Relevant Image Features for Vernier Acuity

Abstract

The extraction of relevant image features is a key part of discrimination learning (E Gibson, 1969 Principles of Perceptual Learning and Development) and the identification of those features is necessary for the understanding of observer performance. Two mechanisms are thought to limit Vernier acuity judgments: orientation-selective and local-sign mechanisms (Waugh and Levi, 1993 Vision Research33 539 – 552; Beard et al, 1997 Vision Research37 325 – 346). The linear component of relevant image features can be determined for a Vernier task by adding external noise to the image and then averaging the noises separately for the four types of stimulus/response trials. The Vernier stimulus consisted of two short, dark, horizontal lines presented within low-contrast white noise. Two spatial separations were tested: nearly abutting and a wide horizontal separation. The task was to determine if the target lines were aligned or offset in the vertical direction. The noises were averaged separately for the groups of trials, corresponding to each of the four possible stimulus/response combinations (eg, stimulus=offset, response=aligned). The sum of the two ‘not aligned’ images was then subtracted from the sum of the ‘aligned’ images to obtain an overall image. We then computed a weighted average of adjacent pixels, smoothing the image and fostering visualisation of the relevant feature pattern. Our image analysis supports the view that as the vernier line separation narrows, oriented features play a larger role in the Vernier discrimination.