Somatosensory Targets Research Articles

Opposed optimal strategies of weighting somatosensory inputs for planning reaching movements toward visual and proprioceptive targets.

Behavioral studies have suggested that the brain uses a visual estimate of the hand to plan reaching movements toward visual targets and somatosensory inputs in the case of somatosensory targets. However, neural correlates for distinct coding of the hand according to the sensory modality of the target have not yet been identified. Here we tested the twofold hypothesis that the somatosensory input from the reaching hand is facilitated and inhibited, respectively, when planning movements toward somatosensory (unseen fingers) or visual targets. The weight of the somatosensory inputs was assessed by measuring the amplitude of the somatosensory evoked potential (SEP) resulting from vibration of the reaching finger during movement planning. The target sensory modality had no significant effect on SEP amplitude. However, Spearman's analyses showed significant correlations between the SEPs and reaching errors. When planning movements toward proprioceptive targets without visual feedback of the reaching hand, participants showing the greater SEPs were those who produced the smaller directional errors. Inversely, participants showing the smaller SEPs when planning movements toward visual targets with visual feedback of the reaching hand were those who produced the smaller directional errors. No significant correlation was found between the SEPs and radial or amplitude errors. Our results indicate that the sensory strategy for planning movements is highly flexible among individuals and also for a given sensory context. Most importantly, they provide neural bases for the suggestion that optimization of movement planning requires the target and the reaching hand to both be represented in the same sensory modality.

Effector movement triggers gaze-dependent spatial coding of tactile and proprioceptive-tactile reach targets

Reaching in space requires that the target and the hand are represented in the same coordinate system. While studies on visually-guided reaching consistently demonstrate the use of a gaze-dependent spatial reference frame, controversial results exist in the somatosensory domain. We investigated whether effector movement (eye or arm/hand) after target presentation and before reaching leads to gaze-dependent coding of somatosensory targets. Subjects reached to a felt target while directing gaze towards one of seven fixation locations. Touches were applied to the fingertip(s) of the left hand (proprioceptive-tactile targets) or to the dorsal surface of the left forearm (tactile targets). Effector movement was varied in terms of movement of the target limb or a gaze shift. Horizontal reach errors systematically varied as a function of gaze when a movement of either the target effector or gaze was introduced. However, we found no effect of gaze on horizontal reach errors when a movement was absent before the reach. These findings were comparable for tactile and proprioceptive-tactile targets. Our results suggest that effector movement promotes a switch from a gaze-independent to a gaze-dependent representation of somatosensory reach targets.

Segmental and suprasegmental errors in Spanish learning cochlear implant users: Neurolinguistic interpretation

This study aims at investigating the impact of cochlear implant (CI) use for phonological development. The main participants were a group of 14 deaf children who had received their CIs in the second year of life, and who had been wearing them for 24 months. A group of normally hearing (NH) children aged 24 months old was also evaluated. Data was obtained from a non-word repetition (NWR) task. Various segmental and suprasegmental measures were obtained from the NWR data. The CI children scored significantly below the controls for one feature (i.e. place of articulation) and for segment substitutions. Suprasegmental analyses revealed that the CI children made fewer errors with unfooted syllables and more stress errors than the NH children. Stress errors were correlated with segmental/feature errors in the CI children exclusively. We conclude that CI users struggle to acquire consonants, which may cascade into further prosodic deficits. The results are interpreted in terms of a motor control model of speech production. We suggest that CIs provide sufficient information to learn rudimentary auditory representations for syllables; however, such auditory representations might not be detailed enough to implicitly derive the somatosensory consequences of the individual consonants. • We investigate how cochlear implant (CI) use modulates the acquisition of phonology. • Place of articulation, consonant substitution and stress errors were frequent. • The substitution of sibilants and syllable omissions were relatively infrequent. • CIs provide sufficient information to learn the auditory targets of syllables. • But CI users struggle to implicitly derive the somato-sensory targets of consonants.

Spatio-temporal measures of electrophysiological correlates for behavioral multisensory enhancement during visual, auditory and somatosensory stimulation: A behavioral and ERP study

Multisensory enhancement, as a facilitation phenomenon, is responsible for superior behavioral performance when an individual is responding to cross-modal versus modality-specific stimuli. However, the event-related potential (ERP) counterparts of behavioral multisensory enhancement are not well known. We recorded ERPs and behavioral data from 14 healthy volunteers with three types of target stimuli (modality-specific, bimodal, and trimodal) to examine the spatio-temporal electrophysiological characteristics of multisensory enhancement by comparing behavioral data with ERPs. We found a strong correlation between P3 latency and behavioral performance in terms of reaction time (RT) (R = 0.98, P <0.001), suggesting that P3 latency constitutes a temporal measure of behavioral multisensory enhancement. In addition, a fast RT and short P3 latency were found when comparing the modality-specific visual target with the modality-specific auditory and somatosensory targets. Our results indicate that behavioral multisensory enhancement can be identified by the latency and source distribution of the P3 component. These findings may advance our understanding of the neuronal mechanisms of multisensory enhancement.

