Lecture # 25

transcript

Lecture #25

Motion detection and eye movement4/30/13

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Final exam Q

• For next Tuesday, please write 1 final exam questionBased on 1 lecture or conceptShould be a different topic from your wikiList critical pointsWrite study question to distill key pointWrite synthetic question for exam and answer it

Study questions will get postedBest 4 synthetic Q will be on exam : choose 1 of 4 to answer

Final exam format

• Part 1I will post 10 questions on web ahead of exam. You will pick 3 of 5 to answer on exam

• Part 2I will post your study questions on web. The best 4 synthetic questions will be on exam as pick 1 of 4

• Part 3One question which will cover eye development and evolution lectures

Wiki project

• Final wiki project is due next Thursday3 major sections

Try to avoid or at least explain medical jargon Make it readable – do not sound like encyclopediaInclude references in the textHave someone else read it

Or finish it and then wait a day to proof it

Wiki grading rubric

• Introduction Total 40 pts• Section 1• Section 2• Section 3• Facts correct• Use of literature• Figures cited• Writing style• Interesting presentation• Extra effort

Eye movements

• Eyes are in constant motionKeeps receptors from being saturated with same signalIf stare at same thing for long time - scene would fade

Eye movements

• SaccadesRapid eye movements to sample different parts of a scene

• Smooth eye pursuitsSlower smooth motions that follow slow moving objectsNot possible for static image

• Vergence movementEyes move together as focus in on object

Eye movements

Saccades

Smooth pursuit

Vergence

Two other reflexes

• Optokinetic reflexEyes sense motion of object and move to keep object centeredObject moves clockwise - Eyes move clockwise

• If no moving object - hard to make eyes move smoothlyEyes want to saccade

Two other reflexes

• Vestibular-ocular reflexSense motion of head and move eyes in opposite direction to compensateUse semi-circular canals to detect motion and feed to eye muscles

Move head while fixated on object

Eye movements 2

Optokinetic reflex

Vestibulo-ocular reflex

Saccades are not random - take eyes to regions of interest

Fig 7.15

Making a cup o’ tea

Stabilize gaze

Eyes compensate for head movement so that gaze can remain stable or fixate on certain objects

Other animals also use eye saccades

• Coordinate eye and head movementsSaccade and then fixate

Birds can’t move eyes as much so use head motion = head saccade

• Spend 50% of time with head fixed while rest of body moves

• Creates bobbing motion of head

Cronin et al 2005

Cranes in motion

Head angle fixates and then changes rapidly

Cronin et al 2005 Supp Fig 1

Eye movements

• Constant small jerky movementsRefresh image so retinal image doesn’t saturate

• Larger saccadesUp to 1000°/s per motion

• Number of saccades3 per second

Eye movements

• How do we deal with rapid saccades?You don’t notice them

Eye movements

• How do we deal with rapid saccades?You don’t notice them

• Saccadic suppression Suspend visual activity during saccade Suppress input to brainBut no suppression during smooth pursuit

• Spend 10% of time in saccade1.5 hrs per day being blind!!

Why use saccades and fixation?

• Avoid loss of resolution due to motion blurBlur occurs if object moves > 1 receptor diameter / response timeHuman -

Receptor 0.016 deg acceptance angleResponse time = 20 ms = 0.02 sBlur if motion > 1°/s

Why use saccades and fixation?

• Detect motion of objects against steady background

• Detect motion of self against background

Eye movements

6 muscles move the eye, work in pairs

Cranial nerves send control to eye muscles

Oculomotor nucleus sends output to four eye muscles (cranial nerve III)

Trochlear nucleus to superior oblique (cranial nerve IV)

Abducens nucleus to lateral rectus (cranial nerve VI)

Brain regions controlling eye movement

The motor nuclei get input from superior colliculus through gaze centers:

Vertical / horizontal

SC gets input from retinal ganglion cells

FEEDBACK

If stimulate superior colliculus, eyes will move

Motion perception question?

