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SPECIAL ISSUE GIBSON S AFFORDANCES 337, animals are thought to construct the world that they live in and including perception and movement by analyzing situations in. understand In Gibson s view people and animals are attuned volving continuous interactions such as cascade juggling Beek. to variables and invariants of information in their activities as Turvey 1992 and pursuit of a batted fly ball by a baseball. they interact as participants with other systems in the world that outfielder Michaels Oudejans 1992. we inhabit The framing assumptions of ecological psychology are one. Gibson s view of perception has been difficult for many cog form of a general theoretical stance which can be called situa. nitive scientists e g Fodor Pylyshyn 1981 Ullman 1980 tivity theory Greeno Moore 1993 in which cognitive pro. Vera Simon 1993 to understand I believe this is because cesses are analyzed as relations between agents and other sys. Gibson s reasoning involves some quite general framing as tems This theoretical shift does not imply a denial of individual. sumptions about activity and cognition that differ from those of cognition as a theoretically important process It does however. mainstream cognitive science He expressed some of these more involve a shift of the level of primary focus of cognitive analyses. general ideas in the 1970s in discussion of the concept ofqffbr from processes that can be attributed to individual agents to. dances L J Gibson 1977 1979 1986 and the task of devel interactive processes in which agents participate cooperatively. oping a systematic general ecological psychology is being car with other agents and with the physical systems that they in. ried on by a growing group of investigators e g Shaw Turvey teract with If in analyzing those interactive processes one con. Mace 1982 Turvey 1990 1992 cludes that some of them factor conveniently into aspects that. can be attributed to the environment and aspects that can be. Situativity and the Concepts of Affordance and Ability attributed to individual minds that will be a useful and produc. tive result Those of us who are developing situativity theory. Bickhard and Richie 1983 argued that Gibson s thinking however believe that the factoring assumption should not be. evolved from a view of perception as encoding features of the taken as a general methodological and theoretical commitment. environment toward a more general view of perception as an Research in ecological psychology has focused mainly on re. aspect of a person s or animal s interaction with the environ lations of agents with physical systems and environments In. ment The encoding view which is still prevalent in informa other research processes of communication and reasoning are. tion processing psychology involves analyzing cognition in also being approached in ways that are inconsistent with fac. terms of a factoring assumption that supports analyses of toring assumptions that have typically been made Clark and. different stages of cognitive processing in relative isolation from Schaefer 1989 Schegloff 1972 and others have analyzed. each other conversations as interactive collaborations in which contribu. A claim of ecological psychology as I understand it is that tions such as references to a place are considered as successful. the interactions among aspects of cognition and behavior are joint actions rather than as events that occur when one person. sufficiently subtle and complex that our prevalent factoring uses a referring term Many studies of socially organized prob. strategy is scientifically unproductive As Turvey has put it lem solving and reasoning in complex environments have been. The types of phenomena that should lead the way must be conducted including a study by Suchman and her associates. drawn from perception in the service of action and from action Brun Cottan et al 1991 of the ground operations of an air. in the service of perception Turvey 1992 p 86 When per line and studies by Hutchins 1991 in press of processes of. ception motor movement memory reasoning or whatever is calculating a military ship s position as it enters a harbor and of. studied as a separate factor one hopes that the conclusions one remembering to adjust the wing flaps of a commercial aircraft. takes from those studies apply in situations where other factors as its speed decreases during a landing Symbolic representa. have significant roles Although there have been occasional ob tions of information in these situations are very tangible and. jections to the factoring assumption e g by Dewey 1896 the theoretical analyses that turn out to be productive are at the. Lashley 1951 factoring of processes especially into events level of functions that are accomplished by groups of people. occurring outside and inside the mental system has been a interacting with each other rather than of hypothetical mental. persistent methodological commitment of psychological re representations constructed by and operated on by individuals. search Gibson was already suspicious that perception and the The view of problem solving that assumes a process of search. observer s movement did not factor neatly in the perception of in a symbolic problem space consisting of representations of. motion when he wrote The visual perception of objective mo an initial state a goal and a set of operators