Responses In Somatosensory Cortex Research Articles

Cortical hyperexcitability in response to preserved spinothalamic inputs immediately after spinal cord hemisection

Chronic injury of the main somatosensory pathways ascending along the spinal cord - the dorsal columns and the spinothalamic tract - can produce both changes in the organization of cortical somatotopic maps and neuropathic pain. Little is known, however, about the early neurophysiological changes occurring immediately after injury. We bilaterally recorded the neural activity of the hindpaw representation of the primary somatosensory cortex evoked by stimuli delivered to the hindpaws before and immediately after a thoracic spinal cord hemisection in anesthetized rats. This unilateral spinal cord injury allowed us to separately investigate the cortical effects of deafferenting the dorsal column (stimuli ipsilateral to the hemisection) or the spinothalamic tract (stimuli contralateral to the hemisection). The hemisection produced immediate bilateral changes in the cortical responses evoked by stimuli delivered to the hindpaw ipsilateral to the hemisection (deafferented dorsal column): an expected loss of classical short-latency cortical responses, accompanied by an unexpected appearance of long-latency activations. At the population level, these activations reflected a progressive stimulus-induced transition of the hindpaw somatosensory cortex from up-and-down states to a sustained activated state. At the single-cell level, these cortical activations resembled the “wind-up” typically observed – with the same type of stimuli – in the dorsal horn cells originating the spinothalamic tract. Virtually no changes were observed in the responses evoked by stimuli delivered to the hindpaw contralateral to the hemisection (deafferented spinothalamic tract). These results suggest that spinal cord hemisection immediately produces an abnormal hyperexcitability of the primary somatosensory cortex in response to preserved spinothalamic inputs from the hindpaw. This immediate cortical hyperexcitability could be important to understand the long-term development of cortical reorganization and neuropathic pain after incomplete spinal cord lesions.

Effects of somatosensory stimulation and attention on human somatosensory cortex: An fMRI study

It is well known that primary and non-primary areas of human somatosensory cortex are involved in the processing of adequate deviant/rare stimuli and omission of frequent stimuli. However, the relative weight and interaction of these variables is poorly known. This functional magnetic resonance imaging (fMRI) study tested the hypothesis that somatosensory stimulus processing and attention especially interact in non-primary somatosensory areas including secondary somatosensory cortex (SII) and insula. To test this hypothesis, responses of somatosensory cortex were mapped during four conditions of an oddball paradigm: DELIVERED COUNT and IGNORE (count or ignore deviant/rare electrical stimuli, respectively); OMITTED COUNT and IGNORE (count or ignore the rare omission of frequent electrical stimuli, respectively). The deviant/rare and frequent electrical stimuli were delivered to median and ulnar nerve, respectively. It was observed that contralateral (left) primary somatosensory responses were not markedly modulated by the mentioned deviant/rare events. Furthermore, contralateral SII and insula responded to all but not OMITTED IGNORE (purely attentive) condition, whereas ipsilateral (right) SII responded to all conditions. Finally, ipsilateral insula responded to the COUNT (attentive) conditions, regardless of the physical presence of the deviant/rare stimuli. The results suggest that in somatosensory modality, bilateral SII and left (contralateral) insula reflect complex integrative processes of stimulus elaboration and attention, whereas right (ipsilateral) insula mainly sub-serves active attention to deviance within a sequence of somatosensory stimuli.

Negative Blood Oxygen Level Dependence in the Rat:A Model for Investigating the Role of Suppression in Neurovascular Coupling

Modern neuroimaging techniques rely on neurovascular coupling to show regions of increased brain activation. However, little is known of the neurovascular coupling relationships that exist for inhibitory signals. To address this issue directly we developed a preparation to investigate the signal sources of one of these proposed inhibitory neurovascular signals, the negative blood oxygen level-dependent (BOLD) response (NBR), in rat somatosensory cortex. We found a reliable NBR measured in rat somatosensory cortex in response to unilateral electrical whisker stimulation, which was located in deeper cortical layers relative to the positive BOLD response. Separate optical measurements (two-dimensional optical imaging spectroscopy and laser Doppler flowmetry) revealed that the NBR was a result of decreased blood volume and flow and increased levels of deoxyhemoglobin. Neural activity in the NBR region, measured by multichannel electrodes, varied considerably as a function of cortical depth. There was a decrease in neuronal activity in deep cortical laminae. After cessation of whisker stimulation there was a large increase in neural activity above baseline. Both the decrease in neuronal activity and increase above baseline after stimulation cessation correlated well with the simultaneous measurement of blood flow suggesting that the NBR is related to decreases in neural activity in deep cortical layers. Interestingly, the magnitude of the neural decrease was largest in regions showing stimulus-evoked positive BOLD responses. Since a similar type of neural suppression in surround regions was associated with a negative BOLD signal, the increased levels of suppression in positive BOLD regions could importantly moderate the size of the observed BOLD response.

