Chapter 1 Notes PDF

INTRODUCTION TO SENSATION AND PERCEPTION THE PHYSICAL WORLD How do we know the sky is blue? How do we know it is cold outside? How do we know where a bird is calling from? SENSATION AND PERCEPTION Sensation Perception Ability to det...

INTRODUCTION TO SENSATION AND PERCEPTION THE PHYSICAL WORLD How do we know the sky is blue? How do we know it is cold outside? How do we know where a bird is calling from? SENSATION AND PERCEPTION Sensation Perception Ability to detect a stimulus Giving meaning to the detection of a stimulus Results in a private experience Allows the individual to interpret, react, or understand an experience Senses are structures specialized for receiving signals from the environment The brain receives sensory signals Senses send information about those signals to Processing of sensory signals gives rise to the brain for processing perception, or the experience of meaning in response to a stimulus THE FIVE SENSES HOW MANY SENSES DO WE HAVE? THE FIVE SENSES… Taste Touch Vision Hearing Smell Taste Touch Vision Hearing Smell Gut taste Vestibular Cold? receptors? system? Itch? Tickle? Interoception? METHODS 1. Thresholds FOR MEASURING 2. Scaling PERCEPTION 3. Signal Detection Theory 4. Time Course of Perception 5. Sensory Neuroscience 6. Computational Models 1. THRESHOLDS Thresholds – methods of evaluating the minimum and maximum levels of stimulation that can be detected by our senses 1. THRESHOLDS Gustav Fechner – founder of experimental psychology Examined the connection between mind and matter – the internal experience of the external world Derived a branch of science based on three key ideas: 1. Dualism: the mind has a separate existence from the external world of the body 2. Materialism: matter is all that exists, and all things, including the experience of mind and consciousness, are the result of interactions of bits of matter 3. Panpsychism: the mind exists as a property of all matter 1. THRESHOLDS Defined psychophysics – the science of quantifying the relationship between physical events and psychological experience This science introduced the main concepts used to assess and understand sensation and perception: Two-point threshold: minimum distance at which two stimuli can be distinguished Just noticeable difference: the smallest detectable difference between two stimuli, or the minimum amount of change that can be correctly judged as different from a reference stimulus Absolute threshold: minimum amount of stimulation needed for a person to detect a stimulus 50% of the time 1. THRESHOLDS How do we measure thresholds? 1. Method of constant stimuli: many stimuli, ranging from rarely to almost always perceivable, are presented one at a time. 2. Method of limits: The magnitude of a single stimulus, or the difference between two stimuli, is varied incrementally until the participant responds differently 3. Method of adjustment: nearly identical to the method of limits, only the participant controls the stimulus magnitude directly 1. THRESHOLDS – METHOD OF CONSTANT STIMULI Many trials of stimuli presenting different magnitudes are presented to measure the point at which the stimulus of interest is detected 50% of the time is then measured. Why 50%? Why not as soon as a difference is measured at all? Variability – we are sometimes more and sometimes less sensitive to stimuli As this method measures subtle judgements of perception, it requires multiple exposures at each level to ensure that failures or successes were not accidental or due to the influences of other factors Often hundreds or even thousands of trials are needed to get an accurate measurement Example: Muller-Lyer threshold experiment Carefully examine each image and indicate whether the lines are the same length or not 1. THRESHOLDS – METHOD OF CONSTANT STIMULI 1. THRESHOLDS – METHOD OF LIMITS Method of constant stimuli is simple, but inefficient Method of limits varies stimulus intensity in alternating increasing and decreasing order When ascending, participants are asked to indicate the moment they can detect the stimulus When descending, participants are asked to indicate the moment when they can no longer detect the stimulus The levels are then plotted to see where the crossover points appear The average of these points is taken to get the threshold value 1. THRESHOLDS Absolute threshold – our ability to tell the presence of a stimulus from the absence of a stimulus Difference threshold – our ability to judge a difference or change between two stimuli 1. THRESHOLDS Ernst Weber – anatomist and physiologist interested in understanding how people perceive changes in the world around