Neuronal Coding in the Retinaand Fixational Eye Movements

Friday Seminar TalkNovember 6, 2009

Christian Mendl Tim Gollisch Lab

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

ganglion cells

Experimental Setup

Fixational Eye Movements

source: Martinez-Conde laboratory

• Constant feature of normal vision

• Visual perception fading• Enhancement of spatial

resolutionRiggs 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)

Meister M, Lagnado L and Baylor DA. Concerted signaling by retinal ganglion cells. Science (1995)

Martinez-Conde S et al. Microsaccades counteract visual fading during fixation. Neuron (2006)

Fixational Eye Movements II

Eye movements of the turtle during fixation

• Periodic component at approximately 5 Hz• Imitating fixational eye movements →

retina better encoder• Neurons synchronize more

Greschner, Ammermüller et.al. Nature Neuroscience (2002)

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

stimulus period

Encoding the Spike Train

stimulus-locked binning

unlockedbinning

Encoding spike patterns

→ 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 binningstimulus-locked binning

individual cells

cell pairs

Population Code: Synergy

Synergy versus mutual information for several recordings

unlocked binningstimulus-locked binning

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

Boehringer Ingelheim FondsFoundation for Basic Research in Medicine