Adaptation to prevailing stimuli is a ubiquitous property of the visual system that optimizes its dynamic range. The perceived difference in orientation of successively presented lines of similar orientation is exaggerated and the perceived shape of an object is influenced by previously experienced shapes. Change in perceived shape is assumed to arise through the adaptation of shape detectors. Here we consider an alternative: adaptation within a substrate of local oriented line detectors resulting in enhanced shape contrast in similar shapes. We show that the perceived shapes of a spatially coincident circle and Cartesian grid can be manipulated independently by adaptation to geometrically transformed copies of themselves. The same transformation was applied to the circle and the grid to create the adaptors; therefore, the specificity of the effects of adaptation demonstrates that the visual system adapts to the shape of objects rather than applying transformations to the reference frame of the visual field. The tilt aftereffect predicts local changes in perceived orientation, and fields of such local effects can often account for the global change in perceived shape of complex objects, including faces.

Radial frequency (RF) patterns, paths deformed from circular by a sinusoidal modulation of radius, have proved valuable stimuli for investigation of visual shape processing. Their utility relies upon evidence that thresholds for detection of modulation decrease, as cycles of modulation are added, at a rate that cannot be accounted for by the improving probability of detection of any single cycle (probability summation). This has been interpreted as indicative of global processing. Recently Mullen, Beaudot, and Ivanov (2011), using low contrast RF patterns viewed in cosine phase through a Gaussian window, demonstrated the existence of a local cue to modulation that was more salient than the global shape cue present in sectors of RF patterns. The experiments reported here investigate why this cue has not previously obscured global integration of shape information in RF patterns. Using stimuli modulated in sine phase, Experiment 1 showed that the presence of a circular sector of path, used to complete a partially modulated RF pattern, does not raise thresholds, contrary to the suggestion of Mullen et al. (2011). Experiment 2 demonstrated integration for high and low contrast RF patterns viewed in sine phase through a Gaussian window and Experiment 3 showed the same for patterns in cosine phase if the use of a local phase specific curvature cue was precluded. Effective use of local curvature in the test comparison, then, requires knowledge of pattern orientation to define the sign of curvature. Experiment 4 demonstrated global processing of shape information for a range of radial frequencies and also showed that the local maximum gradient with respect to circular within an RF pattern covaries with threshold. This implies that it is this cue, or one that covaries linearly with it, that is integrated across cycles of modulation by the global processing mechanism.

No abstract

Sinusoidal modulation of radial speed around a circular path of tangentially oriented Gabor patches results in the percept of modulation of the radius. These patterns have been called motion radial frequency (RF) patterns. Sensitivity to these patterns has been attributed to global summation of local speed by mechanisms, analogous to those proposed to explain sensitivity to spatial RF patterns, which are sensitive to particular radial frequencies of speed modulation. We demonstrate that: Adaptation to spatial RF patterns results in a phase specific after effect in motion RF patterns and vice versa; the rate of change of perceived displacement of a Gabor patch with a moving grating with increasing speed of that grating, is independent of whether the patch is or is not incorporated into a circular path; in the absence of local motion direction cues, global integration across 1, 2 and 3 cycles of motion and spatial RF modulation conforms to a power function with the same index; and finally that the magnitude of the motion position illusion is sufficient and necessary to account for sensitivity to motion RF patterns. We therefore propose that motion RF patterns are analyzed by the same mechanisms responsible for sensitivity to spatial RF patterns using perceived rather than actual local positions of the Gabor patches.

Observers make sense of scenes by parsing images on the retina into meaningful objects. This ability is retained for line drawings, demonstrating that critical information is concentrated at object boundaries. Information theoretic studies argue for further concentration at points of maximum curvature, or corners, on such boundaries [1]–[3] suggesting that the relative positions of such corners might be important in defining shape. In this study we use patterns subtly deformed from circular, by a sinusoidal modulation of radius, in order to measure threshold sensitivity to shape change. By examining the ability of observers to discriminate between patterns of different frequency and/or number of cycles of modulation in a 2x2 forced choice task we were able to show, psychophysically, that difference in a single cue, the periodicity of the corners (specifically the polar angle between two points of maximum curvature) was sufficient to allow discrimination of two patterns near their thresholds for detection. We conclude that patterns could be considered as labelled for this measure. These results suggest that a small number of such labels might be sufficient to identify an object.

