Bilateral Stimuli Research Articles

Naso-Temporal Asymmetry for Signals Invisible to the Retinotectal Pathway

Monocular viewing conditions show an asymmetry between stimuli presented in the temporal and nasal visual fields in their efficiency for automatically triggering eye saccades and grasping attention. For instance, observers free to make a saccade to one of two stimuli presented together orient preferentially to the temporal stimulus. Such naso-temporal asymmetry (NTA) has been assumed to reflect the asymmetry in the retinotectal pathway to the superior colliculus. We tested this hypothesis using S cone stimuli, which are invisible to the magnocellular and retinotectal pathways. The observed NTA in choice saccades to bilateral stimuli was no less present for S cone stimuli than for luminance stimuli. Additionally, the amplitude of the NTA can be enhanced when S cone signals are added to luminance signals. These results suggest that behavioral NTA in humans is not diagnostic of retinotectal mediation. Furthermore, we found no asymmetries in latency, suggesting that the NTA in saccade choice does not originate simply from a bottom-up asymmetry in any low level visual pathways.

Interlateral asymmetry in the time course of the effect of a peripheral prime stimulus

Evidence exists that both right and left hemisphere attentional mechanisms are mobilized when attention is directed to the right visual hemifield and only right hemisphere attentional mechanisms are mobilized when attention is directed to the left visual hemifield. This arrangement might lead to a rightward bias of automatic attention. The hypothesis was investigated by testing male volunteers, wherein a “location discrimination” reaction time task (Experiments 1 and 3) and a “location and shape discrimination” reaction time task (Experiments 2 and 4) were used. Unilateral (Experiments 1 and 2) and unilateral or bilateral (Experiments 3 and 4) peripheral visual prime stimuli were used to control attention. Reaction time to a small visual target stimulus in the same location or in the horizontally opposite location was evaluated. Stimulus onset asynchronies (SOAs) were 34, 50, 67, 83 and 100 ms. An important prime stimulus attentional effect was observed as early as 50 ms in the four experiments. In Experiments 2, 3 and 4, this effect was larger when the prime stimulus occurred in the right hemifield than when it occurred in the left hemifield for SOA 100 ms. In Experiment 4, when the prime stimulus occurred simultaneously in both hemifields, reaction time was faster for the right hemifield and for SOA 100 ms. These results indicate that automatic attention tends to favor the right side of space, particularly when identification of the target stimulus shape is required.

Screening Effect Diverts the Swimming Directions from Diaphototactic to Positive Phototactic in a Disk-shaped Green Flagellate Mesostigma viride¶

We found diaphototactic behavior (i.e. the cells swim perpendicularly with respect to the incident light) in a strain with colorless eyespot of a unicellular disk-shaped green flagellate Mesostigma viride. Lacking pigments completely in the eyespot, the screening effect in this strain was due only to the central part of the chloroplast whose cross section was thin. The diaphototaxis was most obvious when unilateral green stimulus light (520–580 nm) was given, whereas positive phototaxis appeared when given blue light (430–490 nm). The choice between diaphototaxis and (ordinary) phototaxis depended entirely on the transmission (%T) of the cell body against each wavelength of the stimulus: the green light penetrated well (%T > 90%), whereas the blue light was considerably shaded by the chloroplast (50% < %T < 70%). The fraction of positive phototactically behaving cells against each wavelength was in proportion to the front-to-back contrast value obtained at each individual wavelength. The fraction of diaphototaxis was inversely proportional to it. In addition, bilateral stimulus irradiations to wild-type cell with colored eyespot provided useful information about the principle of the diaphototactic steering.

Interhemispheric interactions and redundancy gain: tests of an interhemispheric inhibition hypothesis

In simple reaction time (RT) tasks, responses are faster when stimuli are presented to both the left and right visual hemifields than when a stimulus is presented to a single hemifield. Paradoxically, this redundancy gain with bilateral stimuli is enhanced in split-brain individuals relative to normals. This article reports three experiments testing an account of that enhancement in which normals' responses to bilateral stimuli are slowed by interhemispheric inhibition. In simple RT tasks, normal participants responded bimanually to left, right, or bilateral visual stimuli. In choice RT tasks, they responded to each stimulus with one hand, responding bimanually only when both stimuli were presented. Measurements of response forcefulness (Experiment 1) and electroencephalographic activity (Experiments 2 and 3) showed no evidence of the correlation patterns predicted by the hypothesis of interhemispheric inhibition. The results suggest that such inhibition is unlikely to be the explanation for enhanced redundancy gain in split-brain individuals.