combined with. tion and subjective movement J J Gibson 1954 That lack a problem solver s domain specific knowledge and strategy for. of factoring became a major point of his more general theory of planning has been challenged in studies of reasoning and deci. perception J J Gibson 1966 and has been a key issue in the sion making in activities of trying to get malfunctioning photo. development of ecological psychology for example in the anal copying machines to work Suchman 1987 of grocery shop. ysis of information that is available to a person or animal mov ping and food preparation by American adults Lave 1988 of. ing through a spatial environment Cutting Springer Braren selling produce candy and other commodities by young Bra. Johnson 1992 Lee 1980 An important feature of the eco zilian street merchants Carraher Schliemann Carraher. logical view involves a shift in situations that are taken as para 1988 Saxe 1991 of making inferences about quantitative. digmatic cases of cognition Rather than building a theory of properties of a physical system that behaves according to linear. perception on analyses of situations with stationary observers functions by middle school and high school students Greeno. and building a theory of action on analyses of situations where Moore Mather 1993 and of solving or constructing expla. an agent tries to reproduce a movement of an object in space nations of algebra word problems Hall 1990 These studies. ecological psychologists are working to build a theory of activity have taken an interactivist view of reasoning considering it as. 338 JAMES G GREENO, an interaction of the problem solver with material systems that volves the affordance of an aperture for a person to walk from. include meaningful symbols and considering the interpretation one side of a partition to the other The affordance provided by. of the symbols meanings as an important process to be un an aperture is a function of its width and the ability of a person. derstood An aspect of this approach was anticipated by J J to move through the aperture depends on the person s width. Gibson and E J Gibson 1955b who argued against Postman Warren and Whang s analysis of this affordance focused on an. 1955 that study of learning to perceive symbols should focus interesting feature of behavior in the range of aperture and per. on processes of differentiation Symbols like natural objects son widths in which the affordance varies They measured. must be differentiated or identified in order to be carriers of whether participants walked straight through apertures which. meaning They come in sets not singly And it is quite possible varied in width or turned their shoulders The frequency of. that the meaning of a symbol in the mathematico logical sense shoulder turning decreased as the width of the aperture in. is given by its univocality within the set Gibson Gibson creased There were two groups of participants one group of. 1955b pp 449 450 greater than average size and the other group of smaller than. A proposal by Neisser 1992 is particularly relevant to the average size The psychometric functions of shoulder turning. perception of symbols Neisser has argued that one needs to frequency versus aperture size differed between the two groups. distinguish two kinds of perceptual processes which he calls when aperture size was plotted in ordinary physical units. direct perception and recognition Direct perception which When aperture size was plotted as a ratio of the physical width. provides information for orientation and locomotion in space of the aperture to the width of the participant s shoulders the. occurs in dynamic interaction with the environment Recogni two psychophysical functions were nearly identical. tion which provides information for identifying and classifying. objects and events is more effective when the observer can ac Affordances and Abilities in Situation Theory. cumulate information about the features of an object or ar. rangement A significant shift in theoretical perspective can be aided by. If we choose not to factor behavior into the process categories and sometimes requires use of a different formal system Tur. of perception memory movement reasoning decision making vey 1992 and his associates have used the theoretical formal. and so on one then needs theoretical terms for referring to as isms of dynamical systems theory productively in analyzing sys. pects of the phenomena and systems at the level of agent situa tems such as juggling and the gaits of animals. tion interactions Gibson s concept of affordance is a key pro Another system that provides theoretical and formal support. posal The idea is quite straightforward In any interaction in for an interactivist psychology is situation theory Barwise. volving an agent with some other system conditions that enable Perry 1983 Devlin 1991 which includes a significant refor. that interaction include some properties of the agent along with mulation of logic One example of the kind of shift that situa. some properties of the other system Consistent with his empha tion theory proposes is its characterization of the meaning of. sis on understanding how the environment supports cognitive symbolic expressions such as sentences In standard logic and. activity Gibson focused on contributions of the physical sys linguistics the meaning of a sentence is a relation between the. tem The term affordance