Gravitation and the Hemispheric Specialization of the Brain in Bilateral Symmetry Determination

We studied the influence of weightlessness on bilateral symmetry detection during prolonged space flight. Supposing that weightlessness may affect visual information processing by the right and left hemispheres in different ways, we studied this phenomenon with regard for the part of the visual field where to a stimulus was presented (the sight fixation center or the left/right half of this field). We used two types of stimuli, i.e., closed figures (polygons) and distributed figures formed by dots. There was a distinct difference between the central and noncentral presentation of stimuli under terrestrial conditions. When a stimulus was presented noncentrally (on the left or right), a manifest dominance of the horizontal axis was observed. However, there was no substantial difference while stimulating the left and right parts of the visual field. This contradicts the hypothesis on hemispheric specialization of the brain in symmetry detection. When stimuli were presented eccentrically, weightlessness did not notably influence information processing. When they were presented centrally, the predominance of the vertical axis in closed figures tended to weaken under the impact of weightlessness. However, this predominance strengthened when multicomponent figures were presented in space. The different influences of weightlessness on perceiving symmetry of stimuli of different types shows that it may be detected at various levels with different degrees of using nonvisual sensory information.     

Distributed attention and focused attention: different modalities of use

In the present experiments two different attention modalities were investigated: focused and distributed attention. In Experiment 1 manual choice reaction times to lateralized visual stimuli were measured. Visual stimuli (rectangles or squares) were presented in one of six positions along the horizontal axis. In between each position a digit 1 to 5 was permanently displayed. The subjects were instructed to switch their attention to one of the five digits and to respond as fast as possible to the stimulus target flashed either on the right or on the left of the focus of attention. The only difference in Experiment 2 was that the digit stayed on 100 msec only. The results show the importance of a visual cue (Exp. 1) for focused attention. In the absence of a permanent visual cue (Exp. 2) only distributed attention can be employed.     

Numerical Magnitude Affects Temporal Memories but Not Time Encoding

Previous research has suggested that the perception of time is influenced by concurrent magnitude information (e.g., numerical magnitude in digits, spatial distance), but the locus of the effect is unclear, with some findings suggesting that concurrent magnitudes such as space affect temporal memories and others suggesting that numerical magnitudes in digits affect the clock speed during time encoding. The current paper reports 6 experiments in which participants perceived a stimulus duration and then reproduced it. We showed that though a digit of a large magnitude (e.g., 9), relative to a digit of a small magnitude (e.g., 2), led to a longer reproduced duration when the digits were presented during the perception of the stimulus duration, such a magnitude effect disappeared when the digits were presented during the reproduction of the stimulus duration. These findings disconfirm the account that large numerical magnitudes accelerate the speed of an internal clock during time encoding, as such an account incorrectly predicts that a large numerical magnitude should lead to a shorter reproduced duration when presented during reproduction. Instead, the findings suggest that numerical magnitudes, like other magnitudes such as space, affect temporal memories when numerical magnitudes and temporal durations are concurrently held in memory. Under this account, concurrent numerical magnitudes have the chance to influence the memory of the perceived duration when they are presented during perception but not when they are presented at the reproduction stage.     

Facilitation and interference components in the Simon effect.

In four experiments we investigated whether the Simon effect consists of both facilitation and interference. In Experiment 1 subjects had to press one of two lateralized keys in response to one of two stimuli (a rectangle or a square). The stimuli were presented at fixation or to the right or left of it. The stimulus-response mappings could be corresponding (i.e., right key--rectangle or square on the right side), non-corresponding (i.e., right key--rectangle or square on the left side), or neutral (i.e., stimulus in the center). Results showed both facilitation and interference effects, but interference was three times greater than facilitation. To test whether the neutral position was favored by visual acuity, Experiments 2, 3 and 4 used slight different displays where visual acuity was the same at every stimulus position. Results indicated that the Simon effect is comprised of facilitation and interference components of the same magnitude. These findings were discussed with reference to dual-route models of the Simon effect.     

