Positron emission tomography imaging in human immunodeficiency virus-1-associated neurocognitive disorders: an interesting case and a review of the positron emission tomography literature.

Human immunodeficiency virus (HIV)-1-associated neurocognitive disorder can manifest with a variety of neurologic, cognitive, and behavioral impairments. We report a case of a 49-year-old non-HIV risk woman with an occult HIV infection who posed a diagnostic challenge as she suffered from a HIV-1-associated neurocognitive disorder with predominant motor symptoms mimicking upper motor neuron disease. Functional imaging using F-18 fluorodeoxyglucose positron emission tomography provided evidence of involvement of several cerebral regions which exhibited a distinct pattern of relative cerebral hypermetabolism (subcortical, brainstem, and cerebellar regions) and hypometabolism (sensorimotor cortex, mesiofrontal, and mesiotemporal areas) and functionally corresponded to the clinical symptoms. The results of the positron emission tomography scan are discussed in comparison with the current positron emission tomography literature and future perspectives are illustrated.

Evaluation of thalamocortical impulse propagation in the akinetic Rigd type of Parkinson's disease using high-frequency (600 Hz) SEP oscillations.

Human median nerve somatosensory evoked potentials exhibit low-amplitude, high-frequency (600 Hz) oscillations (HFOs) superimposed onto the primary cortical response 'N20.' Previous EEG-studies indicated the HFOs to reflect in part activity generated at the thalamus and within the thalamocortical radiation. Expecting impairment of thalamocortical impulse propagation in Parkinson's disease (PD) the present study aimed to explore the performance of the HFOs in a cohort of PD patients in comparison to an exactly age- and sex-matched group of controls. To avoid motor interference and to minimize the influence of central tremor oscillators to the HFOs we selected PD patients suffering from an akinetic rigid type of the disease. We recorded multichannel somatosensory evoked potentials and applied an advanced analysis combining source and time frequency analysis. Low frequency and HFO signals showed no significant differences between PD patients and the control group, neither in the source waveforms evaluation nor in the time frequency analysis. Contrasting two former studies indicating enhanced HFOs in nonselected PD patients the present results differ most probably due to the lack of motor interference effects and the missing impact of central tremor-oscillators on the signals by selecting patients without tremor. THEME: Sensory systems. TOPIC: Somatosensory cortex and thalamocortical relationships.

The impact of stimulus properties on low- and high-frequency median nerve somatosensory evoked potentials.

Questioning whether stimulation properties in median nerve somatosensory evoked potentials show interactions multichannel recordings were performed in a three-factorial repeated measures design with the parameters (i) eyes opened versus eyes closed, (ii) stimulation intensity above motor threshold versus intensity sub motor threshold, (iii) stimulation rate 0.5 Hz versus 9 Hz resulting in somatosensory evoked potentials recorded during eight different conditions. Varying the stimulation intensity revealed an impact on the amplitude and the latency of the N20 source activity and on the amplitude, the duration and the number of peaks of the high frequency oscillatory (HFOs) sources. Modifying the stimulation rate lead to an effect on the amplitude and latency of the N20 and on the amplitude of the high-frequency oscillatory sources. The condition opened/closed eyes had an impact on the duration and number of high-frequency oscillatory. No relevant interactions between the stimulation properties were found. In consequence, varying one stimulus parameter already leads to a saturation of the low as well as high-frequency somatosensory evoked potentials components. Thus, the careful choice of stimulation parameters is a condition precedent for reasonable data interpretation.

Cerebral processing of spontaneous reversals of the rotating Necker cube.

The cerebral processing of spontaneous perceptive reversals of the rotating Necker cube was studied in humans by combined functional MRI and electroencephalography. These reversals prefer certain positions of the Necker cube and can be studied without external reference of the perception. Functional MRI revealed six bilaterally active regions in the visual, parietal, and premotor cortex. A new method determined phase-locked electroencephalography-activations in the regions of interest and showed a significant stimulus-locked activity that started in the left Brodmann area 18. This activity started 38 ms after passing the symmetric position of the Necker cube and spread along the dorsal stream. We suggest that a further portion of the event-related potential signal reflects additional top-down processing, dependent on the position of the Necker cube.

Comparative source localization of electrically and pressure-stimulated multichannel somatosensory evoked potentials.

The purpose of the study was to determine if there is a difference in the determination of the cortical hand area by dipole source estimation after artificial and natural stimuli. In principle, there are advantages of both methods: pressure stimulation is less invasive and compatible to fMRI, whereas electrical stimulation can be applied with higher stimulus rates and elicits sharper waveforms. Electrical and pressure stimulation was performed simultaneously on the thumb and fifth finger on eight healthy volunteers. The somatosensory evoked potentials after electrical stimulation showed sharper peaks and higher amplitudes than the pressure stimulated potentials. For the two stimulus qualities, cortical source positions of thumb and fifth finger separated significantly in the vertical z-axis. Both methods deliver reliable stimulation and therefore allow separate source localization of thumb and fifth finger. For cortical plasticity studies, peripheral somatosensory stimulation is of great importance. According to these findings, the choice of method, electrical or mechanical stimulation, may depend on practical criteria.

Short-term cortical reorganization by deafferentation of the contralateral sensory cortex.

The topographic arrangement of the human primary somatosensory cortex following deafferentation of the contralateral cortex has been investigated by means of dipole source analysis. Somatosensory-evoked potentials were obtained by electrical stimulation of digit 1 and digit 5 of the left hand before and after anesthesia of digits 2-4 of the right hand during different terms of attention. Anesthesia induced an expansion of the three-dimensional distance between digits 1 and 5. This suggests intercortical plasticity modulated between bilateral primary somatosensory cortical areas, which is unaffected by spatial attention. These changes occur rapidly and are probably mediated by disinhibition of intercortical connections, leading to hyperexcitability of the primary sensory cortex that is contralateral to the region undergoing deafferentation.