refers to whatever it is about the en sentence and conditions in the world that the sentence asserts. vironment that contributes to the kind of interaction that oc generally focusing on conditions in which the sentence is true. curs One also needs a term that refers to whatever it is about In situation theory the meaning of a sentence is a relation be. the agent that contributes to the kind of interaction that occurs tween situations The meaning of a spoken sentence is a relation. I prefer the term ability although Shaw et al 1982 preferred between the situation in which the sentence is uttered and the. to coin the term effectivity for that concept I believe my use of situation about which uttering the sentence conveys informa. the term ability is also synonymous with Snow s 1992 use of tion Analysis of a sentence s meaning includes specifying the. the term aptitude epistemic connection that the speaker has with the situation. Affordances and abilities or effectivities or aptitudes are in that the sentence refers to Relativizing the meanings of sen. this view inherently relational An affordance relates attributes tences this way has very salutary effects including resolution of. of something in the environment to an interactive activity by an the classical philosophical puzzles about reference known as the. agent who has some ability and an ability relates attributes of liar paradox Barwise Etchemendy 1987. an agent to an interactive activity with something in the envi In situation theory the concept of constraint plays a key role. ronment that has some affordance The relativity of affordances A constraint is a regularity involving situation types A situa. and abilities is fundamental Neither an affordance nor an abil tion type is a class of situations with objects that have a specified. ity is specifiable in the absence of specifying the other It does property of relation For example reading a b 1 desig. not go far enough to say that an ability depends on the context nates a type of situation where there is something indicated by. of environmental characteristics or that an affordance depends a that is reading something else indicated by b For example. on the context of an agent s characteristics The concepts are you are in a situation that is of this type where a s anchor is. codefining and neither of them is coherent absent the other you and b s anchor is this article A constraint is a dependency. any more than the physical concept of motion or frame of ref relation between situation types For example. erence makes sense without both of them reading a b 1 printed b 1 seeing a b 1. As Gibson s idea of affordances has been developed in re. search it seems most productive when it is treated as a graded is a constraint that says that if one thing is reading another thing. property rather than as a property that is or is not present A and the thing being read is printed then the thing that is reading. beautifully simple example by Warren and Whang 1987 in it is also seeing it. SPECIAL ISSUE GIBSON S AFFORDANCES 339, Situation theory provides a way of thinking about knowing Affordance conditions for this constraint include the shape and. how to do things in its concept of attunement to constraints other mechanical features of the steering wheel which is de. Barwise 1989 attributed the idea of attunement to J J Gib signed to afford rotary movement and the mechanical arrange. son and Gibson 1966 1982 attributed it to Lashley 1951 ment of the steering system that translates rotary motion of the. Several analyses e g Barwise Perry 1983 Israel Perry steering wheel into turning the wheels of the car Ability condi. 1991 have considered attunement to constraints as a basis for tions for this constraint include the driver s ability to grasp the. making inferences For example attunement to the constraint wheel and to move her or his arms in the plane of the steering. that smoke means fire i e a situation of the type that has wheel If we were to focus an analysis on a more general func. smoke is also a situation of the type that has fire will support tional action such as having the car stay in a traffic lane as the. an inference by someone who perceives smoke that there is a road turns ability conditions would include perceptual abilities. fire there to see the lane markers or the edge of the road in relation to the. Attunement to constraints also can play an important role in car s movement in the space and perceptual motor coordina. analyses of skilled activity For example in steering a car the tion of that dynamic visual information with the action of turn. driver is attuned to subtle and complex constraints that relate ing the steering wheel. changes in the direction of the car s forward motion with Viewed in this general way the concepts of affordance and. amounts of turning the steering wheel The driver s actions of ability can be used in any analysis of activity and some interest. exerting force on the wheel have effects of making the wheel ing implications are suggested For example participants in. turn more or less depending on the force that is applied and of conversations utter words and phrases that they expect will re. making the direction of the car change more or less depending sult in reference to objects places properties events and so on. on the amount the wheel is turned People who share a linguistic practice are attuned to a