Effect of Phase Reversals on Perception of Complex Visual Stimuli
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Binocular rivalry is an intriguing visual phenomenon which over recent decades has particularly engaged the interest of scientists. This phenomenon is induced when two different images are viewed, one by each eye, with alternations occurring between perceiving one image for a few seconds, followed by the other image for a few seconds. Thus, despite the constant sensory input, there are striking changes in perception. There are several extrinsic factors (e.g. stimulus variables such as contrast, colour, motion) that are well known to influence rivalry, however, much less work has been conducted on the effect of other extrinsic factors such as non-visual stimulation. Recent studies into multimodal influences (e.g. tactile, olfactory, auditory stimulation) on binocular rivalry indicate that interactions occur between the different senses; whereby there are significant changes in how often subjects perceive either of the presented images.
The aim of the study was to further our understanding of the mechanisms involved in rivalry processing, with implications also for understanding how multiple and often conflicting stimuli from the environment are resolved in the human brain. For this purpose, the influence of auditory stimulation on binocular rivalry was explored using unilateral and bilateral auditory stimuli. The investigation was divided into two stages which differed in the frequencies of the auditory stimuli presented and the task involved subjects viewing vertical and horizontal gratings while indicating visual perception in the presence and absence of auditory stimulation.
The present results indicate that auditory stimulation influences binocular rivalry, confirming the interaction between audio and visual perceptions. More specifically, the higher frequency (3000Hz) increased visual temporal rate and the perception of horizontal gratings to a greater extent than the lower frequency (1000Hz). The results further suggest that auditory stimulation can modulate the functional status of the cerebral hemispheres, and consequently impact the perception of visual stimuli.
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The brain constantly has to interpret stimuli from a range of modalities originating from the same or different objects to create unambiguous percepts. The mechanisms of such multisensory processing have been intensely studied with respect to the time window of integration or the effect of spatial separation. However, the neural mechanisms remain elusive with respect to the role of alerting effects and multisensory integration. We addressed this issue by choosing a test paradigm where we could manipulate potentially alerting stimuli and simultaneously activating stimuli independently: We measured the temporal ventriloquism effect in European starlings by using the temporal order judgment paradigm with subjects judging the temporal order of the lighting of 2 spatially separated lights. If spatially noninformative acoustic stimuli were added to the visual stimuli the performance improved when the 2 visual stimuli were flanked by acoustic cues with a small time-offset compared to synchronous presentation. Two acoustic cues presented with asymmetric offsets showed that this effect was mainly driven by the cue trailing the second visual stimulus, while an acoustic cue leading the first visual stimulus had less effect. In contrast, 1 singleton acoustic cue prior to the first visual stimulus, without a second acoustic cue, enhanced performance. Our results support the hypothesis that the first stimulus pair with the leading sound activates alerting mechanisms and enhances neural processing, while the second stimulus pair with the trailing sound drives multisensory integration by simultaneous activation within the temporal binding window. (PsycINFO Database Record
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Visual stimuli are known to influence the perception of auditory stimuli in spatial tasks, giving rise to the ventriloquism effect. These influences can persist in the absence of visual input following a period of exposure to spatially disparate auditory and visual stimuli, a phenomenon termed the ventriloquism aftereffect. It has been speculated that the visual dominance over audition in spatial tasks is due to the superior spatial acuity of vision compared with audition. If that is the case, then the auditory system should dominate visual perception in a manner analogous to the ventriloquism effect and aftereffect if one uses a task in which the auditory system has superior acuity. To test this prediction, the interactions of visual and auditory stimuli were measured in a temporally based task in normal human subjects. The results show that the auditory system has a pronounced influence on visual temporal rate perception. This influence was independent of the spatial location, spectral bandwidth, and intensity of the auditory stimulus. The influence was, however, strongly dependent on the disparity in temporal rate between the two stimulus modalities. Further, aftereffects were observed following approximately 20 min of exposure to temporally disparate auditory and visual stimuli. These results show that the auditory system can strongly influence visual perception and are consistent with the idea that bimodal sensory conflicts are dominated by the sensory system with the greater acuity for the stimulus parameter being discriminated.
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In natural environments the speed of moving objects continually changes. To successfully interact with such objects it is useful to not only focus on the ongoing speed, but to also take speed changes into account. In the present study we were interested in whether area MT - the main motion area in the primate brain - represents not only information of the ongoing speed of a visual motion stimulus but also reflects recent stimulus speed history. We recorded from MT neurons from macaque monkeys during visual stimulation. The stimulus consisted of random dots moving into the preferred direction of the neuron. Stimulus speed changed smoothly over time (either linearly accelerating (condition 1) or decelerating (condition 2). Both conditions contained the same actual speeds, but the speed history differed. We found that the responses of most MT cells were influenced by the recent speed history of the visual stimulus. One main finding was a change in tuning width: The speed tuning was narrower when the stimulus was accelerating than when it was decelerating. This suggests that the system is less sensitive to speed changes when the stimulus smoothly decelerates. We investigated this in a psychophysical experiment with human subjects by determining detection thresholds to speed changes in a smoothly accelerating or decelerating stimulus. The results confirmed the prediction: during smooth deceleration, sensitivity was lower than during acceleration. In summary we found that the recent stimulus speed influences the speed tuning properties of MT neurons and the speed perception of human subjects. This is further evidence that the visual system does not represent snapshots of the ongoing visual stimulation but integrates information over time. This integration is beneficial for survival in an ever changing environment.
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