatic adaptation
The brain habituates to a omnipresent color when the neurons activated to that color become fatigued. Thus discounting the effects of that color on final determinations.
Memory color
Characteristic color of familiar objects affects color perception. People judge the familiar objects to have a richer more saturated color than unfamiliar objects
What is an object?
Objects are the basic units in our representations of the world
How long does visual object recognition take?
50 - 500 ms
Steps in visual object recognition
Segmentation of the visual field
Grouping/Unit formation - most objects are partially occluded
Recovering 3d shapes from particular views (we do not know distance)
Describe and represent shape
Object recognition theory
The brain creates a description of an object from light and then compares that to objects in memory
Recognition and perception relationship
Often used interchangeably now because the processes are difficult to separate
Basic level recognition
objects are categorized into ordinal categories (phone not Trevor’s phone)
Information-processing tasks in visual object recognition
Edge detection edge classification Extracting junctions classifying junctions boundary assignment Grouping unit formation Shape perception
perceptual phenomenon that occurs when a change in a visual stimulus is introduced and the observer does not notice it
Change blindness
You do not notice things you do not deliberately attend to
Inattentional blindness
Doctrine of specific nerve energies
Nature of sensation depends on which sensory fibers are stimulated, rather than how they are stimulated
Goes from retina and carries visual info to thalamus
Optic (II) nerve
Sense processing in the cortex
The cortex often becomes polysensory meaning that information from more than one sense is being combined
Event-related potential
A measure of electrical activity from a subpopulation of neurons in response to particular stimuli that requires averaging many EEG recordings
MEG (Magnetoencephalography)
High temporal and moderate spatial resolution but very expensive
Computed tomography (CT)
Uses X-rays to creates images of slices through volumes of material
fMRI
functional magnetic resonance imaging measures blood oxygen level-dependent signal,
high spatial, poor temporal resolution
PET (positron emission tomography)
An imaging technology that enables us to define locations in the brain where neurons are especially active by measuring the metabolism of brain cells using safe radioactive isotopes
Definition of perception
The act of giving meaning to a detected sensation
Sensation
The ability to detect a stimulus, perhaps to turn that detecting into a private experience
Smallest detectable difference between two stimuli, or the minimum change in a stimulus that enables it to be correctly judged as different from a reference stimuli
Just noticeable difference or difference threshold
Weber’s law
Just noticeable difference is a constant fraction of the comparison stimulus
Fechner’s law
S = k log R, S = psychological sensation, log R = logarithm of physical stimulus level multiplied by constant K
The minimum amount of stimulation necessary for a person to detect a stimulus 50% of the time
Absolute threshold
Method of constant stimuli
The experimenter creates many instances of stimuli with
different intensities and presents these stimuli randomly
Can be somewhat inefficient: Many stimuli are above or
below thresholds, but gives you more information.
Method of adjustment
Similar to the method of limits, except subject controls
the increase/decrease of intensity
II. Not as reliable, but fast & easy
Method of limits
Experimenter begins with a set of stimuli that vary in
intensity, but then increases or decreases the intensity of
these stimuli until the stimuli are able to be perceived, or
lose their ability to be perceived, respectively
-Relies on honest
A psychophysical method in which the participant assigns values according perceived magnitudes of the stimuli
magnitude estimation
Steven’s Power Law
S = al^b S = sensation, a = constant, b = exponent
Color matching study set-up
Subjects were given three knobs that controlled three lights and told to match the lights to a forth color
Trichomatric theory/Young-helmholtz
Only three colors Blue/Green/red
Colors that create contrasts
Blue/Yellow
Green/Red
Red/Green
What explains color contrast (not full) according to trichromatic theory
Neurons in one cone become fatigued after a stimulus created high rate of firing so the other cones have higher activity in contrast
Which color contrasts can be accounted for and which cannot?
blue, yellow = yes
red, green = no
Problems for trichromatic theory
Why are some colors harder to visualize (reddish green)
color contrast of red + green
Color deficiency have a pattern (blue + yellow)
People rarely describe a color as a comination of blue + yellow or red + green
Yellow seems intuitively like a simple color but is actually complex according to trichrome theory
Opponent color theory
Perception of color is based on output of three colors
red-green
blue-yellow
black-white
Hurvich & Jameson (1957) experiment
Subjects are given a knob to add green to blue until it cancels also given knob to add red to green
In the Hurvich & Jameson experiment: which color had no unique version
There was no unique red because it always had a tinge of yellow in it
DeValois 1960
Opponent neurons
Relationship between trichromatic theory & opponent theory
Not mutually exclusive
Trichromatic receptors create response picked up by opponent cells
Cerebral achromatopsia
color blindness - shows that color processing happens in the cortex
How is color represented in the cortex?
