Response of Anterior Parietal Cortex to Cutaneous Flutter Versus Vibration

om 25 Hz vibrasjon (flutter) på huden og reaksjonen i hjernen. 25 Hz øker absorbsjon av molekyler i hjernens tilsvarende område, mens 200 Hz demper absorbsjon. Konsekvensen av demping rel økning er jeg usikker på. Denne studien mener at varme øker absorbsjon og det gjør 25 Hz også. Mulig dette ikke relateres til percusoren siden den vibrerer på 8-15 Hz når vi jobber.

Twenty-five-Hertz (“flutter”) stimulation of a discrete skin site on either the hindlimb or forelimb for 3–30 s evoked a prominent increase in absorbance within cytoarchitectonic areas 3b and 1 in the contralateral hemisphere. This response was confined to those area 3b/1 regions occupied by neurons with a receptive field (RF) that includes the stimulated skin site.

In contrast, same-site 200-Hz stimulation (“vibration”) for 3–30 s evoked a decrease in absorbance in a much larger territory (most frequently involving areas 3b, 1, and area 3a, but in some subjects area 2 as well) than the region that undergoes an increase in absorbance during 25-Hz flutter stimulation.

The increase in absorbance evoked by 25-Hz flutter developed quickly and remained relatively constant for as long as stimulation continued (stimulus duration never exceeded 30 s).

As has been pointed out by others, although the percept of flutter is referred with great accuracy to the actual locus of low-frequency skin stimulation, as frequency is increased (≥50 Hz) the evoked sensation (vibration) often is referred to tissues deep and remote from the actual site of skin contact (Bolanowski et al. 1988; LaMotte and Mountcastle 1975; Mountcastle 1984;Sherrick et al. 1990).

First, previous work showed that an increase in the time of exposure to a vibrotactile stimulus leads to an increase in the spatial contrast in the stimulus-evoked SI global activity pattern (Tommerdahl and Whitsel 1996). Second, a recent psychophysical study (Goble and Hollins 1994) showed that human vibrotactile frequency discriminative capacity improves substantially with prior exposure to stimuli similar in frequency to those to be discriminated. And third, a quantitative electroencephalographic (EEG) study of human subjects reported that the contralateral postcentral response to a spatially discrete cutaneous flutter stimulus becomes more spatially localized with increasing duration of stimulation (Kelly et al. 1997; E. F. Kelly and S. E. Folger, unpublished data).

A: images showing surface vascular pattern (top left), thresholded response to 25-Hz skin stimulation at 9.4 s (bottom left), and images of the response acquired at different times after stimulus onset. B: spike trains recordings (left); peristimulus time (PST) histograms (top right); and spatial histograms showing how mean absorbance varies with distance at 2 different times (2.2 and 7.0 s) after stimulus onset. C, top: spike trains obtained from an SI neuron studied duringpenetration 1 (left), from 2 SI neurons studied duringpenetration 1 (middle), and from 1 SI neuron studied in penetration 3.Horizontal line at top of each spike train raster indicates time of 25-Hz stimulation; for every neuron the skin site stimulated (contralateral radial interdigital pad) was the same site used to evoke the OIS activity pattern shown in A. Response to 1st stimulus shown at top of each raster; response to 15th stimulus shown at bottom. Graphs show the trial-by-trial difference between each neuron’s mean firing rate during vs. after each stimulus presentation (MFRstim − MFRbackground). Vibrotactile stimulus parameters: 25 Hz, 400 μm peak-to-peak, 7-s duration, 45-s interstimulus interval, stimulator probe contacted 2 mm skin site.

Inspection of Fig. 5 reveals that 25-Hz stimulation (left) evoked a localized increase in absorbance (indicated by dark region) primarily confined to area 3b (a small component of the response to 25-Hz stimulation also occupies a neighboring part of 3a insubject 2, and a neighboring part of area 1 in subject 4). Same-site 200-Hz stimulation, however, yielded quite a different result; in all nine subjects studied (the data for 4 subjects are shown in Fig. 5, middle; each image shows the response at 6 s after onset of stimulation), the region of area 3b that had been maximally activated by 25-Hz stimulation exhibits only near-background or slightly lower-than-background (decreased) absorbance values.

Specifically, although both 200 and 25 Hz evoked an absorbance decrease within region 2, the magnitude of the decrease was larger and more rapid at the higher stimulus frequency.

The observations illustrated in Fig. 9, therefore, appear fully consistent with our interpretation of the OIS imaging observations: although 25-Hz stimulation of a skin site elevates the spike discharge activity of neurons within a sector of area 3b throughout the entire period of skin stimulation, same-site 200-Hz stimulation evokes a much more transient elevation of the spike discharge activity of the same area 3b neurons.

Similarly, the evidence obtained in recent functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) human brain imaging studies (Apkarian 1995a,b, 1996;Apkarian et al. 1994; Derbyshire et al. 1996; Coghill et al. 1994) and in optical imaging studies of the somatosensory cortex in anesthetized monkey subjects (Tommerdahl et al. 1996a, 1998) indicate that a noxious skin heating stimulus normally evokes activation of the topographically appropriate region in one cortical territory (area 3a, SII and/or anterior cingulate cortex) and simultaneously suppresses the activity in the corresponding region in other functionally related territories (areas 3b and 1).

Also relevant to the idea that stimulus-evoked mechanoreceptive afferent activity can evoke SI suppression/inhibition is an observation reported in a study of SI neurons in conscious behaving monkeys: Lebedev et al. (1994) found that the mean firing rate (MFR) of SI neurons stimulated on the contralateral palm of the hand at 127 Hz was significantly lower than the rates obtained at 27 and 57 Hz, leading those authors to conclude that the “decrease in MFR of neurons with cutaneous receptive fields (RFs) at 127 Hz may be due to inhibitory mechanisms dependent on stimulus frequency.”

lso, the fact that 2DG metabolic mapping experiments in both monkey and cat have demonstrated that a high-amplitude (0.5–1.0 mm peak-to-peak) 25-Hz sinusoidal skin stimulus evokes a prominent, columnar pattern of above-background 2DG uptake in the topographically appropriate sector of areas 3b and 1 even after preexposure to such stimulation for a prolonged period (for 15 min to >1 h) before administration of the 2DG tracer (Juliano and Whitsel 1981, 1983, 1989; Tommerdahl et al. 1996a),

These published findings lead us to propose that the discovery that 200-Hz stimulation, but not 25-Hz stimulation of the same skin site, suppresses area 3a (for example, see Fig. 6) and the finding that noxious skin heating selectively activates area 3a (Tommerdahl et al. 1996a, 1998) are both consistent with the proposal that it is area 3a, and not areas 3b and 1, which plays the leading role in the perception of noxious skin heating stimuli (Tommerdahl et al. 1996a, 1998).

Til slutt, forklaring om parietal lappen fra Wiki:
The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the dorsal stream of the visual system. This enables regions of the parietal cortex to map objects perceived visually into body coordinate positions. Several portions of the parietal lobe are important in language processing. Just posterior to the central sulcus lies the postcentral gyrus. This area of the cortex is responsible for somatosensation.[1]Somatosensory cortex can be illustrated as a distorted figure — the homunculus (Latin: «little man»), in which the body parts are rendered according to how much of the somatosensory cortex is devoted to them.[2]

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