Elena Gheorghiu

University of Stirling · Department of Psychology

Symmetry perception studies have generally used two stimulus types: figural and dot patterns. Here, we designed a novel figural stimulus—a wedge pattern—made of centrally aligned pseudorandomly positioned wedges. To study the effect of pattern figurality and colour on symmetry perception, we compared symmetry detection in multicoloured wedge patter...

Numerosity estimation around the subitizing range is facilitated by a shape-template matching process and shape-coding mechanisms are selective to visual features such as colour and luminance contrast polarity. Objects in natural scenes are often embedded within other objects or textured surfaces. Numerosity estimation is improved when objects are...

The tilt illusion (TI) describes the phenomenon in which a surround inducer grating of a particular orientation influences the perceived orientation of a central test grating. Typically, inducer-test orientation differences of 5 to 40 degrees cause the test orientation to appear shifted away from the inducer orientation (i.e. repulsion). For orient...

Recognition of simple shapes and numerosity estimation for small quantities are often studied independently of each other, but we know that these processes are both rapid and accurate, suggesting that they may be mediated by common neural mechanisms. Here we address this issue by examining how spatial configuration, shape complexity, and luminance...

Shape-adaptation studies show that surround textures can inhibit the processing of contours. Using event-related potentials (ERP), we examined the time-course of neural processes involved in contour-shape and texture-shape processing following adaptation to contours and textures. Contours were made of Gabor strings whose orientations were either ta...

Studies have revealed that textures suppress the processing of the shapes of contours they surround. One manifestation of texture-surround suppression is the reduction in the magnitude of adaptation-induced contour-shape aftereffects when the adaptor contour is surrounded by a texture. Here we utilize this phenomenon to investigate the nature of th...

The human visual system is often tasked with extracting image properties such as symmetry from rapidly moving objects and scenes. The extent to which motion speed and symmetry processing mechanisms interact is not known. Here we examine speed-tuning properties of symmetry detection mechanisms using dynamic dot-patterns containing varying amounts of...

Recent studies have shown that limiting the lifetime of pattern elements improves symmetry detection, potentially by increasing the number of element locations. Here, we investigate how spatial relocation, luminance contrast modulation and lifetime duration of elements affect symmetry perception in dynamic stimuli. Stimuli were dynamic dot-patterns...

Recent studies have suggested that temporal dynamics rather than symmetrical motion-direction contribute to mirror-symmetry perception. Here we investigate temporal aspects of symmetry perception and implicitly, its temporal flexibility and limitations, by examining how symmetrical pattern elements are combined over time. Stimuli were dynamic dot-p...

Electrophysiological studies of symmetry have found a difference wave termed the Sustained Posterior Negativity (SPN) related to the presence of symmetry. Yet the extent to which the SPN is modulated by luminance-polarity and colour content is unknown. Here we examine how luminance-polarity distribution across the symmetry axis, grouping by luminan...

Visual illusions cut across academic divides and popular interests: on the one hand, illusions provide entertainment as curious tricks of the eye; on the other hand, scientific research related to illusory phenomena has given generations of scientists and artists deep insights into the brain and principles of mind and consciousness. Numerous thinke...

The human visual system has specialised mechanisms for encoding mirror-symmetry and for detecting symmetric motion-directions for objects that loom or recede from the observers. The contribution of motion to mirror-symmetry perception has never been investigated. Here we examine symmetry detection thresholds for stationary (static and dynamic flick...

In biological vision, contextual modulation refers to the influence of a surround pattern on either the perception of, or the neural responses to, a target pattern. One studied form of contextual modulation deals with the effect of a surround texture on the perceived shape of a contour, in the context of the phenomenon known as the shape aftereffec...

Aim: Contour shape processing is selective to luminance polarity (Gheorghiu & Kingdom, 2006, Vis.Res) and surround textures inhibit the processing of contour shapes (Gheorghiu & Kingdom, 2014, Vis.Res). Energy models of texture processing suggest that textures are not selective to luminance polarity (Malik & Perona, 1990). Here we investigate the n...