Transcranial direct current stimulation applied over the somatosensory cortex - differential effect on low and high frequency SEPs.

OBJECTIVE: Transcranial direct current stimulation (tDCS) has an influence on the excitability of the human motor cortex measured by motor evoked potentials (MEPs) after transcranial magnetic stimulation. Low and high frequency (HFOs) components of somatosensory evoked potentials (SEPs) were studied questioning whether a comparable effect can be observed after applying tDCS to the human somatosensory cortex. METHODS: Multichannel median nerve SEPs were recorded before and after applying tDCS of 1mA over a period of 9min with the cathode placed over the somatosensory cortex and the anode over the contralateral forehead and vice versa in a second session. The source activity of the N20, N30 and HFOs was evaluated before and after application of tDCS. RESULTS: After cathodal tDCS to the somatosensory cortex we found a significant reduction of the N20 source amplitude while there was no effect after anodal stimulation. For the N30 component and HFOs no change in source activity was observed. CONCLUSIONS: Corresponding to the results for the motor cortex a sustained reduction of the excitability of the somatosensory cortex after cathodal tDCS was shown. SIGNIFICANCE: We demonstrated differential effects of tDCS on the high and low frequency components of SEPs confirming the hypothesis of locally and functionally distinct generators of these two components.

High-frequency somatosensory thalamocortical oscillations and psychopathology in schizophrenia.

Human cortical somatosensory evoked potentials (SEPs), which are presumably generated in afferent thalamocortical and early cortical fibers, reveal a burst of superimposed early (N20) high-frequency oscillations (HFOs), around 600 Hz. There is increasing evidence of an imbalance of thalamocortical systems in schizophrenic patients. In order to assess correlations between somatosensory evoked oscillations and symptoms of schizophrenia, we investigated median nerve SEPs in 20 inpatients and their age-matched and gender-matched healthy controls using a multichannel EEG. Dipole source analysis and wavelet transformation were performed before and after application of a 450-Hz high-pass filter. In schizophrenics, the maximum HFOs occurred with a significantly prolonged latency. There was also a higher amplitude (energy) in the low-frequency range of the N20 component compared with the controls. Importantly, amplitudes (energy) of HFOs were inversely correlated with symptoms of formal thought disorder and delusions. Alterations of the thalamocortical somatosensory signal processing in schizophrenia with absence of an early HFO - assumed to be of inhibitory nature - could indicate a dysfunctional thalamic inhibition with increased amplitudes of N20, paralleled by enhanced positive schizophrenic symptoms.

Patterns of disturbed impulse propagation in multiple sclerosis identified by low and high frequency somatosensory evoked potential components.

In human median nerve somatosensory evoked potentials (SSEPs), high frequency (600 Hz) oscillations (HFOs) are superimposed onto the low frequency SSEP component N20. High frequency oscillations are generated both in deep axon segments of thalamo-cortical projection neurons and at the primary somatosensory cortex. The present study aimed to test the hypothesis that HFOs might be more sensitive to temporal dispersion caused by demyelinating lesions in multiple sclerosis (MS) than the N20. The authors recorded HFOs in median nerve SSEPs in 50 patients with definite MS and in 30 healthy controls. Three patterns of SSEP alterations were found: (1) abolished HFOs with either normal (11% of stimulated limbs), or delayed N20 (16% of stimulated limbs); (2) an attenuation of N20 amplitude with preserved HFOs (13%); and (3) a mixture of both patterns (21%). The first pattern--normal N20 with abolished HFOs--indicates that the HFOs are a sensitive marker of slight demyelination. The second pattern is suggestive of a mainly axonal lesion type, while the third pattern points to a combined axonal/demyelinating process or a conduction block. Analysis of HFOs allows identification of slight demyelinating processes in MS patients in whom the N20 SSEP component remains unaffected. The HFOs provide a tool to distinguish different patterns of disturbed impulse propagation.

Functional dissociation of a subcortical and cortical component of high-frequency oscillations in human somatosensory evoked potentials by motor interference.

To identify the possibly divergent impact on early and late high-frequency oscillations (HFOs) in human somatosensory evoked potentials (SEPs), we have studied motor interference effects on the HFOs, and the relevance of such effects for the controversy concerning their origins. While the late HFO is thought to be generated in the somatosensory cortex, there is an ongoing discussion whether the early burst is of cortical or subcortical origin. Movements of the index finger were performed in parallel with median nerve SEP recordings. The intracortically generated N20-SEP and the late HFO were attenuated by the motor task, while the brainstem low-frequency P14-SEP and the early HFO remained unaffected. These differing effects are consistent with a generation of the early HFOs by cortical presynaptic activity in terminals of the thalamocortical projection, and confirm a postsynaptic intracortical origin of the late burst subcomponent.

Spatial direction of attention enhances right hemispheric event-related gamma-band synchronization in humans.

Gamma-band oscillations are related to sensory information processing and attention. To further illuminate the relationship of gamma-band activity and selective somatic attention, we have studied the effects of direction of attention on the stimuli of somatosensory evoked potentials. Responses during focused attention to stimulation of the right median nerve and ignored stimulation at the right tibial nerve were compared with responses of ignored stimulation of the right median nerve but focused attention to stimulation at the right tibial nerve. Subtraction of the data demonstrated a significant predominantly right hemispheric event-related synchronization in the gamma-band by attention to the right median nerve in the time period between 280 and 325 ms post-stimulus. This finding implicates the involvement of a right temporoparietal network in selective spatial attention.