them Weber’s Law: the ability of a person to detect a difference between a standard and comparison stimulus depends on the value of the standard stimulus The just noticeable difference (JND) between the comparison and standard stimuli was always a constant fraction of the standard stimulus’ value This ratio holds true for all stimuli across all stimulus differences except for extreme stimuli (those at the very boundaries of our sensory ranges) 1. THRESHOLDS Terms: Standard – the stimulus value that remains constant when evaluating ability to detect a difference between two stimulus values Comparison – the stimulus value that changes when evaluating the ability to detect a difference between two stimulus values WHICH PERSON WOULD NOTICE THE DEPOSIT OF $1,000,000 MORE? $10 $100,000,000,000,000 WHICH DIFFERENCE IS MORE NOTICEABLE? $100,000,000,000,000 + $100 + $1 = $101 $1,000,000,000,000 = $101,000,000,000,000  THEY ARE EQUALLY NOTICEABLE The difference between Difference between $100 $100,000,000,000,000 and and $101 is 1% $101,000,000,000,000 is 1% 1. THRESHOLDS Weber’s Law: The just noticeable difference between a comparison stimulus and the standard stimulus is a constant fraction of the of the standard stimulus This means that larger standard values will have larger JNDs and smaller standard values will have smaller JNDs 1. THRESHOLDS Fechner adapted Weber’s Law to make it universal: S = k log R S = psychological sensation Log R = the logarithm of the physical stimulus level K = the constant WHAT DOES THAT MEAN??? As the intensity of a physical stimulus increases, our sensitivity requires a bigger change in the stimulus intensity to notice a change That’s why you would immediately notice a million dollars suddenly appearing in your bank account, but Elon Musk would not.  2. SCALING Physical stimuli are not equivalent to the perceptual experience Example: Sound frequency and pitch Sound frequency = the number of times a soundwave repeats in one second Pitch = the perceived quality of “highness” or “lowness” of a tone In general, as frequency increases, so does pitch, but: we can increase frequency without a perceived difference in pitch as frequencies approach the sensory boundaries, perceived pitch no longer changes 2. SCALING Magnitude Estimation: Ask observers to rate the experience of physical stimulus changes, assigning values to the perceived magnitudes of the stimulus 2. SCALING Steven’s Power Law: S = aIb S = the sensation of the stimulus a = constant to correct for units I = intensity of the physical stimulus b = an exponent 2. SCALING For stimuli that have exponents of less than one Steven’s Law works the same as Fechner’s or Weber’s Law – the sensation of the stimulus grows less rapidly than the physical measure of the stimulus When exponent values are greater than one, Fechner and Weber no longer accurately predict the sensory experience of the physical stimulus so Steven’s Law must be used Electric shock – increasing the intensity of an electric shock by a factor of 4 increases the sensation of pain by a factor of 128 2. SCALING Cross-modality matching: an observer matches the perceived intensity of a stimulus in one sensory modality to a stimulus presented in a separate sensory modality Example: adjust the brightness of a lamp to match the perceived loudness of an auditory tone 2. SCALING Not all people experience stimuli the same way PROP – a molecule that is found in brussels sprouts, turnip, cabbage, and other veggies People are divided into three genetic groups: Non-tasters - equate the taste sensation to a gentle finger tap on the arm Medium tasters - equate the taste sensation to a mild headache Supertasters - equate the taste sensation to the most intense pain imaginable 3. SIGNAL DETECTION THEORY EVEN IN THE QUIETEST NOISE IS NOT JUST SOUND ENVIRONMENT THERE IS ALWAYS BACKGROUND NOISE 3. SIGNAL DETECTION THEORY Some terms to know: Signal: the stimulus you are trying to detect Noise: distractor stimuli that mask or hide the signal External noise: other stimuli in the environment that interfere with signal detection Internal noise: stimuli generated by your own nervous system 3. SIGNAL DETECTION THEORY Imagine your friend bet you $100 you couldn’t pull a spare key out of the junk drawer on the first try without looking. The key is the signal The other items cluttering the drawer are the noise Items in the junk drawer (noise) LESS MORE Spare key + items in the junk drawer (signal + noise) LESS MORE LESS MORE  Criterion LESS MORE 3. SIGNAL DETECTION THEORY When the response is above the criterion value, it is treated as though the signal is present. When the response