Rate effect and mismatch responses in the somatosensory system: ERP-recordings in humans

In the first experiment, somatosensory event-related potentials (SERPs) were recorded to tactile pulses and vibration bursts applied to the left middle finger in trains of 4-8 stimuli with 1 s intervals. In addition to the negative N140 deflection, also the positive P50, P100 and P300 waves attenuated in amplitude with stimulus repetition. These decreases were immediate, there being no marked further amplitude attenuation after the second stimulus. The locus of this rate effect appears not to be the primary SI or SII, but rather prefrontal cortices or some deeper structures. In the second experiment, vibratory stimuli of different frequencies or at different skin sites were presented using the oddball paradigm. When the deviant stimulus was a high-frequency vibration burst, it elicited a distinct N250 deflection, probably analogous to the auditory N2b. When the deviation was a change in the stimulation site, no N250 deflection but instead an extra negativity between 100-200 ms latency, probably analogous to the auditory mismatch negativity, was observed.

Visuospatial mnemonic load modulates event-related slow potentials

Electroencephalographic slow wave potentials were recorded during the performance of visuospatial working memory tasks. The aim was to study the effects of varying mnemonic loads on slow potentials, and to dissociate the contribution of mnemonic and motor components. Subjects were tested with three spatial delayed matching-to-sample tasks in which the mnemonic load varied while the preparatory motor demands remained constant. The delay-related slow potential was more negative during the tasks in which the subjects had to memorize the locations of six or four stimuli than when only one location had to be memorized. Significant differences between the slow potentials in the tasks with different mnemonic loads were recorded at frontal and temporal recording sites. Since the preparatory motor requirements were similar in all tasks, the modulation of slow potentials reflects working memory processing rather than motor preparatory activity.

Is the somatosensory N250 related to deviance discrimination or conscious target detection?

Effects of attention to, and probability of sudden changes in, repetitive stimuli on somatosensory evoked potentials (SEP) were studied. Low - (30 Hz) and high-frequency (140 Hz) vibratory stimuli were delivered in random order to the middle finger of the left hand with different presentation probabilities in different blocks. Also ignore conditions were administered. In the ignore conditions, the probability had no effect on SEPs. However, when the standard stimuli were omitted, the "deviants" elicited small N140 and P300 deflections not observed in response to deviants when standards were also present. In the attention conditions, deviant stimuli (targets) elicited large N250 and P300 deflections which increased in amplitude with a decreased target probability. However, when subjects counted infrequently presented "deviants" alone (standards omitted) the enhanced N140 and the P300 with shortened latency were elicited, but no N250 wave could be found. At the ipsilateral side, a distinct N200 deflection was seen which could be the N250 with a shorter latency because of an easier task (detection instead of discrimination). The results might be interpreted as suggesting that the somatosensory N250 is related to conscious detection of target stimuli.

Auditory and somatosensory event-related brain potentials in early blind humans

Previous event-related potential (ERP) studies have suggested a possible participation of the visual cortex of the blind in auditory processing. In the present study, somatosensory and auditory ERPs of blind and sighted subjects were recorded when subjects were instructed to attend to stimuli of one modality and to ignore those of the other. Both modalities were stimulated with frequent ("standard") and infrequent ("deviant") stimuli, which differed from one another in their spatial locus of origin. In the sighted, deviant stimuli of the attended modality elicited N2 type of deflections (auditory N2b and somatosensory N250) over the lateral scalp areas. In contrast, in the blind, these ERP components were centroposteriorly distributed, suggesting an involvement of posterior brain areas in auditory and somatosensory stimulus discrimination. In addition, the mismatch negativity, elicited by deviant auditory stimuli even when the somatosensory stimuli were attended, was larger in the blind than in the sighted. This appears to indicate enhanced automatic processing of auditory stimulus changes in the blind. Thus, the present data suggest several compensatory changes in both auditory and somatosensory modalities after the onset of early visual deprivation.

A spatial oculomotor memory-task performance produces a task-related slow shift in human electroencephalography

Electroencephalographic (EEG) deflections in humans related to the performance of memory-guided saccades were studied in this work. The EEG deflections were recorded during 2 spatial oculomotor delayed response tasks in which the subject was instructed to make a saccade either to the right or to the left depending on the spatial location of the cue which had been shown in the beginning of the delay period. The EEG deflections were compared to those recorded during a control task in which the subject also made a saccade to the right or to the left after a delay but the requirement to keep spatial information actively in mind was minimized. A slow delay-related shift was recorded during all task conditions. The slow shift was positive in the most frontal and negative in the more posterior recording sites. The negative slow shift in the more posterior recording sites was larger in the memory tasks than in the control task. Since the memory and the control tasks differed mainly in their requirement to hold spatial information in mind it is suggested that the difference in the magnitude of slow shifts between the memory and the control tasks reflects neural activity related to spatial working memory. But although the oculomotor responses in all tasks were similar, the preparatory activities for the impending eye movements may not have been similar and in addition to working memory may have contributed to the observed differences in the slow shifts.

