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)

Amacrine cellsAmacrine cells

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

* 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

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

Catching the ACTIONCatching the ACTION

Illustration of the emphasis of moving objects against background.

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

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

a: incoherently

b: coherently

c: Gray background

d: drift of 450µm/s

BT: brisk transient

DS: direction selective

LED:local edge detection

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

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’

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

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

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

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

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

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

For your eyes only!

Uzumaki ampan” Prof. Akiyoshi’s homepage

Do you see a bulge and Do you see a bulge and motion?motion?

Count the black spots !Count the black spots !

Do you see a motion ?

Optical IllusionOptical Illusion

http://psy.ucsd.edu/~sanstis/motion.html

http://psy.ucsd.edu/~sanstis/SASlides.html