Is 'yes' if the internal response exceeds this criterion.Ĭriteria can range from being very conservative to avoid making misses, to being liberal to avoid making false alarms. SDT is based on the idea that a subject chooses a 'criterion' level of internal response so that the decision on any trial SignalPDF = NormalPDF(x,signalMean,variance) NoisePDF = NormalPDF(x,noiseMean,variance) The PsychToolbox provides us with a function 'NormalPDF' which is simply a Gaussian function normalized so that its total Our normal distributions will be 'unit normals'. To keep things simple, we'll choose a variance of 1, so that Let's choose some numbers and draw the PDF's for illustration. Will be normally distributed with equal variances but different means. In the simplest case, 'noise' and 'signal' trial distributions Probability distribution function (PDF) along that dimension. (a 'signal trial) will lead to a weak response, causing the subject to respond 'no' - a 'miss' trial.įormally, the internal representation is along a single dimension, and the response to a given stimulus is described by a Or, sometimes a trial containing a stimulus Response that the subject will claim to see a stimulus, causing a false alarm. Sometimes the noise in a trial containing no stimulus (a 'noise' trial) will produce a large enough This means that the internal response to any stimulus will varyįrom trial to trial. Variablity in the neuronal representation of the stimulus. The noiseĬan either be external, caused by variability in the number of photons in the flash, for example or internal, caused by intrinsic The idea behind SDT is that there is 'noise' in the system that interferes with perfect performance on the task. Is a very dim flash of light in the dark. On each trial and the subject must respond 'yes' or 'no' to indicate whether or not anything was seen. The simple 'yes/no' forced choice experimentīefore getting to the 2AFC experiment, we'll start with a simpler version where either no stimulus or a weak stimulus is presented Estimating D-Prime from simple forced-choice.The simple 'yes/no' forced choice experiment.A rather general model based on the concept of signal uncertainty is presented which attempts to explain this relationship. The usefulness of this concept is illustrated by considering the shape of the psychophysical function-the function relating the detectability of the signal to its intensity. The concept of ideal observer is reviewed with special emphasis on the assumptions of the derivation. The measure of signal detectability appears to be independent of the psychophysical procedure when the physical parameters of signal and noise are held constant. The theory provides a technic of analysis which allows one to obtain a quantitative estimate of both factors. By treating the threshold process as an instance of hypothesis testing, two determinants of the process are recognized: (1) the detectability of the signal and (2) the criterion level of the observer. The paper discusses how statistical decision theory has been used to analyze the auditory threshold process. Detection theory is treated as a combination of two theoretical structures: decision theory and the concept of ideal observer. This paper presents a fairly complete review of detection theory as it is applied to certain psychoacoustic data.
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