chapter 3 - Sensation and perception 2013
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Transcript of chapter 3 - Sensation and perception 2013
Sensation & Perception
What’s the difference?Sensation
Detecting stimuli from the body or environment
PerceptionOrganizing sensations into meaningful patterns
StimulusForm of energy that can affect sense organs
Sensation
Sensory receptorsDetect stimuli & convert energy into neural impulsesReceptors are designed to serve very specific functions
Thresholds and Stimulus Change
ThresholdsThere is a minimum amount of any given sensation that has to be present for us to notice itAbsolute threshold
This is the minimum amount of a stimulus that is necessary for us to notice it 50% of the time
Sensory adaptationIf a stimulus is unchanging, we become desensitized to itKeeps us focused on changes, not constants
VisionElectromagnetic energy
Long wavelengths: AC circuits, radio waves, infrared raysShort wavelengths: visible light, X-rays, UV & gamma raysOther animals can see other segments of the spectrum of electromagnetic energy
Bees can see ultraviolet rays and blue-violet, but not redPit vipers can see infrared rays Dogs can’t see all the colors that humans can (no red)
Our Visible Spectrum Red Orange Yellow Green Blue Indigo
Violet
Hue determines colorDepends on length of the distance from one peak to the next on the wave
Intensity determines brightnessDepends on amplitude of the wave
TransductionThe process where the eye converts electromagnetic energy (light) into nerve impulses
The Eye
Parts of the EyeCornea
Light is initially focused by this transparent covering over the eye
PupilLight enters the eye through this opening
IrisMuscle connected to the pupil that changes its size to let in more or less lightEveryone has a unique iris (thus it is a new security technique being employed by some organizations)
Parts of the EyeLens
This flexible disk under the cornea focuses light onto the back of the eyeAccommodation
Flexibility of the lens allows eye muscles to adjust light from objects at various distances away
RetinaLight reflected from the lens is received by this sheet of tissue at the back of the eyeContains the receptors that convert light to nerve impulses
How we see color: ConesRetinal cells that respond to particular wavelengths of light, allowing us to see color
Most of our cones are located on the fovea, which gives us the sharpest resolution of visual stimuli3 types of cones, each sensitive to different light frequencies
How we see in the dark: Rods
Retinal cells that are very sensitive to light but only register shades of gray (i.e., no color)Rods are located everywhere in the retina except in the fovea
Rods allow us to see at night without strong light – this is why we see less color at night
Because of where the rods are on the retina, we see best at night without light in the periphery of our vision
Dark adaptation
Rods & Cones
Optic NerveFrom the receptor cells in the retina, the converted impulse from light is directed to the optic nerve
This is the large bundle of nerve fibers that carry impulses from the retina to the brainIt sits on the retina but contains no cones or rods, so this is where you experience a ‘blind spot’
We aren’t aware that we have a blind spot because our brain completes patterns that fall across our blind spot and because our eyes are constantly moving (‘filling’ it in)Pg 149 in book - an activity to demonstrate this
Processing of Visual Information
RetinaProcesses electrical impulses, starts to encode and analyze sensory information (at the most basic level)
Optic NerveNeurons pick up the messages from retina, transmit to the thalamus, then on to the visual cortex, then on to more specified areas
Visual perception Principles
During organisation and interpretation the visual cortex applies rules to understand the information. We tend to automatically use these without any conscious effort.
There are 3 categories:Gestalt principlesDepth principlesPerceptual constancies
Gestalt Principles of VisionFigure-ground
We recognize objects (figures) by distinguishing them from the background (ground)
ClosureWe tend to fill in gaps in a figure
SimilarityParts of an image that look alike tend to be grouped together
ProximityParts of an image that are near one another tend to be grouped together
Depth PrinciplesHow is it that we perceive a 3-dimensional world when our eyes only project a 2-dimensional image on our retinas??!Our brain uses different cues to perceive depth. There are 2 types:
Binocular depth cues (convergence, retinal disparity)Monocular depth cues (accommodation, pictorial cues – linear perspective, interposition, texture gradient, relative size, height in the visual field
Binocular depth cuesOur brain uses both eyes working together to provide information about depth and distance (this is especially important for objects that are close).
Convergence The brain detecting depth and distance from the changes in the tension in the eye muscles that occur when the two eyes turn inward to focus on an object
Retinal disparityRefers to the difference in the location of the visual image on the retina (due to the angle of the view)
Monocular depth cuesMonocular cuesOur brain also uses information from the stimulus that requires the use on only one eye.AccommodationPictorial cues
Linear perspective InterpositionTexture gradient Relative sizeHeight in the visual field
Accommodation Involves the automatic adjustment of the shape of the lens to focus an object in response to changes in how far away that object is.
Monocular – pictorial depth cues
Linear perspectiveIs the apparent convergence of parallel lines as they recede (go back ) into the distance
InterpositionOccurs when one object partially block or covers another and therefore the one behind is perceived as further away
Texture gradientThe gradual reduction of detail that occurs in a surface as it recedes into the distance
Relative sizeThe tendency to visually perceive the object that produces the larger image on the retina as being closer
Height in the visual field Refers to the location of objects in out field of vision, whereby object that are located closer to the horizon are perceived as being more distant than objects located further from the horizon
Perceptual Constancies - Vision
The image of an object on your retina can very in size, shape, and brightness
But we still continue to perceive the object as stable in size, shape and brightness
Size constancyShape constancyBrightness constancy
Size Constanc
yThe tendency to view an object as constant in size despite changes in the size of its image on the retina (as we move)
Shape Constanc
yThe tendency to see an object as retaining its form despite changes in orientation
Brightness
Constancy
The tendency to perceive an object as maintaining its level of brightness in relations to its surroundings