Unit 4: Sensation and Perception
Section 1: Psychophysics
Section 2: Vision
Section 3: Hearing
Section 4: Taste and Smell
Section 5: Touch
Section 6: Other Systems
Basic Concepts
· Sensation is the stimulation of the sense organs.
· Perception is the selection, organization, and interpretation of sensory input.
Section 1: Psychophysics
Ref: pp. 126-131 5e; 124-128 6e
Psychophysics is what early psychologists called the study of how physical stimuli are translated into psychological experience.
Thresholds
· A threshold is the dividing point in the intensity of a stimulus between being detected and not being detected.
· An absolute threshold is the minimum intensity of a stimulus for it to be detected 50% of the time.
Just Noticable Difference (JND)
· A JND is the smallest difference between in an amount of stimulation that can be sensed.
· Weber's Law (Ernst Weber): The size of the JND is a constant proportion to the size of the initial stimulus (e.g., the JND for weight is 1/30 of the original stimulus).
Signal Detection Theory
· Signal Detection Theory: detecting stimuli involves decision processes as well as sensory factors. Both decision and sensory processes are influenced by a variety of factors.
· Signal detection theory assumes that a person has set criteria for how strong a stimulus must be before reacting and that perception is influenced by "noise" (irrelevant stimuli).
Subliminal Perception
· Subliminal Perception involves perceiving a stimulus without being consciously aware of it.
· Although subliminal perception exists, it has little effect on people's attitudes or behaviors.
Sensory Adaptation
· Sensory Adaptation is the gradual decline in sensitivity to a stimulus.
· Sensory adaptation is automatic. It allows people to notice changing stimuli but ignore constant stimuli.
Section 2: Vision
Ref: pp. 131-156 5e; 129-154 63
Light
· Light is a form of radiation that travels as a wave.
· The amplitude of light affects its brightness.
· The wavelength of light affects its color.
· Purity refers to how varied the mix of light is.
The Eye
· Light enters the eye through the cornea and is focused on the retina at the back of the eye. Images on the retina are upside down.
· The lens changes shape to focus on objects. Common problems like nearsightedness and farsightedness are caused by defects in the lens.
· The iris is the colored muscle surrounding the pupil. The iris contracts to make the pupil smaller to allow less light into the eye.
Ref: Fig 4.7 (5e); Fig. 4.7 & 4.9 (6e)
The Retina
· The retina is neural tissue at the back of the eye. It absorbs light, processes images, and sends visual information to the brain.
· The optic disk is the part of the retina where the optic nerve connects the eye to the brain. This causes a blind spot in everyone's vision. The optic nerve consists of the axons of the retinal neurons.
· The visual receptors in the retina are called rods and cones.
· Cones are involved in daylight vision and color vision.
· Rods are involved in night vision and peripheral vision.
· The fovea is the center of the retina where the cones are concentrated.
· Dark adaptation is the process by which the eyes become more sensitive to light in low illumination; light adaptation is the process by which the eyes become less sensitive to light in high illumination.
Ref: Fig. 4.9 (5e)
Vision and the Brain
· The optic nerves "cross over" at the optic chiasm and send the visual signals to each side of the brain.
· The optic nerve goes to
1. a part of the thalamus called the lateral geniculate nucleus (LGN) for processing
· The LGN is the primary visual pathway. It is divided into
a. the magnocellular channel (where is the object?)
b. the parvocellular channel (what is the object?)
1. a part of the midbrain called the superior colliculus for coordination with other senses.
· This is called parallel processing.
· Most visual information is passed on to the visual cortex to the occipital lobe.
· After the visual cortex, the information is sent to specialized parts of the brain, e.g., to the temporal lobe to recognize faces.
Ref: Fig. 4.12 (5e), Fig.4.13 & 4.15 (6e)
Color Vision
· Color is the psychological interpretation of light.
· Color is determined by wavelength (hue), amplitude (brightness), and purity (saturation).
