Sensation & Perception

KEY TERMS:

Sensation - detecting stimuli from the body or environment

Perception - organizing sensations into meaningful patterns

Stimulus - Form of energy that can affect sense organs

Psychophysics  - study the relationship between stimuli & our psychological response to them

Sensory receptors - cells designed to serve very specific functions such as to detect stimuli & convert energy into neural impulses

Thresholds - a minimum amount of any given sensation that has to be present for us to notice it

Absolute threshold -This is the minimum amount of a stimulus that is necessary for us to notice it 50% of the time

Sensory adaptation - If a stimulus is unchanging, we become desensitized to it. Keeps us focused on changes, not constants

Just noticeable difference (JND) - smallest difference in amount of stimulation that a specific sense can detect

Weber’s law - states that the size of the JND is a constant proportion of the initial stimulus

VISION 

Hue determines color
Intensity determines brightness
Transduction - the process where the eye converts electromagnetic energy (light) into nerve impulses

The Eye 

Parts: 
Cornea - light is initially focused by this transparent covering over the eye
Pupil - Light enters the eye through this opening
Iris - Muscle connected to the pupil that changes its size to let in more or less light
TRIVIA: Everyone has a unique iris (thus it is a new security technique being employed by some organizations)
Lens - This flexible disk under the cornea focuses light onto the back of the eye. It has capacity of
Accommodation - Flexibility of the lens allows eye muscles to adjust light from objects at various distances away
Retina - Light reflected from the lens is received by this sheet of tissue at the back of the eye; it contains the receptors that convert light to nerve impulses
Cones - retinal cells that respond to particular wavelengths of light, allowing us to see color and are located mostly on the fovea, which gives us the sharpest resolution of visual stimuli
          Theories on Seeing Colors
         A. Trichromatic theory - our eyes have three types of sensors (Red, blue & green receptors)
         B. Opponent-process theory - receptors respond to pairs of colors (White-black / red-green / yellow-blue)
Rods - retinal cells that are very sensitive to light but only register shades of gray (i.e., no color); are located everywhere in the retina except in the fovea (we see best at night without light in the periphery of our vision) and allow us to see at night without strong light (this is why we see less color at night)
Optic Nerve - this is where the converted impulse from light is directed from the receptor cells in the retina; it is a large bundle of nerve fibers that carry impulses from the retina to the brain and sits on the retina but contains no cones or rods, so this is where you experience a ‘blind spot’
(TRIVIA: 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) as discussed in Gestalt Principles of Perception.)

Processing of Visual Information
The retina processes electrical impulses. It encodes and analyzes sensory information (at the most basic level) then in the optic nerve, the neurons pick up the messages from retina, transmit to the thalamus, then on to the visual cortex, then on to more specified areas.

Gestalt Principles of Vision
Figure-ground - we recognize figures (objects) by distinguishing them from the background
Proximity - Marks that are near one another tend to be grouped together
Closure - we tend to fill in gaps in a figure
Similarity - Marks that look alike tend to be grouped together
Continuity - Marks that tend to fall along a smooth curve or a straight line tend to be grouped together

Depth Perception - the ability to perceive a 3-dimensional object when our eyes only project a 2-dimensional image on our retinas
Different cues to perceive depth:
Binocular disparity - since we use both our eyes to focus on an image, the angles used by each eye to put the image on the fovea of our retina is used by the brain to perceive distance
Monocular cues - Our brain also uses information from the stimulus that does not involve our use of both eyes
Motion - specifies distance of an object based on its movement
Motion parallax - objects that are closer to us move farther across our field of view than do objects that are in the distance
Texture gradient - progressive changes in surface texture that signal distance
Linear perspective - parallel objects seem to get closer together as they get farther away

Perceptual Constancy - 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 constancy - The tendency to view an object as constant in size despite changes in the size of its image on the retina (as we move)
Shape constancy - The tendency to see an object as retaining its form despite changes in orientation
Color constancy - The tendency to view an object as retaining its color despite changes of brightness in the environment

AUDITION

Pitch - Frequency of air waves
Loudness/volume - Amplitude of air waves

The Ear

http://www.ndt-ed.org/EducationResources/HighSchool/Sound/Graphics/Ear.gif

Outer ear: Pinna
& external auditory canal
Middle ear: Eardrum (tympanic membrane), hammer, anvil & stirrup
Inner ear: Oval window, cochlea (basilar membrane inside)

The Hearing Process
Air waves move the tympanic membrane (eardrum), which moves the hammer, anvil and stirrup (these all amplify the air wave and pass it on)to the basilar membrane in the cochlea. Here, different frequencies are transduced via hair cells (i.e., the receptors of the ear) into nerve impulses that are sent to the auditory cortex of the brain.

Theories on Hearing
There are two theories on transduction in basilar membrane (i.e., how we perceive sound):
Frequency Theory states that neural impulses are stimulated more with higher frequencies of air waves
More plausible for small frequencies, rather than high frequencies because we can hear freqs higher than the maximum rate of neural firing (1,000 neurons a second)
Place Theory states that different frequencies of air waves activate different places along the basilar membrane

TACTILE - SENSE OF TOUCH

• Skin is the body’s largest sensory organ. Millions of skin receptors mix and match to produce specific perception. 
• Four basic types of sensations are: Pressure, warmth, cold, and pain. Skin senses pain and warns us of impending danger. 
• Our brain releases endorphins which are neurotransmitters that have a pain-killing effect.
• Gate-control theory explains that pain impulses can be inhibited by closing of neural gates in the spinal cord. 

Kinesthetic sense 

•  provides info about position of joints, muscles, limbs; gives us control over body movements

Vestibular sense 

•  provides info about body’s orientation relative to gravity and head’s position in space; helps us maintain balance
• relies on semicircular canals in the inner ear

Olfaction 

• sense of smell through detecting molecules in the air  
• Olfactory receptors are built so that only molecules with particular shapes will fit in particular receptors
• Receptors send neural signals to the brain, passing the thalamus (memory) and the limbic system (emotions) along the way this is why odors often trigger emotional memories 

Gustation 

• sense of taste through detecting molecules of substances that have dissolved in saliva
• the sense of taste combines with the sense of smell to produce perception of flavor of food
• Research suggests that neural impulses for both senses converge to some degree in brain area associated with the perception of flavor
• When the sense of smell is blocked, we have a harder time detecting most flavors

Taste buds - clusters of hair-like receptor cells; within each bud is a cluster of 50 to 150 receptor cells
TRIVIA: We have about 10,000 taste receptor cells (most on tongue)
Four types of taste: sweet, sour, salty & bitter


References:

Myers, D. (2005) General Psychology