RT Journal Article
A1 Chubb, Charles
A1 Landy, Michael S.
T1 Measuring visual mechanism sensitivity
JF Journal of Vision
JO Journal of Vision
YR 2007
DO 10.1167/7.9.360
VO 7
IS 9
SP 360
OP 360
SN 1534-7362
AB Poirson and colleagues (JOSA A, 7, 783ā789, 1990) presented a simple and compelling argument to show that one cannot determine the sensitivity of mechanisms that underlie discrimination performance if thresholds lie on an ellipsoid (and they almost always do, at least within measurement error). In brief, the ellipsoidal model assumes that stimuli p and q are n-dimensional (n=3 for color vision, but is typically far larger for spatial visual stimuli), and that discrimination performance is based on their difference d=pāq as processed by m mechanisms. If the mechanism sensitivities to each dimension are summarized in an mXn matrix M, the ellipsoidal model predicts that performance is a function of |Md|=|RMd| for any orthonormal (e.g., rotation) matrix R. That is, any such rotation applied to the mechanisms results in an alternative model that makes the same predictions. On the other hand, Chubb, Landy & Econopouly (Vis Res, 44, 3223ā3232, 2004) claimed to have isolated and measured the sensitivity of an individual texture-sensitive mechanism: the blackshot mechanism. How can this be? The method used by Chubb and colleagues requires one to use a high-dimensional space of stimuli and to find a relatively high-dimensional subspace of stimuli in which almost all dimensions of that subspace trade off linearly in controlling discrimination performance. If these conditions hold, one can be practically certain that a single mechanism determines discrimination performance within that subspace. And, by trading off stimulus power in that subspace with power in other directions, one can measure the complete sensitivity function of the isolated mechanism. Robustness of the method to small violations of its assumptions will be discussed.
RD 5/6/2021
UL https://doi.org/10.1167/7.9.360