• Place on paper in front of youFixate on dotMove pen from left to right as stare at dotSee pen move

• Follow motion of pen with eyesSee pen moveImage of dot moves across retinaDo you perceive it as moving?

Distinguish eye movement and object movement

Movement of image on retina

Eye movement

So is object moving?

What are you seeing?

Yes Yes

Yes No

No Yes

Distinguish eye movement and object movement

Movement of image on retina

Eye movement

So is object moving?

What are you seeing?

No No No Stable object

Yes Yes No Survey stable object

Yes No Yes Object must be moving

No Yes Yes Following moving object

Comparator

• Motor systems send signal to eyes to move and message to comparator that eyes are moving

• Compensate for eye movement so other parts of brain do not interpret retinal image change as motion

Wolfe Fig 7.16

Motion perception

• Needed to detect motionDirectionSpeed

• Special circuits Peripheral retina (M ganglion cells)

Magnocellular layers of LGN primary visual cortex (V1)

middle temporal lobe (MT)

Wiring - motion detector

Use two cells to detect object first at point A and then point B

A detects object before B. Motion neuron, M doesn’t get signal from both at same time

Motion neuron, M stimulated by large stationary object

Cell X signals only if signal from both D and B

Detects motion from left to right

Not sensitive to motion right to left

Cell D delays signal

Q. How could a neuron be a delay cell?

Cell D delays signal Cell X signals only if signal from both D and B

Adding a 2nd cell makes for an additional synapse. This will add a few ms delay.Having a longer axon can also build in a delay time.These delays are known in fly eyes and auditory systems.

Simulation

Motion circuitry

• Can detect:Motion directionSpeed

• Circuitry works for actual moving object or object that

Appears at ADisappearsReappears at B

= Apparent motion

Apparent motion

Perception of apparent motion

• Apparent motion changes with speed at which alternatesOptimal frequency

Too fast - flickersToo slow - hopsJust right - motion appears smooth

Apparent motion

Aperture problem• Viewing moving image through an aperture

How do we know which parts correspond through time?Mask or aperture makes correspondence uncertain

Correspondence problem

Which dot does the blue one correspond to in the next frame?

A or B?

Global motion detection solves correspondence problem

Global motion detectionBy comparing output of all 4 motion sensors, the global sensor can only come to one conclusion

Motion is down and to right

Motion correspondence

Other ways to test for motion sensors

• Motion detection is in middle temporal lobe (MT)

• Train monkeys to detect dots movingCan detect even if only a few % are in motionLesion parts of MTMonkeys then need many more dots to detect the motion

Dot motion

Using motion to navigate - optic flow

• Motion of visual field tells if moving in space• Focus of expansion

Locate non-moving part of scene - where you are headedUse motion vs lack of motion to set direction

Motion of visual field

Other ways to test for motion sensors

• In other monkeysStimulate cells of MT sensitive to motionMonkeys report moving dots even if none are there

• In humans can use fMRIFunctional magnetic resonance imagingDetermine which neurons / brain regions are active

Detect by increased oxygen delivery to working neurons (blood oxygen-level dependent, BOLD fMRI)

Detecting motion in humans - motion aftereffects

• If watch motion in one direction for a while, demon (neuron) will get tired out

• If stop motion, get reverse motion sensation

Motion after effects

Second order “motion”

• Can get apparent motion by inverting black and white dots

Second order motion

Peripheral drift illusion• If stimulate peripheral vision with stepped lightness

gradients, detect motion black dark graywhite light gray

Intensity

Peripheral drift illusion

More motion from step vs graded shading

More motion from short vs long segments

Color can enhance motion for some viewers

Kitaoka snakes - more motion from curved edges

Kitaoka snakes - is smaller better?

Mechanics

• Lightness gradient stimulates velocity detectors

• These work together to stimulate global motion detectorsTell your brain that something is moving

Michael Bach

Arthur Shapiro - America University

Lecture # 25

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