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Sensation & Perception On chapters 7 & 8 Gray 4th edition © Kip Smith, 2003 Sensation - Topics Basics Taste and Smell Touch and Pain Hearing The ear and cochlea Getting aural information to the brain Vision The eye and retina Getting visual information to the brain © Kip Smith, 2003 Perceptual Basics Thresholds Top down & bottom up processing © Kip Smith, 2003 Sensation ≠ The picking up of information from the environment and the body Perception The transformation of input signals into action potentials We experience 5 modalities of external sensation © Kip Smith, 2003 Coming aware of information from the environment The transformation of action potentials from multiple modalities into a coherent representation of the environment Basics - Absolute thresholds The limits of perception The lowest level of a stimulus, X, that provokes a response from the sensory system Vision: the faintest detectable light Sound: the quietest detectable sound © Kip Smith, 2003 Difference thresholds The limits to the perception of change The smallest increment of a stimulus, DX, that provokes a response from the sensory system Weber’s Law Pressure: © Kip Smith, 2003 Q = Dc/c X = the weight of some object DX = the smallest weight that makes that object feel heavier Another threshold Vision: We see 24 “frames” per second Anything faster appears continuous Movies project at 32 (?) frames per second and convince us the motion is continuous © Kip Smith, 2003 Subliminal signals Signals that are below threshold Question: Can we be influenced by signals that are below threshold? © Kip Smith, 2003 Top Down vs. Bottom Up Top Down From the whole to the parts Perception is driven by context or expectation © Kip Smith, 2003 Bottom Up From the parts to the whole Perception is driven by features of the stimulus What is this? Describe it © Kip Smith, 2003 Please read this aloud THE CAT IN THE HAT © Kip Smith, 2003 An ancient Buddhist parable It was six men of Indostan, to learning much inclined, Who went to see the elephant (though all of them were blind), That each by observation might satisfy his mind. The first approached the elephant, and, happening to fall Against his broad and burly side, at once began to call: "I see," said he, "the elephant is very like a wall!" The second, feeling of the tusk, cried, "Ho! What have we here So very round and smooth and sharp? To me 'tis mighty clear This wonder of an elephant is very like a spear!" © Kip Smith, 2003 Top down or bottom up ? The third approached the animal, and, happening to take The squirming trunk within his hands, thus boldly up and spake, "I see," said he, "the elephant is very like a snake!" The fourth reached out his eager hand and felt about the knee: "What most this wondrous beast is like is mighty plain, " said he, "'Tis clear enough the elephant is very like a tree!" The fifth, who chanced to touch the ear, said, "E'en the blindest man Can tell what this resembles most. Deny the fact who can, This marvel of an elephant is very like a fan!" © Kip Smith, 2003 End of the parable The sixth no sooner had begun about the beast to grope, Than, seizing on the swinging tail that fell within his scope, "I see," said he, "the elephant is very like a rope!" And so these men of Indostan disputed loud and long, Each in his own opinion exceeding stiff and strong, though each was partly in the right, and all were in the wrong! So oft in group endeavors, the members of the team Rail on in utter ignorance of what each other mean, As if it were an elephant not one of them has seen. © Kip Smith, 2003 Taste and Smell The chemical senses Gatekeepers to the insides © Kip Smith, 2003 Taste and Smell Intimately related Most of the flavor of food is sensed not in the mouth but in the nose Sensory interaction Pinch your nose and apple = potato Smell is unique No thalamic waystation © Kip Smith, 2003 Taste Signal Receptors Spontaneously regenerate every two weeks Stuff that enters your body Taste buds on tongue Neural pathway © Kip Smith, 2003 Sweet Salt Bitter Sour Thalamus to the medulla Smell Signal Receptors Neural pathway © Kip Smith, 2003 Stuff that enters your body Little hairs in the back of the nose Directly to the temporal lobe: no thalamic waystation Smell Each receptor responds to a limited number of chemicals There are thousands of receptors & thousands of chemicals There is no one-to-one dedicated circuit © Kip Smith, 2003 Smell The receptors in your nose are every bit as good as those in your dog’s nose Why does your dog have a better sense of smell? The dog has a thousand times as many sensors © Kip Smith, 