The extrinsic/intrinsic classification of two-dimensional motion signals with barber-pole stimuli.
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The perceived direction of different barber-pole stimuli was assessed by adjusting an arrow on the screen. The terminator ratio (TR: number of terminators moving along the long side divided by the number of terminators moving along the small side) was either one or three. In this latter case, the aperture orientation was either vertical or horizontal. The grating was either in the same plane as the aperture (intrinsic condition) or behind the aperture--the frame containing the aperture had a crossed disparity relative to the grating--(extrinsic condition). A nested design with 120 observers was used for the whole study. Five grating orientations were intermingled within any session. With a terminator ratio of three, the results depend strongly on the aperture's orientation. When the rectangular aperture is horizontal, the perceived direction of an intrinsic grating is horizontal (the typical barber-pole illusion), whereas it is only slightly biased towards orthogonal one-dimensional (1D) motion signals (Vp) in the extrinsic condition. When the aperture is vertical, the perceived direction in the intrinsic condition is largely biased toward Vp, and on average it is close to Vp in the extrinsic condition. In this latter case, however, analysing the distributions of responses shows that many responses do not lie around Vp but are clustered near vertical or horizontal. This motion capture depends on the grating's orientation. With a terminator ratio of one, motion capture is present in both the extrinsic and intrinsic conditions. Moreover, a global bias toward horizontal is observed: this horizontal bias is much larger in the extrinsic condition. Altogether, these results suggest that binocular disparity alone is a weak determinant of the extrinsic/intrinsic classification of two-dimensional (2D) motion signals compared to the occlusion cues provided by unpaired regions in binocular images. Second, truly extrinsic 2D motion signals are not suppressed but rather actively compete against each other to capture the 1D motion signals. This results in a perceptual multistability which is much stronger with extrinsic signals. Finally, given the inherent multistability of barber-pole stimuli, high-level factors can alter the strength of this competition and prime any of the 2D motion signals. (+info)
Global motion processing is not tuned for binocular disparity.
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An important goal of the visual system is the segmentation of image features into objects and their backgrounds. A primary cue for this is motion: when a region shares the same pattern of motion it is segregated from its surround. Three experiments were carried out to investigate whether the segmentation of image features on the basis of motion information is facilitated by the addition of binocular disparity. Coherence thresholds were measured for the discrimination of the global direction of motion of random dot kinematograms (RDKs) in which the relative disparity of the signal and noise dots was manipulated. When the signal dots were embedded in a three dimensional cloud of noise dots, coherence thresholds were similar to those measured when signal and noise dots were both presented with zero disparity. However, when the signal dots were separated from the noise dots in depth, global motion processing was strongly facilitated. These results were considered in terms of two models, one in which global motion is processed by disparity tuned mechanisms, the other in which the discrimination of the direction of motion is mediated by an attention-based system. It was concluded that global motion processing is not tuned for binocular disparity and that the facilitation of the discrimination of direction provided by binocular disparity in certain circumstances reflects the role of an attention-based system. (+info)
A simple saliency model predicts a number of motion popout phenomena.
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Visual search for a moving target among stationary distractors is more efficient than searching for a stationary target among moving distractors, and searching for a fast target among slow distractors is more efficient than vice versa. This indicates that the ease of search for a target with a particular motion is not determined simply by the difference between target and distractor velocities. We suggest a simple model for predicting ease of search for a unique motion, based upon a quantitative measure of target saliency. Essentially, search will be easier the more the target motion deviates from the general pattern of velocities in the scene. Our model predicts a number of well-known motion search phenomena, and suggests that one control for target saliency as well as target discriminability when drawing conclusions about visual system mechanisms from search experiments. (+info)
Mechanisms of perceptual learning.
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Systematic measurements of perceptual learning were performed in the presence of external or stimulus noise. In the new external noise method (Dosher, B, & Lu, Z.-L. (1997). Investigative Ophthalmology and Visual Science, 38, S687; Lu, Z.-L., & Dosher, B. (1998). Vision Research, 38, 1183-1198), increasing amounts of external noise (white Gaussian random noise) is added to the visual stimulus in order to identify mechanisms of perceptual learning. Performance improved (threshold contrast was reduced) over days of practice on a peripheral orientation discrimination task--labelling Gabor patches as tilted slightly to the right or left. Practice improvements were largely specific to the trained quadrant of the display. Performance improved at all levels of external noise. The external noise method and perceptual template model (PTM) of the observer identifies the mechanism(s) of performance improvements as due to stimulus enhancement, external noise exclusion, or internal noise suppression. The external noise method was further extended by measuring thresholds at two threshold performance levels, allowing identification of mixtures in the PTM model. Perceptual learning over 8-10 days improved the filtering or exclusion of external noise by a factor of two or more, and improved suppression of additive internal noise--equivalent to stimulus enhancement--by 50% or more. Coupled improvements in external noise exclusion and stimulus enhancement in the PTM model may reflect channel weighting. Perceptual learning may not reflect neural plasticity at the level of basic visual channels, nor cognitive adjustments of strategy, but rather plasticity at an intermediate level of weighting inputs to decision. (+info)
Rod-cone-interactions in deuteranopic observers: models and dynamics.
