Neuroimaging evidence for cortical involvement in the preparation and in the act of swallowing

This study employed whole head magnetoencephalography and synthetic aperture magnetometry to investigate the cortical topography of the preparation and the execution of volitional and reflexive water swallowing and of a simple tongue movement. Concerning movement execution, activation of the mid-lateral primary sensorimotor cortex was strongly lateralized to the left during volitional water swallowing, less strongly lateralized to the left during reflexive water swallowing, and not lateralized at all during tongue movement. In contrast, the preparation for both volitional water swallowing and tongue movement showed a bilateral activation of the primary sensorimotor cortex. No activation was seen prior to reflexive water swallowing. Activation of the left insula and frontal operculum was observed only during both the preparation and the execution of volitional water swallowing. These new findings suggest a left hemispheric dominance for the cortical control of swallowing in humans.

How atypical is atypical language dominance?

Atypical, right-hemisphere language dominance is poorly understood. It is often observed in patients with brain reorganization due to lesions early in life. It can also be encountered in seemingly normal individuals. We compared the patterns of neural language activation in 7 individuals with left- and 7 with right-hemisphere language dominance, none of whom had any evidence of brain lesions. We speculated that incongruencies in the activation patterns in atypical, right-hemisphere language dominance could indicate a reorganized neural language system after undetected early brain damage. Functional magnetic resonance imaging analysis of brain activation during phonetic word generation demonstrated (1). no increased activation in the subdominant hemisphere in right compared to left language dominance, (2). a similar variability in the pattern of activation in both groups, and (3). a mirror reverse pattern of activation in right- compared to left-hemisphere dominant subjects. These findings support the view that in individuals with an unrevealing medical history right-hemispheric dominance constitutes a natural rather than an abortive variant of language lateralization.

[Language acquisition and statistical learning]

Statistical learning is a basic mechanism of information processing in the human brain. The purpose lies in the extraction of probabilistic regularities from the multitude of sensory inputs. Principles of statistical learning contribute significantly to language acquisition and presumably also to language recovery following stroke. The empirical database presented in this manuscript demonstrates that the process of word segmentation, acquisition of a lexicon, and acquisition of simple grammatical rules can be entirely explained through statistical learning. Statistical learning is mediated by changes in synaptic weights in neuronal networks. The concept therefore stands at the transition to molecular biology and pharmacology of the neuronal synapse. It still remains to be shown if all aspects of language acquisition can be explained through statistical learning and which regions of the brain are involved in or capable of statistical learning. Principles of effective language training are obvious already. Most important is the massive, repeated interactive exposure. Conscious processing of the stimulus material may not be essential. The crucial principle is a high cooccurrence of language and corresponding sensory processes. This requires a more intense training frequency than traditional aphasia treatment programs provide.

[Neuropsychological disorders in amyotrophic lateral sclerosis]

Amyotrophic lateral sclerosis (ALS) has traditionally been assumed to be a purely motor disturbance. It is now recognized that 2-5% of patients with ALS develop frontotemporal dementia. Additionally, neuropsychological analysis and functional imaging suggest that a proportion of patients with classical ALS also have neuropsychological impairment. On postmortem examination, ALS patients classified as not demented showed atrophy of the frontotemporal cortex. Conversely, in patients with frontotemporal dementia without known motor impairment, atrophy of spinal and bulbar neurons were found on postmortem examination. It is still not known whether patients with ALS and neuropsychological impairment form a distinct subgroup or if they are part of a continuous spectrum that runs from pure motor impairment to pure neuropsychological impairment.

When finding words becomes difficult: is there activation of the subdominant hemisphere?

Language-related activation has been observed in the right cerebral hemisphere by functional imaging in dysphasic patients who had partially recovered from a left hemispheric ischemic stroke with aphasia. It has been cautioned that, because dysphasic patients have difficulties in retrieving words, a right-hemisphere activation could be the result of an unspecific increase in global brain activation because of an increased effort. To test this hypothesis, we increased the difficulty of finding words in a word completion task in healthy subjects (n = 14) and measured hemispheric activation by functional transcranial Doppler sonography (fTCD). The sensitivity of fTCD for this approach was validated with an established motor paradigm by detecting a steady increase in bilateral cerebral perfusion in parallel to increasing the speed of finger tapping. Conversely, in the linguistic task, increasing the difficulty of word completion did not change task related perfusion of the dominant or subdominant hemisphere (repeated measurement ANOVA: P = 0.8). These results demonstrate that difficult to perform word searches do not lead to an additional involvement of the subdominant hemisphere. This suggests that after stroke, language-related activation of the subdominant hemisphere is not simply an effort-related effect.