Perceptual learning in sensorimotor adaptation

Motor learning often involves situations in which the somatosensory targets of movement are, at least initially, poorly defined, as for example, in learning to speak or learning the feel of a proper tennis serve. Under these conditions, motor skill acquisition presumably requires perceptual as well as motor learning. That is, it engages both the progressive shaping of sensory targets and associated changes in motor performance. In the present study, we test the idea that perceptual learning alters somatosensory function and in so doing produces changes to human motor performance and sensorimotor adaptation. Subjects in these experiments undergo perceptual training in which a robotic device passively moves the subject's arm on one of a set of fan-shaped trajectories. Subjects are required to indicate whether the robot moved the limb to the right or the left and feedback is provided. Over the course of training both the perceptual boundary and acuity are altered. The perceptual learning is observed to improve both the rate and extent of learning in a subsequent sensorimotor adaptation task and the benefits persist for at least 24 h. The improvement in the present studies varies systematically with changes in perceptual acuity and is obtained regardless of whether the perceptual boundary shift serves to systematically increase or decrease error on subsequent movements. The beneficial effects of perceptual training are found to be substantially dependent on reinforced decision-making in the sensory domain. Passive-movement training on its own is less able to alter subsequent learning in the motor system. Overall, this study suggests perceptual learning plays an integral role in motor learning.

Valid Cues for Auditory or Somatosensory Targets Affect Their Perception: A Signal Detection Approach

We investigated the effects of focusing attention towards auditory or somatosensory stimuli on perceptual sensitivity and response bias using a signal detection task. Participants (N = 44) performed an unspeeded detection task in which weak (individually calibrated) somatosensory or auditory stimuli were delivered. The focus of attention was manipulated by the presentation of a visual cue at the start of each trial. The visual cue consisted of the word "warmth" or the word "tone". This word cue was predictive of the corresponding target on two-thirds of the trials. As hypothesised, the results showed that cueing attention to a specific sensory modality resulted in a higher perceptual sensitivity for validly cued targets than for invalidly cued targets, as well as in a more liberal response criterion for reporting stimuli in the valid modality than in the invalid modality. The value of this experimental paradigm for investigating excessive attentional focus or hypervigilance in various non-clinical and clinical populations is discussed.

How efficient is the orienting of spatial attention to pain? An experimental investigation

This study investigated how efficient spatial attention was oriented to pain in 2 experiments. Participants detected whether painful (pain group) or nonpainful (control group) somatosensory stimuli were delivered to the left or right hand. Each stimulus was preceded by a visual cue presented near to the stimulated hand (valid trial), the opposite hand (invalid trial), or centrally between hands. To examine both exogenous and endogenous orienting of attention, the spatial predictability of somatosensory targets was manipulated. In the first experiment, visual cues were nonpredictive for the location of the pain stimulus, as a result of which, orienting was purely exogenous, i.e., resulting from the occurrence of the visual cue at the location of somatosensory input. In the second experiment, visual cues were spatially predictive, as a result of which, endogenous control was added, i.e., attention driven by expectations of where the somatosensory target will occur. The results showed that only in experiment 1 was spatial attention oriented more efficiently to painful compared with nonpainful somatosensory stimulation. This effect was due to faster responses on valid relative to baseline trials (engagement), rather than slower responses on invalid relative to baseline trials (disengagement), and was significantly correlated with self-reported bodily threat. In experiment 2, prioritization of the pain location was probably overridden by task strategies because it was advantageous for participants’ task performance to attend to the cued location irrespective of whether stimulation was painful or not. Implications of these findings for theories of hypervigilance and attentional management of pain are discussed.