The effect of predictability on subjective duration

Events can sometimes appear longer or shorter in duration than other events of equal length. For example, in a repeated presentation of auditory or visual stimuli, an unexpected object of equivalent duration appears to last longer. Illusions of duration distortion beg an important question of time representation: when durations dilate or contract, does time in general slow down or speed up during that moment? In other words, what entailments do duration distortions have with respect to other timing judgments? We here show that when a sound or visual flicker is presented in conjunction with an unexpected visual stimulus, neither the pitch of the sound nor the frequency of the flicker is affected by the apparent duration dilation. This demonstrates that subjective time in general is not slowed; instead, duration judgments can be manipulated with no concurrent impact on other temporal judgments. Like spatial vision, time perception appears to be underpinned by a collaboration of separate neural mechanisms that usually work in concert but are separable. We further show that the duration dilation of an unexpected stimulus is not enhanced by increasing its saliency, suggesting that the effect is more closely related to prediction violation than enhanced attention. Finally, duration distortions induced by violations of progressive number sequences implicate the involvement of high-level predictability, suggesting the involvement of areas higher than primary visual cortex. We suggest that duration distortions can be understood in terms of repetition suppression, in which neural responses to repeated stimuli are diminished.     

Effects of Auditory Stimuli in the Horizontal Plane on Audiovisual Integration: An Event-Related Potential Study

This article aims to investigate whether auditory stimuli in the horizontal plane, particularly originating from behind the participant, affect audiovisual integration by using behavioral and event-related potential (ERP) measurements. In this study, visual stimuli were presented directly in front of the participants, auditory stimuli were presented at one location in an equidistant horizontal plane at the front (0°, the fixation point), right (90°), back (180°), or left (270°) of the participants, and audiovisual stimuli that include both visual stimuli and auditory stimuli originating from one of the four locations were simultaneously presented. These stimuli were presented randomly with equal probability; during this time, participants were asked to attend to the visual stimulus and respond promptly only to visual target stimuli (a unimodal visual target stimulus and the visual target of the audiovisual stimulus). A significant facilitation of reaction times and hit rates was obtained following audiovisual stimulation, irrespective of whether the auditory stimuli were presented in the front or back of the participant. However, no significant interactions were found between visual stimuli and auditory stimuli from the right or left. Two main ERP components related to audiovisual integration were found: first, auditory stimuli from the front location produced an ERP reaction over the right temporal area and right occipital area at approximately 160–200 milliseconds; second, auditory stimuli from the back produced a reaction over the parietal and occipital areas at approximately 360–400 milliseconds. Our results confirmed that audiovisual integration was also elicited, even though auditory stimuli were presented behind the participant, but no integration occurred when auditory stimuli were presented in the right or left spaces, suggesting that the human brain might be particularly sensitive to information received from behind than both sides.     

Visible Persistence of Single-Transient Random Dot Patterns: Spatial Parameters Affect the Duration of Fading Percepts

Visible persistence refers to the continuation of visual perception after the physical termination of a stimulus. We studied an extreme case of visible persistence by presenting two matrices of randomly distributed black and white pixels in succession. On the transition from one matrix to the second, the luminance polarity of all pixels within a disk- or annulus-shaped area reversed, physically creating a single second-order transient signal. This transient signal produces the percept of a disk or an annulus with an abrupt onset and a gradual offset. To study the nature of this fading percept we varied spatial parameters, such as the inner and the outer diameter of annuli (Experiment I) and the radius and eccentricity of disks (Experiment III), and measured the duration of visible persistence by having subjects adjust the synchrony of the onset of a reference stimulus with the onset or the offset of the fading percept. We validated this method by comparing two modalities of the reference stimuli (Experiment I) and by comparing the judgments of fading percepts with the judgments of stimuli that actually fade in luminance contrast (Experiment II). The results show that (i) irrespective of the reference modality, participants are able to precisely judge the on- and the offsets of the fading percepts, (ii) auditory reference stimuli lead to higher visible persistence durations than visual ones, (iii) visible persistence duration increases with the thickness of annuli and the diameter of disks, but decreases with the diameter of annuli, irrespective of stimulus eccentricity. These effects cannot be explained by stimulus energy, which suggests that more complex processing mechanisms are involved. Seemingly contradictory effects of disk and annulus diameter can be unified by assuming an abstract filling-in mechanism that speeds up with the strength of the edge signal and takes more time the larger the stimulus area is.     