great. Many constraints only hold conditionally and conditional many constraints that include conventions of reference Such. constraints are often used in analyses For example the con constraints have the following form. straints that relate turning a steering wheel to changing the di. use of a term refer to something, rection of a car s motion are conditional on among other things. the car s steering mechanism being intact Constraints of this kind are obviously conditional They only. The idea of conditional constraints provides a way to charac hold when participants in the conversation are attuned to a. terize affordances and abilities quite clearly A skilled practi shared set of constraints Many constraints also depend on the. tioner s actions have effects that are functional in the activity In conversational setting. other words there are constraints of the following form Affordance conditions for reference constraints include prop. erties of the language that the participants use For example. action by agent good effects in situation just as a car s steering wheel shaped so the driver can grasp and. turn it affords changing the car s direction a term in a lan. where good effects are outcomes that are needed or desirable guage composed of phonemes that a speaker can pronounce. for a broader activity to be successful Affordances and abilities. affords referring to an object or property that the term is used. can be thought of as conditions in which the constraints of suc to designate Abilities to speak and perceive the terms of the. cessful performance hold language are among the ability conditions required for con. As a simple example consider moving from a hallway into a. straints of reference to apply in conversational activity. room in a building An action that accomplishes that is walking. Affordances for reference constraints also are present in situ. into the room which has the desired effect that the person is in. ations where conversations take place Most obviously when the. the room because of the action The relevant constraint is as. objects we want to refer to are in the situation their presence is. an affordance for ostensive reference for example by pointing. walk into the room be in the room and saying That one In Clark and Wilkes Gibbs s 1986. study of collaborative reference two participants were given. Affordance conditions for this constraint include the presence identical sets of cards with tangram pictures One of the partic. of a doorway that is wide enough to walk through as well as a ipants had the cards arranged in a sequence and the task of the. path along a supporting surface If there is no doorway or if the pair was to communicate so the other participant s cards were. floor is too weak to support the person s weight at some point in the same sequence The task was made challenging by pre. along the path or if there is a large obstacle somewhere along venting the participants from seeing each others cards because. the pathway the constraint does not apply Ability conditions a screen blocked their views. for the constraint to include the ability to walk along the path The participants could accomplish the task because their. including the perceptual ability to see the doorway and the co shared language had terms to designate properties of shapes and. ordination of vision with motor activity needed to move toward locations such as triangle next to the square and familiar. and through the doorway forms that could be recognized as similar to some of the tan. In the more complicated situation of driving action out gram forms Over a few trials the pairs developed conventions. come constraints for the driver include moving her or his arms that enabled them to refer to the various cards more economi. some amount while holding the wheel with the outcome that cally In the terms of my theoretical proposal here they devel. the car s direction changes by some amount oped elements of a communicative practice including con. straints of reference that had not been available to them ini. movement of arms car changes direction tially,340 JAMES G GREENO. Analyses of reasoning also can be framed in these terms dependencies between variables e g if the spool is larger the. Along with several students and other colleagues I have been block moves farther on each turn and relations between the. working toward a situativity theoretic analysis of reasoning blocks positions such as how much one block gains on the. about quantitative properties of a physical system that we de other. signed for research purposes a few years ago The system which As I understand it situativity theory does not imply that hu. we call winches has two tracks in which small metal blocks man activity does not use symbols and its domain is not limited. are pulled by strings that wind around spools when a handle is to processes in which symbols play no role It does treat symbols. turned Distances along the tracks are shown by 1 in unit rul as a special kind of resource for cognition however and at. ers and different sizes of spools are varied to provide different tempts to analyze the ways in which symbols function as com. distances per turn of the handle We have set up situations with ponents of the situations that people are in when they reason. the blocks in some starting positions connected to spools of and communicate Hall s 1990 study of algebra students and. some