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We studied the interactions between rods and L-cones in deuteranopic human observers by stimulating the photoreceptors independently. Thresholds were determined using a PEST procedure for different ratios of rod to L-cone modulation without modulating the S-cones. Modulation frequency was either 2 or 10 Hz and the retinal illuminance ranged from 4.7 to 470 td (10.9-1090 scot td). We measured at 2, 7.5 and 20 degrees retinal eccentricity. The threshold data could be described by a model based on a vector addition of responses originating in the rods and the L-cones. The relative strength of rod signals relative to the L-cone signals increased with increasing retinal eccentricity and decreasing retinal illuminance. At 20 degrees eccentricity, rod and cone signals were of about equal magnitude at retinal illuminances as high as 470 td. Temporal frequency did not have a large effect on the ratio of rod to L-cone signal strength. (+info)
Use of an early nonlinearity to measure optical and receptor resolution in the human infant.
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We measured the resolution of the optics and receptoral processes in human infants. To do so, we recorded visual-evoked potentials (VEPs) to sampled sinewave gratings, stimuli that generate highly visible distortion products at a nonlinearity early in the retina. We varied the spatial frequency content of the stimulus to determine the frequencies that can be transmitted through the optics and receptors and thereby generate distortion products. Data were collected from adults and 2- to 7-month-old infants. The results indicated that the resolution of the infants' optical/receptoral processes was within a factor of two of adults' even at the earliest ages tested. These first stages of processing, therefore, do not explain infants' poor performance in many visual tasks, or restrict the types of visual stimuli affecting more central mechanisms that undergo experience-dependent development. (+info)
Spatial facilitation by color and luminance edges: boundary, surface, and attentional factors.
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The thresholds of human observers detecting line targets improve significantly when the targets are presented in a spatial context of collinear inducing stimuli. This phenomenon is referred to as spatial facilitation, and may reflect the output of long-range interactions between cortical feature detectors. Spatial facilitation has thus far been observed with luminance-defined, achromatic stimuli on achromatic backgrounds. This study compares spatial facilitation with line targets and collinear, edge-like inducers defined by luminance contrast to spatial facilitation with targets and inducers defined by color contrast. The results of a first experiment show that achromatic inducers facilitate the detection of achromatic targets on gray and colored backgrounds, but tend to suppress the detection of chromatic targets. Chromatic inducers facilitate the detection of chromatic targets on gray and colored backgrounds, but tend to suppress the detection of achromatic targets. Chromatic spatial facilitation appears to be strongest when inducers and background are isoluminant. The results of a second experiment show that spatial facilitation with chromatic targets and inducers requires a longer exposure duration of the inducers than spatial facilitation with achromatic targets and inducers, which is already fully effective at an inducer exposure of 30 ms only. The findings point towards two separate mechanisms for spatial facilitation with collinear form stimuli: one that operates in the domain of luminance, and one that operates in the domain of color contrast. These results are consistent with neural models of boundary and surface formation which suggest that achromatic and chromatic visual cues are represented on different cortical surface representations that are capable of selectively attracting attention. Multiple copies of these achromatic and chromatic surface representations exist corresponding to different ranges of perceived depth from an observer, and each can attract attention to itself. Color and contrast differences between inducing and test stimuli, and transient responses to inducing stimuli, can cause attention to shift across these surface representations in ways that sometimes enhance and sometimes interfere with target detection. (+info)
Stochastic resonance improves signal detection in hippocampal CA1 neurons.
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Stochastic resonance (SR) is a phenomenon observed in nonlinear systems whereby the introduction of noise enhances the detection of a subthreshold signal for a certain range of noise intensity. The nonlinear threshold detection mechanism that neurons employ and the noisy environment in which they reside makes it likely that SR plays a role in neural signal detection. Although the role of SR in sensory neural systems has been studied extensively, its role in central neurons is unknown. In many central neurons, such as the hippocampal CA1 cell, very large dendritic trees are responsible for detecting neural input in a noisy environment. Attenuation due to the electrotonic length of these trees is significant, suggesting that a method other than passive summation is necessary if signals at the distal ends of the tree are to be detected. The hypothesis that SR plays an important role in the detection of distal synaptic inputs first was tested in a computer simulation of a CA1 cell and then verified with in vitro rat hippocampal slices. The results clearly showed that SR can enhance signal detection in CA1 hippocampal cells. Moreover, high levels of noise were found to equalize detection of synaptic signals received at varying positions on the dendritic tree. The amount of noise needed to evoke the effect is compared with physiological noise in slices and in vivo. (+info)