Vascular Origins of BOLD and CBV fMRI Signals: Statistical Mapping and Histological Sections Compared

Comparison of 3T blood oxygenation level dependent (BOLD) and cerebral blood volume (CBV) activation maps to histological sections enables the spatial discrimination of functional magnetic resonance imaging (fMRI) signal changes into different vascular compartments. We use a standard gradient echo–echo planar imaging technique to measure BOLD signal changes in the somatosensory cortex in response to whisker stimulation. Corresponding changes in CBV were estimated following the infusion of a super-paramagnetic contrast agent. We imaged in a tangential imaging plane that covered the cortical surface. Images were associated with post mortem histological sections showing both the surface vasculature and cytochrome oxidase stained whisker barrel cortex. We found a significant BOLD signal change in the large draining veins which occurred in the absence of a corresponding CBV change. Results suggest that in the venous drainage system, ~3mm distant from the area of activity, there is a robust change in blood oxygen saturation with little or no volume change. CBV changes are localised over the somatosensory barrel cortex and overlying arterial supply, supporting the theory that CBV changes are greater in the arterial than in the venous vasculature. This work investigating BOLD signal and underlying hemodynamics provides more information on the vascular origins of these important neuroimaging signals.

84. Human cerebral somatosensory cortex response to mechanical stimulation using a new non-magnetic device

84. Human cerebral somatosensory cortex response to mechanical stimulation using a new non-magnetic device

Cerebral cortical thickness in a subject lacking large myelinated afferents

Inter-individual differences in brain anatomy can result from, for example, development and aging, mental illness, conditions such as chronic pain, and learning. We previously reported psychophysical and functional MRI studies on a 56 year old female subject who suffered polyradiculitis and polyneuropathy at age 31, resulting in a specific loss of large myelinated (A-beta) fibres below the level of the nose. The subject had a lack of activity in somatosensory cortices in response to light tactile stimulation, but the insula cortex was activated.

Effects of the N-methyl-D-Aspartate receptor antagonist dextromethorphan on vibrotactile adaptation

BackgroundPrevious reports have demonstrated that short durations of vibrotactile stimuli (less than or equal to 2 sec) effectively and consistently modify both the perceptual response in humans as well as the neurophysiological response in somatosensory cortex. The change in cortical response with adaptation has been well established by a number of studies, and other reports have extended those findings in determining that both GABA- and NMDAR-mediated neurotransmission play a significant role in the dynamic response of somatosensory cortical neurons. In this study, we evaluated the impact that dextromethorphan (DXM), an NMDAR antagonist, had on two distinct vibrotactile adaptation tasks.ResultsAll subjects, both those that ingested 60 mg DXM and those that ingested placebo, were evaluated for their amplitude discriminative capacity between two simultaneously delivered vibrotactile stimuli both with and without 3 conditions of pre-exposure to adapting stimulation. The results demonstrated that the perceptual metrics of subjects who ingested 60 mg DXM were significantly altered from that of controls when the amplitude discrimination task followed one of the conditions of adapting stimulation. Without the condition of pre-exposure to an adapting stimulus (or stimuli), there was little difference between the observations obtained from the subjects that ingested DXM and controls. Peak impact on subject response occurred at 60 min post-ingestion, whereas the scores of controls who ingested placebo were not impacted.ConclusionThe results – that DXM blocks vibrotactile adaptation – is consistent with the suggestion that NMDAR-mediated neurotransmission plays a significant role in the perceptual adaptive response. This finding is also consistent with neurophysiological findings that report observations of the effects of NMDAR block on the SI cortical response to repetitive vibrotactile stimulation.

