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Hartline Experiment

Hartline Experiment. Limulus eye has ommotidia containing one receptor each. Each receptor sends a large axon to the brain. Output of one receptor was inhibited by light shining on a neighboring receptor (lateral inhibition). Ganglion cell receptive fields.

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Hartline Experiment

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  1. Hartline Experiment • Limulus eye has ommotidia containing one receptor each. • Each receptor sends a large axon to the brain. • Output of one receptor was inhibited by light shining on a neighboring receptor (lateral inhibition).

  2. Ganglion cell receptive fields • Receptive field - region of visual field that cell responds to. • Center-surround receptive field On-center, off-surround Off-center, on-surround - - - + + - - - - + + - + - + - + - - + - + + - - + + - - - + + - + - - - + - - - - + - - - + +

  3. Problem • Plot the response of an array of on-center ganglion cells to this image. - - - - - - + + - - - - Ganglion cells

  4. Ganglion cells as computational devices • Write a mathematical function that calculates firing rate of cell from luminance pattern. • 1st guess • Increase in firing rate = weighted sum of intensities within receptive field. • Problem 1 - Adaptation • Problem 2 - dark regions in inhibitory region actually excite cell

  5. Ganglion cells as computational devices • Solution • Increase in firing rate = weighted sum of local contrast values within receptive field. • Local contrast C(x,y) = I(x,y) / M - 1

  6. Ganglion cells as computational devices • Solution • Increase in firing rate = weighted sum of local contrast values within receptive field. • Local contrast C(x,y) = I(x,y) / M - 1 Intensity

  7. Ganglion cells as computational devices • Solution • Increase in firing rate = weighted sum of local contrast values within receptive field. • Local contrast C(x,y) = I(x,y) / M - 1 Mean intensity Intensity

  8. Ganglion cells as computational devices • Solution • Increase in firing rate = weighted sum of local contrast values within receptive field. • Local contrast C(x,y) = I(x,y) / M - 1 Mean intensity Intensity Local contrast

  9. Intensity Contrast

  10. Is firing rate of on-center ganglion cell above or below resting level? 3 2 1

  11. Contrast coding • Response of ganglion cells is invariant to changes in illumination level • This breaks down at extremes (e.g. at night)

  12. - + - - Experiment: What is the response of a ganglion cell To a sine wave grating?

  13. - + - - Ganglion cell receptive field 1 cycle / deg. 2 cycles / deg. 4 cycles / deg. 8 cycles / deg. 16 cycles / deg. Assume: A bright bar of a grating is centered in receptive field. Plot the firing rate of this cell as a function of grating frequency.

  14. - + - - Simple cell receptive field - 1 cycle / deg. 2 cycles / deg. 4 cycles / deg. 8 cycles / deg. 16 cycles / deg. Plot firing rate of cell as a function of grating frequency

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