The role of color in the visual perception of mirror-symmetry is controversial. Some reports support the existence of color-selective mirror-symmetry channels, others that mirror-symmetry perception is merely sensitive to color-correlations across the symmetry axis. Here we test between the two ideas. Stimuli consisted of colored Gaussian-blobs arr...

The role of color in mirror-symmetry perception is controversial: while some reports indicate that mirror-symmetry is color selective, others suggest that symmetry is merely sensitive to color correlations. Here we test between the two ideas as well as examine the role of attention in color-based symmetry perception. Stimuli consisted of a fixed nu...

Contextual modulation refers to the effect of texture placed outside of a neuron’s classical receptive field as well as the effect of surround texture on the perceptual properties of variegated regions within. In this minireview, we argue that one role of contextual modulation is to enhance the perception of contours at the expense of textures, in...

Journal of Vision August 2014, Vol.14, 63. doi:10.1167/14.10.63 http://jov.arvojournals.org/article.aspx?articleid=2143925&resultClick=1 Abstract Aim: Mirror symmetry is a ubiquitous feature in visual scenes, especially in biological objects, and is believed to be encoded by specialized visual mechanisms. Here we examine whether perceived mirror...

Two sub-systems characterize the early stages of human colour vision, the 'L-M' system that differences L and M cone signals and the 'S' system that differences S cone signals from the sum of L and M cone signals. How do they interact at suprathreshold contrast levels? To address this question we employed the method used by Kingdom et al. Journal o...

Prolonged exposure to an oriented line shifts the perceived orientation of a subsequently observed line in the opposite direction, a phenomenon known as the tilt aftereffect (TAE). Here we consider whether the TAE for line stimuli is mediated by a mechanism that integrates the local parts of the line into a single global entity prior to the site of...

Example TAE obtained with non-static, one alternating polarity adaptor and white test. (MOV)

Example TAE obtained with non-static, white adaptor/test. (MOV)

Aim: Prolonged exposure to an oriented line oppositely shifts the perceived orientation of a subsequently observed line, a phenomenon known as the tilt aftereffect (or TAE). The TAE transfers across luminance contrast polarity (Magnussen & Kurtenbach, 1979, Perception 8(5), 523-528), suggesting that either the polarities of local detectors are disc...

We investigated the role of spatial arrangement of texture elements in three psychophysical experiments on texture discrimination and texture segregation. In our stimuli, oriented Gabor elements formed an iso-oriented and a randomly oriented texture region. We manipulated (1) the orientation similarity in the iso-oriented region by adding orientati...

Contour-shape coding is color selective (Gheorghiu & Kingdom, 2007a) and surround textures inhibit the processing of contour shapes (Gheorghiu & Kingdom, 2011; Kingdom & Prins, 2009). These two findings raise two questions: (1) is texture-surround suppression of contour shape color selective, and (2) is texture-shape processing color selective? To...

Evidence that contour-shapes and texture-shapes are processed by different mechanisms included the finding that contour-shape aftereffects are reduced when the adaptation stimulus is a texture made of contours rather than a single contour. This phenomenon has been termed texture-surround suppression of contour-shape, or TSSCS. How does TSSCS operat...

Studies have shown that spatial aftereffects increase with eccentricity. Here, we demonstrate that the shape-frequency and shape-amplitude aftereffects, which describe the perceived shifts in the shape of a sinusoidal-shaped contour following adaptation to a slightly different sinusoidal-shaped contour, also increase with eccentricity. Why does thi...

Aim. Although evidence suggests that contour-shapes and texture-shapes are processed by different mechanisms, they nevertheless interact in an important way. Specifically, textures can inhibit the processing of the shapes of contours they surround; this is termed ‘texture-surround suppression of contour-shape’. How does this suppression operate and...

We investigated the role of spatial arrangement of texture elements in three psychophysical experiments on texture discrimination and texture segregation. In our stimuli, oriented Gabor elements formed an iso-oriented and a randomly oriented texture region. We manipulated (1) the orientation similarity in the iso-oriented region by adding orientati...

