5/4/14 2:14 PM- Presence of animated motion in Point-light displays is almost instantaneously detected by the visual system.Adults need about 100ms to identify point-light human walkerCan discriminate different animals by their typical patterns of motion- To humans, variety of info is also conveyed by point-light animationsnature of actiongenderidentity of actorcant be extracted with a single frame of animation, which would only be a set of unconnected and meaningless pointsmust mean that a complex structure-from-motion analysis is rapidly carried out by the visual system, extracting and integrating form to motionnot dependent of reversal, velocity, or maskingcan also still detect biological movements in scrambled biological motion displays, even though no known animal species can be identified with it.- Explanations of extracting form and action from these displays fall into two classesEvent-from-form explanation visual processes first extract form and then determine actionEvent-from-dynamics explanations based on temporal info for action and argue that the most useful info is that about dynamics, the force acting on objects.- Biological motion perception dramatically affected by display inversionPerformance drops dramatically in when displays are presented upside-down.Less accurate at detecting walking on hands when display was turned upside down than when it was upright so orientation of gravity, not form, seemed crucial for detection.Direction of scrambled biological motion displays retained unless inverted.DevelopmentPerception of biological motion readily available to 3-4 year old childrenPerception disrupted after display inversion in 6y/o and adultsBy 3 months, infants discriminate both point-light displays of biomechanical motion from displays of identical absolute motion with scrambled spatial relations and upright v. Inverted point light moving displays.4 months prefer upright over inverted figures5 months discriminate rigid point light walker display from one win which local rigidity is perturbed, but not only as long as it is presented upright.2 month old infants did not show preference for biological motion displays -> shows that perception of locomotion in point-light displays requires some visual experience or some maturation.No study has investigated sensitivity to Johanssons biological motion displays or effect of inversion at birth.ChicksVisually naive chicks and first exposure to point-light animations depicting walking adult hen or nonbiological motion stimulus -> prefer bio to non bioEven when displays of different animals (cats)But hen> cats> non bioSupports conclusion that sensitivity to biological motion depend on general mechanisms for detection and extraction of invariants of biological motion displays, and would not therefore be limited by species-specific onstraints.Evidence that inversion effect would be independent from experience, which was interpreted as causd by the fact that gravity may constitute a predisposed additive parameter playing a crucial role in biological motion perception.PurposeInvestigate origin of sensitivity to biological motion in humans at birthSame test on newborn chicks used w/ 2 day old newbornsUse of hen-walking animations -> ruled out potentially remote possibility that newborns may have had any previous experience with the kind of motion depicted in the stimuli used.Three experimentsEach on separate group of newbornsIf sensitivity to biological motion is experience independent, then expect babies to discriminate biological from nonbiologocial patterns and to exhibit spontaneous preference for biological stimulusInvestigate origins of display inversion by testing whether a prefercen for upright displays is present at birth and is therefore experience-independent.Resultsexp 1 test whether newborns were capable of discriminating after exposure to a pattern of 13 moving elements representing a walking hen from same pattern of elements moving in a random manner23 frames used to cover an animals entire step sequencesequence was looped and projected onto a computer screen.Random sequence produced using the function random movement and rotation in macromedia director mxHalf the newborns habituated to biological motion, other half to nonbiological motionNon significant t test between total fixation times in the two groupsAfter habituation -> visual preference probed for habituation stimulus v. Novel stimulusLooked longer @ novel stimulus Mean preference for novel stimulus was higher than chanceNo significant difference between mean novelty preference score of newborns habituated to nonbiologcal v. Habituated to biological stimulus.Exp 2 tested spontaneous preference for bio v. Nonbio motion12 newborns presented with same biological and nonbiological motion stimuli used in exp 1.Average total fixation times transformed into percentages to see if significantly different from chance% of total fixation time newborns psent looking at the bio stimulus was 62% and differed significantly from chance levels.8/12 preferred walking hen to random motionno significance between age and preference scorefavors idea that naive newborns show preference for biological motion. Such preference would be largely intrinsicExp 3 tested for presence at birth of biological motion inversion effect13 newborns presented with upright or inverted motion animation sequences picturing walking henpercentage of time newborns spent looking at upright was 58%, above chance11/13 preferred uprightresults showed that newborns showed a significant preference for upright animation sequenceDISCUSSIONJohansson suggested that detection of biological motion oculd be an intrinsic capacity of the visual systemUp until this study, developmental studies have been unable to unequivocally address inversion effect in human subjects because results obtained with infants could always be accounted for by either innate or learning mechanismsIn this study, showed that newborn babies are able to discriminate between two point-light displays of bio or nonbio motionsFound that preference was also orientation specificThis study disentangles question about role of previous experience since newborns dont have experienceCant eclude fetal experience of vestibular motion cues and postnatal experience influencing sensitivity to low-level features of bio motionCant say that sensitivity to dynamic info would be limited to local info because local dynamic relationships are identical in upright v. Inverted experiments, but prefer upright.Babies must either rely on some unlearned representation of an upright walker, or the visual system uses soem general and local dynamic information to constrain the organization of biological motion displaysSuch dinformation would permit differentiation between upright and an inverted version of the same biological motion displayBest candidate of possible constraint = gravityViolation of gravitational constraint would result in failure at discriminating such displaysHas not been testedMore specific hypothesis involve presence of a visual filter selectively tuned to characteristic motion of the limbs of an animal in locomotionThis mechanism would provide a general detection system for presence of terrestrial vertebratesPrevious studies suggest that our sensitivity to certain effects of gravity develops gradually during infancyBut it may be that preference for correct gravitational constraints within the motion of a living being may preexist.Findings conflict with previous results for newborns using stimulus that depicted motion of one single point-light that was described as bio or non bio based on kinematic specification, as opposed to a real animal walkingalso conflict with Fox and McDaniel who reported no evidence for discrimination of bio motion in 2 month old babies.Further empirical data is necessary to finally establish that preference is intrinsic and specificSpecificity of sensitivity towards bio motion patterns could be assessed by contrasting bio motion with other types of nonbio motion (rigid object motion)Role of local info may be tested by using phase or spatially scrambled bio motionIf confirmed such predisposed mechanism would enable newborns to detect and preferentially attend to, the movement of biologically relevant signals in his environmentCompatible w/ Human First Hypothesis, which posits either that humans identify objects and separates conspecificts by using their different properties or that detection of kind properties, guiding distinction between the two different classes of animate/inanimate, may be a feature of the system rendered directly available by the architecture of the brain very early in life.Empirical evidence on infant perception supports this conclusion by showing that ability to discriminate a wide array of properties is not limited to basic low-level stimulus features but rather extends to complex properties that could allow newborns to uniquely single out conspecificsResults of this study suggest that bio motion could be one of the perceptual properties that could allow humans to distinguish living creatures from other obejcts and identify conspecifics from birth

Predisposition for Biological motion in newborn baby5/4/14 2:14 PM

5/4/14 2:14 PM