Ermentrout : "Double or Nothing: Phosphenes and the periodic driving of cortex"


    Bard Ermentrout
    Department of Mathematics
    University of Pittsburgh
    Wednesday 9 March 2011, 16:00
    Seminar Room, Wellcome Trust Centre for Neuroimaging (FIL)
    12 Queen Square, London, WC1N 3AR
    Double or Nothing: Phosphenes and the periodic driving of cortex
    In this talk, I examine two different types of phosphenes - patterns in the visual systems evoked from within it. I first study contour phosphenes in which direct stimulation of the eyeball coupled with a moving bar in the visual field produces slowly propagating waves. The mechanism appears to be due to period doubling which produces an intrinsic bistability. Using averaging, I analyze the dynamics of a one-dimensional analog. In the second part of the talk, I study flicker-induced hallucinations in which diffuse stroboscopic light is capable of  evoking spatial patterns in the visual field. I use Floquet theory and symmetric bifurcation theory to explain experiments that indicate different patterns are seen with different temporal frequencies.
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    The lively mind of the child revels in the manifold stimuli of the external world. Who does not remember, if only dimly, such games from that beautiful time? One of them, which could keep us busy at a more serious age, is as follows: I stand in bright sunlight with closed eyes and face the sun. Then I move my outstretched, somewhat separated, fingers up and down in front of the eyes, so that they are alternately illuminated and shaded. In addition to the uniform yellow-red that one expects with closed eyes, there appear beautiful regular figures that are initially difficult to define but slowly become clearer. When we continue to move the fingers, the figure becomes more complex and fills the whole visual field (Jan Purkinje, 1819)
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