Sensation & Perception

Sensation & Perception
Chapter 4 Sensation & Perception The slides presented here are not to be the only means of studying for the chapter test. You will still need to read and take notes for both the reading and the lecture.

Sensation and Perception
Sensation – An early stage of perception in which neurons in a receptor create an internal pattern of nerve impulses that represent the conditions that stimulated it – either inside or outside the body Perception – A process that makes sensory patterns meaningful and more elaborate

How Does Stimulation Become Sensation?
The brain senses the world indirectly because the sense organs convert stimulation into the language of the nervous system: neural impulses

Transduction Transduction – Transformation of one form of energy into another – especially the transformation of stimulus information into nerve impulses Receptors – Specialized neurons that are activated by stimulation and transduce (convert) it into a nerve impulse Stimulation Transduction Sensation Perception

Transduction Sensory pathway – Bundles of neurons that carry information from the sense organs to the brain

Sensory Adaptation Sensory adaptation – Loss of responsiveness in receptor cells after stimulation has remained unchanged for a while

Thresholds Absolute threshold – p.114 table 4.1 The minimum amount of stimulation required in order for a stimulus to be detected. Difference threshold – Smallest amount by which a stimulus can be changed and the difference be detected 50 percent of the time (also called just noticeable difference – JND)

Thresholds Weber’s law – (Ernest Weber) (ever notice how commercials are always louder than the show you were just watching?) The principle that, to perceive their difference, two stimuli must differ by a constant % (rather than by a constant amount) The size of the JND is proportional to the intensity of the stimulus – so, the JND is large when the stimulus intensity is high, and the JND is small when the stimulus intensity is low. 2 lights must differ by 8% in intensity 2 objects by 2% in intensity 2 tones by .03%

Thresholds – Weber’s In Real Life
This also parallels some of our life experiences. If the price of a .50 cent candy bar goes up .05 cents, shoppers would probably notice it; similarly, if the cost of a $50,000 Mercedes goes up $5,000, potential buyers would raise an eyebrow. In both cases, the price went up by 10%; our thresholds for detecting differences are a roughly constant proportion of the original stimulus.

Signal Detection Theory
Signal detection theory – Assumes there is no single absolute threshold and that detection depends on perceptual judgment as a combination of sensation and decision-making processes. Noise in the dark Stimulus event RAS alerts your brain – NOISE! Neural activity Have I heard that before? Comparison with personal standard It’s a cat on the roof; no need to get up. Action (or no action)

Subliminal Persuasion
Studies have found that subliminal words flashed briefly on a screen can “prime” a person’s later responses No controlled research has ever shown that subliminal messages delivered to a mass audience can influence people’s buying habits

How Are the Senses Alike? How Are They Different?
The senses all operate in much the same way, but each extracts different information and sends it to its own specialized processing region in the brain

The Anatomy of Visual Sensation
Retina – Light-sensitive layer at the back of the eyeball Photoreceptors – Light-sensitive cells in the retina that convert light energy to neural impulses Rods – Sensitive to dim light but not colors Cones – Sensitive to colors but not dim light Fovea – Area of sharpest vision in the retina

Afterimages – What happens when retina is fatigued?
If you fatigue the cells in the retina, it will cause an effect known as an “afterimage.” These are sensations that linger after the stimulus is removed. Positive afterimages are caused by a continuation of the receptor and neural processes following stimulation (e.g. seeing the flash of a light bulb after it goes off; sparklers on 4th of July. Negative afterimages are caused by the opposite or the reverse of the original stimulus. This is best explained by the Opponent Process Theory of Color. Opponent Process Theory of Color states that when you are seeing one color, you are suppressing or inhibiting the opposite color from being seen (e.g. sensing red, not sensing green; sensing blue, not sensing yellow; sensing black, not sensing white). Colors come in OPPOSITE pairs.

Optic nerve – Bundle of neurons that carries visual information from the retina to the brain Blind spot – Point where the optic nerve exits the eye and where there are no photoreceptors

Transduction of Light in the Retina

Visual cortex – Part of the brain – the occipital cortex – where visual sensations are processed Color – Psychological sensation derived from the wavelength of visible light – color, itself, is not a property of the external world

Neural Pathways in the Human Visual System

How the Visual System Creates Color
Electromagnetic spectrum – Entire range of electromagnetic energy, including radio waves, X-rays, microwaves, and visible light Visible spectrum – Tiny part of the electromagnetic spectrum to which our eyes are sensitive

How the Visual System Creates Color
Copyright © Allyn and Bacon 2006

The Ways of Sensing Colors
Trichromatic Theory: The earliest stage of color sensation that states that colors are sensed by three different types of cones sensitive to light in the red, blue, and green wavelengths.

