Position jitter and undersampling in pattern perception

The present paper addresses whether topographical jitter or undersampling m ight limit pattern perception in foveal, peripheral and strabismic amblyopi c vision. In the first experiment, we measured contrast thresholds for dete cting and identifying the orientation (up, down, left, right) of E-like pat terns comprised of Gabor samples. We found that detection and identificatio n thresholds were both degraded in peripheral and amblyopic vision; however , the orientation identification/detection threshold ratio was approximatel y the same in foveal, peripheral and amblyopic vision. This result is somew hat surprising, because we anticipated that a high degree of uncalibrated t opographical jitter in peripheral and amblyopic vision would have affected orientation identification to a greater extent than detection. In the secon d experiment, we investigated the tolerance of human and model observers to perturbation of the positions of the samples defining the pattern when its contrast was suprathreshold, by measuring a 'jitter threshold' (the amount of jitter required to reduce performance from near perfect to 62.5% correc t). The results and modeling of our jitter experiments suggest that pattern identification is highly robust to positional jitter. The positional toler ance of foveal, peripheral and amblyopic vision is equal to about half the separation of the features and the close similarity between the three visua l systems argues against extreme topographical jitter. The effects of jitte r on human performance are consistent with the predictions of a 'template' model. In the third experiment we determined what fraction of the 17 Gabor samples are needed to reliably identify the orientation of the E-patterns b y measuring a 'sample threshold' (the proportion of samples required for 62 .5% correct performance). In foveal vision, human observers are highly effi cient requiring only about half the samples for reliable pattern identifica tion. Relative to an ideal observer model, humans perform this task with 85 % efficiency. In contrast, in both peripheral vision and strabismic amblyop ia more samples are required. The increased number of features required in peripheral vision and strabismic amblyopia suggests that in these visual sy stems, the stimulus is underrepresented at the stage of feature integration . (C) 1998 Elsevier Science Ltd. All rights reserved.