Radial frequency adaptation suggests polar-based coding of local shape cues

Abstract

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.

Norms are Not the Norm: Testing Theories of Sensory Encoding Using Visual Aftereffects

“…Critically, the aftereffect induced by a horizontally compressed adaptor was larger when disparity cues were added (via stereo goggles), resulting in an adapting image that was consistent with viewing an uncompressed shape from an angle (Bell et al, 2008). This result is consistent with our data, although the studies are only weakly analogous.…”

“…Bell, Dickinson, and Badcock (2008) showed that radial frequency aftereffects can transfer partially between adaptors and tests with different horizontal compressions, consistent with a transfer between shapes viewed from different perspectives.…”

“…Until now, our knowledge of shape aftereffects had been consistent with a neural substrate in which shapes are represented by the magnitudes and positions of points of maximal local curvature in the retinal image, indicating V4 as a plausible substrate (Bell et al, 2008;Gheorghiu & Kingdom, 2007;Pasupathy & Connor, 2002;Poirier & Wilson, 2006;Suzuki, 2003). The influence of shape constancy operations are not, however, evident until later stages in the visual hierarchy (Janssen et al, 1999;Kourtzi & Kanwisher, 2001;Tanaka, 1996Tanaka, , 2003.…”

Measuring Sensitivity to Viewpoint Change with and without Stereoscopic Cues

“…A RID can be simulated by appropriate transformation of images using software packages or by writing one's own code 8,10 . Readers wishing to know more about RID can consult relevant texts and websites concerned with viewpoint transformations, or learn about built-in functions in the image processing software of packages such as those listed in our Materials.…”

“…Changes in viewpoint can occur either when the observer, or the viewed object, moves. Importantly, the viewpoint cost is often reduced by the introduction of stereoscopic cues to the observer [1][2][3][4]6,8 . It is, however, important to distinguish between the rotation of 3D and planar objects.…”