Degree of language lateralization determines susceptibility to unilateral brain lesions

Language is considered a function of either the left or, in exceptional cases, the right side of the brain. Functional imaging studies show, however, that in the general population a graded continuum from left hemispheric to right hemispheric language lateralization exists. To determine the functional relevance of lateralization differences, we suppressed language regions using transcranial magnetic stimulation (TMS) in healthy human subjects who differed in lateralization of language-related brain activation. Language disruption correlated with both the degree and side of lateralization. Subjects with weak lateralization (more bilaterality) were less affected by either left- or right-side TMS than were subjects with strong lateralization to one hemisphere. Thus in some people, language processing seems to be distributed evenly between the hemispheres, allowing for ready compensation after a unilateral lesion.

D-amphetamine does not improve outcome of somatosensory training

BACKGROUND

D-amphetamine has been shown to affect early stages of stroke recovery, and may have a beneficial effect on functions when administered later after stroke.

OBJECTIVE

To test D-amphetamine effects on skill acquisition after the acute or subacute stages of stroke, when lesion-related structural changes have consolidated.

METHODS

Sixteen healthy subjects were treated with D-amphetamine during a 4-week training of tactile frequency discrimination in a placebo-controlled, double-blind design.

RESULTS

All subjects improved significantly in tactile temporal acuity. However, improvement did not differ in subjects treated with or without D-amphetamine.

CONCLUSION

No beneficial effect of D-amphetamine on somatosensory training improvements was found in healthy subjects.

Language and spatial attention can lateralize to the same hemisphere in healthy humans

BACKGROUND

Disorders of language classically occur after left brain lesions, and disorders of spatial attention after right brain lesions. It is unclear whether the hemispheric dissociation of functions is a fixed pattern of brain organization.

OBJECTIVE

The authors determined whether lateralization of language and lateralization of spatial attention also dissociate in people with atypical (i.e., right hemispheric) language dominance.

METHODS

The authors selected 10 subjects with typical, i.e., left hemispheric, and 10 with atypical, i.e., right hemispheric, language representation on a random basis from a sample of 326 healthy volunteers examined with functional transcranial Doppler sonography (fTCD) for language dominance. In these subjects, hemispheric lateralization of cerebral perfusion during a line bisection task was determined with fTCD.

RESULTS

The authors found a dissociation between dominance for language and spatial attention in all but four subjects. In the latter subjects, there was a significant lateralization to the right hemisphere for both tasks. The four subjects showed normal intellectual, linguistic, and spatial performance, with normal EEG and MRI scans of the brain.

CONCLUSION

Even in the absence of brain pathology, the same hemisphere can be dominant in control of both language and spatial attention.

Latency of auditory evoked field deflection N100m ruled by pitch or spectrum?

The auditory evoked field (AEF) in response to pure tones of 250 and 1000 Hz and a complex tone with a periodicity of 4 ms (composed of the frequencies 1000, 1250, 1500, 1750, and 2000 Hz), corresponding to a pitch of 250 Hz, was recorded with a 37-channel neuromagnetometer system. The intensity was 60 dB sensation level (SL). Two different stimulus durations were examined in 12 subjects: 500 ms (long tones) and 100 ms (short tones). The stimulus onset asynchrony (SOA) was uniformly distributed between 3 and 4 s for the long tones and between 0.8 and 1.2 s for the short tones. Each subject was investigated four times, to assess the intraindividual variability. The mean latency of the AEF deflection N100m turned out to be similar for the long and the short tones: about 98 and 87 ms for the pure tones of 250 Hz and 1000 Hz, respectively, and 95 ms for the complex tone with a pitch of 250 Hz. However, a great interindividual variability was observed, exhibiting no consistent relationship between the N100m latencies for the three different tones, except that the response to the pure tone of 1000 Hz generally occurred earlier. In conclusion, this study does not support the proposal that the N100m latency represents a code for pitch, although a low pitch appears to be a factor favoring a longer N100m latency.

