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Chapter 2

Chapter 2. Outline. Linear filters Visual system (retina, LGN, V1) Spatial receptive fields V1 LGN, retina Temporal receptive fields in V1 Direction selectivity. Linear filter model. White noise stimulus. Fourier transform. A: Stimulus is velocity profile;

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Chapter 2

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  1. Chapter 2

  2. Outline • Linear filters • Visual system (retina, LGN, V1) • Spatial receptive fields • V1 • LGN, retina • Temporal receptive fields in V1 • Direction selectivity

  3. Linear filter model

  4. White noise stimulus

  5. Fourier transform

  6. A: Stimulus is velocity profile; B: response of H1 neuron of the fly visual system; C: rest(t) using the linear kernel D(t) (solid line) and actual neural rate r(t) agree when rates vary slowly. D(t) is constructed using white noise H1 neuron in visual system of blowfly

  7. Deviation from linearity

  8. 5 types of cells: Rods and cones: photo-transduction into electrical signal Lateral interaction of Bipolar cells through Horizontal cells. No action potentials for local computation Action potentials in retinal ganglion cells coupled by Amacrine cells. Note G_1 off response G_2 on response Early visual system: Retina

  9. Lateral geniculate nucleus (LGN) cells receive input from Retinal ganglion cells from both eyes. Both LGNs represent both eyes Neurons in retina, LGN and visual cortex have receptive fields: Neurons fire only in response to higher/lower illumination within receptive field Neural response depends (indirectly) on illumination outside receptive field Pathway from retina via LGN to V1

  10. Simple and complex cells • Cells in retina, LGN, V1 are simple or complex • Simple cells: • Model as linear filter • Complex cells • Show invariance to spatial position within the receptive field • Poorly described by linear model

  11. Neighboring image points are mapped onto neighboring neurons in V1 Visual world is centered on fixation point. The left/right visual world maps to the right/left V1 Distance on the display (eccentricity) is measured in degrees by dividing by distance to the eye Retinotopic map

  12. Retinotopic map

  13. Retinotopic map

  14. Visual stimuli

  15. Nyquist Frequency

  16. Spatial receptive fields

  17. V1 spatial receptive fields

  18. Gabor functions

  19. Response to grating

  20. Temporal receptive fields • Space-time evolution of V1 cat receptive field • ON/OFF boundary changes to OFF/ON boundary over time. • Extrema locations do not change with time: separable kernel.

  21. Space-time receptive fields

  22. Space-time receptive fields

  23. Space-time receptive fields

  24. Direction selective cells

  25. Complex cells

  26. Retina and LGN receptive fields

  27. Retina and LGN receptive fields

  28. Comparison model and data

  29. Constructing V1 receptive fields • Oriented V1 spatial receptive fields can be constructed from LGN center surround neurons

  30. Summary • Linear filters • White noise stimulus for optimal estimation • Visual system (retina, LGN, V1) • Visual stimuli • V1 • Spatial receptive fields • Temporal receptive fields • Space-time receptive fields • Non-separable receptive fields, Direction selectivity • LGN and Retina • Non-separable ON center OFF surround cells • V1 direction selective simple cells as sum of LGN simple cells

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