Dynamic characteristics of multisensory facilitation and inhibition

Multimodal integration, which mainly refers to multisensory facilitation and multisensory inhibition, is the process of merging multisensory information in the human brain. However, the neural mechanisms underlying the dynamic characteristics of multimodal integration are not fully understood. The objective of this study is to investigate the basic mechanisms of multimodal integration by assessing the intermodal influences of vision, audition, and somatosensory sensations (the influence of multisensory background events to the target event). We used a timed target detection task, and measured both behavioral and electroencephalographic responses to visual target events (green solid circle), auditory target events (2kHz pure tone) and somatosensory target events (1.5±0.1mA square wave pulse) from 20 normal participants. There were significant differences in both behavior performance and ERP components when comparing the unimodal target stimuli with multimodal (bimodal and trimodal) target stimuli for all target groups. Significant correlation among reaction time and P3 latency was observed across all target conditions. The perceptual processing of auditory target events (A) was inhibited by the background events, while the perceptual processing of somatosensory target events (S) was facilitated by the background events. In contrast, the perceptual processing of visual target events (V) remained impervious to multisensory background events.

Auditory Modulation of Somatosensory Spatial Judgments in Various Body Regions and Locations

The spatial modulation effect has been reported in somatosensory spatial judgments when the task-irrelevant auditory stimuli are given from the opposite direction. Two experiments examined how the spatial modulation effect on somatosensory spatial judgments is altered in various body regions and their spatial locations. In experiment 1, air-puffs were presented randomly to either the left or right cheeks, hands (palm versus back) and knees while auditory stimuli were presented from just behind ear on either the same or opposite side. In experiment 2, air-puffs were presented to hands which were aside of cheeks or placed on the knees. The participants were instructed to make speeded discrimination responses regarding the side (left versus right) of the somatosensory targets by using two footpedals. In all conditions, reaction times significantry increased when the irrelevant stimuli were presented from the opposite side rather than from the same side. We found that the back of the hands were more influenced by incongruent auditory stimuli than cheeks, knees and palms, and that the hands were more influenced by incongruent auditory stimuli when placed at the side of cheeks than on the knees. These results indicate that the auditory-somatosensory interaction differs in various body regions and their spatial locations.

The effects of tongue loading and auditory feedback on vowel production

This study investigated the role of sensory feedback during the production of front vowels. A temporary aftereffect induced by tongue loading was employed to modify the somatosensory-based perception of tongue height. Following the removal of tongue loading, tongue height during vowel production was estimated by measuring the frequency of the first formant (F1) from the acoustic signal. In experiment 1, the production of front vowels following tongue loading was investigated either in the presence or absence of auditory feedback. With auditory feedback available, the tongue height of front vowels was not modified by the aftereffect of tongue loading. By contrast, speakers did not compensate for the aftereffect of tongue loading when they produced vowels in the absence of auditory feedback. In experiment 2, the characteristics of the masking noise were manipulated such that it masked energy either in the F1 region or in the region of the second and higher formants. The results showed that the adjustment of tongue height during the production of front vowels depended on information about F1 in the auditory feedback. These findings support the idea that speech goals include both auditory and somatosensory targets and that speakers are able to make use of information from both sensory modalities to maximize the accuracy of speech production.

Spatio-temporal dynamics of reach-related neural activity for visual and somatosensory targets

The parieto-frontal network plays a crucial role in the transformations that convert visual information into motor commands for hand reaching movements. Here we use electroencephalography to determine whether the planning of reaching movements to visual and somatosensory targets involves a similar spatio-temporal pattern of neural activity. Subjects performed reaching movements toward spatial locations defined either by visual (light-emitting diode) or somatosensory (vibration of a fingertip of the contralateral hand) stimuli. To identify the activations associated with sensorimotor transformations, we subtracted the event-related potentials recorded in a “static” task (the stimuli were presented but no movement was initiated) from those recorded in a “reach” task (a reach had to be initiated toward the spatial location of the stimuli). In the visual condition, reach-related activities were observed over parietal, premotor and sensorimotor areas contralateral to the reaching hand. Activation was first observed over parietal areas 140 ms after stimulus onset and progressed to frontal areas. The proprioceptive condition recruited a similar set of structures as for visual targets. However, the temporal pattern of activity within these cortical areas differed greatly. Activity was sustained over premotor and sensorimotor areas throughout the reaction time interval, occurring simultaneously with the parietal activation. These results suggest that a common cortical network serves to transform visual and somatosensory signals into motor commands, but that the interactions between the structures of this network differ. This raises the possibility that different coordinate frames are used to encode the motor error for the two target modalities.