Electrophysiological correlates of attention-spreading in visual grouping

This study examined event-related potentials in a sustained-attention task that involved bilateral stimulus arrays, which were connected or unconnected by a line. Spatial attention was reflected by a large amplitude of posterior event-related potentials at the hemisphere contralateral, rather than ipsilateral, to the attended hemi-field. The earliest attention effect (P1, 100-160 ms poststimulus) was not affected by connectedness. The subsequent attention effect (N2pc, 190-300 ms) was observed when the target feature appeared in the attended hemi-field, whereas this effect was not seen in the early phase (190-250 ms) in the connected condition. The results indicate that the lateralized event-related potential reflects transient attention-spreading in association with perceptual grouping.

Effects of stimulus-driven synchronization on sensory perception

BackgroundA subject's ability to differentiate the loci of two points on the skin depends on the stimulus-evoked pericolumnar lateral inhibitory interactions which increase the spatial contrast between regions of SI cortex that are activated by stimulus-evoked afferent drive. Nevertheless, there is very little known about the impact that neuronal interactions – such as those evoked by mechanical skin stimuli that project to and coordinate synchronized activity in adjacent and/or near-adjacent cortical columns – could have on sensory information processing.MethodsThe temporal order judgment (TOJ) and temporal discriminative threshold (TDT) of 20 healthy adult subjects were assessed both in the absence and presence of concurrent conditions of tactile stimulation. These measures were obtained across a number of paired sites – two unilateral and one bilateral – and several conditions of adapting stimuli were delivered both prior to and concurrently with the TOJ and TDT tasks. The pairs of conditioning stimuli were synchronized and periodic, synchronized and non-periodic, or asynchronous and non-periodic.ResultsIn the absence of any additional stimuli, TOJ and TDT results obtained from the study were comparable across a number of pairs of stimulus sites – unilateral as well as bilateral. In the presence of a 25 Hz conditioning sinusoidal stimulus which was delivered both before, concurrently and after the TOJ task, there was a significant change in the TOJ measured when the two stimuli were located unilaterally on digits 2 and 3. However, in the presence of the same 25 Hz conditioning stimulus, the TOJ obtained when the two stimuli were delivered bilaterally was not impacted. TDT measures were not impacted to the same degree by the concurrent stimuli that were delivered to the unilateral or bilateral stimulus sites. This led to the speculation that the impact that the conditioning stimuli – which were sinusoidal, periodic and synchronous – had on TOJ measures was due to the synchronization of adjacent cortical ensembles in somatosensory cortex, and that the synchronization of these cortical ensembles could have been responsible for the degradation in temporal order judgment. In order to more directly test this hypothesis, the synchronized 25 Hz conditioning stimuli that were delivered during the initial TOJ test were replaced with asynchronous non-periodic 25 Hz conditioning stimuli, and these asynchronous conditioning stimuli did not impact the TOJ measures.ConclusionThe results give support to the theory that synchronization of cortical ensembles in SI could significantly impact the topography of temporal perception, and these findings are speculated to be linked mechanistically to previously reported co-activation plasticity studies. Additionally, the impact that such synchronizing conditioning stimuli have on TOJ – which can be measured relatively quickly – could provide an effective means to assess the functional connectivity of neurologically compromised subject populations.

Fundamental properties of the N2pc as an index of spatial attention: Effects of masking.

Masking is an important tool in many paradigms used to study the cognitive architecture. The N2pc is an electrophysiological event-related potential (ERP) that has been used as a tool to study the deployment of visual spatial attention. The aim of this paper was to study the effects of masking on the N2pc. Two stimuli were presented on the screen, one to left and one to right of fixation, and subjects reported the identity of one of them. The targets could be discriminated both by their category (letters vs. digit) and by their colour (pink vs. green). Backward masking was produced by presenting a second pair of bilateral stimuli after the offset of the first pair. The second pair of stimuli consisted of characters of the same colour and category as in the first pair. Forward masking was produced by using the very same stimuli as in the backward masking condition, but by instructing subjects to report the second stimulus. The forward mask trials had longer response times compared to no-mask trials, and backward mask trials had even longer response times, and also a higher error rate. Although the different masking procedures lead to clear behavioural effects, the N2pc was not affected, suggesting that the deployment of visual spatial attention, per se, was not affected by pattern masking. A sustained posterior contralateral negativity (SPCN) following the N2pc was also found (300 ms post-target, and beyond), and the amplitude of the SPCN was strongly modulated by the number of presented stimuli and the duration of the SPCN was positively correlated with RT in the behavioural task. We hypothesize that the SPCN reflects neural activity associated with the passage of information through visual short-term memory.