· Subtractive color mixing occurs when wavelengths are removed from light, e.g., by mixing paints.
· Additive color mixing occurs when wavelengths are added, e.g., by mixing colored spotlights.
· The trichromatic theory of color vision proposed that the eye has three types of receptors for color vision (red, blue, green). This was the first theory and it explains both color mixing and color blindness (inability to distinguish colors).
· The opponent process theory of color vision proposed that color vision was the result of antagonist responses to three pairs of colors (red & green, blue & yellow, black & white). This explained complementary colors (colors that are mixed to produce gray) and afterimages.
· Current understanding of color vision mixes both these theories. The eye has three types of cones, each sensitive to different wavelengths. Opponent processes do occur in the retina, LGN, and visual cortex for red & green and blue & yellow.
Ref: Fig. 4.15-20 (5e); 4.16-21 (6e)
Perceiving Patterns & Objects
· People's visual perception is highly subjective. The same visual stimulus can produce different perceptions for different people.
· Perceptual Set: people's perceptions are influenced by their expectations--people are more ready to perceive visual stimuli in certain ways.
· Feature Analysis or Bottom-Up Processing occurs when a person pays attention to details (features) to perceive an object.
· Top-Down Processing occurs when a person recognizes the object or pattern before looking at details.
· Gestalt Principles explain how we perceive stimuli in terms of top-down processing.
· Phi phenomenon: illusion of movement when still pictures are presented in rapid succession (like a film).
· Figure and Background: we separate objects from the background.
· Proximity: objects that are close together we perceive as a group.
· Similarity: objects that look alike are perceived as a group.
· Continuity: we tend to follow straight lines in looking at things.
· Simplicity: we tend to interpret stimuli in the simplest ways.
· Closure: we tend to fill in missing visual elements.
· Distal Stimuli are the actual 3D objects that exist. Proximal Stimuli are the 2D images on the retina which we see.
· Because our proximal stimuli are always changing (consider the changes in the proximal stimulus of a door opening from rectangle to trapezoid), we create perceptual hypotheses to understand what we see (i.e., though the shape of the proximal image changes on our retinas, we understand that the distal stimulus--the door--does not change).
· Perceptual Constancy is our tendency to understand stable perception despite changing sensory input.
· An optical illusion involves an apparent discrepancy between the appearance of a visual stimulus and its physical reality. Some examples are the Ames room (p.155), impossible figures (like in M.C. Escher's art), and the moon illusion.
Ref: Fig. 4.22-29, 4.36-4.43 (5e); Fig. 4.22-33, 4.38-44 (6e)
Depth Perception
· Depth perception is the interpretation of visual cues to determine how distant objects are.
· Binocular cues are clues about distance based on having two eyes.
· Retinal Disparity: the difference between retinal images of objects within 25 feet.
· Convergence: the eyes converge as they focus on closer objects.
· Monocular cues are clues about distance based on either eye alone.
· Motion Parallax: objects at different distances move across the retina at different speeds.
· Linear Perspective: lines converge in the distance.
· Texture Gradient: details are harder to see in the distance.
· Interposition: nearer objects block objects that are farther away.
· Relative Size: objects seem smaller than distant objects
· Height in Plane: distant objects are higher in our field of vision than nearer objects.
· Light and Shadow suggest distance
· There is evidence that pictorial depth cues (monocular cues 2-7) are culturally influenced.
Ref: Fig. 4.32 & 33 (5e); 4.34 & 35 (6e)
Ref: pp.157-161 (5e); 154-159 (6e)
Sound
· Sound waves are vibrations of molecules.
· Sound waves are characterized by amplitude (loudness), wavelength (pitch), and purity (timbre).
· Wavelength is measured in hertz (Hz).
· Humans can only hear wavelengths between 20 and 20,000 Hz.
· Amplitude is measured in decibels (dB).
· Loudness doubles approximately every 10 dB.
· Hearing loss can be caused over time by sounds as low as 70 dB.
