martedì 31 luglio 2012

CORSO ESTIVO DI BIOETICA 2012: NEUROBIOETICA (IV)


Neurobioetica: La Persona al centro
delle Neuroscienze, Etica, Diritto e Società

Roma,  2 luglio 2012
Modulo 1 : Fondamenti ed Etica delle Neuroscienze

Con questo post mi focalizzerò su un approfondimento neuroscientifico relativo alla prima parte della conferenza del professor Orzi.

Si tratta di una recente pubblicazione uscita lo scorso gennaio 2012 sulla prestigiosa rivista Nature Reviews Neuroscience (vol. 13, 63-70). Svenja Borchers et al. utilizzando la stimolazione elettrica corticale, tentarono di “mappare” diverse funzioni cerebrali superiori.

Questo è un tipico esempio di approccio bottom-up che dalla stimolazione di aree cerebrali osserva i cambiamenti comportamentali corrispondenti.

Riporto ora le parti salienti dell’articolo e alcuni commenti.   

Nature Reviews Neuroscience 13, 63-70 (January 2012) | doi:10.1038/nrn3140

Opinion: Direct electrical stimulation of human cortex — the gold standard for mapping brain functions?

by Svenja Borchers, Marc Himmelbach, Nikos Logothetis & Hans-Otto Karnath

Abstract

Despite its clinical relevance, direct electrical stimulation (DES) of the human brain is surprisingly poorly understood. Although we understand several aspects of electrical stimulation at the cellular level, surface DES evokes a complex summation effect in a large volume of brain tissue, and the effect is difficult to predict as it depends on many local and remote physiological and morphological factors. The complex stimulation effects are reflected in the heterogeneity of behavioural effects that are induced by DES, which range from evocation to inhibition of responses — sometimes even when DES is applied at the same cortical site. Thus, it is a misconception that DES — in contrast to other neuroscience techniques — allows us to draw unequivocal conclusions about the role of stimulated brain areas.

Introduction.

Since its first use in the late nineteenth century (Box 1), intraoperative application of electrical current to the cortex in patients with tumours or epilepsy has become a standard technique during brain surgery for inferring the function of brain areas in humans. While the patient performs motor, language or cognitive tasks under controlled conditions, the cortical surface is stimulated with a bipolar electrode to provoke reproducible, transient changes in behaviour.

Krause used this technique to produce the first detailed map of the organization of human motor cortex1, 2, whereas Cushing used it to characterize the sensory responses evoked by direct electrical stimulation (DES) of the postcentral gyrus3. Since then, DES has been used to characterize the representation of many other functions, including cognitive ones. For example, the representation of language functions was mapped to the human left hemisphere4, 5, 6, 7, 8, whereas spatial processing of visual objects9 and spatial orienting and exploration10 were mapped to the human right hemisphere.


Box 1 | The rise of a new technique

Direct electrical stimulation (DES) had been practised in animals during the second half of the nineteenth century67, and was first applied in a human by Robert Bartholow in 1874. By inserting electrodes along the exposed brain of a woman with a basal cell carcinoma, Bartholow observed muscular contractions of contralateral limbs and found that no pain was experienced by the brain substance proper. Following this, there were reports by Victor Horsley68, Charles Sherrington69, Harvey W. Cushing3, Fedor Krause1, 2 and several other clinicians who applied the technique to surgically treat brain tumours and localize epileptic foci. By the early 1900s, DES was generally approved for use and the first map of the human motor cortex1, 2 was made by Fedor Krause, based on results of faradic stimulation of the cortex (see the figure, part a).



The patient's epileptogenic area was identified by attempting to reproduce the patient's usual type of seizure by applying DES. In 1934, the technique was improved when Otfrid Foerster and Hans Altenburger established intraoperative electrocorticography (ECoG). This technique allowed them not only to apply but also to record electrical activity from the brain surface70. At around this time, Wilder Penfield and his colleagues applied DES (see the figure, part b, which shows the sites of peri-operative brain stimulation and recording during awake surgery of a patient performed by Penfield around 1949) to map motor and sensory areas, leading to the famous diagrams of the motor and sensory homunculi57, 59. These authors confirmed Krause's earlier findings in relation to motor cortex1, 2 (see the figure, part a). The control of electrical stimulation parameters has improved considerably throughout the past century (for example, a move from using unipolar electrodes2 to using bipolar electrodes8, 57, and from using the surgeon's tongue to test stimulation intensity2, 71 to using modern stimulation protocols). Nevertheless, the results on the behavioural level seem to be quite comparable, although more recent studies allow more precise localization of functions. Importantly, the homunculi mapped by Penfield and Boldrey57 suggested a rather simple and strictly separated organization, but these authors in fact reported an overlap between the sensory and motor cortices as well as between cortical areas representing different body parts. Only recently it was shown that the functional organization of the sensorimotor cortex does indeed seem to be more characteristic of mosaicism than of somatotopy72. Figure part a is reproduced from Ref. 2. Figure part b is reproduced courtesy of W. Feindel, Montreal Neurological Institute (MNI) Neuro History Archives, Canada.



The seemingly clear causality between stimulation and behaviour has led some to conclude that DES represents the “gold standard for brain mapping” in systems neuroscience11. In this line, recent results of experimental DES in humans9, 12 were well received in hotly debated areas of cognitive neuroscience, such as the role of parietofrontal networks in motor intention and visuospatial awareness.

But is DES the straightforward method that it seems to be?

We will argue that DES evokes a complex summation effect in a large volume of brain tissue that is difficult to predict. This does not lessen the relevance of this technique as a clinical tool in neurosurgery, but has implications for the use of DES findings in building neurocognitive models of brain functions. Misinterpreting the effects of DES could erroneously bias our conclusions and hypotheses on such functions. In this Perspective, we review both the effects of DES on the cellular level and its local and remote effects when it is used for surface stimulation during brain surgery.


........ (continua)

Articolo completo: clicca qui.

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