Behavioural relevance of atypical language lateralization in healthy subjects

In most humans, language is lateralized to the left side of the brain. It has been speculated that this hemispheric specialization is a prerequisite for the full realization of linguistic potential. Using standardized questionnaires and performance measures, we attempted to determine if there are behavioural correlates of atypical, i.e. right-hemispheric and bilateral, language lateralization. The side and degree of language lateralization were determined by measuring the hemispheric perfusion differences by functional transcranial Doppler ultrasonography during a word generation task in healthy volunteers. Subjects with left (n = 264), bilateral (n = 31) or right (n = 31) hemisphere language representation did not differ significantly with respect to mastery of foreign languages, academic achievement, artistic talents, verbal fluency or (as assessed in a representative subgroup) in intelligence or speed of linguistic processing. These findings suggest that atypical hemispheric specialization for language, i.e. right-hemisphere or bilateral specialization, is not associated with major impairments of linguistic faculties in otherwise healthy subjects.

Functional reorganization of the human primary somatosensory cortex after acute pain demonstrated by magnetoencephalography

The somatosensory system is capable of functional reorganization following peripheral denervation or training. Studies on human amputees with phantom limb pain provided evidence that these reorganizational changes are modulated through nociceptive input. In the present study we used magnetoencephalographic recordings of six healthy volunteers to assess whether acute pain by itself causes a reorganization of the primary somatosensory cortex. After the induction of an intense experimental pain at the thenar of the left hand by intradermal injection of capsaicin, the extent of the cortical hand representation and the distance between the hand representation and the localization of the lip decreased. A likely mechanism for this acute reorganization is that pain induced hyperresponsiveness of the left thenar to tactile input from neighboring body sites.

Handedness and hemispheric language dominance in healthy humans

In most people the left hemisphere of the brain is dominant for language. Because of the increased incidence of atypical right-hemispheric language in left-handed neurological patients, a systematic association between handedness and dominance has long been suspected. To clarify the relationship between handedness and language dominance in healthy subjects, we measured lateralization directly by functional transcranial Doppler sonography in 326 healthy individuals using a word-generation task. The incidence of right-hemisphere language dominance was found to increase linearly with the degree of left-handedness, from 4% in strong right-handers (handedness = 100) to 15% in ambidextrous individuals and 27% in strong left-handers (handedness = -100). The relationship could be approximated by the formula: f1.gif" BORDER="0">. These results clearly demonstrate that the relationship between handedness and language dominance is not an artefact of cerebral pathology but a natural phenomenon.

Assessment of hemispheric language lateralization: a comparison between fMRI and fTCD

The cerebral blood flow velocity (CBFV) in the basal arteries during a word-generation task was assessed by functional transcranial Doppler ultrasonography (fTCD) and by functional magnetic resonance imaging (fMRI). The study investigates how event-related CBFV modulations in the middle cerebral artery (MCA) relate to regional cerebral blood flow (rCBF) changes. Both fMRI and fTCD were used in 13 subjects (7 men, 6 women, aged 21 to 44 years). The maximum difference of relative CBFV changes between the left and right MCA during the word-generation task was used as the language laterality index (LIfTCD). For the fMRI examination during the nearly identical language task, the corresponding index was defined by LIfMRI = 100(N(L) - N(R))/(N(L) + N(R)), where N(L) and N(R) refer to the numbers of voxels activated in the left and right hemisphere, respectively. The evoked CBFV changes expressed by LIfTCD and the corresponding laterality index, LIfMRI, estimated by fMRI showed a close linear relation (regression analysis: r = 0.95, p < 0.0001). The results of this study demonstrate that language-related velocity changes in the MCAs relate to rCBF increases in a linear fashion. Since the laterality indices assessed by fMRI and fTCD are in such close agreement both techniques can therefore be used in a complementary way.

Optimized activation of the primary sensorimotor cortex for clinical functional MR imaging

BACKGROUND AND PURPOSE

One application of functional MR imaging is to identify the primary sensorimotor cortex (M1 and S1) around the central sulcus before brain surgery. However, it has been shown that undesirable coactivation of nonprimary motor areas, such as the supplementary motor area and the premotor area, can interfere with the identification of the primary motor cortex, especially in patients with distorted anatomic landmarks. We therefore sought to design a simple functional MR imaging paradigm for selective activation of the primary sensorimotor cortex.

METHODS

Different paradigms using finger tapping for motor activation were examined and compared with respect to the distribution of activated voxels in primary and nonprimary cortical areas. Studies were conducted in 14 healthy volunteers using a blood oxygen level-dependent multislice echo-planar imaging sequence.