is below the criterion value, it is treated as though the signal is absent. This leads to four possible outcomes: You do not pull your hand out when you grab a coin (a correct rejection or true negative) You pull your hand out when you grab the key (a hit or true positive) You pull your hand out when you grab a coin (false alarm or false positive) You do not pull your hand out when you grab the key (a miss or a false negative) Correct Rejection Criterion LESS MORE Hit Criterion LESS MORE False Alarm Criterion LESS MORE Miss Criterion LESS MORE 3. SIGNAL DETECTION THEORY Sensitivity: a value that defines the ease with which an observer can tell the difference between the presence and absence of a stimulus or the difference between stimulus 1 and stimulus 2 Sensitivity is measured using d’ (pronounced d prime) When the two distributions almost completely overlap there is no sensitivity - you cannot tell noise alone from the noise + signal When the two distributions have very little overlap there is high sensitivity – you can easily tell the difference between the noise alone and the noise + signal 3. SIGNAL DETECTION THEORY False alarms as likely as hits Almost all hits, nearly 0 false alarms LESS MORE LESS MORE LESS MORE 3. SIGNAL DETECTION THEORY If we hold sensitivity (d’) constant, we can change the pattern of errors made  Minimize misses 50% misses Maximize misses LESS MORE LESS MORE LESS MORE Maximize false alarms 50% false alarms Minimize false alarms 3. SIGNAL DETECTION THEORY Receiver operating characteristic (ROC): the graphical plot of the hit rate as a function of the false alarm rate. Chance performance will fall along the diagonal Good performance (high sensitivity) “bows out” towards the upper left corner Plotting the ROC curve allows one to predict the proportion of hits for a given proportion of false alarms, and vice versa. Changes in criteria move performance along a curve but do not change the shape of the curve 4. TIME COURSE OF PERCEPTION Most methods focus on IF a stimulus can be detected Sometimes we want to know WHEN a stimulus is detected 4. TIME COURSE OF PERCEPTION Time course of perception involves several distinct events Stimulus detection Stimulus processing Conscious awareness of stimulus identification Complete processing of stimulus meaning 4. TIME COURSE OF PERCEPTION Researchers measure the time required to detect a target stimulus by adding a following "masking" stimulus The masking stimulus is assumed to terminate perceptual analysis of the target The time from the start of the stimulus to the start of the mask is called the Stimulus Onset Asynchrony (SOA) The SOA needed to perceive the target stimulus depends on what we are trying to perceive The longer the SOA, the more information we are able to process 4. TIME COURSE OF PERCEPTION 50ms (1/20 of a second) is all we need to get the general idea of an image 50ms allows us to identify basic categories of familiar objects (e.g. chair) We need longer to identify specific members of a category (e.g. kitchen chair) The first image was presented for 1/100 of a second (10ms) The second image was presented for 1/20 of a second (50ms) The third image was presented for 1/2 of a second (500ms) 4. TIME COURSE OF PERCEPTION Sound, touch, and light: 200-250 ms Vestibular (sense of head movement): over Sometimes detection is not enough – we 400 ms want to know how long it takes to react to Reaction times can vary within a sensory a stimulus system: Simple reaction time: the time it takes to Vibration on skin: within 250 ms respond to the presentation of a stimulus Detecting a thermal stimulus: up to 1 sec due This value is different for each sensory to slow heat flow process. system More complex acts of perception can take longer.  5. SENSORY NEUROSCIENCE 5. SENSORY NEUROSCIENCE Signals from the environment are detected by mechanical senses These signals are then converted into electrochemical signals The electrochemical signals travel from our sense organs to the brain for processing Johannes Müller discovered that sensation depends on which sensory fibers are stimulated, not how they are stimulated (doctrine of specific nerve energies) 5. SENSORY NEUROSCIENCE Why do you see stars when you hit your head? The mechanical pressure created by the impact causes the vision nerves to fire sending signals to the brain Thanks to Johannes Müller we know that if vision nerves are activated the result is a visual sensation 5. SENSORY NEUROSCIENCE 12 pairs of nerves originate in the brain stem and reach sense organs and muscles through openings in the skull One set for each half of the body Each nerve controls specific