01_Sensory Neuroscience & Measuring Perception.pdf

Sensory Neuroscience SCIENTIFIC METHODS: Thresholds: limits of what can be perceived Scaling: measuring experience Sensory neuroscience: biology of S&P Sensory modalities have primary receiving areas Some are polysensory: information from several senses is combined Resting membrane potentia...

Sensory Neuroscience SCIENTIFIC METHODS: Thresholds: limits of what can be perceived Scaling: measuring experience Sensory neuroscience: biology of S&P Sensory modalities have primary receiving areas Some are polysensory: information from several senses is combined Resting membrane potential = -70 mV Flow of Na+ in / K+ out = depolarization Flow of K+ in / Na+ out = repolarization Membrane potential below - 70mV = hypolarization FIRING RATE Increase the stimulus intensity can increase the firing rate Refractory period Spontaneous activity: AP occurring without stimulation Modern Brain Electrophysiology 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 Visually Evoked potentials (VEP): A measure of electrical activity from a sub population of visual neurons in response to a visual stimulus Magnetoencephalography (MEG): A technique, similar to EEG, that measures changes in magnetic activity across populations of many neurons in the brain Brain Imaging Computerized tomography (CT): An imaging technology that uses X-rays to create images of slices through volumes of material 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 Positron Emission tomography (PET): functional neuroimaging technique based on measurement of changes in blood flow associated with brain activity, using a radioactive substance introduced into the blood Functional Magnetic Resonance Imaging (fMRI): A variant of MRI that makes it possible to measure localized patterns of activity in the brain Activated neurons provoke increased blood flow, which can be quantified by measuring changes of oxygenated and deoxygenated blood to strong magnetic fields Blood oxygen level-dependent (BOLD) signal: The ratio of oxygenated to deoxygenated hemoglobin that permits the localization of brain neurons that are most involved in a task Functional near-infrared spectroscopy (FNIRS): brain activity is measured using near-infrared light to estimate cortical hemodynamic activity which occur in response to neural activity Measuring Perception Perceptual Process Perception is a product of evolution Provides information for our survival Types of energy in our environment determines how our senses are developed Can only sense a limited range of energy Sense of reality is limited Types of senses 1. Vision/sight 2. Audition/hearing 3. Tactile perception/touch 4. Proprioception/body perception 5. Nociception/pain perception 6. Thermoreception/temperature 7. Balance 8. Body movement 9. Olfaction/smell 10. Gustation/taste Perceptual Process - cycle Environmental stimulus All the energy we could perceive Attended/distal stimulus Small subset of the environmental stimulus  Stimulus on the receptors (i.e. light on retina) Transduction (i.e. light in, electricity out) Processing (neuron signalling in CNS) Knowledge Existing knowledge affects what we perceive and what we recognize Perception Recognition where perception is familiar Action Back to environment PSYCHOPHYSICS Psychophysics: science of defining quantitative relationships between physical stimuli and subjective experience Invented by Fechner How to Measure Perception Absolute threshold: minimum amount of stimulation required for a person to detect it 50% of the time Method of limits: starting at a high volume and lowering the volume until it cannot be perceived anymore & vice versa & retrieving the mean Method of adjustment: adjust the stimulus using a dial/buttons until it is perceived Quick but not entirely accurate Method of constant stimuli: uses a selection of stimuli that cover a range that is likely to include the absolute threshold Simple yes/no response on one block of trials Psychometric function from many blocks Problem: need to present stimuli many times Staircase method: each stimulus is one level up or down from the previous Each reversal of participant response = direction of the stimulus change also reverses Average intensity across all reversals is an estimate of the absolute threshold Takes less trials than method of constant stimuli and is more accurate than method of adjustment Techniques Difference threshold: “is it the same or different”; calculation with the method of adjustment and constant stimuli Also called just noticeable threshold (JND) averaging between 75% and 