RESULTS

The most selective activation of the primary sensorimotor cortex was obtained with a paradigm combining right-sided finger tapping as the activation condition with left-sided finger tapping as the control condition. Analysis of the signal time course of primary and nonprimary areas revealed that the highly selective primary motor activation was due to it being restricted to contralateral finger movements, as opposed to the nonprimary motor areas, which were activated by ipsilateral, contralateral, and bilateral finger movements alike.

CONCLUSION

When performing functional MR imaging to determine the location of the primary sensorimotor cortex, one should compare unilateral voluntary movements as the activation condition with contralateral movements as the control condition to accentuate activation of the primary motor area and to suppress undesirable coactivation of nonprimary motor areas.

Language lateralization in healthy right-handers

Our knowledge about the variability of cerebral language lateralization is derived from studies of patients with brain lesions and thus possible secondary reorganization of cerebral functions. In healthy right-handed subjects 'atypical', i.e. right hemisphere language dominance, has generally been assumed to be exceedingly rare. To test this assumption we measured language lateralization in 188 healthy subjects with moderate and strong right-handedness (59% females) by a new non-invasive, quantitative technique previously validated by direct comparison with the intracarotid amobarbital procedure. During a word generation task the averaged hemispheric perfusion differences within the territories of the middle cerebral arteries were determined. (i) The natural distribution of language lateralization was found to occur along a bimodal continuum. (ii) Lateralization was equivalent in men and women. (iii) Right hemisphere dominance was found in 7.5% of subjects. These findings indicate that atypical language dominance in healthy right-handed subjects of either sex is considerably more common than previously suspected.

[Neuronal plasticity exemplified by the somatosensory system]

The mammalian brain is capable of a substantial functional reorganization, manifesting on a cortical somatotopical and on a behavioral level. Possible mechanisms are reviewed based on the work by others and ourselves on somatosensory reorganization in humans. The somatosensory system is characterized by divergent projections from the periphery to the cerebral cortex. Changes in synaptic weights allow for reorganization of sensory processing: On one side, limb amputation will result in a representational "invasion" of the differentiated cortex from neighboring regions with concomitant perceptual changes. On the other side, sensorimotor training can increase the representational cortical zone of a limb. Plastic changes can be temporary or persistent. Modulating factors like pain and certain drugs seem to induce a permissive state in the cortex resulting in enhanced reorganization. Thus, specific physical training combined with pharmacoceutical modulation holds promise to improve functional recovery after brain lesions.

Cortical asymmetries of the human somatosensory hand representation in right- and left-handers

Hemispheric asymmetry is known for higher brain functions like language and attention. We tested whether such an asymmetry also exists in the representation of elementary sensory functions. Magnetic source imaging was used to compare the cortical somatosensory hand representation in seven right- and five left-handed individuals. In all right-handers the representation of the dominant hand was larger than the contralateral one in the corresponding hemispheres. In contrast, only two out of five left-handers revealed a larger representation of the dominant left hand compared to the right one. In agreement with previous findings on the lateralization of language and attention, there is a strong correlation between handedness and the extent of the cortical hand representation in right-, but not in left-handers. We conclude that a profound functional hemispheric asymmetry also exists in primary sensory cortices.

Clinical applications of functional MRI at 1.0 T: motor and language studies in healthy subjects and patients

In this article we describe clinical applications of functional MRI (fMRI) at 1.0 T. All experiments were performed on a commercially available 1.0-T system (Magnetom Impact Expert, Siemens AG, Erlangen, Germany) using a blood oxygen level-dependent (BOLD)-sensitive multi-slice EPI technique (TE 66 ms, 4 mm slice thickness, 210 mm field of view, 64 x 64 acquisition matrix). Different paradigms for localization of the motor cortex and for language lateralization were tested in healthy subjects and patients. Methodological considerations concerning the development of the paradigms are also described. In all healthy subjects, motor activation elicited BOLD signal changes in the sensorimotor cortex, permitting identification of primary motor and sensory cortical areas. Furthermore, focal activation of different cortical areas by a language task was possible in 6 of 10 subjects. Nineteen motor studies were performed in 18 patients with supratentorial lesions, in most cases prior to neurosurgical procedures. In 14 studies, fMRI results demonstrated the localization of the motor hand areas relative to the lesion. The results proved valuable for preoperative planning and contributed to therapeutical decisions. We conclude that functional MRI for clinically relevant applications, such as localization of motor and language function, is feasible even at a field strength of 1.0 T without dedicated equipment.

