1,720,971 research outputs found
Collinear motion strengthens local context in visual detection
Full text linkDetection of elongated objects in the visual scene can be improved by additional elements flanking the object on the collinear axis. This is the collinear context effect (CE) and is represented in the long-range horizontal connection plexus in V1. The aim of this study was to test whether the visual collinear motion can improve the CE. In the three experiments of this study, the flank was presented with different types of motion. In particular, the collinear motion aligned with the longitudinal axis of the to-be-detected object: toward or away from it, and the orthogonal motion with a direction perpendicular to the collinear axis. Only collinear motion toward the target showed a robust and replicable empowerment of the CE. This dynamic modulation of the CE likely is implemented in the long-range horizontal connection plexus in V1, but, given that in addition it conveys the time information of motion, there must be a direct feedback in V1 from higher visual areas where motion perception is implemented, such as Middle Temporal (MT). Elongated visual objects moving along their longitudinal axis favor a propagation of activation in front of them via a network of interconnected units that allows the visual system to predict future positions of relevant items in the visual scene
Vigorous Orientation Signal Propagates Best from Collinear Motion
No full textIn the present study, the Poggendorff illusion was tested with four types of stimuli: A moving dot, a moving bar parallel to the inducing lines, a moving bar collinear to the motion trajectory, and static bars as in the classic illusion. Psychometric functions of the alignment task showed that the collinear bar, where orientation and motion trajectory matched, yielded the best alignment performance almost eliminating the illusion; the vertical bar, on the contrary, showed the worst alignment, finally the dot and the static bars led to intermediate alignments. These results demonstrate the interaction between orientation and motion trajectory that likely takes place in the primary visual cortex (V1) where these two signals might be modulated by top-down activity from higher order areas such as the middle temporal (MT). This vigorous orientation-motion trajectory interaction allows extremely accurate positional predictions of moving objects in the visual scene, in particular during occlusion
New ERP methods tailored onto single subject neuroanatomical variability
No full textGebodh and coworkers present an innovative method for strengthening the reliability with which event-related potential (ERP) components are recorded. By tailoring the experimental design to the neuroanatomical singularity of each observer, this method provides more robust and cleaner data
Lateralized readiness potential elicited by undetected visual stimuli
No full textVisual stimuli undetected by normal subjects as a result of masking procedures can nonetheless activate response preparation in motor areas and yield a motor response. An unanswered question is whether the same holds for undetected subliminal stimuli that are not responded to. To answer this question, in this study normal subjects were tested on a simple visual reaction time task with stimuli above, at, or below the psychophysical threshold while the lateralized readiness potential (LRP), i.e. an electrophysiological correlate of premotor activation in the primary motor cortex, was computed. We found a reliable LRP not only for suprathreshold stimuli but also for subthreshold stimuli to which subjects did not respond. The main thrust of this study is that it provides evidence that activation of the motor cortex occurs even with subthreshold visual stimuli and without an overt response
Lateralized readiness potential elicited by undetected visual stimuli.
No full textVisual stimuli undetected by normal subjects as a result of masking procedures can nonetheless activate response preparation in motor areas and yield a motor response. An unanswered question is whether the same holds for undetected subliminal stimuli that are not responded to. To answer this question, in this study normal subjects were tested on a simple visual reaction time task with stimuli above, at, or below the psychophysical threshold while the lateralized readiness potential (LRP), i.e. an electrophysiological correlate of premotor activation in the primary motor cortex, was computed. We found a reliable LRP not only for suprathreshold stimuli but also for subthreshold stimuli to which subjects did not respond. The main thrust of this study is that it provides evidence that activation of the motor cortex occurs even with subthreshold visual stimuli and without an overt response
Unconscious priming by illusory figures: the role of the salient region.
No full textIn this study we provide evidence that unconscious priming can be obtained as a result of the processing of the salient region (SR) of illusory figures and without that of illusory contours (ICs). We used a metacontrast masking paradigm where illusory figures were masked by real figures. In Experiment 1 we found a clear priming effect when participants were asked to discriminate between square and diamond masks preceded by congruent or incongruent illusory square or diamond primes. It is likely that metacontrast impairs the processing of ICs but not of the SR; therefore the above result strongly suggests that the priming effect was specifically related to the processing of the SR. In Experiment 2 participants were tested in the same task as in Experiment 1 with additional primes in which the inducers were presented in the same locations but their shapes were changed so as to modify the global configuration. We termed these primes High, Low, and No Salient Region (HSR, LSR, and NSR, respectively). The HSR condition replicated Experiment 1, whereas in the LSR and NSR conditions the priming effect got progressively smaller. The results of Experiment 1 were replicated with the priming effect significantly larger in the HSR than in all other conditions. It was also larger in the HSR than in LSR condition and smallest but still present in the NSR condition. Taken together, these results indicate that the unconscious processing of only the SR yields a priming effect and that a reduction of the saliency of the SR leads to a reduction of the priming effect, while its elimination does not abolish it
Electrophysiological modulation in an effort to complete illusory figures: configuration, illusory contour and closure effects
No full textFigure recognition process: From the coarse configuration standing from the background to the closure of a meaningful shape, was investigated by ERP technique. ERP components at different latencies from stimulus onset allowed to tap into the figure recognition process at discrete time-points when different cognitive operations take place. In this study, we present two experiments where the support-ratio (SR) of illusory figures was manipulated to vary continuously the recognition of geometrical figures. In the first experiment three shapes were used to vary the SR and the P1 component (80-130 ms) was modulated by the configuration-effect explained, in part for the first time, with the unbalanced physical stimulation between upper and lower visual field. In the second experiment, we used one shape and varied systematically the SR in a discrimination task. The N1 (130-180 ms) and the N2 (230-270 ms) were modulated by two effects: The Ic-effect, represented by the N1, and the closure-effect, represented by the N2, being larger when the SR was small and the discrimination more difficult with respect to large SRs and easier discrimination. These results showed that figure recognition proceeded from the coarse parsing of the visual scene (configuration-effect), through the completion of a set of illusory borders (Ic-effect) to the final assembling of a meaningful shape (closure-effect)
Does subliminal visual perception have an error-monitoring system?
No full textThere is substantial evidence that subliminal stimuli, i.e. stimuli that cannot be perceived consciously, may influence visually guided human behaviour. Two important points require further investigation, namely, the neural bases and the functional capability of unconscious stimulus processing. In this study we aimed at studying one specific aspect of the latter question, i.e. whether unconscious vision can feed into an error-monitoring system in much the same way as conscious vision. We therefore tested whether the event-related potential component known as error-related negativity, which represents the electrophysiological correlate of an error-monitoring system, is produced by unconscious errors. We found an error-related negativity not only for errors committed in a discrimination task with visible stimuli but also for those committed with subthreshold stimuli. Moreover, behavioural analysis showed post-error slowing of reaction time for correct responses following unconscious as well as conscious errors. Thus, the present results provide both electrophysiological and behavioural evidence of an error-monitoring system operating even when stimuli cannot access consciousness
Interhemispheric transfer of visual information in humans: the role of different callosal channels
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