Common neural substrates support speech and non-speech vocal tract gestures

The issue of whether speech is supported by the same neural substrates as non-speech vocal tract gestures has been contentious. In this fMRI study we tested whether producing non-speech vocal tract gestures in humans shares the same functional neuroanatomy as non-sense speech syllables. Production of non-speech vocal tract gestures, devoid of phonological content but similar to speech in that they had familiar acoustic and somatosensory targets, was compared to the production of speech syllables without meaning. Brain activation related to overt production was captured with BOLD fMRI using a sparse sampling design for both conditions. Speech and non-speech were compared using voxel-wise whole brain analyses, and ROI analyses focused on frontal and temporoparietal structures previously reported to support speech production. Results showed substantial activation overlap between speech and non-speech function in regions. Although non-speech gesture production showed greater extent and amplitude of activation in the regions examined, both speech and non-speech showed comparable left laterality in activation for both target perception and production. These findings posit a more general role of the previously proposed “auditory dorsal stream” in the left hemisphere — to support the production of vocal tract gestures that are not limited to speech processing.

Changing Reference Frames during the Encoding of Tactile Events

The mindless act of swatting a mosquito on the hand poses a remarkable challenge for the brain. Given that the primary somatosensory cortex maps skin location independently of arm posture [1, 2], the brain must realign tactile coordinates in order to locate the origin of the stimuli in extrapersonal space. Previous studies have highlighted the behavioral relevance of such an external mapping of touch, which results from combining somatosensory input with proprioceptive and visual cues about body posture [3-7]. However, despite the widely held assumption about the existence of this remapping process from somatotopic to external space and various findings indirectly suggesting its consequences [8-11], a demonstration of its changing time course and nature was lacking. We examined the temporal course of this multisensory interaction and its implications for tactile awareness in humans using a crossmodal cueing paradigm [12, 13]. What we show is that before tactile events are referred to external locations [12-15], a fleeting, unconscious image of the tactile sensation abiding to a somatotopic frame of reference rules performance. We propose that this early somatotopic "glimpse" arises from the initial feed-forward sweep of neural activity to the primary somatosensory cortex, whereas the later externally-based, conscious experience reflects the activity of a somatosensory network involving recurrent connections from association areas.

An artificial neural network model of orienting attention toward threatening somatosensory stimuli

An artificial neural network model was designed to test the threat detection hypothesis developed in our experimental studies, where threat detector activity in the somatosensory association areas is monitored by the medial prefrontal cortex, which signals the lateral prefrontal cortex to redirect attention to the threat. As in our experimental studies, simulated threat-evoked activations of all three brain areas were larger when the somatosensory target stimulus was unattended than attended, and the increase in behavioral reaction times when the target stimulus was unattended was smaller for threatening than nonthreatening stimuli. The model also generated a number of novel predictions, for example, the effect of threat on reaction time only occurs when the target stimulus is unattended, and the P3a indexes prefrontal cortex activity involved in redirecting attention toward response processes on that trial and sensory processes on subsequent trials.

Neural mechanisms of detecting and orienting attention toward unattended threatening somatosensory targets. I. Intermodal effects

Our previous work has identified four components of the somatosensory-evoked potential elicited by painful electrical stimulation of the sural nerve that might index an involuntary process that detects and orients attention toward threatening somatosensory stimuli. These components include a negativity over the central scalp at 70-110 ms poststimulus (CN70-110), a contralateral temporal negativity at 100-180 ms (CTN100-180), a frontocentral negativity at 130-200 ms, and a positive potential at 270-340 ms (the pain-related P2). The results of the endogenous cuing experiment used here suggest that the CN70-110 and CTN100-180 index somatosensory cortex activity that detects a threatening somatosensory stimulus when the subject's attention is focused on another stimulus modality but not another location. The P2, on the other hand, appears to index inferior parietal cortex activity that is specifically involved in orienting spatial attention.

Neural mechanisms of detecting and orienting attention toward unattended threatening somatosensory target stimuli. II. Intensity effects

Negative potentials evoked by painful electrical stimulation of the sural nerve that occur at 100-180 ms poststimulus over the contralateral temporal scalp (CTN100-180) and at 130-200 ms over the fronto-central scalp (FCN130-200) exhibit unusual attention effects. That is, their amplitudes are larger when the painful evoking stimulus is unattended than when it is attended. In this experiment, I show that attention has no effect on the CTN100-180 evoked by a weak, nonthreatening sural nerve electrical stimulus. These data suggest that the generators of the CTN100-180, which include the somatosensory association areas in the parietal operculum, are specifically involved in detecting threatening somatosensory stimuli. The FCN130-200 showed a small increase in the unattended condition, which is consistent with the role of its medial prefrontal cortex generators in monitoring any situation that might require a change in attentional control.

