Recent findings

1. Recent findings • Retinal microcircuit: • Neurons (bipolar and amacrine cells) intervene between: photosensitive cells(rods &cone) and ganglion cells – to shape and compress information • Bipolar cells ( ~ 12 types) :conducts information from photoreceptors to ganglion cells and shape information • Amacrine cells: Inhibiting neurons (at least 29 types)

2. Amacrine cells

3. Functions of amacrine cells • Feedback for retinal gain control : to match ambient illumination and contrast • Starburst cell: to report the direction of moving stimuli • Mechanism to compensate the ballistic eye movement: to avoid blurred image

4. Segregation of object and background motion in the Retina byB.P. Ölveczky S.A. Baccus and M. Meister MIT, USA • * Retina is not a passive receptor • * Microcircuit in retina to detect movement • of an object relative to its background • * Correlated firing in a group of retinal neurons

5. Movements of eye • Fundamental component of vision • Pursuit movements to redirect our gaze --- ballistic • Small eye movements – essential for seeing --- drift • ~ 0.5° (60 cones) at ~ 0.5°/s • visual perception fades with in 0.1 sec --- without drift • Task: Discriminate between motion of an object and eye movements • Movements:‘Pop-out’ and attract our attention

6. Catching the ACTION Illustration of the emphasis of moving objects against background.

7. Eye movements Retina :6-8mm; grating period: 133µm; jitter: step size of 6.7 µm every 15ms; Object region 800 µm (dia.)and background 4.3x3.2mm

8. Detecting motion • Ballistic gaze eye shift : Vision is suppressed • Differentiate between : motion of object and eye movement • Eye movement of the two eyes differ --- complicates • Any detection of motion – easier in retina than in brain • Experiments in the isolated retina of Salamander and rabbit

9. a: incoherently b: coherently c: Gray background d: drift of 450µm/s BT: brisk transient DS: direction selective LED:local edge detection

10. Results of Experiment 1 • Some ganglion cells are highly selective for motion : • They are called “Object Motion Sensitive” (OMS) cells • Responds vigorously for relative motion of object and background • Completely suppressed when background & object move together • Background scene is not important - relative motion is important • Even a slow drift of the object also generates response

11. Results of experiment 2 • a: Increasing object size • Ganglion cells are exited by motion in or near receptive center • Increases up to 250µm • Objects bigger than 1mm has no effect • b: Classic measurement of Flashing spot (1 Hz) is also similar to condition ‘a’

12. Hypothesis: The inhibition from peripheral motion : pulses similar to excitation pulses in the center b: same jitter trajectory to background and center but delay in time Peripheral pulses ~ 100 ms wide c: In salamander retina polyaxonal amacrine cells that responds to coherent jitter and produce ~100ms pulses

13. a: Ganglion cell firing is similar to amacrine cell ( 0 and 180º-- same ) b: Spatial frequency did not matter except when the period is less than 40µm Object motion selectivity is independent of spatial pattern

14. A model of retinal processing - differential motion OMS cell has additive inputs from the nonlinear cells in front of the object and inhibit signal from amacrine cells with similar nonlinear cells underlying the background

15. 200 positions of ganglion cells in salamander retina were probed: Vigorous firing in the region of motion and suppression in rest Supports Motion ‘pop out’ ~ 230 ms to detect motion c: cells in same region fire synchronously d: cell in different regions are asynchronous

16. Motion Illusion By Japanese artist Ouchi Circle appears to float and jitter relative to the background Vertical eye movement in the periphery and horizontal movements in the center Eye executes horizontal and vertical eye movements independently

17. Conclusion • Experiments on retina of rabbit and salamander • Essential building blocks exist in other species ~20% of ganglion cells show nonlinear summation • It is probable that similar ganglion cells with OMS properties exist in many spies including humans • Simple mechanism of motion proposed • Excitation from the receptive field center • Inhibition from similar subunit from the surrounding • Random nature of eye movements that produce transient and sparse activation of both excitation and inhibition network

18. For your eyes only! Uzumaki ampan” Prof. Akiyoshi’s homepage

19. Do you see a bulge and motion?

20. Count the black spots !

21. Do you see a motion ?

22. Optical Illusion http://psy.ucsd.edu/~sanstis/motion.html http://psy.ucsd.edu/~sanstis/SASlides.html