The Myth of Color Sensations,or How Not to See a Yellow Banana

I argue against a class of philosophical views of color perception, especially insofar as such views posit the existence of color sensations. I argue against the need to posit such nonconceptual mental intermediaries between the stimulus and the eventual conceptualized perceptual judgment. Central to my arguments are considerations of certain color illusions. Such illusions are best explained by reference to high-level, conceptualized knowledge concerning, for example, object identity, likely lighting conditions, and material composition of the distal stimulus. Such explanations obviate the need to appeal to nonconceputal mental links in the causal chains eventuating in conceptualized color discriminations.     

Recherche de sensations, désinhibition et pratique de sports à risque : quelques pistes de réflexion

Sensation seeking is at the root of different behaviours. Skydivers, artists, drug addicts and criminals somehow share the same need for stimulation. But are there drug-addicted skydivers? Are there different ways to seek sensations? Studies on risky sport practices and disinhibition are contradictory. While some find risk-taking athletes do not consume substances, others maintain these athletes are indeed the most uninhibited of all. Diversity and the type of activities supposedly depend on what exactly an athlete seeks from a psychological point of view. The analysis of different sensation seekers’ personality traits helps to better understand the choice and role of one or several sources of activation. Based on a review of studies, we will discuss three sensation seeker profiles. Depressed “escapists” primarily seek sensations through substance use in order to regulate their negative affects. This “passive” stimulation mode seems better suited than practising risky sports for these individuals lacking in energy. Conversely, extraverted “hedonists” comfort their positive affectivity by seeking multi-faceted pleasure in risky sports or the “social” use of substances. They are not characterized by negative affects but by alexithymia. Some probably bypass their difficulty to understand their feelings by seeking various readily available sensations that need not be mentally interiorised (purportedly found in disinhibition or risky sports). Finally, “compensatory” types are adventurers who seek sensations in high-risk sports only. They are not characterized by depression, anxiety, disinhibition, extraversion or alexithymia. They seek to enhance and build up their personality by confronting the natural environment and danger. While all athletes seek sensations, these might not be essential to escapist and compensatory types who use them only as a mere means to escape and compensate. Extraverted hedonists seem to be the “true” sensation seekers inasmuch as stimulations are worthwhile in themselves. This strong need for hedonistic sensations might lead to an addictive process, a common answer to psychic sufferings that may also derive from boredom or a need for sensations and pleasure.     

Changes in visual perception at the time of saccades

We frequently reposition our gaze by making rapid ballistic eye movements that are called saccades. Saccades pose problems for the visual system, because they generate rapid, large-field motion on the retina and change the relationship between the object position in external space and the image position on the retina. The brain must ignore the one and compensate for the other. Much progress has been made in recent years in understanding the effects of saccades on visual function and elucidating the mechanisms responsible for them. Evidence suggests that saccades trigger two distinct neural processes: (1) a suppression of visual sensitivity, specific to the magnocellular pathway, that dampens the sensation of motion and (2) a gross perceptual distortion of visual space in anticipation of the repositioning of gaze. Neurophysiological findings from several laboratories are beginning to identify the neural substrates involved in these effects.     

Perceptual Embodiment of Prosthetic Limbs by Transcutaneous Electrical Nerve Stimulation

Objectives: In able‐bodied participants, it is possible to induce a sense of perceptual embodiment in an artificial hand using a visual‐tactile illusion. In amputee patients, electrical stimulation of sensory afferents using transcutaneous electrical nerve stimulation (TENS) has been shown to generate somatic sensations in an amputee's phantom limb(s). However, the effects of TENS on the perceptual embodiment of an artificial limb are not known. Our objective was to investigate the effects of TENS on the perceptual embodiment of an artificial limb in fully intact able‐bodied participants. Materials and Methods: We used a modified version of the rubber hand illusion presented to 30 able‐bodied participants (16 women, 14 men) to convey TENS paresthesia to an artificial hand. TENS electrodes were located over superficial radial nerve on the lateral aspect of the right forearm (1 cm proximal to the wrist), which was hidden from view. TENS intensity was increased to a strong non‐painful TENS sensation (electrical paresthesia) was felt beneath the electrodes and projecting into the fingers of the hand. The electrical characteristics of TENS were asymmetric biphasic electrical pulsed waves, continuous pulse pattern, 120 Hz pulse frequency (rate), and 80 µs pulse duration (width). Results: Participants reported significantly higher intensities of the rubber hand illusion during the two TENS conditions (mean = 5.8, standard deviation = 1.9) compared with the two non‐TENS conditions (mean = 4.9, standard deviation = 1.7), p < 0.0005. Conclusions: Our findings provide initial evidence that TENS paresthesia can be projected into an artificial limb, and this can enhance the sense of perceptual embodiment of an artificial hand. Further exploratory studies involving an amputee population are warranted.     