Color is represented in a distributed way
Synapse efficacy
if the synapse is strong it is sufficient to excite the neuron it connects to
What are the usual dependent variables of psychophysics?
Accuracy and reaction time
Name of a psychophysical method in which the participant assigns values according to perceived magnitudes of the stimuli
Magnitude estimation
The ability to match the intensities of sensations that come from different sensory modalities. Ex: matching brightness of light until it matches loudness of sound
Cross-modality matching
Internal threshold set by observer in signal detection theory
Criterion level response
What is sensitivity in signal detection theory and how does it function
d prime is distance between peak of noise distribution and noise + signal distribution. d’ is 0 if you have no ability to detect signal
Receiver operating characteristic (ROC) curve
x-axis = false alarms, y-axis = hits
Assumptions of SDT
Signal + noise curve & noise curve have same variance and are both normally distributed
Number of cycles of grating per unit of visual angle
Spatial frequency
Cycles per degree
The number of pairs of light and dark bars per degree of visual angle
Aqueous humor
Fluid derived from blood that fills the space immediately behind the cornea and lens.
Vitreous humor
Fluid that fills refracts light and sits between lens and retina
80% of internal volume of the eye
What percent of light that arrives at the cornea reaches the retina?
Around 50%
Accommodation
Process by which the eye changes its focus (the lens gets fatter as gaze is directed toward nearer objects)
Focal distance
Distance between lens and the viewed object
Prebyopsa
Age-related loss of accommodation which makes it difficult to focus on near objects
Emmetropia
When the four optical components of the eye are perfectly matched to the length of the eye ball
Bipolar cell
A retinal cell that synapses with either rods or cones (not both) and with horizontal cells and then passes the signals on to ganglion cells
Amacrine cell
A retinal cell found in the inner nuclear layer that makes synaptic contacts with bipolar cells, ganglion cells and other amacrine cells
Lateral inhibition
Antagonistic neural interaction between adjacent regions of the retina
horizontal cell
A specialized retinal cell that contracts both photoreceptor and bipolar cells
Diffuse bipolar cell
A bipolar retinal cell whose processes are spread out to receive input from multiple cones
Midget bipolar cells
A small bipolar cell in the central retina that receives input from a single cone
P ganglion cells
Receive input from one Midget bipolar cells from cones in the fovea.
Feeds cells in the parvocellular (“small cell”) layer of the lateral geniculate nucleus
70% of ganglion cells
M ganglion cells
Feed the magnocellular layer of the LGN
Compose 8 - 10% of human retina
Receives input from many diffuse bipolar cells
Koniocellular cells
Project to koniocellular layers in the LGN
May be part of primordial blue-yellow system
depolarizes in response to light
ON bipolar cell
Hyperpolarises in response to light
OFF bipolar cell
On-center cell
- Retinal ganglion cell that increases its firing rate when a light shines in the center of its receptor field but decreases its firing rate when the stimuli occurs in the periphery of its receptor field. Turning off the light had the opposite response.
- Gradations of proximity matter
- Contrast matters more than average intensity
Which has larger receptive fields, P-cells or M-cells?
M-cells
Contrast sensitivity function (CSF)
A function describing how the sensitivity to contrast (defined by the reciprocal of the contrast threshold) depends on the spatial frequency (size) of the stimulus
How is the lateral geniculate nucleus (LGN) organized?
Top 4 layers are called parvocellular
Bottom 2 layers are called magnocellular
Left side of LGN receives input from left side of eye
Right side receives input from right side of brain
Layers 1, 4, 6 receive input from right eye
Layers 2, 3, 5 receive input from left eye
What do magnocellular neurons respond to?
respond to large, fast-moving objects
What do parvocellular neurons respond to?
Process detail of stationary targets
Topographical mapping
The orderly mapping of the world in the lateral geniculate nucleus of the visual cortex
Cortical magnification
The amount of cortical area devoted to a specific region in the visual field
Visual crowding
The deleterious effect of clutter on peripheral object recognition
Orientation tuning
The tendency of neurons in striate cortex to respond optimally to certain orientations and less to others
ocular dominance
The property of the receptive fields of straiate cortex neurons by which they demonstrate a preference, responding somewhat more rapidly when a stimulus is presented in one eye than when it is presented in the other
unrelated color
A color that can be experienced in isolation
related color
A color, such as brown or gray, that is seen only in relation to other colors. For example, a “gray” patch in complete darkness appears white
Adapting stimulus
A stimulus whose removal produces a change in visual perception or sensitivity
Neutral point
The point at which a color mechanism is generating no signal. If red-green and blue-yellow mechanisms are at their neural points, a stimulus will appear achromatic
Assumptions that can be made about light
Most light sources are broadband
Spectral compositional curves are usually smooth
Spikes at particular areas are uncommon
Real surfaces tend to be broadband in their reflectances
