Example psychometric function for single observer.

<p>Performance in an 3-interval oddity task follows a Gaussian when error rate is plotted against the test's radial distance (r) along a given orientation θ_i and its complement θ_i+π. Gaussian functions were fit to the data using a maximum likelihood procedure. Any radial test distance containing two or fewer trials was excluded from the fit (examples shown in open red symbols).</p

Motion discrimination contours for a single naïve observer for the 9 standards investigated in Experiment 1.

<p>The results for each individual standard value follow the conventions defined in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102864#pone-0102864-g001" target="_blank">Figure 1</a>. Error bars for each threshold were obtained using a bootstrapping technique and correspond to 95% CIs. Ellipses were fit according to a non-linear least-squares technique.</p

Summary of best-fitting ellipses across the four listeners (L1-L4) studied in Experiment 1.

<p>Two observers (L1, L2) completed all 9 conditions; two others (L3, L4) completed the 3 conditions lying on the major negative diagonal. The horizontal grey lines have length  = ±1 Weber fraction. All ellipses are oriented parallel to the axes of the distance-duration plane. Thus, one-sample t-tests for the mean ellipse orientations associated with the three standards on the major negative diagonal did not differ significantly from vertical (top-left: t(3) = 1.84, p>.10; middle: t(3) = .45, p>.50; bottom-right: t(3) = −0.81,p>.40). The results therefore provide no evidence that speed is used to discriminate test from standard; performance for all observers appears to be governed by separate estimates of distance and duration. The ellipses are stretched parallel to the Y axes, showing that duration discrimination was superior to distance discrimination.</p

The ‘discrimination contours’ technique.

<p>(A) Motion discrimination contours were defined in the distance-duration plane by measuring thresholds along orientations θ_i using an 3-interval oddity task consisting of two identical standard stimuli and one test stimulus, presented in a random order. The test differed from the standards by a given proportion (Weber fraction) of duration and distance; a test with an identical speed to the standards therefore falls anywhere on the thick red ‘iso-speed’ line oriented at θ = 45°. (B) If speed dominates performance, then the ellipse will be oriented obliquely along the iso-speed line θ = 45°. (C) If distance and duration cues are separable and dominate performance, then the resulting motion discrimination contours will be aligned with the cardinal axes and tend to be elliptical. When the major axis is horizontal, distance cues dominate; when the major axis vertical, duration cues dominate.</p

Results of Experiment 2, in which distance and duration noise were added to the standards to force discrimination based on speed.

<p>Each column corresponds to a different listener (L1-L4); each row is a different standard “mean”, corresponding to the standard values given along the major negative diagonal of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102864#pone-0102864-g003" target="_blank">Figures 3</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102864#pone-0102864-g004" target="_blank">4</a>. The results show that the auditory system is sensitive to speed: when distance and duration cues are made uninformative, listeners are able to discriminate stimuli based on speed alone.</p

Comparison of Experiment 1 (without noise) and Experiment 2 (with noise) for the three common standards investigated (see legend).

<p>Without noise, the mean orientation of the major axis of the three ellipses was close to vertical (orientation = 90°). When speed noise was added, the mean was close to the iso-speed line oriented at 45°. Error bars correspond to 95% CIs.</p