Running Head: Auditory Event Perception
Source-Perception LoopRunning Head: Auditory Event PerceptionAuditory Event Perception: The Source Perception Loop for Posture in Human GaitRichard E. Pastore, Jesse D. Flint, Jeremy R. Gaston, & Matthew J. SolomonBinghamton UniversityDepartment of PsychologyP.O. Box 6000Binghamton, NY 13902-6000Phone: (607) 777- 2539Fax: (607) 777- 4890e-mail: pastore@binghamton.edu1
Source-Perception LoopAbstractThere is a small but growing literature on the perception of natural acoustic events, withfew attempts to examine complex sounds, not normally produced in a lab. The currentstudy utilizes a three-stage approach of event perception, examining the source, thesound, and perception, as well as the relationships among these for posture contrast inhuman gait. Understanding the dynamic interactions of source properties, including theposture contrast, allows evaluation of acoustic properties that, directly or indirectly, areposture diagnostic. Listeners performed a within-walker 2IFC judgment of posturecontrast for recorded walking sequences. Our working knowledge of the initial stagesprovides the basis for modeling-based understanding of listener decision-making,resulting in a relatively thorough specification of the full source-perception loop.2
Source-Perception Loop3Auditory Event Perception: The SourcePerception Loop for Posture in Human GaitHumans exhibit extensive abilities to use sounds to identify and monitor events intheir environment, which are often unappreciated and, with the exception of speech,largely unexplored (e.g., Handel, 1989, 1995; McAdams, 1984, 1993). As examples ofabilities, both auto mechanics and physicians use sound to detect possible abnormalitiesand make preliminary diagnoses of probable cause (e.g., Jenkins, 1985), and a similardiagnostic listening strategy can be effective in podiatric medicine (J. Wernick, personalcommunication, May 2004). Although a major long-term goal of hearing research is tounderstand typical auditory perception, the majority of research has investigated only theperception of simple, easily specified laboratory-created stimuli. The resulting large bodyof very basic research has provided important, detailed specifications of the basicfunctioning of the human auditory system (e.g., Moore, 1997; Hartmann, 1998; Yost,1994), but this scientific knowledge often seems to have limited direct relevance forunderstanding the ability to recognize the nature of complex natural source events. Thiscould be either because the findings represent too low a conceptual level, or level oforganization, to be relevant, or because we lack sufficient understanding of natural sourceevent perception.Speech is the one example of a class of complex natural source events that hasbeen studied extensively in terms of the relationship between properties of production(the source event) and the sounds produced, as well as between possible invariantacoustic information and the perceptual categories. One hallmark of more than half acentury of speech research has been the failure to identify invariant acoustic cues for
Source-Perception Loop4features of production, and it may be that complex source events do not readily map intosimple acoustic properties. However, speech perception is often assumed to involve someform of highly specialized, closed module (e.g., Liberman & Mattingly, 1985), possiblyone initially shaped by early experience (e.g., Werker & Tees, 1999). If valid, thesupposition of a unique speech perception mechanism would mean that this body ofresearch would not be relevant to understanding the perception of other types of naturalacoustic events.One alternative perspective, based upon Gibson's ecological conceptualization, isthat all natural acoustic events (including speech) define the structure of the soundsproduced that allows direct perception of the source event (e.g., Fowler, 1991; Fowler &Rosenblum, 1991a,b; Garver, 1993; Vanderveer, 1980). With the basic supposition thatthe proximal stimulus provides direct access to dynamic structural properties of theoriginal source event, a typical study in ecological acoustics seeks to demonstrate thatlisteners can accurately identify properties of the source event (e.g., McAdams, Chaigne,& Roussarie, 2004), yet these studies often provide only limited specification of theacoustic signal (e.g., Klunker-Peck & Turvey, 2000). An alternative perspective is thatlisteners may learn about basic relationships between (sometimes complex) attributes ofsounds and source events that allows identification (or inference) of the nature of sourceevents (e.g., Wildes & Richards, 1988; Mirman, Holt, & McClelland, 2004). Whateverone's theoretical beliefs, it is clear that properties of the original source event are reflectedin, or give structure to, the sounds produced, and that this acoustic information is crucialto the ability to perceive or identify the original source event.
