Motion Perception

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  • Evidence for this includes a patient known as LM who, following a stroke, had great difficulty perceiving certain types of motion. Color vision and acuity remained normal, and there was no difficulty recognizing faces or objects, no difficulty with stereo. But LM cannot see coffee flowing into a cup: appears frozen like a glacier, does not perceive the fluid rising, and often lets the coffee spill or overflows. LM feels uncomfortable in a crowded room or on a street. "People were suddenly here or there, but I have not seen them moving... When I'm looking at the car first it seems far away, but then when I want to cross the road suddenly the car is very near". LM's lesion extends over a substantial region of visual cortex, so one can not localize sharply the regions relevant to LM's motion deficit. This makes it particularly surprising that loss of motion perception can be so cleanly dissociated from other visual abilities.
  • Evidence for this includes a patient known as LM who, following a stroke, had great difficulty perceiving certain types of motion. Color vision and acuity remained normal, and there was no difficulty recognizing faces or objects, no difficulty with stereo. But LM cannot see coffee flowing into a cup: appears frozen like a glacier, does not perceive the fluid rising, and often lets the coffee spill or overflows. LM feels uncomfortable in a crowded room or on a street. "People were suddenly here or there, but I have not seen them moving... When I'm looking at the car first it seems far away, but then when I want to cross the road suddenly the car is very near". LM's lesion extends over a substantial region of visual cortex, so one can not localize sharply the regions relevant to LM's motion deficit. This makes it particularly surprising that loss of motion perception can be so cleanly dissociated from other visual abilities.
  • Area MT is one of the most studied regions of the cortex of the brain, probably second only to V1. Current opinion is that the optical flow field is computed and represented by neurons in area MT. Neurons in MT are selective for motion direction. Neural responses in MT are correlated with the perception of motion. Damage to MT or temporary inactivation causes deficits in visual motion perception. Electrical stimulation in MT causes changes in visual motion perception. Computational theory quantitatively explains both the responses of MT neurons and the perception of visual motion. Well-defined pathway of brain areas (cascade of neural computations) underlying motion specialization in MT.
  • After viewing continuous motion in the same direction for a long time, if you look at a stationary object, it appears to move in the direction opposite to the one you were viewing. This is sometimes called the "waterfall illusion" - if you look at a waterfall for a while, then look at a tree next to it, the tree appears to move upward. The demonstration above shows that this adaptation is local in the retina (to the right of where you were looking, you were adapting to rightward motion, to the left you adapted to leftward, and so on). We take this as evidence for the existence of neurons that are sensitive to motion and selective for the direction of motion, which adapt to the stimulus (analogous to color adaptation after-effects).
  • Role of motion perception:  Motion perception serves lots of helpful functions. Simply detecting that something is moving, draws your attention to it. Segmentation of foreground from background. Compute the 3D shape of an object. Compute the distance to various objects in the scene and estimate the direction in which you are heading within the scene.  For example, hold up two fingers (one on each hand) at different distances, and move your head slowly from side to side while fixating an object on a far wall. Things that are further away slide across the retina more slowly. When there is strong motion on your retina, especially in peripheral regions, you can misattribute that motion and perceive yourself as moving (called "vection"). Movies (especially with large screens as in an IMAX theater) can give this illusion that you are moving. Recognition actions, such as movements of a human (in the "point light displays" shown in class of people walking, dancing, etc., displayed as the motion of a small number of dots attached to the joints of the person).
  • Each arrow represents the speed and direction of motion for each little patch of the visual field. Near points move fast (long arrows), far points move slowly (short arrows). In this example, the arrows point away from a single point called the  focus of expansion  that corresponds to where the observer is heading. The first step in motion perception is for the visual system to estimate optical flow from the changing pattern of light in the retinal image. Then, the 3D motions of observer and objects can be inferred from the optical flow. The optic flow then provides information about the observer's heading and the relative distance to each surface in the world. J. J. Gibson hypothesized that there's sufficient information in the visual stimulus to specify a unique, unambiguous interpretation of 3D motion and depth. Recently, mathematicians have proven that this hypothesis is basically correct. There is a caveat, however: distance and speed are ambiguous (i.e., they trade off). That is, a small, close object when you are moving slowly creates the identical retinal images over time as a large, distant object when you are moving quickly. That's why you need a speedometer in your car. You are lousy at making absolute speed and distance judgements. But, you are very good at relative speed/direction and relative distance.