Neural correlates of the rubber hand illusion in amputees: A report of two cases

One of the current challenges in the field of advanced prosthetics is the development of artificial limbs that provide the user with detailed sensory feedback. Sensory feedback from our limbs is not only important for proprioceptive awareness and motor control, but also essential for providing us with a feeling of ownership or simply put, the sensation that our limbs actually belong to ourselves. The strong link between sensory feedback and ownership has been repeatedly demonstrated with the so-called rubber hand illusion (RHI), during which individuals are induced with the illusory sensation that an artificial hand is their own. In healthy participants, this occurs via integration of visual and tactile signals, which is primarily supported by multisensory regions in premotor and intraparietal cortices. Here, we describe a functional magnetic resonance imaging (fMRI) study with two upper limb amputees, showing for the first time that the same brain regions underlie ownership sensations of an artificial hand in this population. Albeit preliminary, these findings are interesting from both a theoretical as well as a clinical point of view. From a theoretical perspective, they imply that even years after the amputation, a few seconds of synchronous visuotactile stimulation are sufficient to activate hand-centered multisensory integration mechanisms. From a clinical perspective, they show that a very basic sensation of touch from an artificial hand can be obtained by simple but precisely targeted stimulation of the stump, and suggest that a similar mechanism implemented in prosthetic hands would greatly facilitate ownership sensations and in turn, acceptance of the prosthesis.     

Occipital cortex activation by long-term repetitive tactile stimulation is necessary for object recognition in blinds: A case report

Tactile vision has been approached from a variety of angles using different techniques. So far, a certain kind of object (and text) recognition has been shown, though seeing as such has not been achieved yet, and it remains unclear. Trough repetitive passive tactile stimulation perceptual processing is transferred from temporo-parietal to occipital areas, which affects object recognition. We report the results of passive tactile stimulation, as well as rTMS, applied to a 50 year old left handed blind male with over 97% loss of vision, who suffers from Peter’s anomaly and microphthalmia. After 15 weeks of passive tactile stimulation, the subject showed increased activity in occipital areas associated with the development of visual-like perception which remained unchanged after three months without passive tactile stimulation. Inhibitory rTMS over the visual cortex led to noticeable reduction of spatial recognition performance and visual sensations in this subject. Stable changes in occipital cortical activity can be associated with subjective sensations of seeing. Once occipital activation has been achieved, it is necessary for spatial object recognition. Both facts highlight the implication of occipital areas in tactile vision and the cortical plasticity of passive tactile long-term stimulation in people with blindness.     

The Neurophysiological Representation of Imagined Somatosensory Percepts in Human Cortex

Intracortical microstimulation (ICMS) in human primary somatosensory cortex (S1) has been used to successfully evoke naturalistic sensations. However, the neurophysiological mechanisms underlying the evoked sensations remain unknown. To understand how specific stimulation parameters elicit certain sensations we must first understand the representation of those sensations in the brain. In this study we record from intracortical microelectrode arrays implanted in S1, premotor cortex, and posterior parietal cortex of a male human participant performing a somatosensory imagery task. The sensations imagined were those previously elicited by ICMS of S1, in the same array of the same participant. In both spike and local field potential recordings, features of the neural signal can be used to classify different imagined sensations. These features are shown to be stable over time. The sensorimotor cortices only encode the imagined sensation during the imagery task, while posterior parietal cortex encodes the sensations starting with cue presentation. These findings demonstrate that different aspects of the sensory experience can be individually decoded from intracortically recorded human neural signals across the cortical sensory network. Activity underlying these unique sensory representations may inform the stimulation parameters for precisely eliciting specific sensations via ICMS in future work.SIGNIFICANCE STATEMENT Electrical stimulation of human cortex is increasingly more common for providing feedback in neural devices. Understanding the relationship between naturally evoked and artificially evoked neurophysiology for the same sensations will be important in advancing such devices. Here, we investigate the neural activity in human primary somatosensory, premotor, and parietal cortices during somatosensory imagery. The sensations imagined were those previously elicited during intracortical microstimulation (ICMS) of the same somatosensory electrode array. We elucidate the neural features during somatosensory imagery that significantly encode different aspects of individual sensations and demonstrate feature stability over almost a year. The correspondence between neurophysiology elicited with or without stimulation for the same sensations will inform methods to deliver more precise feedback through stimulation in the future.     

