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.

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Background: It is known that persons with dementia (PWD) suffer a variety of difficulties, with increased agitation as well as lowered mood, focus and lucidity being amongst their greatest challenges. Caregivers are also affected and report increased stress levels, as well as lower energy, mood and focus. The aim of the current study was to explore whether engagement in a singing program could result in improvement in these variables for both PWD and Caregivers.

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.

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