Die neue Landkarte des Gehirns

Wo ist das Fach für die peinlichen Erinnerungen, in welche Schublade ordnet unser Gehirn die Familienfotos ein? Untersuchungen per Magnetresonanztomograf ergeben eine sich ständig ändernde Landkarte des menschlichen Denkorgans.

Die Vorstellung, bestimmte Nervenzellen hätten jeweils ganz bestimmte Funktionen im Denken und Fühlen inne, wird von der Wissenschaft längst als überholt angesehen. Auch die Vorstellung einer ausgeprägten Arbeitsteilung zwischen linker und rechter Gehirnhälfte ist nicht mehr als ein Modell. Tatsächlich funktioniert das Denkorgan durch die Vernetzung seiner Bestandteile. Und doch lassen sich mit Hilfe der funktionellen Magnetresonanztomografie Bereiche abgrenzen, in denen bestimmte Vorgänge ablaufen. Eine zusammenfassende Auswahl der jüngsten Ergebnisse.

Magnetresonanztomograf (Philips 3T Achieva). Bild: Wikimedia Commons Das Bild "Modern 3T MRI.JPG" stammt aus der freien Mediendatenbank Wikimedia Commons und steht unter der GNU-Lizenz für freie Dokumentation . Der Urheber des Bildes ist KasugaHuang GNU Free Documentation License Version 1.2, November 2002 Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. 0. PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. 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Der Multimedia-Hub

Im dorsalen medialen präfrontalen Kortex - im Prinzip gleich hinter der Stirn - steckt das Zentrum, das autobiografische Erinnerungen mit Musik und Gefühl verknüpft. Diesen „Multimedia-Hub“ haben Forscher entdeckt, als sie feststellten, dass Alzheimer-Betroffene auch im letzten Stadium der Krankheit stark auf ihnen bekannte Musik reagieren. Das Gebiet ist nämlich eines der letzten, das im Verlauf der Krankheit atrophiert.

Das Einsamkeits-Zentrum

Die temporoparietale Verbindung, der die Funktion des Sich-in-andere-Versetzens zugeschrieben wird, lässt sich offenbar auch als Sitz des Gefühls der Einsamkeit ausmachen: Forscher fanden heraus, dass dieser Bereich bei einsamen Menschen stärker aktiviert wird, wenn diese sich Fotos von Menschen in unangenehmen Situationen ansehen. Das ventrale Striatum hingegen, dem Funktionen im Belohnungssystem zugeordnet werden, wird bei sich nicht einsam fühlenden Menschen stärker aktiviert, wenn diese Fotos von Menschen in angenehmen Situationen betrachten.

Das Aufpass-Zentrum

Der Locus Ceruleus, auch „blauer Kern“ genannt, ist offenbar für den Übergang von einem zerstreuten zu einem aufmerksamen Gehirnzustand zuständig. Das konnten Forscher in einer Studie zeigen, indem sie mit Hilfe eines Medikaments den Zustand des blauen Kerns änderten - das Ergebnis lässt auf eine Behandlungsmöglichkeit etwa für Autismus oder Schizophrenie hoffen.

Das Lernzentrum

In einem Teil der Großhirnrinde vollzieht sich anscheinend ein Großteil der Lernprozesse, wenn es um Faktenwissen geht. Dabei, auch das haben fMRI-Aufnahmen ergeben, ändern sich die Erfolg versprechenden Lernstrategien mit dem Alter: Während achtjährige Kinder vor allem durch positives Feedback lernen, können Zwölfjährige (aber auch Erwachsene) schon mit negativer Kritik umgehen. Bei jüngeren Kindern verpufft negatives Feedback noch.

Das Mitleidzentrum

Wenn wir mit jemandem, der unter Schmerzen leidet, Mitleid empfinden, registriert das gleich eine ganze Reihe von Hirnarealen - und zwar interessanterweise dieselben, die auch für die Schmerzempfindung zuständig sind. Das schließt den somatosensorischen Kortex ein, die temporo-parietale Verbindung und die Amygdala. Wenn aber erkennbar ist, dass der leidende Mensch absichtlich verletzt wird, kommen zusätzlich die Hirnregionen ins Spiel, die sich mit sozialer Interaktion und moralischer Beurteilung befassen.

Das Navigationszentrum

Das GPS-Organ des menschlichen Gehirns wird von einem Teil des Hippocampus gebildet. Eine Studie an Londoner Taxifahrern zeigte, dass die über lange Jahre erworbene Kenntnis von 250.000 Straßen der Stadt den aktivierten Teil des Hippocampus stark vergrößert - selbst bei Busfahrern war von diesem Phänomen nichts festzustellen.

Das Ich-Zentrum und das Du-Zentrum

Wenn wir versuchen, die Gedanken anderer Menschen zu erahnen, werden dieselben Gehirngebiete hochgefahren wie beim Nachdenken über uns selbst - nämlich Teile des ventromedialen präfrontalen Kortex. Mit einer Besonderheit: Das gilt nur für Menschen, die uns ähnlich sind. Betrachten wir hingegen jemanden als politisch, sozial oder religiös andersartig, dann beurteilen wir diesen eher aus unseren Beobachtungen und Erfahrungen heraus.

Das Glaubenszentrum, das Unglaubenszentrum und das Unsicherheitszentrum

Gott im Gehirn zu finden, ist den Forschern bisher nicht gelungen - im Gegenteil, Versuche an Nonnen zeigen, dass sich Religiosität im Hirn sehr unterschiedlich manifestiert. Es gibt aber Areale, die typischerweise mit Glauben, Unglauben und Unsicherheit zu assoziieren sind, glauben die Forscher. Dazu untersuchte man in einer Studie Probanden, denen Wissen vorgelegt wurde, das sie entweder glaubten, nicht glaubten oder nicht beurteilen konnten. Der Unterschied zwischen Glauben und Unglauben zeigte sich demnach vor allem anhand von Aktivitäten im Frontallappen, und zwar unabhängig vom Inhalt des geglaubten (oder nicht geglaubten). Unglauben zeigte sich zudem besonders in den Bereichen, die man sonst Gefühlen wie Schmerz und Unbehagen zuordnet, wie sie etwa bei negativen Sinneseindrücken aus dem Geschmacks- oder Geruchssinn auftreten. Waren die Probanden hingegen unsicher, aktivierten sie vor allem ihren anterioren cingulären Kortex, der sich um Konfliktauflösung und Fehlererkennung kümmert.

Das Schönheitszentrum

Die Beuteilung von Schönheit vollzieht sich in zwei unabhängigen, nicht exklusiven Prozessen: zur Erkennung „objektiver“ Schönheit (etwa an Skulpturen mit Proportionen im „Goldenen Schnitt“) aktivieren wir die Insula. Die subjektive Perspektive, die Vergleiche und das „gefällt mir“ ermöglicht, verleiht uns hingegen die rechte Amygdala.

Morgen in Teil 2: Wo verwalten wir Sucht und Furcht, welche Nervenzellen rechnen? Der zweite Teil einer kleinen Landkarte des Gehirns, die aktuelle Untersuchungen per Magnetresonanztomograf aufgestellt haben.

(Matthias Gräbner)