Cortical tuning: a function of anticipated stimulus intensity

We investigated the activation of the brain during anticipation of tactile stimuli by continuous cerebral blood flow velocity (CBFV) monitoring with bilateral transcranial Doppler sonography. A forced choice paradigm was performed where a first group of subjects (n=16) was expecting suprathreshold and a second group (n=19) was anticipating threshold tactile stimuli to the index finger after a cueing tone. During the anticipation of suprathreshold stimuli the CBFV always exhibited a significantly stronger increase in the right hemisphere than in the left, even when stimuli were anticipated at the right index finger. Conversely when stimuli at perception threshold were expected, the respective contralateral hemisphere showed a significantly stronger perfusion increase. These data show that preparatory activation of the brain during stimulus anticipation is dependant on the expected stimulus intensity.

Cerebral hemodynamic response to generalized spike-wave discharges

PURPOSE

Data in the literature concerning metabolic demand during generalized spike-wave activity (gSW) are conflicting. We investigated instantaneous changes in cerebral blood flow velocities (CBFV) in both middle cerebral arteries (MCAs) by transcranial Doppler sonography (TCD) during gSW paroxysms recorded by scalp EEG.

METHODS

In 13 patients, CBFVs in both MCAs were averaged, time-locked to the occurrence of the gSW; respiratory rate (RR) and end-expiratory pco2 were measured in one patient.

RESULTS

Nine patients showed significant changes in CBFV during gSW. Four had biphasic flow changes with an initial increase (p < 0.05) and a subsequent decrease (p < 0.01). This was partially paralleled by an increase in RR (p < 0.01) and a decrease in pco2 (p < 0.01). In three patients, an increase in CBFV that preceded the onset of gSW by several seconds was observed, followed by a decrease in CBFV. Two patients showed a significant decrease only of CBFV. Only gSWs of a median duration of >0.8 s were associated with significant changes in CBFV.

CONCLUSIONS

We were able to demonstrate that gSWs of several seconds duration lead to cortical perfusion changes. We suggest that the initial increase of CBFV demonstrated in some patients reflects neuronal activation, whereas the subsequent decrease might in part be due to hyperventilation-induced hypocapnia.

Reproducibility of functional transcranial Doppler sonography in determining hemispheric language lateralization

BACKGROUND AND PURPOSE

Since functional transcranial Doppler ultrasonography (fTCD) allows convenient and fully automated quantification of language lateralization, it seems ideal for longitudinal studies of perfusion changes during deterioration as well as recovery of language functions. However, during serial examinations, the technical, stochastic, and physiological variabilities of cerebral blood flow velocities (CBFV) have to be considered. Therefore, before fTCD is accepted as a tool for evaluation of changes in lateralization in the diseased state, its reliability in healthy subjects needs to be determined.

METHODS

We performed fTCD during a word generation task based on a previously validated technique with automated calculation of the averaged CBFV differences in the middle cerebral arteries providing an index of lateralization (LI).

RESULTS

(1) The accuracy of the LI as assessed by the confidence interval was better than 1% of the mean hemispheric difference. (2) On repeated examination, LIs obtained from 10 subjects showed a high test-retest reproducibility (Pearson product moment correlation coefficient r = 0.95, P < 0.0001). (3) On 10 repeated assessments of LI in the same subject, no practice effects were detected.

CONCLUSIONS

Functional TCD is a suitable and very robust tool for the longitudinal quantitative measurement of cerebral language lateralization.

Plasticity of plasticity? Changes in the pattern of perceptual correlates of reorganization after amputation

We report a follow-up study on seven arm amputees in whom magnetic source imaging had originally revealed a strong correlation between the amount of cortical invasion of the deafferented cortex and the amount of pain evoked sensation mislocalized to the phantom limb. This re-examination was performed in order to corroborate the phenomenon of mislocalization. On follow-up examination for mislocalization 4 weeks later, a close correlation had remained between the original amount of cortical representational reorganization of the amputation zone (at the first examination) and the number of sites from where painful stimuli evoked sensations referred to the phantom limb, i.e. the amount of perceptual mislocalization, at the second examination. However, contrary to our expectation, the topography of referred sensation had completely changed in every patient. These results suggest that while the overall extent of reorganization is a rather stable phenomenon, the concomitant changes in the pattern of sensory processing are not. This may be due to the fact that alterations of sensory processing are not hardwired, but are rather mediated by an extensive and interconnected neural network with fluctuating synaptic strengths. This mechanism may be of importance for neurological rehabilitation.