· Sounds louder than 120 dB can cause pain as well as damage.
· Timbre is the purity of the wavelengths, e.g. a note with the same loudness and played on two different instruments can produce different timbres.
Ref. Fig. 4.44-45 (5e), 4.45-46 (6e)
Sensory Processing in the Ear
· The external ear channels the sound vibrations in the ear to the eardrum, a taut membrane covering the middle ear.
· The middle ear converts the vibrations of the eardrum to three tiny bones called ossicles. They are the hammer, the anvil, and the stirrup.
· The inner ear consists mainly of the cochlea, a fluid-filled tunnel that conveys the vibrations of the ossicles to the hairs in the basilar membrane. The movement of the hairs is translated into neural impulses.
· The neural impulses go through the thalamus to the auditory cortex in the temporal lobes.
Ref. Fig. 4.46 (5e), 4.47 (6e)
Theories of Auditory Perception
· Place Theory holds that different parts of the basilar membrane respond to different pitches.
· Frequency Theory holds that pitch is understood by the frequency of the vibration of the entire basilar membrane.
· Psychologists now know that frequency depends on both place coding (different parts of the basilar membrane respond more to different sounds) and to frequency coding.
· Frequency is achieved through the volley princple, i.e. groups of neurons fire in succession.
· Under 1000 Hz is place coding, 1000-5000 Hz is place and frequency coding, and over 5000 Hz is frequency coding.
Ref: Fig. 4.47 (5e), 4.48 (6e0
Auditory Localization
· Auditory localization is locating the source of a sound.
· This is achieved through the difference in intensity and timing of the sound waves reaching the ears, e.g., a sound on your right will be more intense in your right ear (because your left ear is blocked by your body and is facing the other direction) and will reach your right ear before your left ear.
Ref. 4.48 (5e), 4.49 (6e)
Ref. pp. 162-164 (5e), 159-162 (6e)
Taste
· Gustatory receptors are found in the taste buds in the tongue.
· They detect soluble chemicals.
· Neurons then communicate this to the thalamas, then to the cerebral cortex.
· There are four primary tastes: sweet, sour, bitter, and salty.
· Taste preferences are the result of social processes.
· Taste and smell are extensively related.
Ref: Fig 4.49-50 (5e), 4.50-51 (6e)
Smell
· Olfactory receptors are called olfactory cilia.
· They detect chemicals dissolved in the mucus of the upper part of the nose.
· Neurons communicate this directly to the olfactory bulb in the cerebral cortex.
· Humans can distinguish 10,000 smells.
Ref: Fig. 4.51 (5e), 4.52 (6e)
Ref: pp.165-167 (5e), 162-164 (6e)
Touch
· Skin has at least six types of receptors for different functions.
· When pressure is applied, nerves communicate this through the spine to the brainstem.
· The tactile pathway goes through the thalamus to the somasensory cortex in the parietal lobe.
· When temperature is normal, both "warm" and "cold" receptors fire simultaneously.
· When the temperature increases, the "warm" receptors fire more rapidly and the "cold" receptors stop.
· Signals from the thermal receptors travel to the brain through the spine on the slow pathway on which pain travels.
· Pain is transmitted through either the fast pathway or the slow pathway.
· The fast pathway transmits localized, sharp pain on nerves called A-delta fibers.
· The slow pathway transmits less localized, aching pain on nerves called C fibers.
· The perception of pain is subjective and is influenced by many factors, including psychological and cultural factors.
· Psychologists are still studying the effects of endorphins and neural activity on the perception of pain.
Ref: Fig. 4.52-53 (5e), 4.53-54 (6e)
Ref: pp.167-168 (5e) 164-165 (6e)
· The kinesthetic system monitors the positions of the parts of the body.
· Most kinesthetic signals use the same pathways as tactile signals.
· The vestibular system responds to gravity to monitor the body's position in space.
· The vestibular system uses part of the auditory system.
Ref: 4.54 (5e)