Figure 1.
Stimuli and trial procedure from Experiment 1.
(a) An example of the stimuli from the first experiment (with the contrast of distractors reduced for illustrative purposes). Observers had to identify which side of the image contained a structured contour. In this case the contour is surrounded by near-perpendicular elements, which generally enhance detectability. Note that the random path on the right was generated in essentially the same way as the structured contour – except that the orientation of path-elements was randomised prior to presentation. Because of this the orientation of distractors surrounding the random-path is comparable to the context of the structured contour in that elements are near-perpendicular to the contour-spine used to generate the random path. (b) A typical trial of Experiment 1: the test stimulus, which contained a structured contour either on the right or on the left (here the first and the last elements of the path are shaded in red to assist the reader in finding the contour) was immediately followed by a mask with randomly oriented elements. This display persisted until observers had made a response.
Figure 2.
(a) Tolerance to orientation jitter for patients with SZ (red) and healthy controls (blue), measured with random, near-parallel and near-perpendicular surrounds. Black horizontal lines represent mean tolerance. Patients generally tolerate less orientation jitter than controls and are not affected by near-parallel contexts. (b) Log-ratios between tolerance with organized and random surrounds (i.e. log[near-parallel/random] and log[near-perpendicular/random]). Patients show less disruption from near-parallel surrounds compared to controls.
Figure 3.
Examples of the stimuli from the second experiment, where observes had to judge the orientation of the central Gabor (clockwise or anticlockwise of vertical).
(a) Isolated target condition. (b) Random flankers condition. (c) Contour-fragment condition.
Figure 4.
(a) Mean orientation discrimination thresholds for patients with SZ (red) and non-clinical controls (blue) in the three conditions tested in Experiment 2 (isolated Gabor, random flankers, contour-fragment). Black horizontal lines represent mean orientation thresholds. Note that in this graph higher y-values indicate poorer performance, whereas in Figure 2a higher y-values indicate better performance. Patients’ thresholds in the isolated-Gabor condition are doubled compared to controls’, indicating reduced sensitivity to local orientation. (b) Log-ratios between thresholds in the isolated and crowded stimuli (i.e. log[isolated/random] and log[isolated/contour]). Both in the random flankers condition and in the contour-fragment condition patients show less crowding compared to controls.