The modulation of pain by attention. Insights arising from neurophysiological studies

It is generally acknowledged that pain is a complex perceptive phenomenon that depends not only on the physical properties of the nociceptive stimulation but also on the subject’s emotional and cognitive state. The central nervous system has its own resources to control pain and cope with external sources of danger. Among such internal abilities, attention plays a major role: taking attention away from the nociceptive signal and focusing it on other perceptive objects decreases nociceptive processing and resulting pain. Neurophysiological studies have succeeded in describing the spatio-temporal organisation in the human brain of the attentional processes involved in the modulation of pain. On the one hand, focusing attention on information from another sensory modality or on another body area can bias nociceptive processing by decreasing brain responses in somatosensory cortices (top-down attention). On the other hand, other attentional mechanisms controlled by the anterior cingulate area allow attention to be involuntarily attracted by the pain itself in order to process a potential source of danger (bottom-up attention). Pain control seems to depend on a fine balance between various attentional processes.

MEG event-related desynchronization and synchronization deficits during basic somatosensory processing in individuals with ADHD

BackgroundAttention-Deficit/Hyperactivity Disorder (ADHD) is a prevalent, complex disorder which is characterized by symptoms of inattention, hyperactivity, and impulsivity. Convergent evidence from neurobiological studies of ADHD identifies dysfunction in fronto-striatal-cerebellar circuitry as the source of behavioural deficits. Recent studies have shown that regions governing basic sensory processing, such as the somatosensory cortex, show abnormalities in those with ADHD suggesting that these processes may also be compromised.MethodsWe used event-related magnetoencephalography (MEG) to examine patterns of cortical rhythms in the primary (SI) and secondary (SII) somatosensory cortices in response to median nerve stimulation, in 9 adults with ADHD and 10 healthy controls. Stimuli were brief (0.2 ms) non-painful electrical pulses presented to the median nerve in two counterbalanced conditions: unpredictable and predictable stimulus presentation. We measured changes in strength, synchronicity, and frequency of cortical rhythms.ResultsHealthy comparison group showed strong event-related desynchrony and synchrony in SI and SII. By contrast, those with ADHD showed significantly weaker event-related desynchrony and event-related synchrony in the alpha (8–12 Hz) and beta (15–30 Hz) bands, respectively. This was most striking during random presentation of median nerve stimulation. Adults with ADHD showed significantly shorter duration of beta rebound in both SI and SII except for when the onset of the stimulus event could be predicted. In this case, the rhythmicity of SI (but not SII) in the ADHD group did not differ from that of controls.ConclusionOur findings suggest that somatosensory processing is altered in individuals with ADHD. MEG constitutes a promising approach to profiling patterns of neural activity during the processing of sensory input (e.g., detection of a tactile stimulus, stimulus predictability) and facilitating our understanding of how basic sensory processing may underlie and/or be influenced by more complex neural networks involved in higher order processing.

BOLD fMRI and somatosensory evoked potentials are well correlated over a broad range of frequency content of somatosensory stimulation of the rat forepaw

Electrical stimulation of the rat paw is commonly used to study the hemodynamic, metabolic and neuronal mechanisms of functional MRI (fMRI) responses in somatosensory cortex. Several groups have reported good correlation between the blood oxygenation level-dependent (BOLD) fMRI signal and somatosensory evoked potentials (SEPs) using short, typically 300 μs, square stimulation pulses. The spectral power of these short pulses is evenly distributed over a wide range of frequencies and thus the effects of the frequency content of the stimulation pulse on fMRI responses have not been previously described. Here, the effects that different stimulation pulse waveforms with a range of frequency content have on neuronal activity, as measured by SEPs, and on the amplitude of the BOLD fMRI signal in rat somatosensory cortex are investigated. The peak-to-peak SEP amplitudes increased as the power in the high frequency harmonics of the different pulse waveforms increased, using either triangular or sinusoidal stimuli waveforms from 9 Hz to 180 Hz. Similarly, BOLD fMRI response increased with increased high frequency content of the stimulation pulse. There was a linear correlation between SEPs and BOLD fMRI over the full range of frequency content in the stimulations.

