recent findings
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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Recent findingsRecent findings
Retinal microcircuit:
1. Neurons (bipolar and amacrine cells) intervene between: photosensitive cells(rods &cone) and ganglion cells – to
shape and compress information
2. Bipolar cells ( ~ 12 types) :conducts information from photoreceptors to ganglion cells and shape information
3. Amacrine cells: Inhibiting neurons (at least 29 types)
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Amacrine cellsAmacrine cells
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Functions of amacrine cellsFunctions of amacrine cells
1. Feedback for retinal gain control : to match ambient illumination and contrast
2. Starburst cell: to report the direction of moving stimuli
3. Mechanism to compensate the ballistic eye movement:to avoid blurred image
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* 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
byby
B.P. B.P. ÖÖlveczky S.A. Baccus and M. Meisterlveczky S.A. Baccus and M. MeisterMIT, USAMIT, USA
Segregation of object and background Segregation of object and background motion in the Retinamotion in the Retina
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Movements of eyeMovements 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
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Catching the ACTIONCatching the ACTION
Illustration of the emphasis of moving objects against background.
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Eye movementsEye 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
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Detecting motionDetecting motion
1. Ballistic gaze eye shift : Vision is suppressed
2. Differentiate between : motion of object and eye movement
3. Eye movement of the two eyes differ --- complicates
4. Any detection of motion – easier in retina than in brain
Experiments in the isolated retina of Salamander and rabbit
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a: incoherently
b: coherently
c: Gray background
d: drift of 450µm/s
BT: brisk transient
DS: direction selective
LED:local edge detection
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Results of Experiment 1Results 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
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Results of experiment 2
a: Increasing object size
1. Ganglion cells are exited by motion in or near receptive center
2. Increases up to 250µm
3. Objects bigger than 1mm has no effect
b: Classic measurement of Flashing spot (1 Hz) is also similar to condition ‘a’
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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
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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
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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
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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
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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
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ConclusionConclusion
Experiments on retina of rabbit and salamander
1. Essential building blocks exist in other species ~20% of ganglion cells show nonlinear summation
2. 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
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For your eyes only!
Uzumaki ampan” Prof. Akiyoshi’s homepage
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Do you see a bulge and Do you see a bulge and motion?motion?
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Count the black spots !Count the black spots !
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Do you see a motion ?
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Optical IllusionOptical Illusion
http://psy.ucsd.edu/~sanstis/motion.html
http://psy.ucsd.edu/~sanstis/SASlides.html