sizes and asked questions For example the red block may teachers solving word problems provided particularly instruc. be at 9 connected to a 3 in circumference spool with the tive examples of ways that problem solvers construct symbolic. blue block at 0 attached to a 6 in spool An example of the representations that facilitate their reasoning locating symbols. questions we asked students is Will the blue block ever be that relate to each other in convenient spatial arrangements. ahead of the red block If so after how many turns Student. participants were quite successful in answering such questions Perception of Affordances. and they used an interesting variety of methods Some answers. were obtained in ways that involved direct interaction with the Discussions of Gibson s concept of affordance have been. physical device by pointing to successive positions along the plagued by confusion about where to locate the reference of the. rulers where the blocks would be after different numbers of term For example is the affordance that a chair provides for. turns Some answers used another physical medium paper and sitting a property of the chair a property of the person who sits. a pencil to record symbols for the positions the blocks would on it or perceives that he or she could sit on it or something. be in after successive turns Some answers were obtained using else It seems clear to me that Gibson s intention was that the. arithmetic relations and operations either spoken aloud as a affordance is a property of whatever the person interacts with. student worked them out or reported retrospectively after an but to be in the category of properties we call affordances it has. answer was given To analyze these processes we have developed to be a property that interacts with a property of an agent in. hypotheses about ways in which inferences are made about both such a way that an activity can be supported. ordinal and numerical properties of the blocks motions in Affordances are in this view preconditions for activity as I. which the physical presence of the winches provide significant believe is made clearer when they are treated as conditions for. affordances as do physical symbols for numbers that are on the constraints The presence in a situation of a system that pro. rulers or are constructed by the participants by writing Greeno vides an affordance for some activity does not imply that the. et al 1993 In answering the questions students provided evi activity will occur although it contributes to the possibility of. dence that they were attuned to constraints of the winches op that activity Additional conditions include aspects of the activ. eration such as the constant distance that each block moved on ity of an agent in the situation having to do with motivation and. each turn and the correspondence between the physical dis perception Motivation to engage in some action is related to. tances moved in a sequence of turns and arithmetic relations of what the agent is doing at a more general level For example if. adding or multiplying numbers a person is engaged in an activity of going to attend a class then. Affordances for the activities of reasoning were provided by the action of moving into the classroom is a functional part of. the physical presence of the winches so students could point to that activity That will make the person attentive to aspects of. positions along the track and we hypothesize mentally simu the environment that could provide an affordance for moving. late events of moving the blocks Affordances were provided as into the classroom such as the doorway from the hall into the. well by the physical resources for writing symbolic representa classroom. tions in the form of tables Affordances also were provided by Consistently with his view of perception as a process of pick. the conceptual entities and operations of arithmetic which were ing up information directly Gibson characterized affordances. available to the student by virtue of their participation in the as properties of objects and layouts that are specified by infor. social practices of school and other settings where numerical mation in the array of energy that is available to an agent s per. symbols and inferences occur ceptual systems This characterization works well for many. In a subsequent study Moore 1993 observed performance affordances such as the affordance of a doorway for moving to. on these reasoning tasks in different situations one with the the other side of a partition The question of how the affordance. physical winches present one with a computer graphic simula is perceived is a straightforward research problem and experi. tion of the winches along with numerical symbols of the posi ments conducted by Warren and his associates provide elegant. tions of the blocks after each simulated turn and one in which examples of analyses that identify the information that human. tables of numerals are constructed without reference to a sys perceptual systems pick up to detect affordances In the case of. tem to provide a physical or simulated physical semantic in perceiving the affordance of a doorway Warren and Whang s. terpretation Students performance provided evidence that the 1987 analysis indicated that the effective stimulus is the visual. differences among the reasoning environments affected their information that specifies the physical width of the