2003 Pheromones Chemical communication Sexual attraction / receptivity © Kip Smith, 2003 Touch and Pain © Kip Smith, 2003 Touch ≠ Pain Touch is for exploring the environment Pain is a warning signal that something is wrong Leprosy is a disease that, among other things, kills the nerves that carry the pain signal © Kip Smith, 2003 Touch The skin has 4 different kinds of pressure sensors specialized to sense different type of pressure Broad, diffuse pressure Tapping Stretching Vibration Thermoreceptors are different sets of cells © Kip Smith, 2003 Touch The density of pressure sensors varies Why? Lots on the tips of fingers, lips Few on the calf © Kip Smith, 2003 Gate Control Theory of Pain Small fibers carry the pain signal Activity in the small fibers opens a “gate,” and you sense pain Large fibers carry all other pressure signals Activity in the large fibers closes the “gate,” suppressing the pain © Kip Smith, 2003 Hearing From Sound Waves to the Cortex © Kip Smith, 2003 Ear Workings Sound waves Ear drum (tympanic membrane) Middle ear Hammer, anvil, stirrup Inner ear Cochlea Basilar membrane Hair cells © Kip Smith, 2003 Properties of Sound Waves Wavelength The distance between adjacent pressure peaks or troughs Pitch or tone or notes © Kip Smith, 2003 Which notes travel further? High or low? Low notes Why? The lower the note, the longer the wavelength Properties of Sound Waves Amplitude The height between a pressure peak and the adjacent trough Loudness © Kip Smith, 2003 Is there such a thing as too loud? Yes Why? The receptors - hair cells in the cochlea are very sensitive and can be easily damaged How do we … ? Determine Loudness Encoded by the number of neurons firing Localize Sounds Two ears: © Kip Smith, 2003 Sound reaches the two ears differently time differences loudness differences Hearing Pitch - 2 Theories Place Theory Frequency Theory © Kip Smith, 2003 Place theory Frequency theory for pitch Neurons fire at same rate as the frequency of the sound. Example: If a sound wave has 100 pressure peaks per second (100 Hz), then the action potentials will fire 100 times per second. © Kip Smith, 2003 So which is right? Place Th. or Frequency Th.? High Frequencies V. Low Frequencies Place theory Both! Frequency theory Everything else Some combination of the two © Kip Smith, 2003 Hearing, Summary Signal Signal processing apparatus Soundwave Ear Eardrum Middle ear bones Inner ear, cochlea Receptors/ transducers Hair Cells Neural pathway Auditory nerve © Kip Smith, 2003 Vision & Hearing Both the optic and auditory nerves project to the thalamus Information from the right (left) side goes to the left (right) thalamus and to the left (right) hemisphere of the cortex Perceptual information is processed on the contralateral side The auditory cortex is in the temporal lobe The visual cortex in the occipital lobe © Kip Smith, 2003 Vision From Photons to Action Potentials © Kip Smith, 2003 The visual stimulus Photons Light that reflects off a surface in the environment and then passes through the lens of your eye © Kip Smith, 2003 Eyeball © Kip Smith, 2003 Parts of the eye Pupil Iris Lens Humors Retina Fovea Optic nerve Blind spot © Kip Smith, 2003 Topics Distribution of photoreceptors Receptive fields The Retina The back of the eye is called the retina The retina is composed of photoreceptors: rods and cones and neurons: bipolar horizontal ganglia © Kip Smith, 2003 The retina is part of the brain Rods and Cones Rods and cones are photoreceptors They transduce (= convert) light into action potentials They convert photons into neural activity © Kip Smith, 2003 Structure of the retina How the transduction works A photon stimulates a rod or cone The rod or cone generates a cascade of chemical reactions that produce a voltage The voltage triggers an action potential in the attached neuron (a bipolar cell) © Kip Smith, 2003 Visual information The action potential is information of the existence of light at a specific location on the retina Each location on the retina provides information about a specific location in the environment The action potential is information about surfaces in the environment © Kip Smith, 2003 The transmission of information The bipolar cell sends a signal to a ganglion cell (another type of neuron) The ganglion cell sends the signal via the optic nerve to the thalamus (the sensory way station) © Kip Smith, 2003 Differences between rods and cones Sensitivity vs. Resolution: Rods share ganglion cells Cones do not high sensitivity, low resolution low sensitivity, high resolution Color Rods are insensitive to color There are 3 types of cones, sensitive to 3 different