PSY100H1 Chapter Notes - Chapter 5: Motion Perception, Face Perception, Edge Detection
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CHAPTER 5: SENSATION, PERCEPTION, AND ATTENTION
othis poing has NO rods or codes → blind spot
ooptic nerve splits into two part that cross at OPTIC CHIASM
causes all info from left side of visual space to be projected on right hemisphere of
brain, vice versa
first synapse of majority of ganglion cells in thalamus, from there to primary visual
-visual sensory neurons “tuned” to particular types of information in light
- some respond best to color, shape, directions of motion→ gives neuron SPECIFIC receptive field:
population of sensory receptors that influences activity in a sensory neuron, region within which cell
responds to given stimuli.
VISUAL receptive fields located on a specific region of retina or visual space.
-ganglion cells have receptive fields sensitive to edges (abrupt changes in brightness or color)
and insensitive to uniform regions, large areas of same color or brightness.
oRetina filters out uniform because little useful information
oCompressed image sent to brain emphasizes borders
-adjacent photoreceptors in retina inhibit one another
e.g. illusion of round grey dots at intersection of white lines against dark background
object looks lighter against a black background than white one, called simultaneous
emphasizes changes in visual stimuli
imp for finding boundaries of objects
Color of light
-determined by wavelength (400-700 nm)
-but wrong to equate physical wavelength with perceived color
-color categorized by: hue, brightness, saturation
-hue: distinctive characteristics of color, depends on light’s wavelength when reaches eye
-brightness: perceived intensity or luminance of color, depends on amount of light
odifferent from lightness! A psychological dimension determined by its brightness relative to
obecause of simultaneous contrast, two greys with same brightness can different in lightness
depending on surrounding levels of brightness
olightness more important for visual appearance, but not physical property
-saturation: purity of color, varies according to mixture of wavelengths in stimulus
-color determined not only just wavelength but MIXTURE of wavelengths (SPECTRAL PATTERN) in
-TWO ways to produce spectral pattern: SUBTRACTIVE or ADDITIVE mixture
oSubtractive color mixing: mixture occurs within stimulus itself, physical not psychological.
E.g. mixing paints. Occurs because color determined by pigments.
Red, yellow, blue are the subtractive primary colors. Get black if mix all 3.
-additive color mixing: what you see is determined by interaction of wavelengths with
receptors in eye, PSYCHOLOGICAL process
-three primaries law of color: any color can be created by combining 3 wavelengths, one
from long-wave (red), one from middle (green-yellow), one from short (blue-violet)
oexact colors are ARBITRARY
oadditive primary colors: Red, green, blue. Get WHITE light if mix.
oPrism rainbow: because different wavelengths refract at different angles
-starts in cone cells, transducer light into neural impulses
oTHREE types of cones: S(blue)M(green)L(red)
oColor of stimulus determined by how much of each type of cone activated (ratio)
oE.g. YELLOW light stimulates M and L, hardly S
-some colors seem to be “opposites”: red and green, blue and yellow
-due to retinal ganglion cells
osome excited by L cones but inhibited by M cones (red and green are opposites)
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osome excited by S cones, but inhibited by L&M (blue and yellow opposites) color-opponent
ganglion pg 179
respond MOST to BOUNDARIES and LESS to UNIFORM regions
-what you perceive vastly different from pattern of stimulation on retina
-perception research: about how does the brain extract a stable representation of world from
info sense provide?
Primary sense regions of cerebral cortex
-smell and taste in frontal lobe
-from thalamus extend axons to primary auditory cortex (called A1 for first auditory area) in
-here code frequency (pitch)
-has tonotopic organization: neurons at rear end respond best to LOW frequencies (fog horn),
neurons at front end to HIGH frequencies (train whistle)
-encoding by location
-secondary auditory areas also tonotopic
-primary somatosensory cortex (called S1) in parietal lobe
-S1 has somatotopic organization: connected areas of body tend to be next to each other in
-Back or calves have little area, lips and fingers have a lot
-up to half of cerebral cortex participate in visual perception
-primary visual cortex (V1) is in occipital lobe
-neuronic pathways preserves spatial relationships
oretinotopic organization: systematic ordering so that adjacent areas in retina correspond
to adjacent areas in V1
othe more forward along V1, the more peripheral (away from center) portion of visual field
-hierarchical processing: processing occurs in a number of stages, first of which performs
analysis of retinal image by extracting elementary features (primitives, e.g. lines, angles,
colors, curves, motion). Different neurons process diff primitives.
-Hubel and Wiesel: firing rate of simple cells decreases as orientation of line segment rotated
away from preferred orientation
Visual areas beyond V1 form two parallel processing streams:
-ventral stream, low includes occipital and temporal lobes, object perception and recognition
like color and shape, “what” pathway (identify object)
-dorsal stream, high occipital and parietal lobes, spatial perception “where pathway”
Carbon monoxide poisoning damaged visual system, esp “what” pathway
-Could not recognize faces, objects. But could by their voice/touch.
-Object agnosia: inability to recognize objects
-conscious perception of objects is impaired: no awareness she is taking in any visual info about
objects she sees
Filling-in: constructive nature of perception
-early visual areas, such as V1, fill in the constant region info that was thrown out at level of
ganglion cells e.g. blind spot
-happens automatically, “cognitively impenetrable”
First step in processing a form is encoding the features that compose it.
-visual search task: an experiment used to study form perception in which an observer tries to
detect a target stimulus among an array of distracter stimuli
-Found pop-out: phenomenon when simple stimuli are used, subjects take same amount of time
to find the target whether there are few or many distracters.
-Simple scene elements processed all at one, oddball stands out
-Suggest vast amount of low-level info is automatically detected at the same time
-principles of perceptual organization
-perceptions are different from the sum of their constituent sensations
-“gestalt” in psych means “organized whole”
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