doorway. reasoning activity Apparently the physical or simulated scaled to the eye height of the observer. winches afforded reasoning to a greater degree about ordinal In addition to affordances for orientation and locomotion in. SPECIAL ISSUE GIBSON S AFFORDANCES 341, space it is likely that many affordances for individual actions in properties represented implicitly in mental simulations rather. conversations and other interpersonal interactions are perceived than being explicitly stated in propositions. directly that is without need for mediating symbolic represen. tations In conversations pauses facial expressions and other Conclusions. gestures provide information that influences the opportunities. for each participant s actions Analyses of the auditory and vi J J Gibson s theorizing in the 1950s in his own writing. sual energy characteristics that specify these affordances have Gibson 1954 and with E J Gibson J J Gibson E J Gib. not been done to my knowledge Even so there is strong evi son 1955a 1955b reflected two general commitments One. dence that mediating symbolic representations are not required commitment was that important phenomena should not be left. for their perception because infants are responsive to important out of the psychology of perception simply because they were. features of these social stimuli well before they have developed difficult to study with available experimental methods Another. the capability of forming symbolic representations that denote commitment was that concepts in the theory of perception. them e g Tomasello Kruger Ratner in press should not be subsumed under the apparently broader psycho. In the broad meaning that I am proposing for the concept of logical concept of stimulus response association because the. affordance it would be inappropriate to consider direct percep view of perception and perceptual learning that results from. tion of affordances as a denning characteristic J J Gibson that subsumption is theoretically less adequate than another. 1979 1986 used mailboxes as an example He proposed that a view involving differential attunement. mailbox provides an affordance for posting letters and it surely Gibson did not however treat the psychology of perception. does that It seems likely that for most people in most if not all as a compartment closed off from the rest of psychology In his. circumstances the process of cognizing that affordance in later theorizing he worked toward a broader view of behavior. cludes classifying the physical object as a mailbox The infor that would be consistent with a theoretically adequate under. mation required for that classification has to be visually avail standing of perception This view focuses on interactive re. able of course but the process of classification includes I lations of behaving agents with systems in their environments. should think a mental state that has the epistemic status of a In Gibson s view perception is a system that picks up informa. symbol that designates the property of being a mailbox A the tion that supports coordination of the agent s actions with the. oretical possibility that seems reasonable is that mental symbols systems that the environment provides This led him to develop. of that kind are products of the perceptual processes that Neis the idea of affordances the characteristics of objects and ar. ser 1992 regarded as recognition rather than direct percep rangements in the environment that support their contributions. tion to interactive activity and therefore the characteristics of the. What of symbols that are recognized and used in processes of environment that agents need to perceive. making inferences As J J Gibson and E J Gibson 1955b Considerable efforts are being made to develop a general the. recognized symbols need to be identified to be used meaning oretical account of activity considered as interactions of agents. fully In Neisser s 1992 distinction then perception of sym with systems in their environments and significant progress is. bols is a process of recognition rather than a process of direct occurring Some of these efforts in ecological psychology have. perception and if my conjecture about mental symbols is cor used Gibson s idea of affordances quite explicitly Other efforts. rect such recognition creates mental symbols It follows then focused on situations involving social interactions as well as in. that those mental symbols could be available for processes of teractions with physical systems mainly use concepts and. making inferences according to operations that transform sym methods of ethnographic social science We should aspire to a. bolic expressions into other symbolic expressions In our analy theory that merges these perspectives along with the insights. ses of reasoning Greeno et al 1993 we have taken this as an and methods of information processing cognitive science these. assumption People represent with mental symbols in different support analyses of information structures that are the contents. ways just as one does with spoken or written symbols We rep of human symbolic communication and describe the contents. resent with some symbols as propositions and inferences have of other interactions of agents in situations I have suggested in. the form of generating further representations of propositions this commentary some ways in which an extended version of. One can represent with other