Illusory movements of the contralesional hand in patients with body image disorders

OBJECTIVES: In the present study, we assess whether illusory sensations of movement can be elicited in patients with right brain damage (RBD). METHODS: Ten RBD patients (three with disorders of bodily representations) were asked to report whether movements of their right hand induced any illusory somatic or motor sensations. Inquiries on anomalous sensation of movement of the left hand were carried out while subjects: 1) observed the moving hand in a mirror propped vertically along the parasagittal plane; 2) looked directly at the moving hand; 3) looked at the still hand; 4) kept their eyes closed. Twelve healthy subjects served as controls. RESULTS: Movement of the right hand induced a very clear sensation of movement of the left, contralesional hand in two patients affected by body image disorders. Remarkably, this occurred mainly while subjects were looking in the mirror, that is, when conflicts between visual, somatic, and motor information were maximal. In no condition did control subjects report any consistent anomalous evoked movement or sensation. CONCLUSIONS: Illusory movements of the left, plegic hand contingent upon sensorimotor conflicts can be evoked in brain damaged patients with body image disorders.     

Paradoxical heat sensations during moderate cooling of the skin

Paradoxical heat sensations during cooling of the skin were examined in two experiments. In Expt. I the number of occurrences of sensation was studied in 19 naive test subjects (Ss) when cooling from thermal indifference both without and with preceding heating. Without preceding heating 13 Ss reported sensations of paradoxical heat (9.8% of all stimulations). Preheating markedly facilitated the occurrence of the sensations (35% of all stimulations). In Expt. II the effects of cooling velocity (velocities 0.4, 0.7 and 2.0 °C/s) and the type of skin area stimulated (hairy or glabrous skin of the hand) on the thresholds of paradoxical sensations were studied in 4 Ss without and with preheating. Cooling velocity, type of skin area and preheatting had significant effects on the sensation thresholds, the thresholds being the higher (i.e. the sensation appearing at lower stimulation temperatures) the higher the cooling velocity, if the stimuli were applied to the glabrous skin, or if no preheating was used. The results confirm the existence of paradoxical heat sensations during cooling of the skin and suggest that thesensation is mediated by polymodal units supplied by C-fibers.     

Cutaneous sensation of electrical stimulation waveforms

BackgroundSkin sensation is the primary factor limiting the intensity of transcranial electrical stimulation (tES). It is well established that different waveforms generate different sensations, yet transcranial stimulation has been limited to a relatively small number of prototypical waveforms.ObjectiveWe explore whether alternative stimulation waveforms could substantially reduce skin sensation and thus allow for stronger intensities in tES.MethodsWe systematically tested a range of waveforms in a series of 6 exploratory experiments stimulating human adults on the forearm and in one instance on the head. Subjects were asked to rate skin sensation level on a numerical scale from “none” to “extreme”.ResultsHigh frequency (>1 kHz) monophasic square wave stimulation was found to decrease in sensation with increasing duty cycle, baseline, and frequency, but the sensation was never lower than for constant current stimulation. For the purpose of injecting a net direct current (DC), a constant current is optimal. For stimulation with alternating current (AC), sensation decreased with increasing frequency, consistent with previous reports. Amplitude modulation did not reduce sensation below stimulation with constant AC amplitude, and biphasic square waveforms produced higher sensation levels than biphasic sinusoidal waveforms. Furthermore, for DC stimulation, sensation levels on the arm were similar to those reported on the head.ConclusionOur comparisons of various waveforms for monophasic and biphasic stimulation indicate that conventional DC and AC waveforms may provide the lowest skin sensations levels for transcutaneous electrical stimulation. These results are likely generalizable to tES applications.     