Color Blindness Color blindness – Vision disorder that prevents an individual from discriminating certain colors

Hearing: If a Tree Falls in the Forest...
The Physics of Sound Frequency – Number of cycles completed by a wave in a given amount of time Low Frequency High Frequency

Hearing: If a Tree Falls in the Forest...
The Physics of Sound Amplitude – Physical strength of a wave High Amplitude Low Amplitude

How Sound Waves Become Auditory Sensations
Tympanic membrane – The eardrum

Cochlea – Where sound waves are transduced

Basilar membrane – Thin strip of tissue sensitive to vibrations Cochlea

Auditory nerve – Neural pathway connecting the ear and the brain

Auditory cortex – Portion of the temporal lobe that processes sounds

The Psychology of Pitch, Loudness, and Timbre
Pitch – Sensory characteristic of sound produced by the frequency of the sound wave (place theory and frequency theory) Loudness – Sensory characteristic of sound produced by the amplitude (intensity) of the sound wave Timbre – The quality of the sound wave that comes from both pitch and loudness (mom’s voice vs. Mrs. B’s voice).

Deafness Conduction deafness – An inability to hear resulting from damage to structures of the middle or inner ear Nerve deafness – An inability to hear, linked to a deficit in the body’s ability to transmit impulses from the cochlea to the brain, usually involving the auditory nerve or higher auditory processing centers

Position and Movement Vestibular sense – Sense of body orientation with respect to gravity Kinesthetic sense – Sense of body position and movement of body parts relative to each other

How Do You Smell? (haha) Olfaction – Sense of smell Olfactory bulbs – Brain sites of olfactory processing Pheromones – Chemical signals released by organisms to communicate with other members of the species

Taste Gustation – The sense of taste
Taste buds – Receptors for taste (primarily on the upper side of the tongue)

The Skin Senses Touch Warmth Cold
All skin senses are registered in the somatosensory cortex of the parietal lobe.

Pain Pain is psychological, not physical!
Gate Control Theory – Close the gate on the sensation that gets to the brain that way I won’t feel the pain! Placebos – Substances that appear to be drugs but are not Placebo effect – A response to a placebo caused by subjects’ belief that they are taking real drugs

What is the Relationship Between Perception and Sensation?
Perception brings meaning to sensation, so perception produces an interpretation of the external world, not a perfect representation of it

The Machinery of Perceptual Processing
Percept – Meaningful product of a perception Feature detectors – Cells in the cortex that specialize in extracting certain features of a stimulus Binding problem – A major unsolved mystery in cognitive psychology, concerning the physical processes used by the brain to combine many aspects of sensation to a single percept

Bottom-Up and Top-Down Processing
Bottom-up processing – Analysis that emphasizes characteristics of the stimulus, rather than internal concepts Top-down processing – Emphasizes perceiver's expectations, memories, and other cognitive factors

Perceptual Constancies
Perceptual constancy – Ability to recognize the same object under different conditions, such as changes in illumination, distance, or location

Perceptual Ambiguity and Distortion
Illusions – Demonstrably incorrect experience of a stimulus pattern, shared by others in the same perceptual environment Ambiguous figures – Images that are capable of more than one interpretation

Perceptual Illusions Do you see or ?

Perceptual Illusions The Herman Grid Do you see small gray squares between the black squares?

Theoretical Explanations for Perception
Learning-based inference – View that perception is primarily shaped by learning, rather than innate factors Perceptual set – Readiness to detect a particular stimulus in a given context

Some illusions are learned!

Here is what you are seeing

The Gestalt Approach Gestalt psychology – View that much of perception is shaped by innate factors built into the brain Figure – Part of a pattern that commands attention Ground – Part of a pattern that does not command attention; the background

The Gestalt Approach Closure – Tendency to fill in gaps in figures and see incomplete figures as complete

The Gestalt Laws of Perceptual Grouping
Similarity Proximity Continuity Common fate Prägnanz

Law of Prägnanz A BIRD IN THE THE HAND

How do we perceive depth?
The Visual Cliff

Monocular Cues – Depth cues that do not depend on having two eyes. Binocular Cues – Depth cues that depend on having both eyes.

