Bernstein Newsletter

Perception of motion

The appearance of a spot of light on the retina causes sudden activation of millions of neurons in the brain within tenths of milliseconds.

At the first cortical processing stage, the primary visual cortex, each neuron thereby receives thousands of inputs from both close neighbors and further distant neurons, and also sends out an equal amount of outputs to others. Scientists from the Ruhr University Bochum and the Bernstein Group for Computational Neuroscience have now described the complex interplay between individual cells in a computer model that is based on far reaching interactions between cortical neurons. The model explains how neurons can be preactivated by their neighbors, so that stimuli can be processed more rapidly.The starting point of the study was a phenomenon called “line motion illusion” in psychology: if an observer looks at a square followed in fast succession by an elongated bar, he will perceive a gradual movement, as if the bar is drawn out from the location of the previously flashed square. This illusion arises because the presentation of the square leads to an initially local brain activation that spreads quickly as a travelling wave. At first, this wave remains subthreshold and hence cannot be perceived consciously. If shortly after, however, a second, horizontal bar-shaped stimulus appears, the activity can become supra-threshold. Scientists around Dirk Jancke have now successfully imple-mented these complex interaction dynamics in a computational model. A so-called neural field was used in which the impact of each model neuron is defined by its distance-dependent interac-tion radius: close neighbors are strongly coupled, and neurons that are farther apart are gradually less interacting. Two layers, one excitatory and one inhibitory, are recurrently connected such that a local input leads to transient, focal activity that propagates quickly and then gradually fades out. In such a model, the long-range interactions lead to a pre-activation of distant nerve cells.Such pre-activation may play an important role during the processing of moving objects. Given that neuronal processing takes time, the brain always receives information about the external world with a certain delay. In order to counterbalance such delays, pre-activation may serve a “forewarning” of neurons that represent locations ahead of an object trajectory and thus, may enable a more rapid crossing of firing thresholds to save important processing time.The model offers a mathematical framework of how the brain operates beyond a simple passive mapping of external events and how it conducts “interactive” information processing, leading, in limit cases, to what we call illusions. The future challenge will be to implement neural fields for more complex visual stimulus scenarios. Here, it may be an important advantage that this model class allows abstraction from single neuron activity and provides a mathematically handable description in terms of interactive cortical network function.

Link to Newsletter

http://homepage.ruhr-uni-bochum.de/Dirk.Jancke/page4_bernsteinnewsletter1210en_de.pdf (english/deutsch)