Selective averaging of argon laser induced pre-pain and pain related cortical responses

The argon laser was found to be suitable for pre-pain and pain stimulation. The visible (488 and 515 nm) argon laser light penetrates the skin and is absorbed by melanin and haemoglobin. Argon laser stimuli of different intensities were perceived differently, and could be classified into 3 pre-pain and 3 pain classes. The pre-pain sensations were either perceived as warmth, weak pin prick, or weak pin prick followed by warmth. The pain sensations were perceived as painful pin pricks of different intensities. Single cortical responses to argon laser stimuli were averaged selectively, according to this perceptual classification, and characteristic pre-pain and pain related cortical responses were recorded. The sensation of warmth was related to a late cortical deflection, 700-800 ms after stimulus onset, whereas the pin prick related response appeared with a latency of 300-400 ms. Pre-pain related responses were only recorded when selective averaging according to perception was used. The amplitude of the selectively averaged pain related cortical responses correlated with the subjective sensation of pain. Selective averaging of pre-pain and pain related single responses may prove useful in studying the cortical projection of different perceptions or modality patterns, and to investigate the function of the thermal and nociceptive pathways and their interactions.     

Small nerve fiber selectivity of laser and intraepidermal electrical stimulation: A comparative study between glabrous and hairy skin

ObjectivesIn clinical neurophysiology practice, various methods of stimulation can be used to activate small-diameter nociceptive cutaneous afferents located in the epidermis. These methods include different types of laser and intraepidermal electrical stimulation techniques. The diffusion of the stimulation in the skin, inside or under the epidermis, depends on laser wavelength and electrode design, in particular. The aim of this study was to compare several of these techniques in their ability to selectively stimulate small nerve fibers.MethodsIn 8 healthy subjects, laser stimulation (using a CO₂ or Nd:YAP laser) and intraepidermal electrical stimulation (using a micropatterned, concentric planar, or concentric needle electrode), were applied at increasing energy or intensity on the dorsal or volar aspect of the right hand or foot. The subjects were asked to define the perceived sensation (warm, pinprick, or electric shock sensation, corresponding to the activation of C fibers, Aδ fibers, or Aβ fibers, respectively) after each stimulation. Depending on the difference in the sensations perceived between dorsal (hairy skin with thin stratum corneum) and volar (glabrous skin with thick stratum corneum) stimulations, the diffusion of the stimulation inside or under the epidermis and the nature of the activated afferents were determined.ResultsRegarding laser stimulation, the perceived sensations turned from warm to pinprick with increasing energies of stimulation, in particular with the Nd:YAP laser, of which pulse could penetrate deep in the skin according to its short wavelength. In contrast, CO₂ laser stimulation produced only warm sensations and no pricking sensation when applied to the glabrous skin, perhaps due to a thicker stratum corneum and the shallow penetration of the CO₂ laser pulse. Regarding intraepidermal electrical stimulation using concentric electrodes, the perceived sensations turned from pinprick to a combination of pinprick and electrical shocks with increasing intensities. Using the concentric planar electrode, the sensations perceived at high stimulation intensity even consisted of electric shocks without concomitant pinprick. In contrast, using the micropatterned electrode, only pinprick sensations were produced by the stimulation of the hairy skin, while the stimulation of the glabrous skin produced no sensation at all within the limits of stimulation intensities used in this study.ConclusionsUsing the CO₂ laser or the micropatterned electrode, pinprick sensations were selectively produced by the stimulation of hairy skin, while only warm sensation or no sensation at all were produced by the stimulation of glabrous skin. These two techniques appear to be more selective with a limited diffusion of the stimulation into the skin, restricting the activation of sensory afferents to the most superficial and smallest intraepidermal nerve fibers.     

Event-related brain potentials during orienting to auditory and visual stimulation in spinal cord injured humans.

In intact humans, deprivation of somatosensory and kinesthetic sensations result in significant alterations in perception and information processing. There have been very few studies to discover if the loss of sensation with spinal cord injury (SCI) in humans affects perceptual operations. We hypothesized that the SCI participant would either exhibit arousal, perceptual, and information processing alterations similar to experimentally sensory deprived subjects (who provide the closest human analogue), or that the somatosensory cortex would show reorganization for the processing of other modalities of stimulation. The subjects consisted of 16 paraplegic, 13 quadriplegic, and 22 non SCI controls. Subjects received an auditory orienting task consisting of a 500 Hz tone presented 20 times each at 66, 75, 88, and 101 db and a visual orienting task incorporating light flashes of 115, 123, 131, and 140 lux presented 20 times each. EEG information processing data were recorded from C3 and C4 for 100 msec prior to and 500 msec post stimulation. Information processing variables, analyzed as event-related potentials, indicated that the somatosensory cortex of SCI groups had a flattened response to auditory stimulation. The control group had a significantly larger P2 component. We concluded that these data signified that the somatosensory cortex did not reorganize function in response to chronic deafferentation nor was the SCI subject hyperresponsive to stimulation.     