Hemispheric Asymmetry in the Auditory Facilitation Effect in Dual-Stream Rapid Serial Visual Presentation Tasks

Even though auditory stimuli do not directly convey information related to visual stimuli, they often improve visual detection and identification performance. Auditory stimuli often alter visual perception depending on the reliability of the sensory input, with visual and auditory information reciprocally compensating for ambiguity in the other sensory domain. Perceptual processing is characterized by hemispheric asymmetry. While the left hemisphere is more involved in linguistic processing, the right hemisphere dominates spatial processing. In this context, we hypothesized that an auditory facilitation effect in the right visual field for the target identification task, and a similar effect would be observed in the left visual field for the target localization task. In the present study, we conducted target identification and localization tasks using a dual-stream rapid serial visual presentation. When two targets are embedded in a rapid serial visual presentation stream, the target detection or discrimination performance for the second target is generally lower than for the first target; this deficit is well known as attentional blink. Our results indicate that auditory stimuli improved target identification performance for the second target within the stream when visual stimuli were presented in the right, but not the left visual field. In contrast, auditory stimuli improved second target localization performance when visual stimuli were presented in the left visual field. An auditory facilitation effect was observed in perceptual processing, depending on the hemispheric specialization. Our results demonstrate a dissociation between the lateral visual hemifield in which a stimulus is projected and the kind of visual judgment that may benefit from the presentation of an auditory cue.     

Distortions of subjective time perception within and across senses

Background

The ability to estimate the passage of time is of fundamental importance for perceptual and cognitive processes. One experience of time is the perception of duration, which is not isomorphic to physical duration and can be distorted by a number of factors. Yet, the critical features generating these perceptual shifts in subjective duration are not understood.

Methodology/Findings

We used prospective duration judgments within and across sensory modalities to examine the effect of stimulus predictability and feature change on the perception of duration. First, we found robust distortions of perceived duration in auditory, visual and auditory-visual presentations despite the predictability of the feature changes in the stimuli. For example, a looming disc embedded in a series of steady discs led to time dilation, whereas a steady disc embedded in a series of looming discs led to time compression. Second, we addressed whether visual (auditory) inputs could alter the perception of duration of auditory (visual) inputs. When participants were presented with incongruent audio-visual stimuli, the perceived duration of auditory events could be shortened or lengthened by the presence of conflicting visual information; however, the perceived duration of visual events was seldom distorted by the presence of auditory information and was never perceived shorter than their actual durations.

Conclusions/Significance

These results support the existence of multisensory interactions in the perception of duration and, importantly, suggest that vision can modify auditory temporal perception in a pure timing task. Insofar as distortions in subjective duration can neither be accounted for by the unpredictability of an auditory, visual or auditory-visual event, we propose that it is the intrinsic features of the stimulus that critically affect subjective time distortions.     

Effect of music on motor reaction time and interhemispheric relations

Influence of music on motor reaction time (RT) was studied. Warning and triggering stimuli were presented either in the left or in the right visual fields. RT was recorded when playing classical or variety music; control sessions were not accompanied by music. Music shortened RT, and its stimulating effect was the stronger the longer were the initial RTs without music. The influence of variety music was more effective than of the classic one. RT was shortened more when the triggering stimulus was presented in the left visual field. This phenomenon is considered to be an evidence of predominant influence of music on the right cerebral hemisphere due to greater activation from emotional structures.     

Size as a parameter for tuning visual attention]

The ability of visual attention to tune to the stimulus size (when this size could not be described by spatial frequencies) was studies. Sinusoidal gratings with frequencies of 1.5, 3, and 6 cycle/degree were used as test stimuli. All these stimuli consisted of 3 periods, consequently, they had different sizes: 2 x 2, 1 x 1, and 0.5 x 0.5 degrees. Three reference stimuli had the same sizes but were constructed as a superposition of all the test frequencies. The reference stimulus of suprathreshold contrast was displayed for 400 ms to the left or to the right of a fixation point at a distance of 3 degrees. After that, the test stimulus of threshold contrast was for 100 ms displayed symmetrically to the fixation point on the other side. Subjects were instructed that the sizes of the reference and test stimuli were the same. It was found that the probability of test detection decreased with increase in the difference between the sizes of the reference and test stimuli. Since in our experiments the spatial frequency could not be used for tuning visual attention, the obtained results suggest that there are specialized mechanisms in the visual system for estimation of the general image size.     

Hemispheric asymetry of the evoked electrical activity of the cerebral cortex to letter and non-verbal stimuli]

Average evoked potentials (AEP) were recorded in practically healthy subjects to "meaningless" figures and letters, presented to different halves of the visual field. Analysis of the amplitudes of AEP late components to verbal and non-verbal stimuli reveals hemispheric asymmetry. A higher amplitude of the late positive evoked response (P300) to a "direct" stimulation both by verbal and non-verbal stimuli (in the contralateral field of vision) is recorded in the left hemisphere than in the right one. Similar stimulation of the right hemisphere does not reveal sucha difference. In the left hemisphere the P300 wave is of a clearly greater amplitude to a "direct" stimulation (contralateral visual field) than to an "indirect" one (ipsilateral visual field), regardless of the nature of the stimulus. No such difference is observed in the right hemisphere. The magnitude of the late negative wave (component N200) to non-verbal stimuli is greater in the right hemisphere both in response to "direct" and "indirect" stimulations. No intrahemispheric difference has been found in the amplitude of late evoked responses of the cerebral cortex to verbal and non-verbal stimuli.     