[Covering the foot sole and heel with pedicled and free transplants]

Thirty patients suffering from defects of the weight-bearing areas of the sole and heel were analyzed employing various methods. Sensibility was analyzed quantitatively. The level of sensibility of the flaps was found to be variable, but it had no effect on soft-tissue stability. Sensory recovery in the flaps was slow and unpredictable. Gait analysis showed that patients supplied with footwear walked relatively symmetrically even though there was a tendency to protect the flap by decreased weight-bearing. Postural control as analyzed by a sway plate was impaired only in patients with flaps having very poor sensibility. Thin skin flaps were found to be most suitable for degloving defects and muscle flaps with split-skin coverage for deep defects. The scapular flap was often too thick and was prone to develop superficial ulcerations on the weight-bearing areas.

Fast decrement with stimulus repetition in ERPs generated by neuronal systems involving somatosensory SI and SII cortices: electric and magnetic evoked response recordings in humans

The effect of stimulus repetition (short trains of stimuli with 1-s inter-stimulus intervals and 15-s inter-train intervals) on both electric and magnetic evoked responses were studied in four subjects. In addition to the later N140 and P300 deflections in electric potentials, a distinct and immediate amplitude decrement was obtained also for the earlier P50 and P100 deflections. The magnetic evoked responses also demonstrated the amplitude decrement for 50 ms (M50) and 100 ms (M100) latency deflections. The time-course and degree of amplitude decrement of the M100 magnetic response corresponded especially well to those of P100 electric deflections. The results thus show the rate effect on electric and magnetic responses at 50 and 100 ms latencies, and further suggest that the electric and magnetic responses, reflecting the activation of somatosensory SI and SII cortical areas at these latencies, respectively, are generated by related neuronal mechanisms.

Human somatosensory evoked potentials to mechanical pulses and vibration: contributions of SI and SII somatosensory cortices to P50 and P100 components

Somatosensory evoked potentials (SEPs) were measured to short tactile pulses and vibratory stimuli applied to the fingertip to determine the characteristics and scalp topography of different early and late SEP components to these types of stimulus. The measurements were obtained from 3 homologous contra- and ipsilateral locations and from the vertex. In 2 subjects the SEPs were measured from 23 recording locations. The subjects were reading during the experiments. The first distinct contralateral response was an anteriorly negative and centrally as well as posteriorly positive peak at about 50 msec latency (P50). Largest P50 responses with shortest peak latencies were measured to single tactile pulses. We suggest that P50 is probably generated in the contralateral SI cortex. The P50 was followed by a distinct negative deflection (N70) in the middle and posterior recording locations on the contralateral hemisphere, which reversed its polarity in the frontal records. This peak was also seen ipsilaterally. At about 100 msec latency a distinct bilateral positive P100 peak was obtained. This peak was most prominent to vibratory stimuli, and especially to high frequency vibration. Comparisons with recent intracortical SEP studies in primates and MEG studies in humans suggest that P100 might be best accounted for by bilateral generators in SII cortices. The early components were followed by a negative N140 wave and by a slow, positive wave with a maximum at about 300 msec. Both waves had an asymmetrical distribution. The N140 wave occurred bilaterally, but was largest contralaterally, and often had two peaks at posterior recording locations. The slow positivity was largest at the vertex and at mid-posterior recording sites.

Separate finger representations at the human second somatosensory cortex

We recorded neuromagnetic responses of the second somatosensory cortex in healthy humans. Cutaneous electrical stimulation of fingers elicited a response around 100 ms, with a field pattern agreeing with activation of the second somatosensory cortex in the upper bank of the Sylvian fissure. In an oddball paradigm, with standards presented to the thumb and deviants (10%) to the middle finger, or vice versa, the second somatosensory cortex responses to deviants were almost three times as high in amplitude as those to standards. A similar amplitude enhancement was obtained when the deviants were presented in the absence of the intervening standards but with the same interstimulus interval. The results indicate that an accurate functional representation of different body areas is maintained at the human second somatosensory cortex.