sensory organs or muscles FUN WITH NERVES Is spicy a flavour? It activates receptors (nerve signals) on the tongue Receptors on the tongue are for flavour…. Right? Wrong! Capsaicin – the chemical responsible for spicy heat in hot sauces – stimulates temperature-sensitive fibers that send a signal to the brain that the temperature is rising FUN WITH NERVES Try this at home: Find a strong mint (containing menthol – e.g. Halls, peppermint candy, Altoids) Enjoy the candy for a minimum of 30 seconds Take a big gulp of cold water FUN WITH NERVES The menthol in the candy activates cold temperature sensors (TRPM8) without changing the temperature One these sensors are activated they become sensitized (more sensitive) to cold stimuli Taking a big gulp of cold water triggers these sensitized nerves to signal a big temperature change to your brain 5. SENSORY NEUROSCIENCE Just as different nerves are dedicated to specific sensory and motor tasks, different areas of the cortex are also dedicated to specific sensory and motor tasks However, there are some areas of the brain that are polysensory, meaning that information from several senses is combined Sensory integration (multisensory integration): the process of combining different sensory signals 5. SENSORY NEUROSCIENCE Let’s say you stubbed your toe: Impact activates pain receptors in your toe The receptors trigger a signal that travels from the sensory neuron in your toe to your spine That neuron then sends signals through the synapse to other neurons The signal travels through multiple neurons to the brain where it is perceived as pain THE ACTION POTENTIAL 5. SENSORY NEUROSCIENCE Neuroscientists can record action potentials and other signals by placing electrodes in or near neurons Different aspects of firing can reveal how neurons receive and transmit information from sense organs through higher levels of the brain One approach is to identify the stimulus that makes a neuron fire the most vigorously 5. SENSORY NEUROSCIENCE What we can measure: How often a cell fires Patterns in how a cell fires Patterns of firing in groups of neurons How much stimulation is needed to get a cell to fire What frequency of stimulation a cell is most sensitive to 5. SENSORY NEUROSCIENCE - EEG Electroencephalography (EEG): A technique that, using many electrodes on the scalp, measures electrical activity from populations of many neurons in the brain Event-related potential (ERP): A measure of electrical activity from a subpopulation of neurons in response to particular stimuli that requires averaging many EEG recordings 5. SENSORY NEUROSCIENCE - MEG Magnetoencephalography (MEG): A technique, similar to EEG, that measures changes in magnetic activity across populations of many neurons in the brain o MEG has the same high temporal resolution as EEG, but it has better spatial resolution. 5. SENSORY NEUROSCIENCE - CT Beam of x-rays are shot through the brain and imaged through absorption much absorption by bone High density less by grey matter still less by fluid Low density Translated into a grey-scale image (2-D) The greater the absorption, the lighter the colour 5. SENSORY NEUROSCIENCE - MRI Magnetic resonance imaging (MRI): An imaging technology that uses the responses of atoms to strong magnetic fields to form images of structures like the brain 5. SENSORY NEUROSCIENCE - FMRI Functional magnetic resonance imaging (fMRI): measures localized patterns of brain activity. Activated neurons provoke increased blood flow, which can be quantified by measuring changes of oxygenated and deoxygenated blood to strong magnetic fields. 5. SENSORY NEUROSCIENCE - FMRI Using statistics, blood flow changes are plotted on the brain in voxels Blood flow changes occur within 2-3mm of the actual site of increased activation, so spatial resolution is high 5. SENSORY NEUROSCIENCE - PET Positron emission tomography Injection with radioactive tracer Release positron Positron + electron = annihilation Generates two photons that travel in opposite directions from the collision site Photons trigger sensors measuring time and location of photon detection 6. COMPUTATIONAL MODELS We can use statistics to explain and predict how the brain receives and transmit signals We then program computers to simulate these processes so we can assess how changes to various factors affect processing 6. COMPUTATIONAL MODELS Models can allow us to ask questions and make predictions about how perception works Different methods/model types allow us to ask different kinds of questions based on how they encode information and how that information is used TUNE IN NEXT WEEK… CHAPTER 2: VISION - FROM LIGHT TO NEURAL SIGNALS