Source-Perception Loop5In recent years, a small, but growing number of studies have begun to investigatespecific aspects of the perception of complex auditory events. These important studiesdiffer significantly in goals, and thus both in the research techniques employed and theanswers achieved. Some studies have investigated the broad classification or grouping oflarge sets of diverse natural and artificial sounds (e.g., Howard & Ballas, 1980, 1981;Ballas & Mullins, 1991; Ballas, 1993). A number of more narrowly focused studies haveinvestigated the identification of a specific source attribute for sounds produced in thelaboratory under controlled conditions (e.g., Giordano, 2005). Lakatos, McAdams, andCaussé (1997), for example, found that listeners could judge the geometric shape of metaland wood bars struck by a mallet, but not the cross-sectional area of metal bars (Lakatos,Gibson, and Cannon, 1997). In a very recent study, Grassi (2005) found that listenerscould judge the size of wood balls dropped on a ceramic plate from a fixed distance.These latter studies focused on well-defined questions, but the acoustic stimuli, whilecomplex, were produced under well-controlled laboratory conditions, and thus variedalmost exclusively as a function of the source property being manipulated.Only a few studies have investigated a specific contrast for natural acousticevents, where the source event and the acoustic stimuli, rather than being highlycontrolled, exhibit variability more typically encountered in one's listening environment.Examples of questions investigated in these studies include material hardness (Freed,1990), gender identification of humans either clapping hands (Repp, 1987) or walking(Li, Logan, & Pastore, 1991), as well as the bouncing versus breaking of objects (Warrenand Verbrugge, 1984). Each of the different types of studies we have summarizedcontributes in different ways to our understanding of natural listening. However,
Source-Perception Loop6investigations of natural listening conditions, best approximated in the last few of thesestudies, poses the greatest difficulties in research design and effective data analysis. As aresult, research procedures and strategies are least well developed. The current auditoryevent perception research goes well beyond these last studies (and, in fact, nearly all ofthe cited studies) in terms of scope, event and stimulus complexity, stimulusspecification, depth of analysis, and conceptual integration. Our study has two primarygoals: the development of a comprehensive framework for research investigating theperception of auditory source events and using that framework to understand perceptionof a specific source event contrast. We describe our conceptual framework first in termsof an example from a laboratory controlled source event contrast then from the focus ofthe current study, the contrast between humans walking in an upright versus stoopedposture.Source-Perception LoopAuditory event perception involves three different, but clearly interrelated andmeasurable, stages of information: the source, the sound, and perception. The sound thatis produced reflects both static and dynamic attributes of the source event. Whether onebelieves that the source event is perceived directly or indirectly, the properties of thesound play a critical role in event perception, and thus, to understand event perception,one needs to investigate the attributes of each of the three information stages along withthe actual and potential relationships among these stages. The strategy in the currentstudy is to carefully map the source-perception loop that starts with the contrasted sourceevent that produces the sounds and ends with listeners using the sounds to judge thenature of the original event. An example helps to illustrate the importance of this loop,
Source-Perception Loop7and allows us to identify differences in research strategy needed to investigate this loopfor natural source events versus more controlled laboratory events .In a recent, excellent study, Grassi (2005) investigated the perception of ball sizefor a set of balls constructed of the same material, but differing in size. The balls weredropped in a controlled fashion (constant distance) on a plate. The task of the listenerswas to judge the size of the ball. Without going into the details provided by Grassi, andrestricting ourselves to the initial contact between the objects, the sound producedreflected the properties and dimensions of the material used to construct the balls and theplate, the contact forces between the balls and the plate, and the surface area of contactbetween the colliding objects. These are all factors in defining the source-sound portionof the source-perception loop. Whether the listeners directly perceive ball size fromsound or infer ball size based upon properties of the sound and some basic knowledgeabout acoustic events that are coupled with assumptions about the properties of the ballsand the plate, judgment is a function of sound properties. However, individual listenersmay differ in their knowledge and their assumptions. Furthermore, listeners may not useall of the available sound properties (e.g., those that change with ball size), and differentlisteners may use different subsets of the available acoustic information (e.g.,Samoylenko, McAdams, and Nosulenko, 1996). Finally, we know from Lutfi & Oh(1997), who used synthetic stimuli and modeling techniques to investigate abilities todiscriminate the material composition of struck bars, that listeners often fail to optimallyuse the information available in the acoustic waveform. We thus differentiate betweenstudying the source-perception loop for a theoretically optimum ideal listener, one who
