a) Information processing in networks close to a critical state
Being at the border between regular and chaotic activity, critical network states display interesting dynamical features such as power-law distributions in size and duration of synchronous events, and they provide a rich repertoire of collective states which are potentially useful for information processing. But what role does critical behavior really play if the brain actively processes stimulus information?
We investigate this question by relating aspects of information processing in the visual system to the dynamics in neural networks close to a critical state. In particular, we focus on mechanisms that link local features in visual scenes into global figures such as contours or textures, and evaluate conditions under which a near-critical dynamics optimizes computational performance. We also study how top-down processes modulate information integration: By integrating physiological evidence with computational work, we have been able to show that selective attention might tune the visual system to a critical state and thereby enhance stimulus representation.
(A) Stimulus discriminability from local field potentials generated by a balanced network of integrate-and-fire neurons (color-coded), in dependence on coupling strengths. Discriminability is maximized near critical states (blue cross) which are characterized by power laws in spike event distributions (blue graphs in panel (B)). In subcritical (green cross) and supercritical regimes (red cross), discriminability is much lower (from: Tomen, N., Rotermund, D., and Ernst, U. (2014). Marginally subcritical dynamics explain enhanced stimulus discriminability under attention. Front. Syst. Neurosci. 8:151. doi: 10.3389/fnsys.2014.00151)
b) Transient dynamics and attentional modulation in visual area MT
The world is a dynamic place, and temporal changes in a scene contain important information relevant for behavior. On the neuronal level, such changes are often represented by transient increases or decreases in activity before firing rates settle into a sustained level. Our studies aim to uncover the mechanisms behind neural transients, and to quantify their impact on information processing. Hereby we closely collaborate with Detlef Wegener (Brain Research Institute, University of Bremen) and Jannis Hildebrandt (Neuroscience Department, University of Oldenburg), to tightly link computational work with physiological data. Currently, we investigate a neural circuit with DIVisive INhibition on Excitatory units (DIVINE-model - panel A), which nicely captures single-unit and population responses of neurons in area MT (panel B and C) to drifting visual stimuli subjected to abrupt changes in drift velocity. When attentional modulation is added as a multiplicative input change to our model, it turns out that the dynamics of the transients, but not the sustained states, correlate nicely with behavior.
(A) Schematic structure of the DIVINE model. (B) and (C) show two exemplary results of fitting the DIVINE model to experimental data of single cells responding to a motion onset in a Gabor stimulus (data from Traschütz A., Kreiter A.K., and Wegener D. (2015) Transient activity in monkey area MT represents speed changes and is correlated with human behavioral performance. J. Neurophysiol. 113: 890-903).