Amplitude-dependency of response of SI cortex to flutter stimulation

Nevner at amplitude (utslag) i percussoren er viktig for utslaget stimuleringen gir i hjernen. Denne tester veldig små amplituder (0,005-0,4mm), men sier at området som aktiveres i hjernen blir tydeligere jo større amplitude.

http://www.biomedcentral.com/1471-2202/6/43

Stimulation of a discrete skin site on the forelimb evoked a prominent increase in absorbance within the forelimb representational region in cytoarchitectonic areas 3b and 1 of the contralateral hemisphere. An increase in stimulus amplitude led to a proportional increase in the magnitude of the absorbance increase in this region of areas 3b and 1 while surrounding cortex underwent a decrease in absorbance.

Correlation maps revealed that as stimulus amplitude is increased, the spatial extent of the activated region in SI remains relatively constant, and the activity within this region increases progressively. Additionally, as stimulus amplitude is increased to suprathreshold levels, activity in the surround of the activated SI territory decreases, suggesting an increase in inhibition of neuronal activity within these regions.

Increasing the amplitude of a flutter stimulus leads to a proportional increase in absorbance within the forelimb representational region of SI. This most likely reflects an increase in the firing rate of neurons in this region of SI.

In general, results from these studies indicate that increases in stimulus intensity are accompanied by increases in the intensity of the evoked signal as well as increases in the activated volume of cortex. As a result these studies predicted that amplitude might be coded not only by the average firing rates of individual SI neurons, but also by the total aggregate of responding neurons.

The results suggest that increasing the amplitude of a skin flutter stimulus evokes a proportionally larger absorbance increase in SI that remains confined to the same SI territory. In addition, it was found that increasing the amplitude of flutter evokes a large decrease in absorbance in the territory that borders the activated region of SI. Neurons in the SI region that demonstrate decreased absorbance in response to flutter stimulation are proposed to undergo stimulus-evoked inhibition and to contribute importantly to the SI processing of high-amplitude skin flutter stimuli.


Areas of high absorbance are indicated by dark patches within each image; regions of high absorbance in each case correspond to the SI locus that represents the stimulated site on the skin.


Spatial histograms of activity at different amplitudes.


Absorbance time course and anatomical registration in SI.

As a result, the observed tendency for absorbance in the same localized region of area 3b to increase with increasing stimulus amplitude (Figures 3 &4) most likely is due to the amplitude-dependence of the average firing rate of neurons in the same region[17].

Regardless of stimulus amplitude, the activated cortical region appears circular in shape and occupies an area approximately 2 mm in diameter (figures 4 &6). Within the ROI average absorbance increases progressively with increasing stimulus duration.

Combined metabolic tracer and neurophysiological studies have shown that the initial response to a repetitive tactile stimulus occupies an extremely large cortical territory. As the repetitive mechanical stimulation is continued, however, the response is quickly sculpted by cortical inhibitory mechanisms, leading to an activity pattern that becomes confined to a relatively restricted region in SI [2527].

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