Seeing Motion: Lecture Notes

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Godwin Randall
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1 Lecture Ntes - 07/13/99 Alex Huk 0. Intrductin Seeing Mtin: Lecture Ntes Mtin perceptin can be brken int 2 main categries: perceiving mtin f bjects in the wrld ("Object mtin"). perceiving mtin f urselves thrugh the wrld ("Observer mtin"). The rganizatin f this lecture reflects this split; nte that the first half des nt crrespnd t an assigned reading (but see Chapter 8 f the curse text fr an advanced treatment, if yu'd like), the secnd half will review a selectin f the material frm Chapter What is mtin? Mtin is ften defined as a change in psitin ver time. Frm high schl, yu may remember the equatin speed = distance/time. Mtin can be brken int tw main cmpnents: directin and speed. Hwever, fr studying visual mtin, there are several ther distinctins that are helpful. Cnsider the variety f types f mtin infrmatin that yu are capable f using in everyday life: 1. Simple translatin: watching a ball thrwn acrss yur field f view, yu can easily perceive that the ball is mving relative t the wrld. 2. Cmplex mtins: verlking a crwded street, with cars, pedestrians, and cyclists all mving in different directins at nce, yu can detect the individual directins and speeds f each persn r vehicle, but can als detect the general "flw" f traffic. 3. Apparent mtin: lking at a nen street sign, where a series f lights flash ne after anther, it appears t mve. 4. Strbscpic mtin: yu see mvement n TV r at the mvies, althugh yu're actually watching a series f static images. 5. Mtin aftereffects: after watching a waterfall fr a few minutes, yu lk at the nearby rcks, and they appear t mve upward, even thugh they're bviusly nt actually changing their psitin ver time. 6. Structure frm mtin: a well-camuflaged animal is impssible t see as lng as it desn't mve. When it des, yu can easily identify it. 7. Optic flw: as yu walk dwn the street, bjects in the wrld change their psitin in yur field f view, and yu are easily able t use this infrmatin t navigate the wrld.

2 8. Induced mvement: sitting n a statinary train and lking ut the windw, the train n the rail next t yurs begins t mve. Yu at first feel like yur train is mving, even thugh it is nt. 9. Eye-mvements: yur eyes are cnstantly mving, making small shaking mtins r lng, smth mtins. In either case, the image falling n yur retina changes, but the wrld seems statinary. 2. Object mtin Let's call examples 1-6 "Object Mtin", since they all invlve bjects changing their psitins ver time. Althugh bjects are really mving ut in the wrld, yur visual system infers mtin. Often this inference is crrect, but smetimes it is nt. Illusins f mvement are therefre ften used t uncver the basic mechanisms that we use t perceive real mtin: Mtin (mvement) aftereffect ("MAE"). After viewing the attached mvie f cntinuus mtin in the same directin, statinary bjects appear t mve in the ppsite directin. Think abut hw the adaptatin f directin-selective neurns in V1 might cntribute t this illusry percept f mvement. Mvement-withut-mtin. (film viewed in class). In these demnstratins, the stimulus als appears mving but never actually changes psitin. This stimulus separates the cntributins f "lcal" and "glbal" mtin-detectin mechanisms: the lcal (r shrt-range) detectrs register mtin (eg, each cntur seems t be mving), but the mre glbal (lng-range) detectrs knw that the larger stimulus is nt actually changing psitin. This suggests a hierarchy f mtin-detectrs in the visual system. In the case f apparent mtin, a stimulus is flashed briefly in ne lcatin and then anther (usually identical) stimulus is flashed in anther (nearby) lcatin. Under certain circumstances, bservers perceive the first stimulus mving t the psitin f the secnd (as if there were nly 1 stimulus, actually mving frm the first psitin t the secnd). This is the well-knwn principle underlying nen marquee lights that yu see at ld theatres and casins. The perceptin f apparent mtin depends crucially n the interval f time between the flashing f the tw stimulus (the "ISI": inter-stimulus interval): If the ISI is < 30 msec (.03 sec), the tw stimuli appear t be simultaneus. If the ISI is between 30 and 60 msec, the first stimulus will appear t mve partially tward the secnd stimulus. If the ISI is > 60 msec, the first stimulus appears t mve smthly and cntinuus t the lcatin f the secnd stimulus (apparent mtin is achieved). When the ISI is lnger (> 200 msec), yu (veridically) perceive the first stimulus being flashed, fllwed by the secnd stimulus being flashed at a different lcatin. Strbscpic mtin: Apparent mtin is 1 type f strbscpic mtin. A mre familiar instance f strbscpic mtin ccurs in mvies and n televisin. As yu knw, film is made f a series f individual, static frames (snapshts), presented rapidly. Early mvies shwed 16 frames per secnd; althugh fast enugh fr strbscpic mtin, peple were still able t detect the flicker between frames (hence the name