Transcript

  • 1. Visual Effects of Motion PerceptionGauri S. Shrestha, M.Optom, FIACLE Optometrist and Lecturer, IOM
  • 2. Moti Motion Perceptionon  When do we perceivepe motion?rc  Changes in spatiale distribution of light, over time,pt can lead to motion perceptioni  Eg; (Watching a person jogging)on  Movement can be Individual or unified Gauri S. Shrestha, M.Optom
  • 3. Motionperception Gauri S. Shrestha, M.Optom 3
  • 4. Moti Types of Motionon  Real Motionpe  Actually occurring motion (A car driving by)r  Apparent or Illusory Motionce  Objects that have the illusion of movingpt  Example: spatially adjacent lights sequentiallyi flashed, a sense of motion is elicitedon  Stroboscopic motion or the Phi phenomenon  Examples; Movie, television and computer monitors  Spatial configuration of each frame is different Gauri S. Shrestha, M.Optom 4
  • 5. Moti Stroboscopic Movementonpe  What time intervals are used to portray movement?rc  <30 msec = no movement is perceivedept  60 msec = realistic, smooth movement perceivedi  Optimum or Beta Movementon  60-200 msec = partial illusion of movement  Pure or Phi Movement Gauri S. Shrestha, M.Optom 5
  • 6. Motionperception Gauri S. Shrestha, M.Optom 6
  • 7. Moti Phi Movementon  the spot appears to move from the firstpe location to the second location withoutrc perception of a change in the intermediatee positions.ption Gauri S. Shrestha, M.Optom 7
  • 8. Moti Induced motionon  It is influenced by the frame of reference.pe  Usually, the larger object appears stationaryrc and the smaller object appears to move.ep  looking at the moon on a partially cloudy,ti windy night. It appears that the moon iso moving through the clouds, although then opposite is the case. Gauri S. Shrestha, M.Optom 8
  • 9. Moti 1st Order vs. 2nd Order Motionon  First Orderpe  Linear exchange of lightr for darkce  Stroboscopic and sinep wave stimulitio  Second Ordern  ‘Global’  More complex stimuli Gauri S. Shrestha, M.Optom 9
  • 10. Moti Random Dot Kinematogramson  Dots moving in randomp directions (Pattern showse no coherence)rce  An increased percentagept of dots begin to move in ai common direction untilo motion in the specifiedn direction is detected Coherence Threshold: smallest percent coherence that results in the perception of motion in a defined direction Gauri S. Shrestha, M.Optom 10
  • 11. Motionperception Gauri S. Shrestha, M.Optom 11
  • 12. Motion  Minimum Displacement Threshold (Dmin)pe  Minimum distance dots must move in a givenrc direction to elicit motion perceptionept  Maximum Displacement Threshold (Dmax)io  Maximum distance the dots can move and stilln elicit motion perception  No more than 1 degree Gauri S. Shrestha, M.Optom 12
  • 13. Moti Processing of Motionon  Primarily Magno Pathway (‘Where’ system,pe Dorsal Processing Stream, Parietal Pathway)rc  Visual Area 5 (V5, MT)ep  Neurons in MT are selective for motion directiont  Electrical stimulation in MT causes changes inio visual motion perceptionn  Cortical area MST is functionally speicalized for optic flow  STS is specialized for recognizing biological motion  Corrolary discharge explains why eye movements Gauri S. Shrestha, M.Optom 13 do not evoke a perception of motion
  • 14. Moti Processing of Motionon  Parvo Pathwaype  Plays a small role in motion perceptionrception Gauri S. Shrestha, M.Optom 14
  • 15. Moti A case study -LMon  A case LM who following a stroke, had greatpe difficulty perceiving certain types of motion.rc  Color vision and acuity remained normal, andep there was no difficulty recognizing faces ort objects, no difficulty with stereo.io  But LM cannot see coffee flowing into a cup:n appears frozen like a glacier, does not perceive the fluid rising, and often lets the coffee spill or overflows. Gauri S. Shrestha, M.Optom 15
  • 16. Moti A case studyon  LM feels uncomfortable in a crowded room or on ap street. "People were suddenly here or there, but sheer have not seen them moving...c  When she is looking at the car first it seems far away,ep but then when I want to cross the road suddenly thet car is very near".io  LMs lesion extends over a substantial region ofn visual cortex, so one can not localize sharply the regions relevant to LMs motion deficit.  This makes it particularly surprising that loss of motion perception can be so cleanly dissociated from other visual abilities. Gauri S. Shrestha, M.Optom 16
  • 17. Motionperception Gauri S. Shrestha, M.Optom 17