Molecular characterization of a novel, widespread nuclear protein that colocalizes with spliceosome components

We report the identification and molecular characterization of a novel type of constitutive nuclear protein that is present in diverse vertebrate species, from Xenopus laevis to human. The cDNA-deduced amino acid sequence of the Xenopus protein defines a polypeptide of a calculated mass of 146.2 kDa and a isoelectric point of 6.8, with a conspicuous domain enriched in the dipeptide TP (threonine-proline) near its amino terminus. Immunolocalization studies in cultured cells and tissues sections of different origin revealed an exclusive nuclear localization of the protein. The protein is diffusely distributed in the nucleoplasm but concentrated in nuclear speckles, which represent a subnuclear compartment enriched in small nuclear ribonucleoprotein particles and other splicing factors, as confirmed by colocalization with certain splicing factors and Sm proteins. During mitosis, when transcription and splicing are downregulated, the protein is released from the nuclear speckles and transiently dispersed throughout the cytoplasm. Biochemical experiments have shown that the protein is recovered in a approximately 12S complex, and gel filtration studies confirm that the protein is part of a large particle. Immunoprecipitation and Western blot analysis of chromatographic fractions enriched in human U2 small nuclear ribonucleoprotein particles of distinct sizes (12S, 15S, and 17S), reflecting their variable association with splicing factors SF3a and SF3b, strongly suggests that the 146-kDa protein reported here is a constituent of the SF3b complex.

Noninvasive determination of language lateralization by functional transcranial Doppler sonography: a comparison with the Wada test

BACKGROUND AND PURPOSE

Functional transcranial Doppler ultrasonography (fTCD) can assess event-related changes in cerebral blood flow velocities and, by comparison between sides, can provide a measure of hemispheric perfusional lateralization. It is easily applicable, insensitive to movement artifacts, and can be used in patients with less than perfect cooperation. In the present study we investigated the validity of fTCD in determining the hemispheric dominance for language by direct comparison of fTCD with intracarotid amobarbital anesthesia (Wada test).

METHODS

fTCD and the Wada test were performed in 19 patients evaluated for epilepsy surgery. By the Wada test, 13 patients were classified as left-hemisphere dominant and 6 as right-hemisphere dominant for language. fTCD was based on the continuous bilateral measurements of blood flow velocities in the middle cerebral arteries and event-related averaging during a cued word generation task previously shown to activate lateralized language areas in normal adults.

RESULTS

In 4 patients fTCD assessment was not possible because of lack of an acoustic temporal bone window. In the remaining 15 candidates, determination of language dominance was concordant with the Wada test in every case. Moreover, the correlation of the lateralization measures from both procedures was highly significant (r=.92, P<.0001).

CONCLUSIONS

This strong correlation validates fTCD as a noninvasive and practical tool for the determination of language lateralization that can be applied for clinical and investigative purposes.

Input-increase and input-decrease types of cortical reorganization after upper extremity amputation in humans

A plastic remodeling of regions in somatosensory cortex has previously been observed to occur in separate experimental paradigms in response to loss of somatosensory input and to increase in input. In this study, both types of cortical reorganization have been observed to occur concurrently in the same adult human nervous system as a result of a single intervention. Following upper extremity amputation, magnetic source imaging revealed that tactile stimulation of the lip evoked responses not only in the area of the somatosensory cortex corresponding to the face, but also within the cortical region that would normally correspond to the now absent hand. This "invasion" of the cortical amputation zone was accompanied by a significant increase in the size of the representation of the digits of the intact hand, presumably as a result of an increased importance of sensory stimulation consequent to increased dependence on that hand imposed by the loss of the contralateral extremity.

Regional cerebral blood flow increases during preparation for and processing of sensory stimuli

Preparing for and processing of sensory stimuli are energy-requiring processes. We attempted to assess the relative contributions of these processes to increases in regional cerebral perfusion. Nineteen healthy right-handed subjects were examined while they were engaged in detecting tactile stimuli to the index finger 5 s after a cueing tone. Cerebral blood flow velocity (CBFV) modulations in the middle cerebral arteries (MCAs) were continuously measured by bilateral simultaneous transcranial Doppler ultrasonography. Tactile stimuli well above threshold per se did not produce a significant, relative CBFV increase in the contralateral MCA. However, when subjects were expecting a threshold tactile stimulus, there was a significant regional increase in CBFV in the hemisphere contralateral to the attended index finger for approximately 15 s, starting within the first seconds after the cueing. This increase was present even before the tactile stimulus was applied and also in sessions when the stimulus was omitted. We conclude that preparation of the cortex causes a stronger regional cerebral blood flow increase than the processing of the tactile stimulus itself.

