Figure 7.

How accuracy and precision of threshold estimates depends on the level defined as threshold performance. The horizontal axis shows Θ, the proportion of correct answers defined as “threshold” (see Equation 8) and (above) the corresponding disparity values in log₁₀ units. The vertical axis on each plot shows the bias in estimated threshold, that is, the difference between the threshold estimate returned after 20 trials and the model's true threshold θ_m, expressed in log₁₀ stereo threshold units. By definition, Ψ(θ_m) = Θ. Black dots show the mean threshold estimate of 2000 simulated staircase procedures, as described in the Methods. Error bars show the standard deviation. The bias is close to zero, and the standard deviation is roughly constant, over a wide range of Θ (∼50%–80%). The staircase assumes the psychometric function given by Equation 9 with b = 4.885/log₁₀ arcsec, λ = 0.03, g = 0.25, and Θ specified by the horizontal axis. The model observer is assumed to have a psychometric function with the same form and the same values of λ and g, but different slope parameters b_M, as specified in each panel.