  • 18. Moti Characteristics of MT (V5)on  Neurons in MT are selective for motion direction.pe  Neural responses in MT are correlated with ther perception of motion.ce  Damage to MT or temporary inactivation causesp deficits in visual motion perception.ti  Electrical stimulation in MT causes changes in visualo motion perception.n  Well-defined pathway of brain areas (cascade of neural computations) underlying motion specialization in MT. Gauri S. Shrestha, M.Optom 18
  • 19. Mo MT neurons receive inputs from direction-selectiveti neurons in V1.on  MT neurons are velocity selective, each respondsp best to a preferred velocity (speed and direction)er within its receptive field, pretty much independent ofc stimulus pattern.ep  By contrast, a direction-elective V1 neuron confoundst motion with pattern. A typical V1 neuron responds toio a particular orientation (edge or bar) moving in an particular direction. The response of the V1 neuron also increases with contrast.  A typical MT neuron, on the other hand, responds to almost any pattern with almost any contrast, as long as it moves with the right velocity. Gauri S. Shrestha, M.Optom 19
  • 20. Mot Demonstration of direction-io selectivity of MT neurons (Movie)n  The electrode was connected to an amplifier,pe and output to a loudspeaker.rc  The audio track allows us to hear theep loudspeaker - each click corresponds to ant action potential.io  This example MT neuron responds strongly ton down-left motion, not at all to up-right motion, and with intermediate firing rates to intermediate directions. Gauri S. Shrestha, M.Optom 20
  • 21. Motionperception Gauri S. Shrestha, M.Optom 21
  • 22. M A columnar architecture in MT for stimulus motion; neuronso with similar motion preferences lie nearby one another, witht an orderly progression from one motion direction to the nextio analogous to orientation columns in V1.nperception Gauri S. Shrestha, M.Optom 22
  • 23. M A characteristics of MT neurons responds: For a neuronsot preferred direction of motion (upward in this case) the neuroni responds more as coherence is increased.onperception Gauri S. Shrestha, M.Optom 23
  • 24. Mo Neurons in area STS respond selectively toti biological motion, like the point-light walkersonperception Gauri S. Shrestha, M.Optom 24
  • 25. Mot Eye movement and motionio perceptionn  How does the visual system keep track ofpe whats moving? You or object?r  Visual images are combined with other information toce inform us about the motion of our eyes, head, andp body.ti  The vestibular system provides information about theon motion of your head and body.  A copy of the eye movement command from the eye movement centers in the brain stem provides information about eye movements.  Vision is combined with vestibular and eye movement signals in Area MST. S. Shrestha, M.Optom Gauri 25
  • 26. Moti Luminance and Coloron  Luminancepe  Adding luminance contrast improves motionrc perceptione  Stronger stimuluspti  Coloron  Not as much of an effect Gauri S. Shrestha, M.Optom 26
  • 27. Moti Luminance and Coloron  How are these aspects studied?pe  Isoluminant gratings-bars have different hues, butrc have the same luminanceept  What has been found in studies?io  Under certain isoluminant conditions, motionn perception is impaired  Perception is weak or not as fast as the actual grating motion Gauri S. Shrestha, M.Optom 27
  • 28. Moti Further Resultson  However, under other isoluminant conditions, thepe perception of motion is not reducedrc  This suggests that the parvo pathway does play ae role in motion perceptionption Gauri S. Shrestha, M.Optom 28
  • 29. Mot Contrastion  The perception of motion and velocity ispe affected by factors in the stimulus unrelatedrc to the motion itself.e  Perceived velocity is diminished as thept contrast is reduced.ion a low contrast object is perceived to move slower than a higher contrast object Gauri S. Shrestha, M.Optom 29
  • 30. Motionperception Gauri S. Shrestha, M.Optom 30
  • 31. Moti Spatial Resolution and Motionon  Dynamic Visual Acuityp  ability to resolve aer moving stimulusc  Resolution is constantep until stimulus velocityt reaches 60-80º/si  Beyond 80º/s, ↓ dynamicon visual acuity This reduction is due to our inability to produce accurate smooth pursuit eye movements at velocities of these magnitudes Gauri S. Shrestha, M.Optom 31
  • 32. Moti Saccadic Eye Movementson  Saccadic Eye Movementspe  Ballistic eye movement between two fixationr pointsce  We do not perceive blur with these movementspti  Saccadic Suppression (Saccadic Omission)on  Vision is suppressed shortly before, during, and shortly after saccadic eye movements  Due to selective suppression of the magno pathway Gauri S. Shrestha, M.Optom 32