AVERAGE: a Windows program for automated analysis of event related cerebral blood flow

Functional transcranial Doppler sonography (fTCD) is used to measure changes in brain perfusion during different states of brain activity by evaluating flow velocities within the major brain arteries. We developed a computer program called AVERAGE which can be used with various TCD devices and allows for subtle quantitative off-line analysis of Doppler flow signals. AVERAGE supports data transformation, heart beat analysis, noise reduction, trigger signal and marker modification, artifact analysis and artifact rejection, as well as data reduction. Perfusion differences and their time course within two different arteries can be analyzed by parametric and non parametric statistical methods. This is important, for instance, in studies on hemispherical dominance during mental task processing. Each data processing step is supported by graphical output. If investigated bilaterally with interhemispheric analysis even minimal activations can reliably be detected and quantified with a sensitivity known from positron emission tomography approaches.

Influence of afferent feedback on isometric fine force resolution in humans

The influence of afferent feedback on isometric fine force resolution was studied in humans. Subjects performed the smallest possible isometric flexion force increments with the index finger while visual, cutaneous, and muscle spindle feedback conditions were varied. In the control conditions with visual feedback, isometric force resolution was finest and independent of cutaneous or muscle spindle feedback. In the absence of visual cues, force resolution was significantly coarser. When agonist muscle spindles were vibrated (100 Hz and 150 Hz), fine force resolution capabilities declined further. Diminution of cutaneous feedback per se did not affect fine force resolution. However, the effect of agonist vibration was attenuated when full cutaneous feedback was available. We conclude that in voluntary isometric contractions the degree of fine force resolution depends on the type of afferent feedback available for calibrating central motor commands. Visual feedback is more powerful than spindle feedback, which is more efficient than cutaneous feedback. The extent to which the central motor command itself contributes to the sensation of force is indirectly implied by reproducible, yet coarser force resolution levels when peripheral information is minimized.

Persistent unihemispheric perceptual impairments in humans following focal seizures

Perception has been linked to a highly coordinated activation of cortical regions whose functional organization and performance is subject to plastic changes. We tested whether chronic repetitive disturbances of the brain by focal epileptic activity have a long-standing detrimental effect on the perceptual performance in the affected hemisphere. Nine patients were examined who had a history of complex partial seizures but no structural cerebral damage on magnetic resonance imaging and no evidence of ongoing epileptic activity on scalp electroencephalography and who had clinically been without seizures for at least 3 days. The side of primary epileptic involvement was determined by seizure semiology (n = 2), focal electroencephalographic slowing (n = 3) or focal abnormality during single photon emission topography (SPECT) (n = 4). The computer controlled psychometric assessment of the somesthetic frequency discrimination revealed that the perception in the hand corresponding to the affected hemisphere was impaired relative to the contralateral hand (P < 0.01), and to the performance of a group of normal controls (P < 0.01). We conclude that mechanisms related to focal epileptic activity can result in regional perceptual decrements even when there is no clinical or surface-electroencephalographic evidence of epileptic discharges. This in turn suggests that somatosensory testing may be of help in localizing, or at least lateralizing an epileptic focus.

Reorganizational and perceptional changes after amputation

The demonstration of cortical representational shifts in adult animals subsequent to deafferentation from amputation or dorsal rhizotomy has spurred attempts to elucidate the perceptual correlates of reorganization. Because the hand is flanked by the face and the trunk on the cortical homunculus it has been suggested that cortical remapping in arm amputees leads to a mislocalization of sensations from these sites to the phantom arm in a systematic manner with modality specific one-to-one topographical correspondence. Therefore, we assessed shifts of representational zones by magnetic source imaging in eight arm-amputees and examined them for referred sensation by somaesthetic stimuli of different modalities at standardized sites. It was found that referred phantom sensations can be evoked from sites on the face and the trunk ipsilateral but also contralateral to the amputation and that the extent of physiological reorganization as revealed by magnetic source imaging strongly correlates with the number of sites, be it ipsi- or contralateral, from where painful stimuli evoke referred sensation. Thus, it seems that the extent of reorganization after amputation is closely related to nociceptive inputs. The mislocalization evoked from both sides of the body, suggesting involvement of bilateral pathways, demonstrates that the perceptual changes go beyond what can be explained by shifts in neighbouring cortical representational zones.

