Eye movements in a computer model
text reproduced from the BCCN Newsletter, October 2008
How we follow objects using our eyes
When we follow a moving object with our eyes, the brain calculates the speed of the object and the eye movement is adjusted accordingly. This alone is already an enormous achievement, but the brain can still do even more. When a car rushes past, it will speed up or slow down more quickly than a pedestrian. Accordingly, our eye movement control reacts to speed changes of fast objects more sensitively than it does to slow moving objects. This is referred to as ‘Gain control’ amongst the experts. The location within the brain in which this gain control is calculated and the neural connections underlying it have been postulated in a mathematical model and experimentally checked by scientists around Ulrich Nuding, Stefan Glasauer and Ulrich Büttner of the Bernstein Center for Computational Neuroscience and the Ludwig-Maximilians-University Munich. Their results are also relevant for the diagnosis of eye movement disorders.
 Transcranial Magnetic Stimulation © Neil G. Muggleton |
Both from behavioral experiments on humans and neuro-physiological studies a great deal is already known about the control of eye movements. It is known, for example, that different cortical brain regions participate in the origin of eye pursuit movement: the MST area in the parieto-temporal cortex and the frontal eye fields (FEFs). Nerve cells in the MST area primarily reflect the speed of eye or target movement, while cells in the FEF mainly react to speed changes. The objective of the Munich scientists was to summarize these findings in a computer model that could explain eye movement control. The model of the scientists simulates the most important circuits required for the control of visual pursuit. In the MST area the speed of the target is calculated in order to compare it and adapt the current eye movement to it. The FEFs represent the postulated location for gain control: here, the sensitivity of eye movement is set for speed changes. The faster an object moves, the greater is the adaptability. ‘With these studies we were able to explain for the first time the potential use of the parallel anatomical pathways in processing,’ says Glasauer. |
In order to check the model, the scientists, together with colleagues from University College London, instructed subjects to pursue a point on a screen with their eyes. In this experiment, the activity of the FEFs was transiently disrupted by transcranial magnetic stimulation, a technology with which specific brain regions can be purposefully influenced for a few seconds. These experiments confirmed the predictions of the model. As long as the observed object moved at a constant speed, a perturbance of the FEFs barely affected eye movement control. The sensitivity of eye movement to speed changes, however, did not increase adequately at higher speeds with a disrupted FEF. In conclusion, gain control is determined in the FEFs depending on the speed of the eye or the target. This change in sensitivity shows interesting parallels with visual attention control in which FEFs also seem to play an important role, and can therefore be considered as an attentional mechanism within the visual pursuit system.
Source / Quelle:
Nuding, U., Ono, S., Mustari, M.J., Büttner, U. & Glasauer, S -- A theory of the dual pathways for smooth pursuit based on dynamic gain control. J Neurophysiol. 2008 Jun;99(6):2798-808.
Nuding, U., Kalla, R., Muggleton, N.G., Büttner, U., Walsh, V. & Glasauer, S. -- TMS evidence for smooth pursuit gain control by the frontal eye fields. Cerebral Cortex. 2008, published online October 1st, 2008, doi:10.1093/cercor/bhn162