Radial Frequency (RF) patterns can be used to study the processing of familiar shapes, e.g. triangles and squares. Opinion is divided over whether the mechanisms that detect these shapes integrate local orientation and position information directly, or whether local orientations and positions are first combined to represent extended features, such...

The shape-frequency and shape-amplitude after-effects, or SFAE and SAAE, are shifts in the perceived shape-frequency and perceived shape-amplitude of a sinusoidal test contour following adaptation to a similar-shaped contour. These shape-effects are the shape analogs of the well-known size after-effect discovered by Blakemore and Sutton (1969), so...

We observed that two disparity — defined depth planes were perceived at a single depth when disparity alternated rapidly between two values. We investigated quantitatively how depth of the plane depended on the temporal frequency of disparity. We alternated the two disparity values between 9 Hz and 35 Hz, which is above the limit of stereomotion. W...

Aim. The shape-frequency and shape-amplitude after-effects (SFAE and SAAE) describe the shifts in perceived shape-frequency/shape-amplitude of sinusoidal-shaped contours following adaptation to contours with slightly different shape-frequencies/shape-amplitudes. When measured using pairs of adaptors/tests positioned above and below fixation, both a...

Most objects in natural scenes are suprathreshold in both color (chromatic) and luminance contrast. How salient is each dimension? We have developed a novel method employing a stimulus similar to that used by B. C. Regan and J. D. Mollon (1997) who studied the relative saliencies of the two chromatic cardinal directions. Our stimuli consist of left...

Aim. The shape-frequency and shape-amplitude after-effects, or SFAE and SAAE, are phenomena in which adaptation to a sine-wave-shaped contour causes a shift in respectively the apparent shape-frequency and shape-amplitude of a test contour in a direction away from that of the adapting stimulus. SFAEs and SAAEs are useful for probing curvature encod...

Aim: Contour shape after-effects have been used to reveal the mechanisms that process and represent curvature-defined shape. Here we use the shape-amplitude after-effect, or SAAE, to explore whether curvature detectors are tuned for the overall orientation of a curve. Methods: We measured the perceived amplitude of curved contours in the upper and...

Aim: The shape-frequency and shape-amplitude after-effects, or SFAE and SAAE, are the phenomena in which adaptation to a sine-wave-shaped contour causes a shift in respectively the apparent shape-frequency and shape-amplitude of a test contour in a direction away from that of the adapting stimulus. We have examined whether the SFAE and SAAE manifes...

Aim. How are 1st-stage inputs to the mechanisms that encode contour curvature combined? One possibility is that they are multiplied, which would increase the mechanism's selectivity to curvature (e.g. Poirier & Wilson, Vis. Res. 2006, 46, 2443–2455). Here we demonstrate a method for revealing multiplication in curvature coding. Suppose a curvature-...

Adaptation to a sinusoidally modulated contour produces a shift in the apparent shape frequency of a subsequently presented test contour, in a direction away from that of the adaptation stimulus. The phenomenon has been termed the ‘shape-frequency after-effect’ or SFAE (Kingdom & Prins, 2005, JOV, 5, 464). We describe an even larger after-effect of...

Aim: We investigated the nature of 1st-order inputs to contour-shape mechanism using the shape-frequency after-effect (SFAE), in which adaptation to a sinusoidally-modulated contour causes a shift in the apparent shape-frequency of a test contour away from that of the adapting stimulus. We measured SFAEs for adapting and test contours that differed...

Aim. The size, or luminance spatial frequency after-effect (LFAE) is the phenomenon in which adaptation to a luminance grating of given spatial frequency causes a shift in the perceived spatial frequency of a grating away from that of the adapting grating (Blakemore & Sutton, 1969, Science, 166, 245–7).The analogous shape-frequency after-effect (or...

We have investigated the global and local motion tuning properties of curvature coding mechanisms using two shape after-effects believed to be mediated by curvature-sensitive mechanisms: the shape-frequency after-effect, or SFAE, and the shape-amplitude after-effect, or SAAE. The SFAE and SAAE are the phenomena in which adaptation to a sine-wave-sh...