Size matters: non-numerical magnitude affects the spatial coding of response

It is known that small and large numbers facilitate left/right respectively (the SNARC effect). Recently, it has been proposed that numerical magnitude is just one example of a range of quantities, which have a common cognitive/neural representation. To investigate this proposition, response congruency effects were explored for stimuli which differed according to their: (a) numerical size, (b) physical size, (c) luminance, (d) conceptual size and (e) auditory intensity. In a series of experiments, groups of undergraduate participants made two-alternative forced choice discriminations with their left or right hands. There were clear interactions between magnitude and responding hand whereby right hand responses were faster for stimuli with (a) large numbers, (b) large physical size, (c) low luminance, and (d) a reference to large objects. There was no congruency effect for the auditory stimuli. The data demonstrate that the response congruency effect observed for numbers also occurs for a variety of other non-numerical visual quantities. These results support models of general magnitude representation and suggest that the association between magnitude and the left/right sides of space may not be related to culture and/or directional reading habits.     

The effect of stimulus duration and motor response in hemispatial neglect during a visual search task

Patients with hemispatial neglect exhibit a myriad of profound deficits. A hallmark of this syndrome is the patients' absence of awareness of items located in their contralesional space. Many studies, however, have demonstrated that neglect patients exhibit some level of processing of these neglected items. It has been suggested that unconscious processing of neglected information may manifest as a fast denial. This theory of fast denial proposes that neglected stimuli are detected in the same way as non-neglected stimuli, but without overt awareness. We evaluated the fast denial theory by conducting two separate visual search task experiments, each differing by the duration of stimulus presentation. Specifically, in Experiment 1 each stimulus remained in the participants' visual field until a response was made. In Experiment 2 each stimulus was presented for only a brief duration. We further evaluated the fast denial theory by comparing verbal to motor task responses in each experiment. Overall, our results from both experiments and tasks showed no evidence for the presence of implicit knowledge of neglected stimuli. Instead, patients with neglect responded the same when they neglected stimuli as when they correctly reported stimulus absence. These findings thus cast doubt on the concept of the fast denial theory and its consequent implications for non-conscious processing. Importantly, our study demonstrated that the only behavior affected was during conscious detection of ipsilesional stimuli. Specifically, patients were slower to detect stimuli in Experiment 1 compared to Experiment 2, suggesting a duration effect occurred during conscious processing of information. Additionally, reaction time and accuracy were similar when reporting verbally versus motorically. These results provide new insights into the perceptual deficits associated with neglect and further support other work that falsifies the fast denial account of non-conscious processing in hemispatial visual neglect.     

The Impact of Attention on Judgments of Frequency and Duration

Previous studies that examined human judgments of frequency and duration found an asymmetrical relationship: While frequency judgments were quite accurate and independent of stimulus duration, duration judgments were highly dependent upon stimulus frequency. A potential explanation for these findings is that the asymmetry is moderated by the amount of attention directed to the stimuli. In the current experiment, participants'' attention was manipulated in two ways: (a) intrinsically, by varying the type and arousal potential of the stimuli (names, low-arousal and high-arousal pictures), and (b) extrinsically, by varying the physical effort participants expended during the stimulus presentation (by lifting a dumbbell vs. relaxing the arm). Participants processed stimuli with varying presentation frequencies and durations and were subsequently asked to estimate the frequency and duration of each stimulus. Sensitivity to duration increased for pictures in general, especially when processed under physical effort. A large effect of stimulus frequency on duration judgments was obtained for all experimental conditions, but a similar large effect of presentation duration on frequency judgments emerged only in the conditions that could be expected to draw high amounts of attention to the stimuli: when pictures were judged under high physical effort. Almost no difference in the mutual impact of frequency and duration was obtained for low-arousal or high-arousal pictures. The mechanisms underlying the simultaneous processing of frequency and duration are discussed with respect to existing models derived from animal research. Options for the extension of such models to human processing of frequency and duration are suggested.     