Source-Perception Loop8uses all of the relevant acoustic information as well as accurate knowledge about thesource, and actual listeners who participate in behavioral experiments.There is one final important distinction between investigations of controlledlaboratory source events and more natural, less restricted, events; the former limits, andthe latter must deal with, the variability in the static and dynamic properties of thedifferent instantiations of the source event. In most of the conditions in the Grassi study,only ball size was varied. In the current study, the stimuli are generated by a number ofhumans walking in an upright and a stooped posture that differ in a number ofuncontrollable ways. In addition to within walker posture contrast differences, there aredifferences across walkers in both static properties (e.g., height, weight, gender, shoesize) and dynamic attributes, some of which are general (e.g., individual biomechanics ofwalking) and others specific to the posture contrast (e.g., degree of change in balanceassociated with posture change). All such across-walker differences have the potential toalter the sounds produced. In terms of the posture difference being investigated, theseindividual differences, at a minimum, represent noise that is added to, and may welldistort, the acoustic signal. In contrast to controlled laboratory events that minimize thisnoise, research on the perception of natural source events needs to deal with the noise,and this need can alter the effectiveness of alternative research strategies and possiblestatistical analyses. Thus, all aspects of the current study would be simpler if we selectedstimuli with clear differences in acoustic properties believed to reflect the posturecontrast. This approach, however, would bias our results and make the study much lessinteresting and important. Instead, other than eliminating walkers whose gait is abnormal(e.g., toe followed by heel impact) and stimuli with extraneous or clearly abnormal
Source-Perception Loop9sounds, we made no attempt to systematically select stimuli; we thus are studying naturalevents.Our current approach to auditory source event perception addresses a number ofdifferent, broad questions relevant to the relationships among the three information stagesin the source-perception loop. The first question concerns the degree to which specificacoustic information differentiates the contrasted source event. The answers to thisquestion allow us to develop parameter-based models for distinguishing between thesource event classes based upon acoustic signal properties, as well as to identify whichaspects of the source event relate to specific differences in the original acoustic events.The second question concerns human decision-making and focuses on the informationprobably used by listeners, either individually or as a group, to distinguish the contrastedsource events. In the current study, developing statistical models that distinguish posturebased upon single and combinations of acoustic parameters allows us to begin to evaluatethe nature and effectiveness of information used by listeners. Finally, comparing leveland pattern of performance for actual listener and stimulus-property-based modelsprovides the foundation for developing strategies to improve actual listener performance,such as eliminating the use of irrelevant or unreliable acoustic properties and encouragingthe use of acoustic properties that better reflect differences across the source events.Auditory Source Event InvestigatedWe had several reasons for deciding to examine the perception of aspects ofhuman gait as a source event. Our previous study (Li, Logan, & Pastore, 1991) hadselected this event class because the recorded stimuli are easily recognized as beinggenerated by humans walking (in contrast to many other natural stimuli; for example,