Increases in Alpha Oscillatory Power Reflect an Active Retinotopic Mechanism for Distracter Suppression During Sustained Visuospatial Attention

Human electrophysiological (EEG) studies have demonstrated the involvement of alpha band (8- to 14-Hz) oscillations in the anticipatory biasing of attention. In the context of visual spatial attention within bilateral stimulus arrays, alpha has exhibited greater amplitude over parietooccipital cortex contralateral to the hemifield required to be ignored, relative to that measured when the same hemifield is to be attended. Whether this differential effect arises solely from alpha desynchronization (decreases) over the "attending" hemisphere, from synchronization (increases) over the "ignoring" hemisphere, or both, has not been fully resolved. This is because of the confounding effect of externally evoked desynchronization that occurs involuntarily in response to visual cues. Here, bilateral flickering stimuli were presented simultaneously and continuously over entire trial blocks, such that externally evoked alpha desynchronization is equated in precue baseline and postcue intervals. Equivalent random letter sequences were superimposed on the left and right flicker stimuli. Subjects were required to count the presentations of the target letter "X" at the cued hemifield over an 8-s period and ignore the sequence in the opposite hemifield. The data showed significant increases in alpha power over the ignoring hemisphere relative to the precue baseline, observable for both cue directions. A strong attentional bias necessitated by the subjective difficulty in gating the distracting letter sequence is reflected in a large effect size of 2.1 (eta2 = 0.82), measured from the attention x hemisphere interaction. This strongly suggests that alpha synchronization reflects an active attentional suppression mechanism, rather than a passive one reflecting "idling" circuits.

Electroconvulsive Therapy Stimulus Dose Expressed as Volume of Seizure Foci

Electrical stimulus dose is typically stated as charge in millicoulombs (mC), representing the number of electrons. However, by overlooking the voltage threshold for neuronal depolarization, charge alone is inappropriate. Any charge can accumulate from subthreshold voltage over time. Stimulus energy represents heat content; without a delivery rate, it is not related to depolarization voltage. The objective was to formulate stimulus dose in accordance with seizure induction physiology. Stimulus dose was expressed as volume of generated seizure foci, a variable dependent on current and charge. A second more detailed model considers brain voltages as related to clinical deep brain stimulation. For the constant current stimuli characteristic of widely available American stimulators, stimulus dose is proportional to current cubed multiplied by charge. This relationship corresponds to the 3 dimensions of a voltage field in space, where the volume above a certain voltage increases with the cube of the voltage radius. As a consequence, any particular charge at 0.9 A (ampere) current has a seizure foci dose approximately 50% higher than the same charge at 0.8 A. The modeling offers an explanation for published measurements comparing instruments of maximum current 0.8 and 0.9 A. The modeling also suggests compensation approaches to be applied to the age-based bilateral stimulus dosing method. An electrical dosing strategy based on "half-age" (2.5 mC/y) is reasonable for 0.9-A stimulation but a "three-quarters age" (3.7 mC/y) is more appropriate for 0.8-A stimulation. The present results can allow doses and seizure thresholds to be compared between different currents, as when substituting one electroconvulsive therapy instrument for another.

Neural consequences of somatosensory extinction

There are currently two main interpretations proposing mechanisms underlying tactile extinction: sensory and attention deficit hypotheses. Kinsbourne proposed an opponent processor model to support the attention deficit hypothesis. He insisted that bilateral hemispheres interact reciprocally through contralaterally oriented vectors, and in patients presenting extinction, balance is impaired, causing inattention. From Kinsbourne's point of view, extinction is not caused by sensory disturbance but inattention, therefore even in extinction patients, simultaneous bilateral stimuli should reach the bilateral primary sensory cortices (SI). Using functional magnetic resonance imaging (fMRI), tactile stimuli were administered to both hands of healthy subjects as well as a tactile extinction patient. The patient with tactile extinction extinguished right palm stimuli following simultaneous palm stimulation. During the fMRI study, we gave tactile stimuli to the right palm, the left palm, and simultaneously to both palms. In normal subjects, simultaneous bilateral stimuli activated the bilateral SI and bilateral secondary sensory cortices (SII). In the patient with right tactile extinction, simultaneous bilateral stimuli activated the bilateral SI along with the bilateral SII and right superior parietal lobule. Our study suggests that activation of SI is insufficient to engender an awareness of sensory stimuli. From the view point of Kinsbourne, stimulus driven activity in one hemisphere suppresses activity in the other hemisphere via callosal connections. Our results support the notion that an undamaged superior parietal lobule in the patient with tactile extinction suppresses the damaged parietal lobe function and causes extinction.

