Prof. Dr. Ralf Galuske
In primary visual cortex, neurons with similar response preferences are grouped into domains forming continuous maps of different stimulus features such as the orientation of a stimulus or its direction of movement.
These maps are a basic property of the processing architecture of the visual cortex. Our research is focussed on the identification of neuronal processes which generate and stabilize these maps. In particular we are interested in the spatiotemporal dynamics of neuronal responses within this well structured cortical network and in the interaction between primary and higher order cortical areas during visual processing. Another line of research is aimed at elucidating mechanisms of experience dependent plasticity in the visual cortex.
To study the layout of cortical maps it is important to record simultaneously the activity of neurons that are distributed over large areas. Therefore, we assess activity by optical imaging of intrinsic signals. With this technique we visualize the local oxygen consumption over a large area of cortex. The local oxygen consumption reflects the neuronal activity very precisely. This technique is combined with several others, such as simultaneous electrophysiological multi-site recording, pharmacological interventions or reversible thermal deactivation of different cortical areas.
1. The visual cortex is interconnected through a dense network of projections with other cortical areas which all interact during the processing of sensory information. So far, research has been focused on the contribution of input arriving from the thalamus to the elaboration of representations in the primary visual cortex. In collaboration with Prof. B. Payne (Boston University, Boston, USA) and Prof. S. Lomber, University of Texas, Dallas, USA) we have examined the role of signals fed back from higher cortical areas into the primary areas. To this end, we combined optical imaging techniques and electrophysiological recordings with reversible thermal deactivation of higher cortical areas and could show that these areas greatly influence the elaboration of responses in the primary visual cortex. Our data show that even the earliest stages in the emergence of cortical representations strongly rely on signals fed back from higher cortical areas.
2. Particularly in early postnatal development the functional and anatomical architecture of the visual cortex is highly susceptible to experience dependent modifications. For example, after occlusion of one eye for a few days (monocular deprivation) this eye will lose its ability to activate the cortex and the respective afferents will eventually be removed. The exact mechanisms through which neuronal activity is translated into the observed structural changes are still unknown. One possibility is that different sets of axons compete for factors whose availability depends on neuronal activity. In a collaboration with Prof. H. Thoenen (MPI for Psychiatry, Munich) and Dr. E. Castrén (University of Kuopio, Kuopio, Finland) we have examined this hypothesis by external addition of different neurotrophic factors during periods of monocular deprivation. These experiments revealed that in particular BDNF (brain derived neurotrophic factor) was able to prevent the disconnection of the deprived eye, indicating that it plays an important role in the competition process underlying experience dependent plasticity.
3. In the adult visual cortex, representations can also undergo experience dependent modifications. In collaboration with Dr. M. Munk we have examined in how far the occurrence of these changes is linked to the general state of the brain. To this end, we repetitively presented oriented visual stimuli and paired the visual stimulation with electrical activation of the mesencephalic reticular formation. This structure in the brain stem controls alertness levels and arousal and has been shown to substantially influence the EEG and the temporal structure of neuronal responses in the visual cortex. Our experiments revealed that adaptive changes in the cortical representation of different stimulus orientations are closely linked to the general state of the animal. Only when repetitive visual stimulation is paired with activation of the reticular formation the representation of these stimuli is enlarged. Without reticular activation repetitive stimulation has the opposite effect on the representations and induces a habituation of responses. Thus, our data imply that the general state and context in which stimuli occur has an influence on their cortical representation and suggest that the occurrence of representational plasticity depends on the temporal structure of the responses evoked by the respective stimuli.