On the mechanism for scale invariance in orientation-defined textures.
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Texture perception is generally found to be scale invariant, that is, the perceived properties of textures do not change with viewing distance. Previously, Kingdom, F. A. A., Keeble, D. R. T., & Moulden, B. (Vision Research, 1995, 35, 79-91) showed that the orientation modulation function (OMF), which describes sensitivity to sinusoidal modulations of micropattern orientation as a function of modulation spatial frequency, was scale invariant--peak sensitivity occurred at a modulation spatial frequency which was invariant with viewing distance when modulation frequency was plotted in object units, e.g. cycles cm-1. We have attempted to determine the mechanism underlying the scale invariant properties of the OMF. We first confirmed that the OMF was scale invariant using Gabor-micropattern textures. We then measured OMFs at a number of viewing distances, while holding constant various stimulus features in the retinal image. The question was which stimulus feature(s) disrupted scale invariance when manipulated in this way. We found that the scale (size) of the micropatterns was a critical factor and that the most important scale parameter was the micropatterns' carrier spatial frequency. Micropattern length and density were shown to have a small influence on scale invariance, while micropattern width had no influence at all. These results are consistent with the idea that scale invariance in orientation-defined textures is a consequence of 'second-stage' texture-sensitive mechanisms being tied in spatial scale selectivity to their 'first-stage' luminance-contrast-sensitive inputs. (+info)
Stereoscopic depth but not shape perception from second-order stimuli.
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Depth can be seen using either linear (first-order) or non-linear (second-order) stereo micropatterns when, in the latter, contrast envelopes contain the disparity information. We examined whether a second-order mechanism can contribute to the perception of 3-D surface shape. Using a variety of different stimulus types, we show that for each, shape is easy to see with linear stimuli. Over a wide range of parameters however, none of our observers perceived shape, however faintly, from the non-linear stimuli. To explore why these elements failed, we simplified our stimulus to a step-edge in depth and measured performance while varying the number of elements. We show how performance declined when more than two non-linear elements were used. We discuss reasons for the limitation found for non-matching elements, including a dissociation for stereopsis between seeing surface shape and depth. (+info)
Simultaneous color constancy: how surface color perception varies with the illuminant.
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In two experiments simultaneous color constancy was measured using simulations of illuminated surfaces presented on a CRT monitor. Subjects saw two identical Mondrians side-by-side: one Mondrian rendered under a standard illuminant, the other rendered under one of several test illuminants. The matching field was adjusted under the test illuminant so that it (a) had the same hue, saturation, and brightness (appearance match) or (b) looked as if it were cut from the same piece of paper (surface match) as a test surface under the standard illuminant. Matches were set for three different surface collections. The surface matches showed a much higher level of constancy than the appearance matches. The adjustment in the surface matches was nearly complete in the L and M cone data, and deviations from perfect constancy were mainly due to failures in the adjustment of the S cone signals. Besides this difference in amount of adjustment, the appearance and surface matches showed two major similarities. First, both types of matches were well described by simple parametric models. In particular, a model based on the notion of von Kries adjustment provided a good, although not perfect, description of the data. Second, for both types of matches the illuminant adjustment was largely independent of the surface collection in the image. The two types of matches thus differed only quantitatively, there was no qualitative difference between them. (+info)
Visual attention modifies spectral sensitivity of nystagmic eye movements.
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If we look out of the window of a travelling train our eyes move rapidly back and forth (saccadic movement). With no attention to individual objects, gaze velocity is low but nystagmic frequency is high (stare nystagmus). If we are interested in individual objects, the angular velocity of gaze is high and the nystagmic frequency low (look nystagmus) (Ter Braak, J.W.G. (1936). Untersuchungen ueber optokinetischen Nystagmus. Archives Neerlandaises de Physiologie de L'homme et des Animaux, 21, 309-376). We show that the spectral sensitivities of the two types of nystagmus differ and that the short-wavelength-sensitive cones significantly contribute only to look nystagmus. (+info)
Peripheral vision and oculomotor control during visual search.
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The present study concerns the dynamics of multiple fixation search. We tried to gain insight into: (1) how the peripheral and foveal stimulus affect fixation duration; and (2) how fixation duration affects the peripheral target selection for saccades. We replicated the non-corroborating results of Luria and Strauss (1975) ('Eye movements during search for coded and uncoded targets', Perception and Psychophysics 17, 303-308) (saccades were selective), and Zelinsky (1996) (Using eye movements to assess the selectivity of search movements. Vision research 36(14), 2177-2187) (saccades were not selective), by manipulating the critical features for peripheral selection and discrimination separately. We found search to be more selective and efficient when the selection task was easy or when fixations were long-lasting. Remarkably, subjects did not increase their fixation durations when the peripheral selection task was more difficult. Only the discrimination task affected the fixation duration. This implies that the time available for peripheral target selection is determined mainly by the discrimination task. The results of the present experiment suggest that, besides the difficulty of the peripheral selection task, fixation duration is an important factor determining the selection of potential targets for eye movements. (+info)
Morphology of transient VEPs to luminance and chromatic pattern onset and offset.
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Characteristics of the visual evoked response to chromatic and luminance-modulated stimuli reflect the activity of underlying neural mechanisms, although selective neuronal activity depends upon stimulus parameters. In the present study, the behaviour of the transient visual evoked response to low spatial and temporal frequency chromatic stimuli is investigated at a range of colour luminance ratios. Our results show that the response to pattern-offset may be used in addition to the pattern-onset response as part of the signature of the evoked response to luminance-modulated or isoluminant chromatic stimuli. (+info)
Texture segregation shows only a very small lower-hemifield advantage.
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Possible hemifield differences in texture segregation were investigated for both simple (Fourier, linear) and complex (non-Fourier, second-order) texture channels. There was only a very small lower-field advantage for texture segregation, consistent with the notion that the major processing in texture segregation is quite low level, perhaps V1. Complex-channel tasks do not show larger hemifield asymmetries than do simple-channel tasks, which suggests that the processes in complex texture channels are not higher level than those in simple. (+info)
Stereoscopic occlusion and the aperture problem for motion: a new solution.
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Recent work has shown that the ability of moving contour terminators to determine the perceived motion of untextured contours is strongly constrained by whether contour terminators are classified as intrinsic (belonging to a moving contour) or extrinsic (belonging to a surface occluding a moving contour). It has also been demonstrated that stereopsis can play a decisive role in this classification. Specifically, Shimojo, Silverman and Nakayama (1989, Vision Research 29, 619-626) argued that the efficacy of stereopsis in classifying moving contour terminators as intrinsic or extrinsic stemmed from the relative depth relationships specified by binocular disparity. Here, evidence is presented which demonstrates that the visual system relies on the presence of unpaired contour terminators to classify stereoscopic contour terminators as extrinsic. The author shows that the tendency to perceive untextured contours translating in a single rectangular aperture in a direction parallel to the longer axis of the aperture (the barberpole illusion) was not abolished by stereoscopic depth differences when the contour terminators were interocularly paired. However, the illusion was abolished when the contours terminators along the longer axis of the aperture were interocularly unpaired. Moreover, contours translated within a square aperture revealed a systematic shift towards the direction of motion signaled by the binocularly paired contour terminators along the horizontal edges of the aperture. These results demonstrate that the classification of stereoscopic contour terminators along an extrinsic-intrinsic dimension results from the presence of local, unpaired contour terminators rather than the relative depth or disparity differences per se, or via the global integration of contour terminators across multiple apertures when multiple apertures are present. (+info)