Imaging the Impact of Cortical Microcirculation on Synaptic Structure and Sensory-Evoked Hemodynamic Responses In Vivo

In vivo two-photon microscopy was used to image in real time dendrites and their spines in a mouse photothrombotic stroke model that reduced somatosensory cortex blood flow in discrete regions of cortical functional maps. This approach allowed us to define relationships between blood flow, cortical structure, and function on scales not previously achieved with macroscopic imaging techniques. Acute ischemic damage to dendrites was triggered within 30 min when blood flow over >0.2 mm2 of cortical surface was blocked. Rapid damage was not attributed to a subset of clotted or even leaking vessels (extravasation) alone. Assessment of stroke borders revealed a remarkably sharp transition between intact and damaged synaptic circuitry that occurred over tens of μm and was defined by a transition between flowing and blocked vessels. Although dendritic spines were normally ~13 μm from small flowing vessels, we show that intact dendritic structure can be maintained (in areas without flowing vessels) by blood flow from vessels that are on average 80 μm away. Functional imaging of intrinsic optical signals associated with activity-evoked hemodynamic responses in somatosensory cortex indicated that sensory-induced changes in signal were blocked in areas with damaged dendrites, but were present ~400 μm away from the border of dendritic damage. These results define the range of influence that blood flow can have on local cortical fine structure and function, as well as to demonstrate that peri-infarct tissues can be functional within the first few hours after stroke and well positioned to aid in poststroke recovery.

Conditioning transcutaneous electrical nerve stimulation induces delayed gating effects on cortical response: A magnetoencephalographic study

The present study was undertaken to investigate after-effects of 7 Hz non-painful prolonged stimulation of the median nerve on somatosensory-evoked fields (SEFs). The working hypothesis that conditioning peripheral stimulations might produce delayed interfering (“gating”) effects on the response of somatosensory cortex to test stimuli was evaluated. In the control condition, electrical thumb stimulation induced SEFs in ten subjects. In the experimental protocol, a conditioning median nerve stimulation at wrist preceded 6 electrical thumb stimulations. Equivalent current dipoles fitting SEFs modeled responses of contralateral primary area (SI) and bilateral secondary somatosensory areas (SII) following control and experimental conditions. Compared to the control condition, conditioning stimulation induced no amplitude modulation of SI response at the initial stimulus-related peak (20 ms). In contrast, later response from SI (35 ms) and response from SII were significantly weakened in amplitude. Gradual but fast recovery towards control amplitude levels was observed for the response from SI-P35, while a slightly slower cycle was featured from SII. These findings point to a delayed “gating” effect on the synchronization of somatosensory cortex after peripheral conditioning stimulations. This effect was found to be more lasting in SII area, as a possible reflection of its integrative role in sensory processing.

Ipsilateral input modifies the primary somatosensory cortex response to contralateral skin flutter.

We recorded the optical intrinsic signal response of squirrel monkey primary somatosensory cortex (SI) to 25 Hz vibrotactile ("flutter") stimulation applied independently to the thenar eminence on each hand and also to bilateral (simultaneous) stimulation of both thenars. The following observations were obtained in every subject (n = 5). (1) Ipsilateral stimulation was accompanied by an increase in absorbance within the SI hand region substantially smaller than the absorbance increase evoked by contralateral stimulation. (2) The absorbance increase evoked by simultaneous bilateral stimulation was smaller (by approximately 30%) than that evoked by contralateral stimulation. (3) The spatiointensive pattern of the SI response to bilateral flutter was distinctly different than the pattern that accompanied contralateral flutter stimulation: with contralateral flutter, the center of the responding region of SI underwent a large increase in absorbance, whereas absorbance decreased in the surrounding region; in contrast, during bilateral flutter, absorbance decreased (relative to that evoked by contralateral flutter) in the central region of SI but increased in the surround. The results raise the possibility that somatosensory perceptual experiences specific to bimanual tactile object exploration derive, at least in part, from the unique spatiointensive activity pattern evoked in SI when the stimulus makes contact with both hands. It is suggested that modulatory influences evoked by ipsilateral thenar flutter stimulation reach SI via a two-stage pathway involving interhemispheric (callosal) connections between information processing levels higher than SI and subsequently via intrahemispheric (corticocortical) projections to the SI hand region.