parts of the color spectrum © Kip Smith, 2003 Color Vision - 2 Theories © Kip Smith, 2003 Tri-Chromatic Theory - Helmholtz Herr Helmholtz (1860s) claimed human vision uses three color receptors sensitive to either red, green, blue and constructed all intermediate colors from combinations of the basic three © Kip Smith, 2003 Tri-Chromatic Theory - Helmholtz Herr Helmholtz (1860s) claimed human vision uses three color receptors sensitive to either red, green, blue and constructed all intermediate colors from combinations of the basic three In the 20th century, the human was found to have three types of cones sensitive to either red, green, or blue © Kip Smith, 2003 Opponent Process Theory Hering Herr Hering (1870s) said the human visual system uses three channels with opposing processes Red vs. Green channel Yellow vs. Blue channel Black vs. White channel © Kip Smith, 2003 Selectively connected ganglia Research has demonstrated that there are pairs of ganglia for each receptive field in the fovea One of the pair is activated by light in the green part of the spectrum The other is activated by light in the red There are similar pairs of ganglia for yellow & blue and for rods (black or white) for each and every foveal field © Kip Smith, 2003 A problem with the Opponent Process Theory How do we see Purple? Pink? Turquoise? Brown? Shades of gray? © Kip Smith, 2003 Getting the signal to the cortex Information in the left visual field is seen on the right side of both retinas. Ganglia on the right (left) side of the retina project to the right (left) hemisphere Information in the world in processed in the contralateral cortical hemisphere © Kip Smith, 2003 The flow of visual information Vision, Summary Signal Signal processing apparatus Photon Eye Receptors/ transducers Neural pathway © Kip Smith, 2003 Pupil Lens Rods & cones Bipolar, horizontal & ganglion cells, optic nerve Perception Topics Gestalt Psychology Depth Perception Perceptual Constancy © Kip Smith, 2003 Gestalt Psychology © Kip Smith, 2003 Figure/Ground The visual system must distinguish an object from its surroundings The object is known as the “figure” The surroundings are known as the “ground” © Kip Smith, 2003 Grouping Principles The visual system must also group pieces of information together Five grouping principles: Proximity Similarity Continuity Closure Connectedness © Kip Smith, 2003 Proximity What do you see? Observation: We tend to group together items that are close together This is the Gestalt Principle of Proximity © Kip Smith, 2003 Similarity What do you see? Observation: We tend to group together items that are physically similar. This is the Gestalt Principle of Similarity © Kip Smith, 2003 Continuity What do you see? Observation: We tend to organize smooth continuous patterns rather than discontinuous patterns This is the Gestalt Principle of Continuity © Kip Smith, 2003 Closure What do you see? Observation: We tend to fill in gaps to make complete objects This is the Gestalt Principle of Closure © Kip Smith, 2003 Connectedness What do you see? Observation: We tend to group things that are connected This is the Gestalt Principle of Connectedness © Kip Smith, 2003 Depth Perception © Kip Smith, 2003 Depth Perception Definition: seeing in 3-D space Seeing depth takes advantage of both: Binocular cues Require both eyes Monocular cues © Kip Smith, 2003 Requires only one eye Binocular Cues Retinal Disparity The eyes receive slightly different views of the world Convergence The eyes turn more toward the center as objects move closer © Kip Smith, 2003 Monocular Cues Relative size Relative motion Interposition Linear perspective Relative clarity Texture gradient Relative height © Kip Smith, 2003 Implied direction of illumination (relative brightness) Relative Motion Far objects appear to move with you; albeit at a slower rate. Fixation Point YOU ARE MOV ING © Kip Smith, 2003 Near objects appear to move in the opposite direction. Linear Perspective © Kip Smith, 2003 Perceptual Constancy © Kip Smith, 2003 Perceptual Constancy Definition: perceiving an object as unchanged although the stimuli we receive has changed Two main categories: Size Constancy Brightness/Color Constancy © Kip Smith, 2003 Size Constancy How does it work? Visual system computes depth and adjusts perceived size accordingly Experiments manipulating depth cues demonstrate that more depth cues generally yields more size constancy © Kip Smith, 2003 Questions? You will be responsible for all the material in this lecture and in Chapters 7 & 8 We have not covered it all in class If you have questions about anything in the chapters, ASK! © Kip Smith, 2003