Now you see it, now you don't: Variable hemineglect in a commissurotomized man

We describe the case of a callosotomized man, D.D.V., who shows unusual neglect of stimuli in the left visual field (LVF). This is manifest in simple reaction time (RT) to stimuli flashed in the LVF and in judging whether pairs of filled circles in the LVF are of the same or different color. It may reflect strong left-hemispheric control and consequent attention restricted to the right side of space. It is not evident in simple RT when there are continuous markers in the visual fields to indicate the locations of the stimuli. In this condition, his RTs are actually faster to LVF than to right visual field (RVF) stimuli, suggesting a switch to right-hemispheric control that eliminates the hemineglect. Neglect is also not evident when D.D.V. responds by pointing to or touching the locations of the stimuli, perhaps because these responses are controlled by the dorsal rather than the ventral visual system. Despite his atypical manifestations of hemineglect, D.D.V. showed evidence of functional disconnection typical of split-brained subjects, including prolonged crossed–uncrossed different in simple reaction time, inability to match colors between visual fields, and enhanced redundancy gain in simple RT to bilateral stimuli even when the stimulus in the LVF was neglected.

Homotopy and heterotopy and the bilateral field advantage in the Dimond paradigm

Pairs of homotopic and heterotopic visual bilateral stimuli and pairs of unilateral visual stimuli were presented to 12 normal right-handed university students requiring a key press if they were of the same form. As predicted from the known histology of the corpus callosum (massive preponderance of homotopic fibers), homotopic presentations yielded significantly faster reaction times than heterotopic stimulations. Bilateral pairs of stimuli were also advantaged in comparison with unilateral trials, replicating Sereno and Kosslyn [Sereno, A. B., & Kosslyn, S. M. (1991). Discrimination within and between hemifields: a new constraint on theories of attention. Neuropsychologia, 29, 659–675]. Moreover, certain attentional processes have never been investigated in the Dimond paradigm and this study provides evidence to the effect that discriminative reaction times to stimulus pairs are strongly influenced by their proximity to the fixation point. In similar previous experiments, the homotopy/heterotopy observation and the bilateral field advantage may have been distorted by that particular confound, as well as several others.

Visual spatial attention tracking using high-density SSVEP data for independent brain-computer communication

The steady-state visual evoked potential (SSVEP) has been employed successfully in brain-computer interface (BCI) research, but its use in a design entirely independent of eye movement has until recently not been reported. This paper presents strong evidence suggesting that the SSVEP can be used as an electrophysiological correlate of visual spatial attention that may be harnessed on its own or in conjunction with other correlates to achieve control in an independent BCI. In this study, 64-channel electroencephalography data were recorded from subjects who covertly attended to one of two bilateral flicker stimuli with superimposed letter sequences. Offline classification of left/right spatial attention was attempted by extracting SSVEPs at optimal channels selected for each subject on the basis of the scalp distribution of SSVEP magnitudes. This yielded an average accuracy of approximately 71% across ten subjects (highest 86%) comparable across two separate cases in which flicker frequencies were set within and outside the alpha range respectively. Further, combining SSVEP features with attention-dependent parieto-occipital alpha band modulations resulted in an average accuracy of 79% (highest 87%).

Local functional state differences between rat cortical columns

Surface evoked potentials (SEPs) during auditory clicks and whisker twitches are usually larger during quiet sleep (QS) over waking and REM sleep. However, SEP amplitudes from single trials fluctuate periodically between high and low values regardless of sleep–wake cycle. To test the hypothesis that state-independent fluctuations represent local functional sleep-like states of individual cortical columns, we examined single trial SEP amplitudes from multiple cortical locations across sleep–wake cycles. Bilateral stimuli produced SEP amplitude fluctuations in each hemisphere that usually covaried ( r = 0.4), but with frequent hemispheric differences. Two neighboring whiskers, twitched simultaneously on the same side, produced highly correlated SEPs in neighboring cortical columns ( r = 0.9) with frequent divergences. We found 50% more disparity during QS over waking, indicating that the differences did not result from recording noise or stimulus inconsistency. Local SEP fluctuations also followed local differences in the delta wave signal during QS ( r = 0.4), suggesting that similar mechanisms may modulate the SEP. The duration of the localized sleep-like (high SEP amplitude) state was dependent on the duration of prior wake-like (low SEP amplitude) state ( r = 0.5), suggesting a use dependence of prior functional state period. Since SEP indicators fluctuated independently from whole animal sleep state, and were frequently different between hemispheres and nearby cortical columns, these data support the theory that sleep-like functional states may be localized to brain regions at least as small as cortical columns.

