Basics of Psychophysics PDF

Receptive Field of a Neuron Neurons in higher brain areas can put together detailed information from multiple neurons to identify the object anywhere on the stimulus surface This means that RFs of these n...

Receptive Field of a Neuron Neurons in higher brain areas can put together detailed information from multiple neurons to identify the object anywhere on the stimulus surface This means that RFs of these neurons are larger and higher resolution – they get the details from earlier in processing and put them together. brain Laying the foundation for our study of the perceptual system Perceptual Receptor Experience cells in sensory Sense surface organs Neurons/Brain Physical Stimulus Information Encoding and Transmission Transduction HOW CAN WE KNOW WHAT’S GOING ON AT EACH STEP?  Methods: Our toolkit Neuronal Level – How does an individual neuron respond to a stimulus Systems Level – How does an area of the brain respond to a stimulus Behavioral Level – How does an organism experience a stimulus  Measuring perception: Activity of a single neuron Signal is electrochemical, so we can measure the change in potential of the cell membrane in order to measure the cell’s activity  What does the signal look like? Each spike is an action potential time Baseline Pressure applied Soft pressure Medium pressure Degree of change in Strong pressure firing rate from baseline indicates signal strength Baseline  What does the signal look like? Baseline Pressure applied Pressure Increasing or decreasing from baseline indicates presence of signal There is a limit: Cannot have infinite number of spikes/sec, cannot have fewer than 0 spikes/sec. What does that mean for experience?  Systems level functional magnetic resonance imagery electroencephalography Perceptual Experience?  PSYCHOPHYSICS: Measuring the relationship between stimulus (physical) and experience (psychological) How sensitive are we to physical stimuli? Thresholds: – Absolute threshold: When do we detect a stimulus? – Difference threshold: When do we detect that one stimulus is different from another?  Measuring absolute thresholds Use method of limits at a range of frequencies, you get audibility curve at absolute threshold (dB SPL) Physical sound intensity Method of Limits Physical Series Series Stimulus #1 #2 Intensity Audibility 51 50 YY YY curve 49 YY 48 NY 47 Y Y 46 Y N 45 N YN 44 NN 43 NN 42 NN Threshold= 45.5 48.5 46.5 44.5 Mean =46.5  Measuring difference thresholds Techniques started in the 19th century W1 W2 Difference threshold 100 g 105 g 5g What is the 200 g 210 g 10 g relationship between 300 g 315 g 15 g stimulus and 400 g 420 g 20 g perception? As stimulus strength increases, so does the amount of difference in the stimulus for an observer to detect that difference Weber’s Law: The Difference Threshold is a constant proportion of the comparison stimulus  Measuring difference thresholds Method of Constant Stimuli Present a stimulus of a single intensity (the “standard”) Present a set of comparison stimuli of different intensities Ask participant to judge whether the comparison stimulus is more intense than standard Obtain the “Just Noticeable Difference” (JND) Intensity of comparison stimulus Why aren’t we perfect detectors? Signal & Noise in Neural Activity The Basics: Summary Physical stimulus, sense organs, receptors, neurons + brain Receptive fields of neurons Measuring perception – Neuron level – Systems level – Behavioral level