Impaired Feedforward Inhibition of the Thalamocortical Projection in Epileptic Ca2+Channel Mutant Mice,tottering

The tottering (tg) mice have a mutation in the CaV2.1 (P/Q-type) voltage-dependent Ca2+ channel alpha(1)2.1 subunit gene. tg mice show not only cerebellar ataxia but also absence epilepsy, which begins at approximately 3 weeks of age and persists throughout life. Similarities in EEG and sensitivity to antiepileptic drugs suggest that tg mice are a good model for human absence epilepsy. Although imbalance between excitatory and inhibitory activity in the thalamocortical network is thought to contribute to the pathogenesis of absence epilepsy, the effect of the mutation on thalamocortical synaptic responses remains unknown. Here we showed imbalanced impairment of inhibitory synaptic responses in tg mice using brain slice preparations. Somatosensory thalamocortical projection makes not only monosynaptic glutamatergic connections but also disynaptic GABAergic connections, which mediate feedforward inhibition, onto layer IV neurons. In tg mice, IPSC amplitudes recorded from layer IV pyramidal cells of the somatosensory cortex in response to thalamic stimulation became disproportionately reduced compared with EPSC amplitudes at later developmental stages (postnatal days 21-30). Similar results were obtained by local stimulation of layer IV pyramidal neurons. However, IPSC reduction was not seen in layer V pyramidal neurons of epileptic tg mice or in layer IV pyramidal neurons of younger tg mice before the onset of epilepsy (postnatal days 14-16). These results showed that the feedforward inhibition from the thalamus to layer IV neurons of the somatosensory cortex was severely impaired in tg mice and that the impairment of the inhibitory synaptic transmission was correlated to the onset of absence epilepsy.

Reduction of somatosensory evoked fields in the primary somatosensory cortex in a one-back task

In the present study, responses of the somatosensory cortex to sensory input of ten human volunteers were investigated during a one-back task with different conditions of attention. During an condition of attention subjects were requested to detect a predefined sequence of tactile stimuli applied to two different fingers of the dominant hand while a series of visual stimuli was presented simultaneously with an asynchronous stimulus-onset to the tactile stimuli. During an condition of distraction subjects received the identical series of visual and tactile stimuli like in the condition of attention but were now requested to detect a predefined stimulus sequence within the visual stimulus domain. In both conditions, somatosensory evoked magnetic fields (SEFs) to the tactile stimuli were recorded by means of a 31-channel magnetoencephalograph (MEG) from subjects' contralateral primary somatosensory cortex. The mean global field power, the dipole strength, the maximum current density, and the first component of the singular value decomposition (SVD) of magnetic fields were used to compare early components of the SEF in the conditions of attention versus distraction. Surprisingly, results revealed significant decreases of measures of all four parameters during the condition of attention as compared to the condition of distraction indicating that early responses of the primary somatosensory cortex became significantly reduced in the condition of attention. We hypothesize that changes in the centre-periphery-relationship of receptive fields in the primary somatosensory cortex may account for this unexpected result.

Neuronal basis of optical imaging signals in sensory cortex

The advent of non-invasive imaging methods such as functional magnetic resonance imaging (fMRI) has made it possible to obtain spatial maps of hemodynamic 'activation' in the human brain under a variety of conditions. However, the indirect and poorly understood nature of the coupling between these hemodynamic signals and the underlying neuronal activity has greatly limited the interpretability of neuroimaging results. In our laboratory, we address the question of coupling between pre- and post-synaptic neuronal activity, and the hemodynamic response in rodent somatosensory cortex in response to a localized tactile stimulus. We use full-field multiple-wavelength (spectroscopic) optical imaging of the intrinsic signals that enables simultaneous measurements of oxyhemoglobin (HbO), deoxyhemoglobin (Hb) and total hemoglobin (HbT) and simultaneous electrophysiological recordings of spiking and synaptic activity. Our results show that

Role of Spike Timing in the Forelimb Somatosensory Cortex of the Rat

The aim of this study was to test the hypothesis that the significance of spike timing in somatosensory processing is not a specific feature of the whisker cortex but a more general characteristic of the primary somatosensory cortex. We recorded ensembles of neurons using microwire arrays implanted in the deep layers of the forelimb region of the rat primary somatosensory cortex in response to step stimuli delivered to the cutaneous surface of the contralateral body. We used a recently developed peristimulus time histogram (PSTH)-based classification method to investigate the temporal precision of the code by evaluating how changing the bin size (from 40 to 1 msec) would affect the ability of the ensemble responses to discriminate stimulus location on a single-trial basis. The information related to the discrimination was redundantly distributed within the ensembles, and the ability to discriminate stimulus location increased when decreasing the bin size, reaching a maximum at 4 msec. In our experiment, at 4 msec bin size the first spike per neuron after the stimulus conveyed almost as much information as the entire responses, so the temporal precision of the code was preserved in the first spikes. Subsequent spikes were less frequent but conveyed more information per spike. Finally, not only the trials correctly classified but also the trials incorrectly classified conveyed information about stimulus location with a similar temporal precision. We conclude that the role of spike timing in cortical somatosensory processing is not an exclusive feature of the highly specialized rat trigeminal system, but a more general property of the rat primary somatosensory cortex.