3 "flicks"). T slve this prblem, film engineers had t play upn nt just apparent mtin but als "visual persistence". T see this phenmenn, wave a pencil quickly in frnt f yur face. Yu'll see the pencil blur, leaving a brief streak behind it at fast speeds. The early frame rate f mvies was t slw t utilize this persistence t build a mre cherent, smth percept, but a change t 24 frames per secnd was adequate, and the "flicking" went away. Current films still shw 24 frames per secnd, but nw flash each frame 3 times, artificially increasing the flicker rate t 72 flashes/sec, which puts mdern film well abve ur ability t detect flicker. 3. Simple mtin detectrs in V1 Hw des ur visual system infer mtin frm the variety f cases described abve? We already knw frm the experiments f Hubel and Weisel that there are directinallyselective neurnsin V1. Building a directinally-selective receptr: when an bject mves, its representatin in the retinal image mves, and therefre stimulates a cnsecutive series f phtreceptrs. See the simple circuit prpsed in the figure based n simple inhibitry cnnectins. See the respnses t 2 different directins f mtin ((a)-(b) fr rightward mtin, (c)-(d) fr leftward). Directin tuning curves: directin-selective cmplex cells fire maximally when a bar f light is mved thrugh their receptive field in a certain ("preferred") directin, and fire less and less as the directin is changed mre and mre frm the preferred. [see tuning curves]. Nte that the directin tuning curves measured fr directin-selective neurns in V1 are measured by passing a bar thrugh the receptive field. The directin f mvement f the bar is assumed t be perpendicular t the rientatin f the bar. Hwever, if the bar passes thrugh the entire receptive field, the mtin f the bar culd actually be in a number f directins. This is knwn as the aperture prblem. The aperture prblem reminds us that the directin and speed f mtin f a bar (r a "grating" pattern f bars) is ambiguus when edges r texture are nt present. Als nte that we, as peple, rarely see mtin thrugh an aperture that is ambiguus-- but the receptive field f each mtinrespnsive neurn is essentially an aperature, s the aperture prblem is smething that ur mtin pathway must always slve. When tw grating patterns with different rientatins are superimpsed, the resulting plaid pattern(usually) appears t mve as a whle. What is the perceived directin? When a plaid grating pattern is passed thrugh the receptive field f directin-selective V1 neurns, the neurns respnd t the individual cmpnents: i.e., if ne f the tw superimpsed gratings is mving in the preferred directin, the cell will fire maximally. These neurns are therefre ften called cmpnent cells. Hwever, ur percept is f a cherent plaid, drifting in a directin that is nt the same as the directin f mtin f either f the cmpnent gratings.