We have investigated the global and local motion tuning properties of curvature coding mechanisms using two shape after-effects believed to be mediated by curvature-sensitive mechanisms: the shape-frequency after-effect, or SFAE, and the shape-amplitude after-effect, or SAAE. The SFAE and SAAE are the phenomena in which adaptation to a sine-wave-sh...

We have used a curvature after-effect, or CAE, to explore whether curvature detectors are tuned for the overall orientation of a curve. CAEs were measured for half-cycle cosine-shaped contours as a function of adaptor contour orientation for a fixed test contour orientation. CAEs (i) were greatest when the adaptor and test contours had the same ori...

We have employed the shape frequency and shape-amplitude after-effects (SFAE and SAAE) to investigate: (i) whether the shapes of illusory and real curves are processed by the same or different mechanisms, and (ii) the carrier-tuning properties of illusory curvature mechanisms. The SFAE and SAAE are the phenomena in which adaptation to a sinusoidal-...

To investigate the residual stereo function of a group of 15 patients with strabismic amblyopia, by using motion-in-depth stimuli that allow discrimination of contributions from local disparity as opposed to those from local velocity mechanisms as a function of the rate of depth change. The stereo performance (percentage correct) was measured as a...

We investigated the binocular properties of curvature-encoding mechanisms using the shape-frequency and shape-amplitude after-effects (or SFAE and SAAE). The SFAE and SAAE refer to the shifts observed in, respectively, the shape-frequency and shape-amplitude of a sinusoidal test contour following adaptation to a contour with different shape-frequen...

Multiplication rather than addition of neural signals is believed to underpin a variety of sensory processes, yet the evidence for multiplication is rare. Here we provide psychophysical evidence for neural multiplication in human visual processing of shape. We show that the curvature of a contour is likely detected by a mechanism that multiplies ra...

The shape-frequency and shape-amplitude after-effects, or SFAE and SAAE, are phenomena in which adaptation to a sinusoidal-shaped contour results in a shift in, respectively, the perceived shape-frequency and perceived shape-amplitude of a test contour in a direction away from that of the adapting stimulus. Recent evidence shows that the SFAE and S...

We investigated whether contour-shape processing mechanisms are selective for color direction using the shape-frequency and shape-amplitude after-effects, or SFAE and SAAE [Gheorghiu, E. & Kingdom, F. A. A. (2006). Luminance-contrast properties of contour-shape processing revealed through the shape-frequency after-effect. Vision Research, 46(21), 3...

The shape-frequency and shape-amplitude after-effects, or SFAE and SAAE, refer respectively to the shifts observed in the perceived shape-frequency and shape-amplitude of a sinusoidal test contour following adaptation to a similar-shaped contour. As with other shape after-effects the shifts are in a direction away from that of the adapting stimulus...

We investigated the first-order inputs to contour-shape mechanisms using the shape-frequency after-effect (SFAE), in which adaptation to a sinusoidally modulated contour causes a shift in the apparent shape-frequency of a test contour in a direction away from that of the adapting stimulus [Kingdom F. A. A., & Prins N. (2005a). Different mechanisms...

We investigated the influence of temporal frequency on binocular depth perception in dynamic random-dot stereograms (DRS). We used (i) temporally correlated DRS in which a single pair of images alternated between two disparity values, and (ii) temporally uncorrelated DRS consisting of the repeated alternation of two uncorrelated image pairs each ha...

The goal of the research presented in this thesis was to investigate temporal properties of disparity processing and depth perception in human subjects, in response to dynamic stimuli. The results presented in various chapters, reporting findings about different temporal aspects of disparity processing, are based on psychophysical experiments and c...

In studies of the temporal flexibility of the stereoscopic system, it has been suggested that two different processes of binocular depth perception could be responsible for the flexibility: tolerance for interocular delays and temporal integration of correlation. None has investigated the relationship between tolerance for delays and temporal integ...