Effect of subthreshold up and down states on the whisker-evoked response in somatosensory cortex.

Changes in spontaneous activity within the cortex recognized by subthreshold fluctuations of the membrane potential of cortical neurons modified the response of cortical neurons to sensory stimuli. Sensory stimuli occurring in the hyperpolarized "down" state evoked a larger depolarization and were more effective in evoking action potentials than stimuli occurring in the depolarized "up" state. Direct electrical stimulation of the thalamus showed the same dependence on the cell's state at the time of the stimulus, ruling out a strictly thalamic mechanism. Stimuli were more effective at triggering action potentials in the down state even during moderate de- or hyperpolarization of the somatic membrane potential. The postsynaptic potential (PSP) evoked from the down state was larger than the up state PSP but achieved about the same peak membrane potential, which was also near the reversal potential of the PSP (about -51 mV). Chloride loading shifted the reversal potentials of both the up state and the whisker-evoked PSP toward a more depolarized membrane potential. In addition, the threshold for action potentials evoked from the down state was lower than for spikes evoked in the up state. Thus the larger PSP from the down state may be caused by its larger driving force, and the state dependence of action potential generation in response to whisker stimulation may in part be related to a shift in threshold. Different mechanisms are therefore responsible for the state-dependence of PSP amplitude and the spike frequency response to the whisker stimulus.

Negative functional response to sensory stimulation and its origins.

Functional imaging studies typically give prominence to positive responses. However, negative changes that accompany activation-induced positive responses are not yet clearly understood. The authors investigated the origin of sustained negative CBF responses that accompanied positive CBF changes. Measurements were made in the rat somatosensory cortex in response to whisker stimulation using laser-Doppler imaging. Flux images indicative of CBF were obtained at rest and during whisker stimulation with a spatial resolution of 200 microm. Large and intermediate blood vessels in the cortical surface exhibiting high flux values were clearly resolved. This greatly reduced the contamination of the tissue pixel volume with macroscopic blood vessels. Regions that responded positively to whisker stimulation were from areas with intermediate to low baseline flux and distinctly away from high flux areas. Stimulation-induced change in signal intensity was the largest in pixels with low baseline flux, presumably from tissue and microvessels. Simultaneously, a sustained decrease in signal intensity was observed in regions with high baseline flux values. The temporal coherence, macrovascular origin, lesser trial-to trial variability, and complete absence of the negative CBF response in the microvascular regions suggest that it may be purely hemodynamic in nature.

The human primary somatosensory cortex response contains components related to stimulus frequency and perception in a frequency discrimination task

Somatosensory stimulation of primary somatosensory cortex (SI) using frequency discrimination offers a direct, well-defined and accessible way of studying cortical decisions at the locus of early input processing. Animal studies have identified and classified the neuronal responses in SI but they have not yet resolved whether during prolonged stimulation the collective SI response just passively reflects the input or actively participates in the comparison and decision processes. This question was investigated using tomographic analysis of single trial magnetoencephalographic data. Four right-handed males participated in a frequency discrimination task to detect changes in the frequency of an electrical stimulus applied to the right-hand digits 2+3+4. The subjects received approximately 600 pairs of stimuli with Stim1 always at 21 Hz, while Stim2 was either 21 Hz (50%) or varied from 22 to 29 Hz in steps of 1 Hz. Both stimuli were 1 s duration, separated by a 1 s interval of no stimulation. The left-SI was the most consistently activated area and showed the first activation peak at 35–48 ms after Stim1 onset and sustained activity during both stimulus periods. During the Stim2 period, we found that the left-SI activation started to differ significantly between two groups of trials (21 versus 26–29 Hz) within the first 100 ms and this difference was sustained and enhanced thereafter (approximately 600 ms). When only correct responses from the above two groups were used, the difference was even higher at later latencies (approximately 650 ms). For one subject who had enough trials of same perception to different input frequencies, e.g. responded 21 Hz to Stim2 at 21 Hz (correct) and 26–29 Hz (error), we found the sustained difference only before 650 ms. Our results suggest that SI is involved with the analysis of an input frequency and related to perception and decision at different latencies.