Search PubMed⌕ Search

Biomedical subjects

Branka Spehar

Publications and source records attributed to Branka Spehar.

11 recordsLinked to original sources
PubMed2005

Dynamics of collinear contrast facilitation are consistent with long-range horizontal striate transmission.

It is well established that activity of striate neurons may be either facilitated or suppressed by visual stimuli presented outside of their classical receptive field (CRF) limits. Whilst two general mechanisms have been identified as candidates for these contextual effects; those based on extra-striate feedback and long-range horizontal striate connections; the physiological data supporting these models is both ambiguous and inconsistent. Here we investigate psychophysically the phenomenon of collinear facilitation, in which contrast detection thresholds for foveally presented Gabor stimuli are reduced via concurrent presentation of remote collinear flankers. Using backward noise masking, we demonstrate that the minimum exposure duration required to induce facilitation increases monotonically with greater target-flanker separation. The inferred cortical propagation velocities of this process (0.10-0.23 ms(-1)) closely correspond with depolarising activity observed to travel across striate cortex of several species. These dynamics strongly suggest that contrast facilitation is mediated via long-range horizontal striate connections. This conclusion complements a recent suggestion that collinear induced long-range suppressive dynamics depend on extra-striate feedback.

Adult↗
PubMed2005

Dynamics of cross- and iso-surround facilitation suggest distinct mechanisms.

Psychophysical studies have found that contrast sensitivity is enhanced by spatially separated flanking stimuli that are collinear with a foveal target. Considerable uncertainty remains, however, about the facilitative effect of other surround configurations. We investigated this by systematically manipulating relative flanker position (target end-zones or side-bands) and orientation (iso-oriented or ortho-oriented targets and flankers) at multiple target-flanker separations. We also examined the effect of a temporal dimension (exposure duration) across combinations of these spatial parameters. We found facilitation in the context of all surround configurations tested, but not at all separations and exposure durations. Interestingly, although the minimum exposure required to induce facilitation (facilitative delay) increased as a function of separation for all configurations (averaged across subjects), the rate at which this occurred depended, not upon flanker position or orientation relative to the target, but the alignment of the flankers relative to each other. By transforming these slopes into striate transmission speeds we estimate that: (i) collinear flanker facilitation matches the slow conduction velocities of long-range (LR) horizontal striate connections and (ii) non-collinear, parallel flanker facilitation correlates with the much faster extra-striate feedforward/feedback connections.

Adult↗
PubMed2005

When are viewpoint costs greater for silhouettes than for shaded images?

Previous studies of object recognition have shown efficient recognition of silhouettes, suggesting that much of the information used to recognize objects resides in the outline. These studies, however, have used objects that contain many components, which provide redundant information. In this study, we examined recognition of silhouettes of less-complex objects, so that redundant information was reduced. We found that viewpoint generalization costs (the decrement of performance when recognizing nonstudied views) were greater for silhouettes than for shaded images, even when the same qualitative components were visible in the outline of both studied and nonstudied views. Thus, silhouettes do not always allow for view generalization as efficiently as do shaded images.

Form Perception↗
PubMed2004

Motion transparency promotes synchronous perceptual binding.

While identified regions of human extrastriate visual cortex are functionally specialized for processing different attributes of an object, the cognitive and neural mechanisms by which these attributes are dynamically bound into integrated percepts are still largely mysterious. Here, we report that perceptual organization influences the dynamics of binding. Specifically, the perception of motion transparency promotes the synchronous perceptual binding of colour and motion, which otherwise exhibits considerable asynchronies. In addition, we demonstrate that perceptual asynchrony can be reinstated by manipulating stereoscopic disparity or speed within the stimulus. Our findings suggest that the phenomenology of colour-motion binding parallels the known physiology of motion processing in area MT of primate visual cortex, supporting the view that the dynamics of perceptual binding is a direct reflection of the time course of the underlying neural processing.

Color Perception↗
PubMed2003

Colour and luminance selectivity of spatial and temporal interactions in orientation perception.

Previous studies have commented upon the similar phenomenology of simultaneous and successive interactions in the perception of orientation. These similarities have been taken as evidence of common mechanisms underlying the simultaneous tilt illusion (TI) and the successive tilt aftereffect (TAE). We measured the TI and TAE for four subjects for combinations of test and inducing stimuli modulated along either the same or orthogonal axes of colour space within the L+M+S, L-M colour-luminance plane. The largest TI and TAE were found when test and inducer were modulated along the same axis of colour space. The TI consistently showed greater selectivity for colour/luminance than the TAE. The results are discussed in relation to the known chromatic properties of the primate visual pathways. Specifically, we suggest that both the TI and TAE involve colour- and luminance-specific neurons in primary visual cortex as well as cue-invariant mechanisms in extrastriate cortex.