Response of SI cortex to ipsilateral, contralateral and bilateral flutter stimulation in the cat

BackgroundWhile SII cortex is considered to be the first cortical stage of the pathway that integrates tactile information arising from both sides of the body, SI cortex is generally not considered as a region in which neuronal response is modulated by simultaneous stimulation of bilateral (and mirror-image) skin sites.ResultsOptical intrinsic signal imaging was used to evaluate the response of SI and SII in the same hemisphere to 25 Hz sinusoidal vertical skin displacement stimulation ("skin flutter") applied contralaterally, ipsilaterally, and bilaterally (simultaneously) to the central pads of the forepaws. A localized increase in absorbance in both SI and SII occurred in response to both contralateral and bilateral flutter stimulation. Ipsilateral flutter stimulation evoked a localized increase in absorbance in SII, but little or no change in SI absorbance. In the forepaw representational region of SI, however, bilateral stimulation of the central pads evoked a response substantially smaller (approximately 30–35% smaller) than the response to flutter stimulation of the contralateral central pad.ConclusionThe finding that the response of SI cortex to bilateral central pad flutter stimulation is substantially smaller than the response evoked by a contralateral flutter stimulus, together with the recently published observation that a region located posteriorly in SII responds with a substantially larger response to a bilateral flutter stimulus than the response evoked from the contralateral central pad, lead us to propose that the SI activity evoked by contralateral skin stimulation is suppressed/inhibited (via corticocortical connections between SII and SI in the same hemisphere) by the activity a simultaneous ipsilateral skin stimulus evokes in posterior SII.

Nonlinear interactions in the visual system: a steady-state VEP study

To study the interaction between different receptive fields in human visual processing, the present study investigated the steady-state evoked potentials (ss-VEPs) by two light emitting diodes (LEDs) lit at the different temporal frequencies, which were presented separately to each hemivisual field of the subjects. Two LEDs set in front of the subject were energized at different temporal frequencies of combinations of 7, 9, 11, and 13 Hz. The subject was asked to look at a fixation point at the midpoint. Nineteen EEGs were recorded and the data were averaged 80–100 times triggered at the peak of a light intensity for each quartile range of phase lag. The phase dependency was studied by comparing steady-state visual evoked potentials (ss-VEPs) and also by using higher-order spectra (HOS). The interactions between background EEGs and ss-VEPs were also investigated. The power spectra of ss-VEPs exhibited peaks at the frequency other than those of stimuli. HOS analysis showed that nonlinear phase coupling between the peak frequencies of ss-VEPs were partially detected. Frequencies of the visual stimuli were suspected to reflect onto the bilateral visual area. The changes in the ss-VEPs triggered by the peak of stimuli were affected by the phase lag between bilateral stimuli. Our results are suggestive of some nonlinear interactions in the visual processing over the different areas.

Ageing and Visual Orienting in Response to Complex Spatial Cues

AbstractAn experiment is reported in which young and older adults performed a visual orienting task and a cue discrimination task. In the visual orienting task symbolic cues (letters) provided predictive information concerning the likely location of a target stimulus. Two letters, drawn from a pool of four ‘valid’ and four ‘invalid’ letters, were presented bilaterally, at either central or peripheral locations. In the cue discrimination task participants were presented with the same bilateral letter stimuli, and indicated whether these represented a valid letter on the right with an invalid letter on the left, or vice versa. In the visual orienting task young adults shifted attention appropriately in response to the letter cues, but older adults failed to do this. Older participants also performed more poorly on the cue discrimination task. However, even older participants who performed as well as young participants on the cue discrimination task failed to orient in response to the letter cues. Results are discussed in terms of effects of ageing on the ability to forge functionally effective links between different task components in the course of learning how to perform new perceptuomotor skills.