4 4. Mtin detectrs in area MT T see the neural mechanisms which underlie ur actual percept f plaid-pattern mtin, we need t leave V1 and head t extrastriate area MT. MT can be defined accrding t the F-A-C-T criteria we have discussed earlier: Functin. MT is ntriusly respnsive t mtin. Cmpare the fmri respnses f varius brain areas t mtin (left) and flicker (right). While mst areas respnd t bth, MT respnds much mre strngly t mtin vs statinary stimuli than flicker vs mean-gray stimuli. This reflects the fact that apprximately 90% f MT neurns are directin-sensitive. Architecture. Staining fr the presence f cytchrme xidase, an enzyme used in metablism, is ften used t help identify different visual areas. See the distinctive patch (quite dark and dense) that is evident in MT. Perhaps mre interestingly, MT has a clumnar architecture, n lnger based n eye-f-rigin (cular dminance clumns) r rientatin preference (rientatin clumns/"pinwheels"), but instead is systematically rganized with regard t the preferrred directin f mtin f neurns. [see the directin-clumns in MT] Cnnectins. The directin-selective neurns in V1 prject (cnnect) t MT neurns. Tpgraphy. Indeed, MT has its wn retintpic map f the wrld (first demnstrated in humans by R Khan and R Dugherty, a graduate student and a pstdc here in the Psych department at Stanfrd). Hwever, the receptive fields f MT neurns are much larger than V1 neurns (i.e., MT-neurn receptive fields cver r "see" a patch f visual space 100 times the size f V1 neurn receptive fields), suggesting that they are able t integrate mre glbal mtin signals. When a plaid pattern is passed thrugh the receptive field f mst MT neurns, the cell respnds t the directin f the plaid, e.g., the perceived directin f mtin, and nt t the directins f mtin f the cmpnent gratings. These cells are called pattern cells [see the different respnses f cmpnent and pattern cells t a plaid pattern] Building a pattern cell: a pattern cell can receive inputs frm a series f cmpnent cells, each f which has a different preferred directin, but all f which have receptive fields cvering the same part f visual space. The respnses f MT neurns have additinal crrespndences t ur cnscius percepts f mtin. Bill Newsme and clleagues (here at Stanfrd) trained mnkeys t perfrm a difficult mtin discriminatin task, judging the directin f mtin f nisy, nt-very cherent mving dt stimuli. Mnkeys view the dts and decide which directin the dts mved. When the experimenters wuld electrically stimulate an MT clumn selective fr upward mtin, the mnkey wuld be mre likely t respnd that the stimulus mved upward.[see the Newsme experimental setup, stimulus, and results]. Human MT and the mtin aftereffect: The time curse f activity in human MT seems t match the time curse f the perceptual MAE (as measured by Rger Ttell at Harvard/MGH). [see the fmri data].