25%, divided by 2 Psychophysical Scaling Does the different threshold depend on the magnitude of the stimulus? YES Weber's Law The smallest change in stimulus that we can detect at different thresholds changes with different stimulus intensities 100g vs 102g (2%) - difficult to notice difference 1000g vs 1,020g (2%) - easy to notice difference Weber's fractions for perception dimension SENSORY DOMAIN PERCEPTUAL FRACTION DIMENSION Taste Saltiness 0.083 Vision Brightness 0.079 Audition Loudness 0.048 Touch heaviness 0.020 Weber’s law = JND = kI I = intensity k= constant Weber's law = JND increases with the increase in intensity of the standard stimulus PSYCHOPHYSICAL SCALING Fechner's alteration of Weber's Law S = k In(I/Io) S = perceived intensity I= natural logarithm of the ratio of stimulus intensity Io = absolute threshold **however, electric shock has the opposite relationship Higher intensity = smaller JND Because it stimulates action potentials Fechner's law = a greater increase in intensity is needed for high-intensity stimuli to produce the same perceived difference in intensity Steven's law S = cln S= perceived intensity c= constant that depends on units used for S and I l= physical intensity n= exponent Stevens' power law = linear or non-linear relationship between stimulus and perceived intensity Steven's Power law exponent SENSORY DOMAIN PERCEPTUAL EXPONENT DIMENSION Taste Sweetness 0.8 Vision Brightness 0.3 Audition Loudness 0.5 Touch Warmth 1.6 Touch Heaviness 1.5 Touch Electric shock* 3.5 Sensory Neuroscience Neuron doctrine: perception depends on the combination of different neurons which code for different attributes of a stimulus Ramon y Cajal Discovery of synapse Detailed drawings of neurons and neural structures Neuron High [K+] and low [N+] Extracellular fluid is opposite Membrane is most permeable to K+ at rest Resting potential of -70mV Action Potential Action potential: spikes generated by electrical signals transmitted by the neuron Generated by the opening of the voltage-gated N+ channels N+ flowing in causes depolarisation N+ channels close and inactivate (refractory period) Voltage-gated K+ channels open, causing repolarisation and temporary hyperpolarisation All-or-nothing law: if the firing of the neuron does not reach the threshold, the neuron will not fire at all Synapses Can be either excitatory or inhibitory ◦ Excitatory: Na+ channels open, generating EPSPs ◦ Inhibitory: Cl- channels open, generating IPSPs Measuring Neural Activity Direct approach recording from peripheral nerves in humans Recording from neurons in the CNS in animals Indirect approach EEG measures the change in potential at the scalp caused by activity or cortical neurons - Contribution from any one neuron is too small to be detected - Many neurons being active together can be measured MEG (magnetoencephalography) measures the changes in magnetic fields caused by neural activity - Can only detect the overall activity of many neurons fNIRS (function near-infrared spectroscopy) measures changes in blood flow and blood oxygenation due to neural activity CT (computed tomography) uses x-rays from multiple angles to create axial images MRI measures changes in the spin of hydrogen atoms in high intensity magnetic fields to create 3D images of neural structures PET uses a radioactive tracer injected into the blood to measure blood flow in the brain fMRI uses MRI to measure the brain oxygen level dependent (BOLD) response - Better spatial resolution Signal detection Noise: random variation in the neural code for the same stimulus Sensory transduction doesn’t always result in the same neural activity (even for the same stimulus) 4 possible outcomes in a signal detection experiment 1. Hit 2. False alarm (type 1 error) 3. Miss (type 2 error) 4. Correct rejection Creating a receiver operating characteristic (ROC) plot X axis = false alarm rate Y axis = hit rate Actual performance rate: hit/false alarm point Chance = 50/50 ** different stimulus intensities will have different ROCs - the higher the intensity, the higher the hit rate Signal detention theory: distinguishes between the ability to detect a stimulus and the willingness to report it Sensitivity: ease with which observer can detect a stimulus (absolute threshold) or distinguish between 2 stimuli (JND) Different observers can have different criteria (internal thresholds) ◦ Can reflect internal bias in favour of a particular response Criterion: internal threshold that is set by the observer Bias: observer tendency to be liberal or conservative in response, indicated by the value of criterion