  • 33. Moti Why have saccadic suppression?on  During saccades, images move across the retinape very quickly (↑temporal frequencies)rc  Magno pathway is most sensitive to ↑temporalep frequenciest  Therefore, this suppression avoids smearing ofio visionn  The suppression mechanism is unknown Gauri S. Shrestha, M.Optom 33
  • 34. Moti The Motion Flow Fieldon  As we move through the world, the objects out inpe the world change in predictable ways.r  two components: Rotational and Translational.ce  Rotational movement: all the components in the movementpt flow fields rotate the same amount around the axisi of rotation regardless of distance.on  Rotation of head  Translational movement: movement  Closer points shift more than further points.  Distance affects both the speed and the direction of the translational component of the motion-flow- field. Gauri S. Shrestha, M.Optom 34
  • 35. Motionperception Gauri S. Shrestha, M.Optom 35
  • 36. Moti Motion Thresholdson  the threshold for detecting motion was measured asp a function of retinal eccentricity.er  The graph shows the data collected with and withoutce correcting lenses that correct for defocus in thep peripheral visual field.tion Gauri S. Shrestha, M.Optom 36
  • 37. Moti Motion Thresholdsonperception Gauri S. Shrestha, M.Optom 37
  • 38. Moti Motion Thresholdson  The thresholds for detecting motion increasepe in the periphery.rc  This increase is smaller when the opticalep correction is used.tion Gauri S. Shrestha, M.Optom 38
  • 39. Moti Motion illusionon  Motion is a perceptual attribute:pe  the visual system infers motion from therc changing pattern of light in the retinal image.ep  the changing pattern of light can give theti illusion of motionon Gauri S. Shrestha, M.Optom 39
  • 40. Moti Motion illusionon  Another example is the motion aftereffect.perception Gauri S. Shrestha, M.Optom 40
  • 41. Motionperception Gauri S. Shrestha, M.Optom 41
  • 42. Moti Role of motion perception:on  Simply detecting that something is moving, drawsp your attention to it.er  Segmentation of foreground from background.ce  Compute the 3D shape of an object.pt  Compute the distance to various objects in the scenei and estimate the direction in which you are headingo within the scene.n  Recognition actions, such as movements of a human Gauri S. Shrestha, M.Optom 42
  • 43. Motionperception Gauri S. Shrestha, M.Optom 43
  • 44. Motionperception Gauri S. Shrestha, M.Optom 44
  • 45. Motionperception Gauri S. Shrestha, M.Optom 45
  • 46. M Each arrow represents the speed and direction of motion forot each little patch of the visual field. Near points move fasti (long arrows), far points move slowly (short arrows).onperception Gauri S. Shrestha, M.Optom 46
  • 47. Moti Optic flowon  The optic flow then provides informationpe about the observers heading and the relativerc distance to each surface in the world.e  J. J. Gibson hypothesized that therespt sufficient information in the visual stimulus toio specify a unique, unambiguous interpretationn of 3D motion and depth. Gauri S. Shrestha, M.Optom 47
  • 48. Moti Optic flowon  That is, a small, close object when we arepe moving slowly creates the identical retinalrc images over time as a large, distant objecte when we are moving quickly.pt  We are very good at relative speed/directionio and relative distance.n Gauri S. Shrestha, M.Optom 48
  • 49. Moti Glaucoma and Temporal Visionon  POAG is primarily diagnosed by ↑IOPp  IOP is often increased, but not alwayser  Not all patients with ↑IOP develop glaucomacep  The ↑IOP damages the ganglion cells and results in visualt field defectsio  We generally do not initiate treatment until defects aren apparent  Substantial ganglion cell damage is done if visual field defects are apparent Gauri S. Shrestha, M.Optom 49
  • 50. Moti Clinical Pointson  Glaucomap  Neural damage is selective for the magno pathwayer and this is where motion is perceivedc  Motion perception tests could be used for earlyep detection of glaucomati  Glaucoma and Random Dot Kinetogramso  D are elevated in glaucoma suspectsn min  Motion coherence thresholds are higher in those regions of the visual field showing Glaucomatous damage compared to the unaffected regions of the visual field Gauri S. Shrestha, M.Optom 50
  • 51. Moti Thanksonperception Gauri S. Shrestha, M.Optom 51