Parietal disruption impairs reflexive spatial attention within and between sensory modalities

Reflexive spatial attention is critical for controlling perception and action. An established body of evidence suggests that mechanisms of spatial attention operate both within and between sensory modalities; however the attentional mechanisms that link modalities in the human brain are unclear. Here we used transcranial magnetic stimulation (TMS) to explore the role of the parietal cortex in coordinating reflexive shifts of spatial attention between vision and touch. In two experiments, healthy participants localised visual and somatosensory targets that were preceded by non-informative visual or somatosensory spatial cues. To determine the role of parietal cortex in spatial orienting, TMS was delivered synchronously with cue onset for 100ms. Results revealed a critical role of the right angular gyrus and supramarginal gyrus in reflexive orienting to visual and somatosensory targets that followed a somatosensory cue. In contrast, the same TMS protocol was ineffective in modulating reflexive orienting based on visual cues. This dependence on cue modality may reflect subcortical redundancy of visual orienting mechanisms. Overall, our results indicate a critical role of the inferior parietal cortex in mediating reflexive shifts of attention within and between sensory modalities.

Modality-Specific Control of Strategic Spatial Attention in Parietal Cortex

The neural basis of selective spatial attention presents a significant challenge to cognitive neuroscience. Recent neuroimaging studies have suggested that regions of the parietal and temporal cortex constitute a "supramodal" network that mediates goal-directed attention in multiple sensory modalities. Here we used transcranial magnetic stimulation (TMS) to determine which cortical subregions control strategic attention in vision and touch. Healthy observers undertook an orienting task in which a central arrow cue predicted the location of a subsequent visual or somatosensory target. To determine the attentional role of cortical subregions at different stages of processing, TMS was delivered to the right hemisphere during cue or target events. Results indicated a critical role of the inferior parietal cortex in strategic orienting to visual events, but not to somatosensory events. These findings are inconsistent with the existence of a supramodal attentional network and instead provide direct evidence for modality-specific attentional processing in parietal cortex.

Latency and accuracy of saccades to somatosensory targets

To examine the accuracy and latency of reflexive saccades to vibratory stimulation of the fingertips made by normal human subjects and to compare the findings to those of visually guided saccades. Eye movements were recorded using infrared oculography. Stimuli were presented via an array of audiometric bone vibrator transducers driven at 250 Hz and positioned at eye level in a darkened room. Target locations were at 0 and +/-5, 10, and 15 deg. Visual stimuli were green LEDs. Saccades were analysed interactively off-line and latency and amplitude measured for both types of saccade. Saccades to tactile stimuli had longer latencies and showed less accurate final eye positions than those to equivalent visual targets; they were unaffected by subject age. Error magnitude for the tactile saccades increased monotonically with increasing target eccentricity; because the fingers remained in a fixed position throughout the testing, this also meant that error was lowest for the thumb and increased with progression outwards towards the ring finger. Visually guided refixations were accurate and differed less than 0.2 deg across target locations. Subject age had no effect on performance. Human subjects may make relatively accurate refixations to tactile targets in the absence of visual cues, but only to certain locations/fingers; others were localised very poorly. Further studies are needed to determine whether finger selection or target location is the primary determinant of accuracy in this task.

Differential expression of nitric oxide in serotonergic projection neurons: neurochemical identification of dorsal raphe inputs to rodent trigeminal somatosensory targets.

The dorsal raphe (DR) is invested with nitric oxide synthase (NOS)-expressing profiles. To characterize the connections of NO-containing cells and further assess neurochemical relationships maintained by DR, the transmitter identity of the raphe projection to the trigeminal somatosensory system was examined. Rats were injected with retrograde tracer into vibrissae-related target areas or with anterograde tracer into DR. NADPH-diaphorase (NADPHd) histochemistry or NOS-immunostaining was combined with serotonin (5HT)- or serotonin transporter (SERT)-immunolabeling to examine: 1) the presence of NO in 5HT-containing axons from DR; 2) the distribution of NO-containing fibers with respect to other nitrergic profiles in the somatosensory system; and 3) the propensity for individual projection neurons in specific subdivisions of DR to colocalize 5HT and NO. Results confirm that "barrel-like" patches can be identified in several adult trigeminal relay nuclei by NADPHd histochemistry and demonstrate that fibers from DR contain 5HT and NO. Observations include a high percentage of cortical midline projection neurons which contained NADPHd (70-80%) and coexpressed 5HT. In contrast, approximately 40% of retrogradely labeled DR-thalamus cells in the lateral wing demonstrated NADPHd or 5HT expression, but not both in the same neuron. Colocalization of NADPHd and 5HT within individual DR projection neurons indicates that: i) DR is a source of nitrergic input to trigeminal structures, and ii) NO and 5HT may be simultaneously released to influence information-processing within somatosensory targets. Disparities in NADPHd expression between retrogradely labeled DR neuronal subpopulations further suggest functional differences in the impact of NO on cortical and subcortical targets.