Phantom digit somatotopy: a functional magnetic resonance imaging study in forearm amputees

Forearm amputees often experience non-painful sensations in their phantom when the amputation stump is touched. Cutaneous stimulation of specific stump areas may be perceived as stimulation of specific phantom fingers (stump hand map). The neuronal basis of referred phantom limb sensations is unknown. We used functional magnetic resonance imaging to demonstrate a somatotopic map of the phantom fingers in the hand region of the primary somatosensory cortex after tactile stump stimulation. The location and extent of phantom finger activation in the primary somatosensory cortex corresponded well to the location of normal fingers in a reference population. Stimulation of the stump hand map resulted in an increased bilateral activation of the primary somatosensory cortex compared with stimulation of forearm regions outside the stump hand map. Increased activation was also seen in contralateral posterior parietal cortex and premotor cortex. Ipsilateral primary somatosensory cortex activation might represent a compensatory mechanism and activation of the non-primary fronto-parietal areas might correspond to awareness of the phantom limb, which is enhanced when experiencing the referred sensations. It is concluded that phantom sensation elicited by stimulation of stump hand map areas is associated with activation of finger-specific somatotopical representations in the primary somatosensory cortex. This suggests that the primary somatosensory cortex could be a neural substrate of non-painful phantom sensations. The stump hand map phenomenon might be useful in the development of prosthetic hand devices.     

The corollary discharge in humans is related to synchronous neural oscillations

How do animals distinguish between sensations coming from external sources and those resulting from their own actions? A corollary discharge system has evolved that involves the transmission of a copy of motor commands to sensory cortex, where the expected sensation is generated. Through this mechanism, sensations are tagged as coming from self, and responsiveness to them is minimized. The present study investigated whether neural phase synchrony between motor command and auditory cortical areas is related to the suppression of the auditory cortical response. We recorded electrocorticograms from the human brain during a vocalizing/listening task. Neural phase synchrony between Broca's area and auditory cortex in the gamma band (35 to ～50 Hz) in the 50-msec time window preceding speech onset was greater during vocalizing than during listening to a playback of the same spoken sounds. Because prespeech neural synchrony was correlated (r = -.83, p = .006), with the subsequent suppression of the auditory cortical response to the spoken sound, we hypothesize that phase synchrony in the gamma band between Broca's area and auditory cortex is the neural instantiation of the transmission of a copy of motor commands. We suggest that neural phase synchrony of gamma frequencies may contribute to transmission of corollary discharges in humans.     

Mitigating Cutaneous Sensation Differences During tDCS: Comparing Sham Versus Low Intensity Control Conditions

BackgroundCutaneous sensations at electrode sites during the administration of direct current brain stimulation may inadvertently influence participants' subjective experience and task performance.ObjectiveThe present study evaluated the utility of a methodological variation that substitutes sham administration with very low intensity (0.5 mA) current delivery.MethodsWe used a 4 × 1 high-definition ring electrode transcranial direct current (HD-tDCS) system to target the left dorsolateral prefrontal cortex (Brodmann's Area 9). Four stimulation conditions were compared in a repeated-measures design: sham 2.0 mA and 0.5 mA intensity, versus active 2.0 mA and 0.5 mA intensity. During stimulation participants performed a cognitive interference task that activates the cingulo-frontal-parietal network, and periodically provided perceived sensation ratings.ResultsWe demonstrate that a relatively low intensity control condition attenuates otherwise large differences in perceived sensation between active and sham conditions. Critically, behavioral task differences maintained between the two active conditions.ConclusionA low intensity control stimulation condition may prove a viable methodological alternative to conventional sham techniques used in repeated-measures designs, though important limitations are discussed.     

