Model-free estimation of a psychometric function

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[x_th,slope] = threshold_slope(pfit,xfit,thresh);

Approximate value of x (=x_th) for which the value of the estimated psychometric function is equal to thresh and the approximate value of slope at x_th.

Input:

pfit: estimated values of the psychometric function

xfit: stimulus levels at which the function was estimated

Optional input:

thresh: criterion level at which to estimate threshold; default is 0.5

Output:

x_th: estimated threshold

slope: estimated value of slope, i.e. derivative of pfit at x_th

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