Source-Perception Loop10sound effects are seldom natural stimuli), and there is both anecdotal and empiricalevidence (Farrington, 1998) that listeners often accurately identify the approach offamiliar individuals from the sound of their footsteps. In addition, our prior research hadtaught us a great deal (in a positive and negative sense) about studying this type of sourceevent. Furthermore, the extensive visual point of light literature (e.g., Johansson, 1973;1986; Cutting, 1978; Cutting & Kozlowski, 1977; Cutting, Proffitt, & Kozlowski, 1978;Runeson & Frykholm, 1983) implies that humans have considerable knowledge about atleast some aspects of the biomechanics of human gait. Finally, there are very extensivemedical (e.g., Cappozzo, 1991; Zatsiorsky, 1998; Proffitt & Kaiser, 1995; Weber &Weber, 1836; Braune & Fischer, 1899; Winter, 1989; 1987; Yamaguchi, Pandy, & Zajac,1991), and podiatric (e.g., Wernick, 1993) literatures on the biomechanics of normal andabnormal human gait that can help guide our specification of the source event.Biomechanics of Human GaitAlthough medical and biomechanical models of human gait are very complex(e.g., Cappozzo, Catani, Croce, & Leardini, 1995; Benedetti, Catani, Leardini, Pignotti, &Giannini, 1998), but do not specify the acoustic consequences of the posture contrastbeing investigated, they do provide a basis for understanding the impact dynamics ofnormal upright stride and the changes that should occur when the walker is stooped. Insimplified terms, a single stride consists of swing and stance phases. In normal gait, thestance phase begins with the heel-strike, then (as the center of gravity moves forward)continues with contact by the entire foot, ending with the heel, followed by the ball andthe toe leaving the ground (Giannini, Catani, Benedetti, & Leardini, 1994). The result is asaddle-shaped temporal pattern or distribution of pressure on the walking surface that, at
Source-Perception Loop11least for a normal gait, differs little across shoe type or size, but does differ with theweight of the walker (Galbraith & Barton, 1970). It is during the stance phase, with thefoot striking the walking surface, that the current task-relevant walking sounds areproduced. In the swing phase, the leg and foot move from a location behind, to under, andthen in front of the center of gravity (Bonnard & Pailhous, 1991), defining the timing anddistance of successive foot-strikes. Although probably programmed independently, thesebroad phases influence each other and are constrained by similar factors (Bonnard,Pailhous, & Danion, 2000). During locomotion, the legs support the upper body (headarm-trunk or HAT system) and propel the body forward (Weber & Weber, 1836). Inaddition to needing support, the HAT system (especially the head) needs to be stable toprovide visual and vestibular information important for locomotion (Holt, Jeng, Ratcliffe,& Hamill, 1995; also, Pozzo, Berthoz, & Lefort, 1990). Human locomotion thus involvesforward progress, support of weight, and maintenance of balance, all of which interact inthe necessary coupling between posture and gait (Kay & Warren, 1998; Diedrich &Warren, 1998) that differs with skeletal structure (e.g., gender).1As will soon be apparent, the change from upright to stooped posture results inreductions in balance or stability that, ultimately, alter the nature and pattern of impactbetween shoe and floor, as well as the shock-absorption for shoe-floor impact. Thesefactors, and the coupling between posture and gait, mean that changes in posture willalter the nature of sounds produced by individual walkers in an upright and stoopedposture, and these across-posture acoustic differences will vary across walkers. We thusnext turn to an analysis of those aspects of gait in a normal upright posture that determinethe balance, impact absorption, and shoe-floor contact properties of the source event.
Source-Perception Loop12This analysis then allows us to contrast the acoustically-relevant consequences of shiftingto a stooped posture.Upright PostureIn a normal upright posture, the back is relatively straight and center of gravitytypically maintained above the hips and anterior to the spine, and locomotion isessentially phasic, stable, balanced, efficient, fluid, and almost automatic. In this posture,the hips rotate on the planted, weight-bearing leg and, except for a small amount of legflexion at the knee during the swing phase of the gait, the legs usually are kept straight asthe hips, and thus weight, is shifted forward from one leg to the other. The result, insimple terms, is a biphasic initial impact that begins with the downward force of the heelon the floor that then couples with the upward force of the floor supporting the heel,followed by the second impact that is largely an upward force as the sole of the shoe isplanted and heel lift-off is begun (Nigg, 1999). In addition, the arms typically add to themaintenance of balance by swinging in a relaxed fashion that is synchronous, but notnecessarily in phase, with hip rotation and leg movement.Because sound is produced as forces are applied through contact between the footand the floor, we focus next on the modification of these impact forces, largely throughchanges in the foot. The principal motions of the foot are supination and pronation.During supination, the arch raises and the bo
perception of auditory source events and using that framework to understand perception of a specific source event contrast. We describe our conceptual framework first in terms of an example from a laboratory controlled source event contrast then from the focus of the current study, the contrast between humans walking in an upright versus stooped
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