Physiological Optics Core Lab 3 October 2018 Basic psychophysical methods: Scott Stevenson Psychophysics refers to the measurement of how a physical stimulus leads to a psychological response, such as “I see it” or “A is more than B.”

A psychophysical procedure is designed to find the weakest stimulus that a person can detect (absolute threshold) or the smallest difference between two stimuli that allows a person to tell they are different (discrimination threshold). Some procedures are designed to measure the perceived intensity of stimuli that are well above threshold.

The theory behind psychophysical methods is that EVERY judgment a person makes is limited by noise. The limiting noise can be variation in the stimulus itself, variation in the nervous system’s response to the stimulus, or a combination of both. In today’s lab, we have noise added to the stimulus on purpose.

Because of noise, a stimulus will not have the same visibility from one test to the next. To find a threshold, a subject is asked the same question many, many times, such as “which is better, A or B?” or “Can you see the target?” Generally, the more responses you get, the more precise and reliable your threshold estimate will be.

Statistical methods are applied to determine the threshold as a summary of all the responses. Responses are summed up to find the probability that a given stimulus level will produce a positive response. When this probability is plotted against a range of stimulus strength, one obtains a Psychometric Function.

Detection, Discrimination, Discriminability Discrimination is seeing a difference between one thing and another thing. Detection is seeing a difference between something and nothing. Signal Detection Theory holds that “nothing” is always something- noise! So detection is a discrimination of signal plus noise from noise alone. Discriminability describes how easily one can reliably discriminate stimuli. The size of the signal, expressed as multiples of the noise standard deviation, is called the discriminability index. We refer to this number as d’ (pronounced “Dee Prime”).

This figure illustrates the concept that the visibility of a target depends on the Signal to Noise ratio. The signal refers to the brightness of the letter E. The Noise refers to the random brightness of the pixels added to it. The curves show the mean and distribution of pixel brightness.

For the lab today, we will try two different psychophysical methods that are commonly used. The Method of Adjustment and the Method of Constant Stimuli.

Method of Adjustment: the subject is allowed to control the stimulus level and is asked to adjust it so that it is just barely visible. This is repeated many times to get a mean and standard deviation.

Method of Constant Stimuli, Yes/No task: the experimenter (or her computer program!) presents stimuli of different strength one at a time, and the subject says “Yes” or “No” for each one. This is

repeated many, many, times and the probability of a “Yes” is plotted against the stimulus level to produce a psychometric function.

As the same question, 100 times.

“Did you see it?”

For each intensity, add up the number of “yes” responses, and plot the totals as a probability curve. The point where the curve crosses a chosen level, like .5, is used as the Threshold Estimate.

Noise makes the curve more shallow.

Method of Constant Stimuli, 2 AFC: the experimenter presents two stimulus arrays, one with the target and the other with just noise. The subject chooses which array has the target. This method is usually the most sensitive, because the subject knows that one of them actually has a target and just chooses whichever one seems stronger.

Today’s lab: We will use a Matlab program called “SignalInNoise” to explore these concepts. The stimulus for this lab will be a letter E buried in Noise. How much brighter does it have to be for you to see it? First you will adjust the brightness of the E until you are confident it is there. You will repeat this 50 times. Then the computer will start presenting the E at different brightness levels (including ZERO) and you will say Yes or No if you see the E or not. Remember, the E is not always there! This is called the Method of Constant Stimuli, Yes/No. Finally, the computer will present two images side by side, and you will say which one has the E. This is called 2 Alternative Forced Choice (Method of Constant Stimuli, 2AFC).

NoiseStandardDeviationis4.Signalsare0,8,32

Data analysis: 1) At the end, you will get a histogram of your 50 adjustment settings, with the mean and standard deviation. 2) You will also get a plot of the probability of “Yes” against the stimulus level. What level gives 50% “Yes” responses? What is the probability of “Yes” at the mean adjustment stimulus level you found from step 1? How often did you say, “Yes” when the stimulus was ZERO? 3) You will also get a plot of the probability of Correct responses vs. stimulus level. What is the guessing rate at zero stimulus? 4) Finally, you will get a plot of the “ideal observer”. This is simply a calculation of the pixel brightness in the left and right image, and the computer chooses whichever is brighter. Note that the computer knows exactly which pixels to count. Here are some references for further reading on psychophysics: Gescheider, G.A., Psychophysics: Method, Theory and Application, Lawrence Erlbaum Associates, Hillsdale, N.J., 1985, pp. 37-59. Dember, W.N. & Warm, J.S., Psychology of Perception, Holt, Rinehart and Winston, New York, 1979, pp. 20-45, 55-64. Engen, T., Psychophysics I. Discrimination and detection. In Kling, J.W. & Riggs, L.A., Experimental Psychology, Holt, Rinehart and Winston, New York, 1971, pp. 11-34. Cornsweet, T.N., The staircase method in psychophysics. Am. J. Psychol. 75: 485-491, 1962. Findlay, J.M., Estimates on probability functions: a more virulent PEST. Percept. Psychophys. 23: 181-185, 1978. Watson, A.B. and Pelli, D.G., QUEST: a Bayesian adaptive psychometric method. Percept. Psychophys. 33: 113-120, 1983. Tyrrell, R.A. and Owens, D.A., A rapid technique to assess the resting states of the eyes and other threshold phenomena: the modified binary search (MOBS). Behav. Res. Meth. Instrum. Comp. 20: 137-141, 1988. Treutwein, B., Adaptive psychophysical procedures. Vision Res. 35: 2503-2522, 1995.