5 5. Observer mtin Infrmatin derived frm visin is a majr guide t ur actins; the perceptin f mtin is especially central t ur wn mvements thrugh the wrld. (see examples 7 and 8 at tp). Optic flw and Heading perceptin: an idea riginally described by JJ Gibsn, the ptic flw field is a representatin f the directin and speed f mtin f the visual field, relative t the bserver. [see an ptic flw field fr a pilt landing a plane]. Nte that near pints mve fast (large arrws) while faraway pints mve slwer (small arrws). All f the arrws pint away frm a central spt, the fcus f expansin, which crrespnds t where the bserver is headed. J.J. Gibsn hypthesized that the visual system estimates the ptic flw frm the chaning pattern f light n the retina, and uses it t estimate the 3D mtins f bservers and bjects. Nte that as bserver mtin becmes mre cmplex (e.g., eye and head mvements while walking) and bjects in the scene mve relative t ne anther (e.g., a bird flies acrss the pilt's field f view), the ptic flw field becmes increasing cmplex. N matter hw cmplex, the ptic flw field prvides infrmatin that underlies many aspects f ur ability t navigate the physical wrld. Aviding cllisins: We are quite gd at detecting bjects that are n a path that will make cntact with us, even as early as 8 days ld. Such "lming" bjects becme increasingly large n the retina as they apprach us, and we appear t make reflexive defensive respnses ("flinching") that are based n a (wisely-cnservative) estimate f "tau", the "time t impact" f the bject. Maintaining balance: We use ptic flw infrmatin, in additin t prpriceptive (= "where we are in space") infrmatin frm jints and muscles, t cmpensate fr slight misbalances. In a "swinging rm" apparatus, the ptic flw can be artificially swayed, and peple make uncnscius cmpensatry leanings in the ppsite directin. Children are especially sensitive t this manipulatin. Adults can cnfirm the imprtance f ptic flw infrmatin t balance by clsing their eyes while standing n ne leg. Induced mtin: Althugh the train example at tp (mtin example #8) ften prduces an illusry feeling f mtin, this usually nly happens when the ther (actually-mving) train is clse. Why desn't induced mtin ccur as ften in cars? See the text fr mre research applying these cncepts t athletics and driving. 6. Eye mvements Our eyes are cnstantly making small, fast mvements, althugh yur perceptin f the wrld isn't nearly s "jumpy". Hwever, there are ways t make yur eye mvements mre bvius; see this dem using afterimages. In fact, as we already learned, we knw that images stabilized n the retina fade. The quick eye-mvements that yu are cnstantly making keep "refreshing" the visual system, s that it keeps detecting change, and nthing fades. There are 2 main types f larger eye-mvements: saccades and smth pursuit. Saccades are fast, pint-a t pint-b, mtins. Smth pursuits, meanwhile, fllw mving bjects

6 gradually. Interestingly, smth pursuit can nly be perfrmed when there's smething t pursue; try mving yur eyes gradually and evenly frm left t right. Yu'll ntice that they jump, and are difficult t cntrl. Nw, mve yur finger acrss yur field f view frm left t right, and fllw with yur eyes. Yu shuld have n truble, nw. Crllary discharge: Despite the nearly-cnstant mtins f ur eyes (as well as ur heads), we perceive a steady wrld. This is likely due t an integratin f visual infrmatin with infrmatin abut eye mvements/psitin and bdy mvements. Usually, this cmpensatry prcess wrks. Hwever, it can be misled, especially when yu dn't cntrl yur eye mvements in the traditinal way. Try gentlypushing n the side f ne eye (keeping the ther clsed): the wrld will appear t jiggle arund, because there isn't an eye-muscle (mtr) signal available t cmpensate fr the mtin f the eye and subsequent mtin f the retinal image. 7. Mtin prcessing after MT: areas MST and STS Area MT sends prjectins t area MST, which appears t prcess mre cmplex mtin patterns. It includes cells that respnd t expansin/cntractin (ptic flw?), and ther cmbinatins f rtatin and translatin. Additinally, there is evidence that MST is the "cmparatr" f crllary discharge, as it receives inputs frm vestibular and mvement areas f the brain. [see sample MST respnses t expansin and rtatin]. Area STS, meanwhile, has neurns that are respnsive t bilgical mtin: see dem in class f "pint-light walkers". This may underlie ur surprisingly-acute abilities t recgnize human-like figures frm very impverished infrmatin (remember the Peter Gabriel vide fr "Sledgehammer"? Yu can identify the dance steps merely frm a few light bulbs stuck n the dancers). 8. Where t nw? Next lecture, we'll examine anther way we perceive the "where" in the visual wrld: seeing the shape and lcatin f bjects in three dimensins.

BROCKTON AREA MULTI-SERVICES, INC. MEDICAL PROCEDURE GUIDE. Date(s) Reviewed/Revised:

BROCKTON AREA MULTI-SERVICES, INC. MEDICAL PROCEDURE GUIDE. Date(s) Reviewed/Revised: Page 1 f 6 Subject: Range f Mtin Exercises Date Develped: 4/2010 PROTOCOL FOR: All trained staff PURPOSE: Range f Mtin (ROM) exercises are very imprtant if an individual has t stay in bed r in a wheelchair.