Spatial features of vibrotactile masking effects on airpuff-elicited sensations in the human hand

It was recently shown that the cutaneous sensitivity to airpuffs is decreased by a low-frequency vibrotactile masker in the hairy skin, and by a low-frequency but especially by a high-frequency masker in the glabrous skin. In the current study, the spatial features of this masking effect were determined in four healthy human subjects, using a reaction time paradigm. The masking effect decreased monotonically with increasing interstimulus distance, and identically in longitudinal and transverse (i.e., lateral) directions in the palm or dorsal surface of the hand. The masking effect was stronger in the glabrous than in the hairy skin, especially in the fingers. In the glabrous skin, the spread of masking effect produced by a high-frequency masker was more extensive than that produced by a low-frequency masker. The mechanical spread of high-frequency vibration was less extensive than that of low-frequency vibration in the skin. In the glabrous skin, a masker applied to the tip of the finger produced a stronger masking effect on sensations in the base of the finger than when the masker was located at the base and the test stimulus was located at the tip. It is concluded that mechanical spread of vibration in the skin is of minor importance in explaining the masking effects. Different peripheral neural mechanisms underlie the airpuff-elicited sensations in the hairy and glabrous skin. The afferent inhibitory mechanisms are stronger for signals coming from the glabrous skin of the fingers than for signals coming from the hairy skin. Furthermore, the peripheral innervation density and size of the cortical representational areas may be of importance in determining the magnitude of the masking effect.

Mechanical sensibility of the sole of the foot determined with vibratory stimuli of varying frequency

The mechanoreceptive properties of the sole of the foot were determined by measuring the detection thresholds to vibratory stimuli of 20, 80, and 240 Hz frequency and 300 ms duration. The thresholds were measured at six different sites on the left sole and at toes 1 and 3 with probes of 2 and 8 mm diameter connected to the moving coil of an electromechanical vibrator. The subject sat in an armchair during the experiments, with the left leg supported horizontally by a vacuum cast positioned on a table. Six subjects participated in the experiments. A simple method of limits was used to make the measurements. Lower average thresholds were obtained with higher vibration frequencies, the average thresholds varying between 40-90 microns at 20 Hz and well below 10 microns at 240 Hz. The major decrease in thresholds occurred between 20 and 80 Hz. Interindividual variability in thresholds was large, but the threshold curves obtained from different subjects and from different stimulation points were of the same general shape. The highest thresholds were measured from the toes, but this regional differences in sensibility was obtained only at the higher vibration frequencies. Comparison of the threshold values at the sole with those found with similar stimuli at the thenar eminence and middle fingertip indicates that the mechanoreceptor mechanisms transmitting information about low-frequency vibration in the sole are similar to those in the palmar skin of the hand.

Mechanical sensibility in free and island flaps of the foot

Mechanical sensibility in 20 free skin flaps and four dorsalis pedis island flaps, used for the reconstruction of foot defects, was analyzed with conventional clinical methods and by determining sensibility thresholds to vibration frequencies of 20, 80, and 240 Hz. To eliminate inter-individual variability, a score was calculated for each frequency by dividing the thresholds determined for each flap with values obtained from the corresponding area on the uninjured foot. The soft tissue stability of the reconstruction was assessed. Patients were divided into three groups according to the scores. In the group of flaps with the best sensibility, the threshold increases were low at all frequencies. In the group with intermediate sensibility, the relative threshold increases were greater, the higher the frequency. In the group with the poorest sensibility, no thresholds were obtained with 240 Hz frequency and the thresholds increases were very high at all frequencies. Sensibility was not related to the length of follow-up time, nor to the type or size of the flap. However, flap sensibility was closely associated with that of the recipient area, where sensibility was usually inferior to that of normal skin. The island flaps generally had better sensibility than the free flaps. There was a good correspondence between the levels of sensibility determined by clinical and quantitative methods. The quantitative data on the level of sensibility obtained with the psychophysical method were found to be reliable and free from observer bias, and are therefore recommended for future studies. The degree of sensibility may have contributed to, but was not essential for, good soft-tissue stability of the reconstruction.

Vibrotactile masking effects on airpuff-elicited sensations vary with skin region in the human hand

Inhibitory interactions between two tactile signals take place predominantly within mechanoreceptive submodality channels. This finding was utilized in the present study to determine the mechanoreceptive channels contributing to tactile sensations elicited by brief airpuff stimuli applied to the hairy and glabrous skin of the human hand. A reaction time paradigm was used to estimate the sensitivity of four subjects to airpuffs without and during continuous vibration (masker) of low (30 Hz) or high (240 Hz) frequency. The sensitivity to airpuffs (test stimuli) was decreased by a low-frequency masker in the hairy skin and by low- and especially by high-frequency maskers in the glabrous skin. The masking effect was enhanced in both skin areas by increasing the intensity of the masker and by decreasing the intensity of the test stimulus. The results suggest that the mechanisms underlying airpuff-elicited sensations consist of the low-frequency channel in the hairy skin, and of both the low- and high-frequency channels in the glabrous skin.