Adaptation in the visual system frequently results in properties of subsequently presented stimuli being repelled along identifiable axes. Adaptation to radial frequency (RF) patterns, patterns deformed from circular by a sinusoidal modulation of radius, results in a circle taking on the appearance of having modulation in opposite phase. Here we used paths of spatially localized gratings (Gabor patches) to examine the role of local orientation adaptation in this shape aftereffect. By applying the tilt aftereffect (TAE) as a function of the local orientation difference between adaptor and test, concomitant with adjustment of local position to accommodate the orientation change and preserve path continuity (Euler's method), we show that a TAE field can account for this misperception of shape. Spatial modulation is also observed spontaneously in a circular path of Gabor patches when the local patch orientations are rotated from tangential to the path. This illusory path modulation is consistent with the path orientation being attracted to the orientation of the patches. This consistent local rule implies a local explanation for the global effect and is consistent with a known illusion with a local cause, the Fraser illusion (FI). A similar analysis to that used for the TAE shows that the Fraser illusion can account for this particular alteration of perceived shape. A model which proposes that local orientations are encoded after considering the activation in a population of neurons with differing orientation tuning can accommodate both effects. It is proposed that these distinct processes rely on the same neural architecture.

The study of shape processing in the human visual system has frequently employed radial frequency (RF) patterns as conveniently manipulable stimuli. This study uses an adaptation paradigm to investigate how local shape information is sampled in the processing of RF contour shapes. Experiment 1 measured thresholds for detecting a fixed mean radius RF contour following adaptation to RF patterns which, in separate conditions, varied in mean radius and radial frequency. Results reveal that, adaptation is strongly tuned for RF over a range of pattern radii, but is not tuned for the number of cycles of radial modulation per visual degree of contour length; a characteristic that changes with both radius and radial frequency. Experiment 2 manipulated the polar angle separation on the fronto-parallel plane between curvature features on a fixed RF by foreshortening the pattern appearance (consistent with a rotation in depth) and shows that RF shape processing is tuned for fronto-parallel separation angles between curvature features. Results were near identical when a stereo rotation cue was added to the perspective modified RF. In the second part of Experiment 2 we showed that RF shape adaptation is also tuned for the polar angular extent of the curvature represented by the lobe at that angle. Collectively, our results indicate that the polar angle at which local curvature features appear, in addition to the angular extent of the curvature feature at that location, are both critical parameters for coding specific RF shapes.

Previously, researchers have used circular contours with sinusoidal deformations of the radius (radial frequency [RF] patterns) to investigate the underlying processing involved in simple shape perception. On finding that the rapid improvement in sensitivity to deformation as more cycles of modulation were added was greater than expected from probability summation across sets of local independent detectors, they concluded that global integration of contour information was occurring. More recently, this conclusion has been questioned by researchers using a method of calculating probability summation derived from signal detection theory (Baldwin, Schmidtmann, Kingdom, & Hess, 2016). They could not distinguish between global integration and probability summation. Furthermore, it has been argued that RF patterns and lines are processed in a similar manner (Mullen, Beaudot, & Ivanov, 2011; Schmidtmann & Kingdom, 2017). The current study investigates these claims using fixed-phase (in which the local elements have spatial certainty) and random-phase (in which the local elements have spatial uncertainty) RF patterns and modulated lines. Thresholds were collected from eight naïve observers and compared to probability summation estimates calculated using methods derived from both high threshold theory and signal detection theory. The results indicate global processing of random-phase RF patterns and evidence for an interaction between local and global cues for fixed-phase RF patterns. They also show no evidence of global integration with modulated line stimuli. The results provide further evidence for the global processing of random-phase RF patterns and indicate that RF patterns and modulated lines are processed differently.