Opposite Distortions in Interval Timing Perception for Visual and Auditory Stimuli with Temporal Modulations

When an object is presented visually and moves or flickers, the perception of its duration tends to be overestimated. Such an overestimation is called time dilation. Perceived time can also be distorted when a stimulus is presented aurally as an auditory flutter, but the mechanisms and their relationship to visual processing remains unclear. In the present study, we measured interval timing perception while modulating the temporal characteristics of visual and auditory stimuli, and investigated whether the interval times of visually and aurally presented objects shared a common mechanism. In these experiments, participants compared the durations of flickering or fluttering stimuli to standard stimuli, which were presented continuously. Perceived durations for auditory flutters were underestimated, while perceived durations of visual flickers were overestimated. When auditory flutters and visual flickers were presented simultaneously, these distortion effects were cancelled out. When auditory flutters were presented with a constantly presented visual stimulus, the interval timing perception of the visual stimulus was affected by the auditory flutters. These results indicate that interval timing perception is governed by independent mechanisms for visual and auditory processing, and that there are some interactions between the two processing systems.     

Human parieto-occipital visual cortex: lack of retinotopy and foveal magnification.

We studied visual representation in the parietal cortex by recording whole-scalp neuromagnetic responses to luminance stimuli of varying eccentricities. The stimuli were semicircles (5.5 degrees in radius) presented at horizontal eccentricities from 0 degree to 16 degrees, separately in the right and left hemifields. All stimuli evoked responses in the contralateral occipital and medial parietal areas. The waveforms and distributions of the occipital responses varied with stimulus side (left, right) and eccentricity, whereas the parietal responses were remarkably similar to all stimuli. The equivalent sources of the parietal signals clustered within 1 cm3 in the medial parieto-occipital sulcus and did not differ significantly between the stimuli. The strength of the parietal activation remained practically constant with increasing stimulus eccentricity, suggesting that the visual areas in the parieto-occipital sulcus lack the enhanced foveal representation typical of most other visual areas. This result strengthens our previous suggestion that the medial parieto-occipital sulcus is the human homologue of the monkey V6 complex, characterized by, for example, lack of retinotopy and the absence of relative foveal magnification.     

Reaction time and interhemispheric interaction

The study was made on healthy adult subjects. The reaction time of the hand (RT) was measured under two conditions: 1) the choice of reaction (right or left hand) is determined by the nature of the warning stimulus; 2) decision on the choice is taken, depending on the second, trigger stimulus. Stimuli are presented at random sequences to different visual fields. The reaction time to the visual signal presented to the visual field ipsilateral to the hand is significantly shorter (by 15 to 26 msec) than to the stimulus in the contralateral visual field. In a simple motor reaction, when no discrimination of trigger stimulus and the decision on the choice of reaction is required, a hemispheric asymmetry of reaction time is manifested: the left hemisphere only responds differently to direct visual stimulation and to that mediated through the contralateral hemisphere.     

Attention samples stimuli rhythmically

Overt exploration or sampling behaviors, such as whisking, sniffing, and saccadic eye movements, are often characterized by a rhythm. In addition, the electrophysiologically recorded theta or alpha phase predicts global detection performance. These two observations raise the intriguing possibility that covert selective attention samples from multiple stimuli rhythmically. To investigate this possibility, we measured change detection performance on two simultaneously presented stimuli, after resetting attention to one of them. After a reset flash at one stimulus location, detection performance fluctuated rhythmically. When the flash was presented in the right visual field, a 4 Hz rhythm was directly visible in the time courses of behavioral performance at both stimulus locations, and the two rhythms were in antiphase. A left visual field flash exerted only partial reset on performance and induced rhythmic fluctuation at higher frequencies (6-10 Hz). These findings show that selective attention samples multiple stimuli rhythmically, and they position spatial attention within the family of exploration behaviors.     

Asymmetry of the latent periods of saccades in human depending on visual space complicity

Saccadic latencies of visually-guided saccades of 10 right-handed subjects with right-leading eyes were studied. Stimulation paradigm was spatially bidimentional, and stimuli were shown along horizontal and vertical meridians. Three traditional single step GAP - NO DELAY - OVERLAP temporal paradigms were used. In the first experiment, each paradigm was applied separately (simple visual space). In the second experiment, all the three paradigms were varied pseudo-random order and equiprobably, which complicated the time parameters of visual stimulation (complicated visual space). Asymmetry of visually-guided saccades along the vertical and horizontal meridians was revealed. The character of this asymmetry varied between subjects. MANOVA showed that the factor of visual space complicity (simple or complicated visual space) affected the latent period of saccades to a greater extent than the factor of stimulus lateralization (stimulus presentation in the left/right or upper/lower visual hemifields).