Conversely exposure-related effects between atmospheric m-xylene concentrations and human body sense of balance

Effects of fixed and fluctuating concentrations of m-xylene in air were studied on human psychophysiological functions. Nine healthy male volunteers were exposed to m-xylene 4 hrs a day, 3 hrs in the morning and 1 hr in the afternoon, with a 40 min. break in between. The subjects were sedentary or exercised at 100 W for 10 min at the beginning of each session in the chamber. The m-xylene concentrations in the air were fixed (8.2 mumol/l; 870 mg/m3) or fluctuated with peaks (5.2-16.4 mumol/l; 550-1740 mg/m3) but the TWA (8 hrs) concentration of m-xylene was always 4.1 mumol/l (435 mg/m3). The two control days, with and without exercise, were similar to the exposure days, but without exposure. Reaction times were measured with a device in which subjects responded to visual and auditory stimuli. Body sway was measured with the subjects' eyes open and closed with a strain gauge transducer platform. Changes in reaction times did not correlate with m-xylene concentrations in the air. Changes of average and maximal body sway were, however, consistently negatively correlated with the intensity of the atmospheric exposure to m-xylene at the beginning of each session in the chamber. Physical exercise combined with exposure to m-xylene improved rather than impaired body sway.

Immediate effects of m-xylene on the human central nervous system

Male volunteers were exposed to m-xylene vapour for 4 h a day either sedentary or with a period of 10 min exercise twice a day with stable (8.2 mumol/l) m-xylene concentrations or fluctuating concentrations with peaks (16.4 mumol/l). The TWA concentration was always the same (8.2 mumol/l). The body balance of the subjects was clearly impaired in the anteroposterior (a-p) direction, especially with their eyes closed during the peaks at rest, whereas it was improved when exercise was included in spite of the higher blood m-xylene concentrations. When on the contrary, the situation was reversed in the lateral direction. Complex audiomotor reactions were impaired after the peaks combined with exercise, whereas changes in simple reaction times were similar in pattern to those found for body balance in the a-p direction. The results suggest a complex interaction between physiological factors and m-xylene and development of adaptation or tolerance during the exposure.

Short-term exposure of human subjects to m-xylene and 1,1,1-trichloroethane

Inhalation exposure of male volunteers to m-xylene (8.2 mumol/l, 200 ppm) and 1,1,1-trichloroethane (TCE) (8.2 and 16.4 mumol/l, 200 and 400 ppm), as well as to a combination of TCE (16.4 mumol/l) and xylene (8.2 mumol/l), induced only slight changes in psychophysiological functions such as body sway and reaction time. Exposure to the 8.2 mumol/l concentration of xylene or TCE tended to improve the performances, whereas TCE exposure at 16.4 mumol/l alone or in combination with xylene tended to have an opposite effect. The results thus suggest a biphasic acute effect of TCE on the central nervous system (CNS) and reveal that xylene and TCE together exhibit neither kinetic interaction nor synergistic nor antagonistic acute effects on the CNS functions studied.

Progressive myoclonus epilepsy is not accompanied by humoral immune response within the central nervous system

CSF and serum from five patients with progressive myoclonus epilepsy taken on two occasions with about one month's interval were examined for intrathecal humoral immune response. The CSF IgG and IgA index values were normal, and no oligoclonal bands were detectable by agarose electrophoresis or polyacrylamide isoelectric focusing (IEF). Immunofixation of IEF separated IgG with four different viruses as possible antigens, and autoradiography did not reveal any intrathecal antibody production. In contradiction to previous reports, no evidence was obtained for a local humoral immune response in progressive myoclonus epilepsy.

Short-term exposure of human subjects to m-xylene and 1,1,1-trichloroethane

Nine healthy male students were exposed to singular atmospheric concentrations of m-xylene (8.2 mumol/l; 200 ppm) or 1,1,1-trichloroethane (TCE) (8.2 and 16.4 mumol/l; 200 and 400 ppm), and also to a combination of xylene (8.2 mumol/l) and TCE (16.4 mumol/l) for 4 h per day at 6-day intervals. The effects of the atmospheric xylene and TCE concentrations on psychophysiological functions such as reaction time, body balance and CFF thresholds were studied. The exposures to xylene alone and to the lower TCE concentrations usually tended to improve the performances, whereas the higher TCE concentration alone or in combination with xylene tended to have an opposite effect, although statistically significant changes in performance, as compared to the control values, were rare. The results thus suggest a biphasic effect of TCE on the central nervous system (CNS), slight stimulation of the CNS at lower and depression at higher TCE concentrations. The results also revealed that xylene and TCE together exhibited neither kinetic interaction nor synergistic nor antagonistic effects on the CNS functions studied.