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  • First I want to explain fixational eye movements, then formulate my research question and research strategy. I will briefly sketch the experimental setup in case you don‘t remember all the details from Vidhya‘s Friday seminar talk.
  • First point: How should the brain interpret the responses? Greschner found synchronized firing -> population code
  • My research strategy derives from the fact that fixational eye movements result in oscillatory shifts of the image on the retina. Imitate these eye movements by a shifting black-white border. Green ellipses denote the receptive fields of 2 ganglion cells; blue arrow shows the shifting border orientation
  • Each dot represents a spike.First idea: work on single cell level, use e.g. spiking rate. But rate approximately constant for both stimuli. Have to use a much higher amplitude as compared to Martin Greschner to elicit any responses.
  • Quantify these correlations by a histogram plot of relative spike timings. Spike timing cross-correlations can provide information about the stimulus; relative spike timings are different for the two stimuli
  • So I created these histogram plots for measured spike timings, as shown in the figure; the 5 angles are color-coded. One observes that the correlations are periodic, showing the same period as the stimulus. Green curve: peak at zero: cells tend to spike synchronously. Red curve: delay.
  • Apply information theory... Upper right: „different patterns for different stimuli?“
  • Quantify population responses by information theory measures. Imutual: stimulus: 5 different angles, spike patterns;either of single cells or cell pairs. Synergycan be positive or negative.
  • Imutual (stimlocked) > Imutual (unlocked), and „additional information“ in stimulus locked case smaller

pptx - TAC Meeting pptx - TAC Meeting Presentation Transcript

  • Neuronal Coding in the Retinaand Fixational Eye Movements
    Christian Mendl
    Tim Gollisch Lab
    Friday Seminar Talk
    November 6, 2009
  • Outline
    Experimental setup
    Review of fixational eye movements
    Research questions and strategy
    A look at the observed data
    Spike timing cross-correlations
    Information theory: entropy, mutual information, synergy, ...
    Summary and outlook
  • Experimental Setup
    ganglion cells
  • Fixational Eye Movements
    Constant feature of normal vision
    Visual perception fading
    Enhancement of spatial resolution
    Martinez-Conde S et al. Microsaccades counteract visual fading during fixation. Neuron (2006)
    source: Martinez-Conde laboratory
    Meister M, Lagnado L and Baylor DA. Concerted signaling by retinal ganglion cells. Science (1995)
    Riggs LA and Ratliff F. The effects of counteracting the normal movements of the eye. Journal of the Optical Society of America (1952)
    Ditchburn RW and Ginsborg BL. Vision with a stabilized retinal image. Nature (1952)
  • Fixational Eye Movements II
    Eye movements of the turtle during fixation
    Greschner, Ammermüller et.al. Nature Neuroscience (2002)
    Periodic component at approximately 5 Hz
    Imitating fixational eye movements -> retina better encoder
    Neurons synchronize more
  • Research Questions
    How can the brain discriminate between various stimuli in the context of fixational eye movements? Optimal decoding strategy?
    Synchronized responses of several retinal ganglion cells -> population code?
  • Research Strategy
    Concrete task: based on spike responses, discriminate 5 different angles
  • Observed Data
    stimulus period: 800 ms
  • Spike Timing Cross-Correlations
  • Spike Timing Cross-Correlations II
    stimulusperiod
  • Encodingthe Spike Train
    unlocked
    binning
    Encodingspikepatterns
    stimulus-locked binning
    -> observer knows the stimulus phase
  • Information Theory
    Mutual information Imutual
    -> How much information („bits“) do the spikes contain about the stimulus
    Synergy
    -> How much additional information is contained in the simultaneous activity of two cells as compared to the individual cells’ responses
  • Mutual Information
    unlocked binning
    individual cells
    stimulus-locked binning
    cell pairs
  • Population Code: Synergy
    unlocked binning
    stimulus-locked binning
    Synergyversus mutual informationfor several recordings
  • Summary
    Fixational eye movements provide information about the stimulus
    If the brain uses individual cells, it needs to know the phase of the fixational eye movements
    For multiple cells, the phase information becomes less important since the cells are synergistic
  • Outlook
    Effect of shorter stimulus periods and smaller amplitudes?
    Try different decoding stategies: optimal patterns, bin sizes?
  • Acknowledgements
    Tim Gollisch Lab
    Tim Gollisch
    Daniel Bölinger
    Vidhya Krishnamoorthy
    Thesis Advisory Board
    • Tim Gollisch
    • Erwin Frey (LMU)
    • Andreas Herz
    • Günther Zeck
    BoehringerIngelheimFonds
    Foundation for Basic Research in Medicine