Perceptual and motor effects elicited by a moving visual stimulus below the forearm: an example of segmentary vection

It has been shown in man that movement of a visual stimulus under the forearm may induce illusory sensations of movement (segmentary vection) and motor activity in the same direction, such that, for example, a sensation of elbow flexion is accompanied by EMG activity in biceps brachialis. The characteristics of these two types of activity, perceptual and motor, are analyzed and compared with analogous phenomena described for the whole body. These comparisons bring out two essential features: the effectiveness of stimuli of small surface area applied to central regions of the retina, and the non-compensatory character of the motor phenomena. The results are discussed with respect to the participation of vision in the control and regulation of limb movements.     

Cortical dynamics of sensorimotor integration during grasp planning

Our sensorimotor interactions with objects are guided by their current spatial and perceptual features, as well as by learned object knowledge. A fresh red tomato is grasped differently than a soft overripe tomato, even when those objects possess the same spatial metrics of size and shape. Objects' spatial and perceptual features need to be integrated during grasping, but those features are analyzed in two anatomically distinct neural pathways. The anterior intraparietal sulcus (aIPS) might support the integration of those features. We combine transcranial magnetic stimulation (TMS) interference, EEG recordings, and psychophysical methods to test aIPS causal contributions to sensorimotor integration, characterizing the dynamics of those contributions during motor planning. Human subjects performing grasping movements were provided with visual information about a target object, namely spatial and pictorial cues, whose availability and information value were independently modulated on each trial. Maximally informative visual cues, irrespective of their spatial or perceptual nature, led to enhanced motor preparatory activity early during movement planning, and to stronger spatial congruency between finger trajectories and target object. Disturbing aIPS activity with single-pulse TMS within 200 ms after object presentation reduced those electrophysiological and behavioral indices of enhanced motor planning. TMS interference with aIPS also disturbed subjects' ability to use learned object knowledge during motor planning. These results indicate that aIPS is necessary for the fast generation of a new motor plan on the basis of both spatial and pictorial cues. Furthermore, as learned object knowledge becomes available, aIPS comes to strongly depend on this prior information for structuring the motor plan.     

Delayed pointing movements to masked Müller-Lyer figures are affected by target size but not the illusion

There is ongoing debate with respect to interpretation of the finding that, in contrast to perceptual size judgments, actions are relatively unaffected by the Müller-Lyer illusion. In normal unrestricted viewing situations observers cannot perform an action directed at an object without simultaneously perceiving the object - this makes it difficult to unequivocally establish whether observed effects are a function of vision for perception, vision for action, a combination of both, or of a single all-purpose visual system. However, there is evidence that observers are capable of performing actions towards objects of which they are not consciously aware, implying that two distinct visual thresholds may exist; one accompanying vision for action and one accompanying vision for perception. To investigate this possibility we created a situation in which visual information was presented below the perception threshold, but above the purported action threshold, allowing examination of action responses independent of contributions from vision for perception. Following a perceptual categorization task, participants performed delayed pointing movements towards briefly exposed masked Müller-Lyer targets of different sizes. When the targets were presented below the perception threshold, participants were unable to discriminate between them, yet their delayed pointing movements were affected by target size (but not the illusion). The results imply that vision for action is functional even after a delay and/or that the pickup of egocentric information is associated with a lower visual threshold than the pickup of allocentric information.     

Functional organization of human supplementary motor cortex studied by electrical stimulation

The presence of somatotopic organization in the human supplementary motor area (SMA) remains a controversial issue. In this study, subdural electrode grids were placed on the medial surface of the cerebral hemispheres in 13 patients with intractable epilepsy undergoing evaluation for surgical treatment. Electrical stimulation mapping with currents below the threshold of afterdischarges showed somatotopic organization of supplementary motor cortex with the lower extremities represented posteriorly, head and face most anteriorly, and the upper extremities between these two regions. Electrical stimulation often elicited synergistic and complex movements involving more than one joint. In transitional areas between neighboring somatotopic representations, stimulation evoked combined movements involving the body parts represented in these adjacent regions. Anterior to the supplementary motor representation of the face, vocalization and speech arrest or slowing of speech were evoked. Various sensations were elicited by electrical stimulation of SMA. In some cases a preliminary sensation of "urge" to perform a movement or anticipation that a movement was about to occur were evoked. Most responses were contralateral to the stimulated hemisphere. Ipsilateral and bilateral responses were elicited almost exclusively from the right (nondominant) hemisphere. These data suggest the presence of combined somatotopic organization and left-right specialization in human supplementary motor cortex.