The images shown on the index page illustrate several of the experimental approaches used in my institute. They represent data taken from real experiments.

The EEG reflects potential changes that occur in a coherent manner in populations of cortical neurons. These signals are recorded non-invasively by electrodes put on the scalp. Modern techniques allow to analyse how the frequency spectrum changes over time in response to stimulus presentation (at time zero). An increase of spectral power occurs in the gamma-band (30-100 Hz) several hundred milliseconds after a target stimulus has appeared on a screen in front of the subject.

Using microelectrodes (see below), action potentials (spikes) can be recorded from small clusters of nerve cells located in the cortex of an experimental animal. When activated by an appropriate sensory stimulus, the cells engage in coherent bursts of spikes (black "needles" marked by red arrowheads). These bursts occur at rather regular intervals, reflecting an oscillatory process in the local network. Frequently, the temporal interval between the bursts is on the order of 20ms, yielding an oscillation frequency around 50 Hz.

The left part of the figure show a microscope view of an exposed part of visual cortex in an anesthetized animal. The curved lines correspond to blood vessels at the cortical surface. Using optical imaging, the preferred orientation of the nerve cells can be determined at each spot in the cortical map. Appropriate superposition of the data yields a colour-coded map of the orientation bands (right panel). This map can be used as a guide for inserting microelectrodes (left) into columns with a particular orientation. If multiple columns with similar orientation preference are recorded simultaneously, neuronal oscillations are often found to be correlated across spatially separate sites. We employ this approach to study neural synchrony at the cellular level.

Active regions of the human brain can be studied non-invasively using fMRI. The method reveals areas that show increased oxygen consumption as a consequence of enhanced neural activation. This allows to localize - with a precision of few millimeters - neural assemblies involved in particular perceptual, cognitive or motor tasks. We apply this method to study activation of brain regions in the context of perceptual selection, attention and conscious awareness. The figure on the right shows a prefrontal area (orange spot) that is more strongly activated during conscious perception of a visual stimulus as compared to a control condition where the same physical stimulus appears on the retina but is not consciously perceived.