Linear Perspective (railroad tracks in Ponzo illusion) Relative Size Cues Texture Gradient/Aerial Perspective Interposition Motion Parallax Monocular Cues – *Depth cues that do not depend on having two eyes *Think Artist’s Drawings! *Only one (mono) eye is required to process depth or distance cues. Not all cues for depth perception require both eyes If you have taken a drawing class, you have learned monocular depth cues. Artists use these cues to imply depth in their drawings. One of the most common cues is linear perspective or relative size (seen in the pictures we saw of the muler-lyer illusion). If two objects that are assumed to be the same size cast two different sized images on the retina, then we judge them to be at different distances (mirrors on the side of your car say “objects in mirror are closer than they appear” because you are seeing an object that you know the size of and are using other objects to judge depth and perception of how close it is to you – the mirror reminds you it is closer than it appears to be in your mirror because that is just a reflection (monocular) and you are not looking at the object with both eyes – you are using the size and shapes of the other objects in the mirror to judge how far away it is (this is also called relative size cues). You could also show an object as being closer by drawing texture on it (texture gradient) because objects that are closer to us have finer details than objects that are farther away. Shading is another monocular cue – shading implies depth and position of objects (lighter is closer, darker is farther away). So, monocular cues are when you use other cues to help you with sense of depth and perception. Copyright © Allyn and Bacon 2006

Linear Perspective – Parallel lines appear to converge toward a vanishing point as they recede in the distance; we don’t need two eyes for this! A B Which box is bigger, A or B? Copyright © Allyn and Bacon 2006

Convergence – as object moves closer to our noses our eyes turn inward and “cross.” Retinal Disparity – (hold finger far away from face and close one eye and then close the other) If an object is far away, the image will be similar, the closer it is, the more disparity there will be. Binocular Cues – Depth cues that depend on having both eyes. This gives us accurate depth perception! Information is processed by each eye and fused to form one image. Binocular cues result from our anatomy – we see the world with two eyes set a certain distance apart and this allows us to perceive depth. Binocular disparity (or retinal disparity) explains what is happening when you hold a finger in front of your eyes and close one eye and look at it with the other. Each eye sees an object from a different angle and the brain gets both images – if the object is farther away, the images will be similar…the closer it is, the more disparity there will be. The other binocular cue is convergence. As an object gets closer, our eyes must move toward each other to keep focused on the object. That doesn’t happen in monocular cues, or if you only had sight in one eye! If the eyes have to converge, the brain knows the object is closer! You need binocular vision (two eyes) in order to throw a ball with accuracy to another person and in order to catch a ball being thrown at you! Copyright © Allyn and Bacon 2006

How do we perceive motion?
Stroboscopic Effect (a movie or flip book) Phi Phenomenon (dancing Christmas lights or a baseball scoreboard that displays fireworks after a homerun is hit – lights are just turning on and off) Autokinetic Effect – In a darkened room, if there is a stationary point of light, the light will appear to move if you stare at it. Visual Capture – When sitting at a traffic light and cars around you move forward, and you feel like your car is moving backward. Stroboscopic Effect - Animation relies on the stroboscopic effect. Take a pad of paper and draw a dot on one corner of the left corner of the page. On the next page, draw a dot in the same spot, but a little more to the right of the one on the previous page. Continue doing this until all the pages are used. Now, flip the pages…and the dot appears to move across the page. Movement is created because of the firing of the neurons in the visual cortex that are designed to detect movement. This is an inaccurate perception because the dot doesn’t move! The phi phenomenon is the illusion of movement in the dancing Christmas lights video we saw at the beginning of the year. The lights appear to move because of the combination of the music and the turning on and off of the lights simultaneously. This is an inaccurate perception because the lights are not dancing! Autokinetic effect is when a stationary point of light appears to move if you stare at it while being in a dark room. That is because of the neurons firing in the retina and the movement of the eye naturally and it creates the illusion of movement when there is none. Visual capture occurs when you are sitting in a stationary car and the cars around you move and you feel like your car is moving! It is an inaccurate perception because your car isn’t moving. Your vision captured the movement and you perceive motion when there is none Copyright © Allyn and Bacon 2006

End of Chapter 4

Sensation & Perception

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