Color Perception↗
PubMed2003

When does illusory contour formation depend on contrast polarity?

Performance on a shape discrimination task was used to investigate when, and to what extent, illusory contour formation depends upon contrast polarity. It was found that shape discrimination performance was markedly worse when contrast polarity reversed within inducers, changing abruptly at the corners, than between inducers. This difference was most evident for stimulus durations from 80-320 ms and corresponded to a doubling of the critical stimulus duration for shape discrimination to reach threshold. At longer stimulus durations the performance was at ceiling level for all configurations regardless of the distribution of contrast polarity variations. These data reconcile previous findings that contrast polarity reversals within inducers disrupt illusory contrast formation at an effective stimulus duration of 120 ms but not 1 s. We conclude that the processes involved in the perception of illusory figures are sensitive to contrast polarity variations in a manner constrained by grouping and completion of the inducing elements into occluded regions.

Contrast Sensitivity↗
PubMed2003

Interactions between color and luminance in the perception of orientation.

At the early stages of visual processing in humans and other primates, chromatic signals are carried to primary visual cortex (V1) via two chromatic channels and a third achromatic (luminance) channel. The sensitivities of the channels define the three cardinal axes of color space. A long-standing though controversial hypothesis is that the cortical pathways for color and form perception maintain this early segregation with the luminance channel dominating form perception and the chromatic channels driving color perception. Here we show that a simple interaction between orientation channels (the tilt illusion) is influenced by both chromatic and luminance mechanisms. We measured the effect of oriented surround gratings upon the perceived orientation of a test grating as a function of the axes of color space along which the gratings were modulated. We found that the effect of a surround stimulus on the perceived orientation of the test is largest when both are modulated along the same axis of color space, regardless of whether that is a cardinal axis. These results show that color and orientation are intimately coupled in visual processing. Further, they suggest that the cardinal chromatic axes have no special status at the level(s) of visual cortex at which the tilt illusion is mediated.

Color Perception↗
PubMed2002

The role of contrast polarity in perceptual closure.

Perceptual closure is one of the principles by which the visual system groups disjointed contour segments belonging to a single object. Recently it has been reported that grouping based on perceptual closure operates only upon contour elements of a consistent contrast sign and is eliminated when closed contours contain contrast reversing segments [Elder, J. & Zucker, S. (1994) A measure of closure. Vision Research, 22, 3361-3369]. The present study re-examines the effect of contrast polarity reversals on perceptual closure with special emphasis on differentiating the effect of the presence of contrast polarity reversals from the effect of their placement along the bounding contour. Results show that perceptual closure exhibits a strong dependence on the distribution of contrast polarity reversals: closed configurations containing intra-figural contrast reversals can be processed rapidly when contrast sign does not change at the corners of two-dimensional shapes.

Adult↗
PubMed2002

Induction in variants of White's effect: common or separate mechanisms?

In White's display the gray target surrounded more by black than white appears darker than the target of the same physical luminance surrounded more by white than black. Several subsequent studies have shown that this effect occurs only when the luminance of the test regions lies between the minimum and maximum luminance values of the inducing stripes. With targets either lighter or darker than both inducing stripes, the direction of the effect is reversed and the effect is known as the 'inverted' White's effect. Views differ on whether the classical and inverted White's effects are mediated by common or separate underlying mechanisms. We varied the aspect ratio of the test and inducing regions in the classical and inverted White's effects. Consistent with previously reported findings, we found that the direction of the classical effect did not depend on the amount of black or white border in immediate contact with the test patch. On the other hand, perceived lightness in the inverted White's effect was affected by such variations, suggesting that induction in classical and inverted White's configurations is governed by different mechanisms. These results confirm the critical importance of the interaction between luminance and geometric relationships in induced brightness.

Color Perception↗
PubMed2002

Modal completion in the Poggendorff illusion: support for the depth-processing theory.

The Poggendorff illusion is one of the most prominent geometrical-optical illusions and has attracted enduring interest for more than a hundred years. Most modern theories explain the illusion by postulating various kinds of distortion of the "test" component of the figure by the context or the inducing component. They make no reference to the importance of processes involved in three-dimensional scene perception for understanding the illusion. We measured the strength of the Poggendorff illusion in configurations containing solid inducing surfaces as opposed to the usual parallel lines. The surface, oblique-line, and background luminances were manipulated separately to create configurations consistent with modal completion of the obliques in front of the surface. The marked decrease in the size of the illusion in conditions favoring modal completion is consistent with claims that perceived spatial layout is a major determinant of the Poggendorff illusion.

Cues↗