Activation of a distributed somatosensory cortical network in the human brain: a dipole modelling study of magnetic fields evoked by median nerve stimulation. Part II: Effects of stimulus rate, attention and stimulus detection

In this study we used a repeated measures design and univariate analysis of variance to study the respective effects of ISI, spatial attention and stimulus detection on the strengths of the sources previously identified by modelling SEFs during the 200 ms following mentally counted left median nerve stimuli delivered at long and random ISIs (Part I). We compared the SEF source strengths in response to frequent and rare stimuli, both in detection and ignoring conditions. This permitted us to establish a hierarchy in the effects of ISI, attention and stimulus detection on the activation of the cortical network of SEF sources distributed in SI and posterior parietal cortex contralateral to stimulation, and in the parietal operculum (SII) and premotor frontal cortex of both hemispheres. In all experimental conditions the SI and parietal opercular sources were the most active. All sources were more active in response to stimuli delivered at long and random ISIs and the frontal sources were activated only in this condition of stimulation. Driving the subject's attention toward the side stimulated had no detectable effect on the activity of SEF sources at short ISI. At long ISIs mental counting of the stimuli increased the responses of all sources except SI. These results suggest that activation of frontal sources during mental counting could reflect a working memory process, and that of posterior parietal sources a spatial attention effect detectable only at long ISIs.

Visual motion activates V5 in dyslexics

A recent functional magnetic resonance imaging (fMRI) study concluded that the motion-specific visual area V5 is not activated in dyslexic subjects. We report here opposing evidence based on whole-scalp neuromagnetic recordings. Apparent-motion stimuli elicited similar activation of V5 in both dyslexic and control subjects, with a trend for longer latencies in dyslexics. Both high- and low-contrast stimuli activated the V5 region in dyslexics. The lack of significant blood flow changes despite modified neuronal synchrony would explain the absence of fMRI signals and the presence of neuromagnetic signals in dyslexic subjects.

Visual awareness of objects correlates with activity of right occipital cortex

In the search for human neural correlates of visual awareness, cortical magnetic responses to coherent and meaningful objects were compared with responses to disorganized and meaningless non-objects when observers tried to detect the coherent objects. Three brief stimulus durations were included to vary the detection rate of the objects. Of the multiple brain regions activated, only the right lateral occipital cortex showed signals correlating with the proportion of correct object detections. The results suggest an important role for this area in visual awareness of objects.

Cortical sources of human short-latency somatosensory evoked fields to median and ulnar nerve stimuli

;;;;;õry evoked magnetic fields were measured with a 122-channel whole-scalp neuromagnetometer from seven healthy adults. Electric stimuli, with an intensity above the motor threshold, were delivered once every 0.5 s alternately to the median and ulnar nerves at the wrist; both wrists were stimulated successively within one session. In most subjects, two distinct neural sources were identified at the contralateral primary somatosensory cortex SI for both stimuli. The first source (M20) peaked at 21-22 ms and indicated activation of area 3b in the contralateral SI hand region. The same source peaked with opposite current direction at 32 ms. The second source (M40) was slightly medial to M20 and exhibited two peaks with the same current direction, first at 25 ms and most prominently at 42 ms. M20 was on average 7 mm more lateral along the central sulcus for median than ulnar nerve stimuli, in agreement with the somatotopic organization of the SI cortex; similar organization for M40 was less clear. These results suggest that M20 and M40 to upper limb stimulation represent activation of distinct neuronal populations in hand SI cortex, presumably in area 3b.

Activation of human mesial cortex during somatosensory target detection task

We recorded somatosensory evoked fields (SEFs) from 10 healthy subjects to ulnar and median nerve stimuli presented at random intervals of 2.4-21.6 s. The subjects either counted the stimuli or ignored them by reading a book. The stimuli activated in both conditions the contralateral SI cortex, the ipsi- and contralateral SII cortices, and the posterior parietal cortex (PPC), in line with earlier observations. In addition, a novel response was observed in nine subjects at 120-160 ms. It was clearly enhanced by attention and was generated in the mesial cortex of the paracentral lobule, close to the end of the central sulcus.

Study of second messenger levels and of sugar catabolism enzyme activities in transformed cells resistant to ionizing radiations

We measured the level of second messengers, the activity of carbohydrate metabolism enzymes, and the resistance to ionizing radiations in normal 32D hematopoietic cells, in v-erbB transformants and in spontaneous transformants. v-erbB and spontaneous transformants were resistant to radiations as compared with their normal counterpart. The second messenger diacylglycerol was elevated in radioresistant clones. Only v-erbB transformants showed increase of the activities of enolase and glucose-6-phosphate dehydrogenase. v-erbB-transformed NIH/3T3 cells, selected as control, showed identical correlation between radioresistance, increase of diacylglycerol, and of enolase and glucose-6-phosphate dehydrogenase activity. These results indicate that increase of diacylglycerol is correlated with resistance to the killing effect of ionizing radiations and could be proposed as a marker of radioresponse.

Depressive symptoms and schizophrenia a psychopharmacological approach

The frequency of depression was assessed in 43 chronic schizophrenic patients during an acute exacerbation phase of schizophrenia. The dexamethasone suppression test was administered to all patients. Depressive symptomatology showed a prevalence from 16.3% for moderate symptoms to 23.3% for mild ones. Depressive symptoms occurred concurrently with the psychotic picture and resolved as the psychosis remitted. Depressive symptoms were not relative to age, sex, duration of illness, DST cortisol levels, drug dosages and extrapyramidal side effects while basal cortisol levels were negatively correlated with basal Hamilton score.

Crossmodal interaction is reflected in vertex potentials but not in evoked magnetic fields

Magnetic responses and slow vertex potentials evoked by tone bursts and median nerve shocks were recorded in healthy humans to study crossmodal interaction. Stimuli were presented either alone, simultaneously or in pairs separated by 0.5 s; the interstimulus interval was 5 s. Simultaneous or preceding stimuli of another modality did not influence the amplitudes of the magnetic responses, which are known to be generated at modality specific projection areas. However, the electric vertex potentials decreased in amplitude in similar situations, suggesting that the electric and magnetic responses reflect neural events of at least partly different brain areas. Our results do not give support for crossmodal interaction at the sensorimotor or auditory cortices.