Heterogeneous Integration of Bilateral Whisker Signals by Neurons in Primary Somatosensory Cortex of Awake Rats

Bilateral single-unit recordings in primary somatosensory cortex (S1) of anesthetized rats have revealed substantial cross talk between cortical hemispheres, suggesting the possibility that behaviorally relevant bilateral integration could occur in S1. To determine the extent of bilateral neural responses in awake animals, we recorded S1 multi- and single-unit activity in head-immobilized rats while stimulating groups of 4 whiskers from the same column on both sides of the head. Results from these experiments confirm the widespread presence of single units responding to tactile stimuli on either side of the face in S1 of awake animals. Quantification of bilateral integration by multiunits revealed both facilitative and suppressive integration of bilateral inputs. Varying the interval between left and right whisker stimuli between 0 and 120 ms showed the temporal integration of bilateral stimuli to be dominated on average by suppression at intervals around 30 ms, in agreement with comparable recordings in anesthetized animals. Contrary to the anesthetized data, in the awake animals we observed a high level of heterogeneity of bilateral responses and a strong interaction between synchronous bilateral stimuli. The results challenge the traditional conception of highly segregated hemispheric processing channels in the rat S1 cortex, and support the hypothesis that callosal cross-projections between the two hemispheres mediate rats' known ability to integrate bilateral whisker signals.

Attentional load and sensory competition in human vision: modulation of fMRI responses by load at fixation during task-irrelevant stimulation in the peripheral visual field.

Perceptual suppression of distractors may depend on both endogenous and exogenous factors, such as attentional load of the current task and sensory competition among simultaneous stimuli, respectively. We used functional magnetic resonance imaging (fMRI) to compare these two types of attentional effects and examine how they may interact in the human brain. We varied the attentional load of a visual monitoring task performed on a rapid stream at central fixation without altering the central stimuli themselves, while measuring the impact on fMRI responses to task-irrelevant peripheral checkerboards presented either unilaterally or bilaterally. Activations in visual cortex for irrelevant peripheral stimulation decreased with increasing attentional load at fixation. This relative decrease was present even in V1, but became larger for successive visual areas through to V4. Decreases in activation for contralateral peripheral checkerboards due to higher central load were more pronounced within retinotopic cortex corresponding to 'inner' peripheral locations relatively near the central targets than for more eccentric 'outer' locations, demonstrating a predominant suppression of nearby surround rather than strict 'tunnel vision' during higher task load at central fixation. Contralateral activations for peripheral stimulation in one hemifield were reduced by competition with concurrent stimulation in the other hemifield only in inferior parietal cortex, not in retinotopic areas of occipital visual cortex. In addition, central attentional load interacted with competition due to bilateral versus unilateral peripheral stimuli specifically in posterior parietal and fusiform regions. These results reveal that task-dependent attentional load, and interhemifield stimulus-competition, can produce distinct influences on the neural responses to peripheral visual stimuli within the human visual system. These distinct mechanisms in selective visual processing may be integrated within posterior parietal areas, rather than earlier occipital cortex.

Temporal order judgment in mice

A temporal order judgment task was developed for mice. After training male mice (C57BL6NCrj, n = 15) to poke their noses into a hole, two stimuli (brief puffs of air) were delivered to the whiskers with a fixed interval of 750 ms in one of four orders: right (R)-left (L), L-R, L-L, and R-R. The mice were rewarded when they oriented their heads toward the first ( n = 5) or second ( n = 10) stimulus after a visual go signal. The mice were trained for up to 50 days. All mice met the criterion for task achievement (daily correct response rate >70% on 3 consecutive days) in response to unilateral stimuli (L-L and R-R), and 9 of the 15 mice met the criterion for task achievement in response to bilateral stimuli (L-R and R-L). The median periods for task achievement were 15 and 34 days for unilateral and bilateral stimuli, respectively. The correct response rate dropped to approximately the chance level after all whiskers had been removed. The nine successful mice were trained further and tested with smaller interstimulus intervals. The probability of right-first judgment plotted against the stimulation interval was fitted with a sigmoid function ( r 2 = 0.92) with asymptotes of 0.29 and 0.73 and a temporal resolution of 160 ms. The sigmoid curve was biased horizontally by 133 ms, reflecting the fact that stimuli delivered simultaneously were judged as left-first rather than right-first. The results show that mice can be trained to judge the temporal order of tactile stimuli delivered to whiskers and that such judgment might be lateralized to the right hemisphere.