martedì 7 agosto 2012

CORSO ESTIVO DI BIOETICA 2012: NEUROBIOETICA (VI)

DES: Direct electrical stimulation of human cortex

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The combined DES and fMRI experiments showed that electrical stimulation of primary visual cortex (V1) elicits positive BOLD responses in topographically matched regions of extrastriate areas.

 
Electrical stimulation has been extensively used to causally link the activity of various brain regions to perception and action, and the technique has recently been applied in electrotherapy (for example, for Parkinson's disease or focal epilepsy) and neural prostheses45.

Considering the variability of the effects of microstimulation mentioned above, it becomes clear that surface DES during surgery evokes a complex summation effect in a large volume of brain tissue that is difficult to predict as it heavily depends on the morphology of the cortical area being stimulated. Determining the net effect of DES thus requires both local and distant neurophysiological measurements under conditions that are comparable to the conditions during neurosurgery in which DES is used. Perhaps surprisingly, such studies are very rare.

In conclusion, neurophysiological and optical imaging studies in humans that applied DES with parameters comparable to those used during neurosurgery have started to elucidate the complicated regional 'processes' surrounding the stimulation site. However, to evaluate whether both excitatory and inhibitory effects occur, it is essential to measure both pre- and postsynaptic sites (see above) in the stimulated area. The indirect measures of brain activity that are currently possible in humans limit the state of knowledge about the cortical changes induced by DES.

Conclusions

The physiology of DES is still far from being completely understood.



The complex effects of electrical stimulation on the cellular level and of cortical surface stimulation are paralleled by a high variability in the behavioural effects of DES.

The different local and remote effects induced by DES and the (unpredictable) dependence on the individual cortical and neuronal morphology and DES parameters are reflected by the heterogeneity of behaviours that are induced by stimulation at one and the same cortical site as well as the number of distant sites at which stimulation results in similar behavioural phenomena.

The evocation of behaviour is often interpreted as evidence for activation of the stimulated cortical neurons, whereas the inhibition of behaviour is attributed to neuronal inactivation.

Although both of these post-hoc interpretations lack a solid physiological foundation, the behavioural heterogeneity instead should remind us that interpretations of DES results, especially those with respect to causality, should be made carefully.

It is a misperception that DES allows us — in contrast to, for example, fMRI and other neuroscience techniques — to draw unequivocal conclusions about the type of processing in the stimulated brain areas. We should not be deluded by the obvious fascination of direct access to the human brain.

Although DES has proven its outstanding value in neurosurgery, a misjudgement about its pros and cons in cognitive neuroscience might erroneously bias our conclusions and hypotheses.

DES is not the gold standard with respect to causality between neuronal activity on the one hand and behaviour on the other, but instead is one method among several in the context of system neurosciences and has its own strength and weaknesses.

 

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