Parallel and serial processing of haptic information in man: effects of parietal lesions on sensorimotor hand function

Recent animal studies have shown that there is an evolutionary shift within the order of primates from parallel to serial processing of haptic information. In an attempt to determine whether there is also evidence of serial processing in humans 10 patients with parietal cortical lesions, three patients with subcortical lesions and one patient after hemispherectomy, were examined. Case-by-case and across subject analysis of lesion type, sensorimotor profile and electrophysiological findings showed that in unihemispheric lesions: (a) there is little impairment of thermesthesia, nociception and vibration sense: (b) two-point discrimination and integrity of the N20 somatosensory component are highly correlated; (c) a loss of the N20 component is accompanied by a severe impairment of stereognosis; (d) conversely, in more posterior lesions astereognosis can occur with an intact N20 component; and (e) if the lesion is in the right hemisphere there is frequently impairment of graphesthesia in both hands. These data are taken to indicate serial processing from SI (as evidenced by an intact N20 component) to posterior parietal cortex allowing progressive spatial and temporal integration. In graphesthesia our data suggest an integrative function of the right parietal cortex for both sides of the body. Other sensory qualities like vibration nociception and thermesthesia are apparently processed in a non-serial, probably parallel way involving both hemispheres. The effects of cerebral lesions in our series suggest parallel as well as serial processing of somesthetic information in man underlying the perception of different haptic features.

Successive activation of both cerebral hemispheres during cued word generation

Most humans have a left hemispheric dominance for language. However, during diagnostic language testing there also is activation of the right hemisphere, which is probably related to attention. To investigate further the role of attention during language production we monitored cerebral blood flow velocities in the middle cerebral arteries by continuous bilateral transcranial Doppler ultrasonography (TCD) in nine right-handed subjects. During cued word generation, a successive activation of the right hemisphere in the preparatory phase, and of the left hemisphere during word searching was demonstrated. These findings indicate that attentional processes precede verbal activation and that the two procedures can be separated by comparative blood flow velocity measurement.

Cortical reorganization in human amputees and mislocalization of painful stimuli to the phantom limb

In human arm amputees, a significant relationship was found between the amount of reorganization in the primary somato-sensory cortex, and the amount of body surface from which painful stimuli evoked sensations that were perceived to be emanating from the now missing extremity, i.e. the phantom limb. This mislocalization could be evoked almost equally from stimulation of either side of the body. Based on these findings obtained by magnetic source imaging and psychophysical testing in eight amputees, it is concluded that the extent of the generally known cortical reorganization contralateral to the amputation is an indicator of more widespread plastic changes in the brain involving bilateral pathways.

Somatosensory evoked potentials elicited by intraneural microstimulation of afferent nerve fibers

Cortical somatosensory evoked potentials (SEPs) after intraneural microstimulation (IMS) of cutaneous and afferent muscle nerve fibers in the median nerve were recorded to study the contribution of different afferent fiber groups to the SEP. Thirty-seven cutaneous fiber bundles, 10 afferent muscle nerve fiber bundles, and 45 single mechanoreceptive afferents of FA I- (n = 12), FA II- (n = 8), SA I- (n = 13), and SA II-type (n = 12) were studied in 29 healthy subjects. IMS of cutaneous fiber bundles evoked cortical responses corresponding to the N20 component after median nerve stimulation in 86% of the fascicles studied, whereas IMS of muscle nerve fiber bundles elicited responses only in 20%. After IMS of single mechanoreceptive afferents of FA I-, FA II-, and SA I-type cortical responses were obtained in all groups in approximately 80% of the stimulated units. The latencies of the SEPs evoked by IMS were comparable to that after compound median nerve stimulation (mean difference 0.58 ms). The N20 amplitudes of SEPs after IMS of cutaneous fiber bundles were on average 28% (n = 32) and for single afferents 22% (n = 30) of that after compound nerve stimulation. It is concluded that the median nerve SEPs evoked by compound median nerve stimulation of the resting hand are dominated by cutaneous rather than muscle afferent input. Furthermore, the fact that selective stimulation of only a few cutaneous afferents produces cortical SEPs of rather high amplitude is compatible with the view that sensory information produced by selective afferent stimulation is differentially gated into the somatosensory cortex.