symbols as simulations and infer Gibson s concept of affordance is involved in one version of this. ences have the form of operating on a model to generate states general theory of activity Gibson s theorizing was seminal in. with properties that correspond to the represented system the development of ecological psychology and is likely to be. A theoretical understanding of mental symbols including seminal in a more general development of a theory of activity. mental states that function as simulations could provide a basis viewed as interactive relations of agents with other agents and. for understanding conceptual entities such as numbers arith physical systems Gibson s affordances then have already been. metic operations and functions Some progress has been made of great value and will support further valuable activity in our. toward theoretical understanding of the ability to form antici field. patory mental models that simulate outcomes of activity in sit. uations Jordan Rumelhart 1992 We need to understand. how that ability includes forming models that satisfy con References. straints that we express and understand in communication with Barwise J 1989 The situation in logic Stanford CA Center for the. statements of propositions This could provide a theory of Study of Language and Information. learning that results in shared mental models that support rea Barwise J Etchemendy J 1987 The liar An essay on truth and. soning about the properties of conceptual entities with the circularity London Oxford University Press. 342 JAMES G GREENO, Barwise J Perry J 1983 Situations and attitudes Cambridge Hutchins E in press How a cockpit remembers its speeds Cognitive. MA MIT Press Bradford Science, Beek P J Turvey M T 1992 Temporal patterning in cascade Israel D Perry J 1991 What is information Report No CSLI. juggling Journal ofExperimental Psychology Human Perception and 91 145 Stanford CA Center for the Study of Language and Infor. Performance 18 934 947 mation, Bickhard M H Richie D M 1983 On the nature of representa Jordan M I Rumelhart D E 1992 Forward models Supervised. tion A case study of James Gibson s theory of perception New York learning with a distal teacher Cognitive Science 16 307 354. Koffka K 1935 Principles of gestaltpsychology New York Harcourt. Brun Cottan E Forbes K Goodwin C Goodwin M Jordan B Brace. Suchman L Trigg R 1991 The workplace project Designing. for diversity and change Videotape Report Palo Alto CA Xerox Lashley K 1951 The problem of serial order in psychology In L A. Palo Alto Research Center Jeffress Ed Cerebral mechanisms in behavior New brk Wiley. Carraher T N Schliemann A D Carraher D W 1988 Mathe Lave J 1988 Cognition in practice Mind mathematics and culture. matical concepts in everyday life In G B Saxe M Gearhart Eds in everyday life Cambridge England Cambridge University Press. Children s mathematics pp 71 87 San Francisco Jossey Bass Lee D N 1980 The optic flow field The foundation of vision Philo. Clark H H Schaefer E F 1989 Contributing to discourse Cog sophical Transactions of the Royal Society of London B 290 169. nitive Science 13 259 294 179, Clark H H Wilkes Gibbs D 1986 Referring as a collaborative Michaels C R Oudejans R R D 1992 The optics and actions of. process Cognition 22 1 39 catching fly balls Zeroing out optical acceleration Ecological Psy. Cutting J E Springer K Braren P A Johnson S H 1992 chology 4 199 222. Wayfinding on foot from information in retinal not optical flow Moore J L 1993 Comparisons of a physical model and computer. Journal of Experimental Psychology General 121 41 72 representations in reasoning and learning about linearfunctions Un. Devlin K 1991 Logic and information Cambridge England Cam published doctoral dissertation Stanford University Stanford CA. bridge University Press Neisser U 1992 June Distinct systems for where and what Rec. Dewey J 1896 The reflex arc concept in psychology Psychological onciling the ecological and representational views of perception Paper. Review 3 357 370 presented at the Fourth Annual Convention of the American Psycho. Fodor J Pylyshyn Z 1981 How direct is visual perception Some logical Society San Diego CA. reflections on Gibson s ecological approach Cognition 9 139 196 Postman L 1955 Association theory and perceptual learning Psy. Gibson E J 1969 Principles of perceptual learning and development chological Review 62 438 446. New York Appleton Century Crofts Saxe G 1991 Culture and cognitive development Studies in mathe. Gibson J J 1954 The visual perception of objective motion and sub matical understanding Hillsdale NJ Erlbaum. jective movement Psychological Review 61 304 314 Schegloff E A 1972 Notes on a conversational practice Formulating. Gibson J J 1966 The senses considered as perceptual systems Bos place In D Sudnow Ed Studies in social interaction pp 75 119. ton Houghton Mifflin New brk Free Press, Gibson J J 1977 The theory of affordances In R Shaw J Brans Shaw R Turvey M T Mace W 1982 Ecological psychology. ford Eds Perceiving acting and knowing Toward an ecological The consequences of a commitment to realism In W Weimer D. psychology pp 67 82 Hillsdale NJ Erlbaum Palermo Eds Cognition and the symbolic processes H Hillsdale. Gibson J J 1982 The problem of temporal order in stimulation and NJ Erlbaum. perception In E Reed R Jones Eds Reasons for realism Se. lected essays of James J Gibson pp 171 179 Hillsdale NJ Erl Snow R E 1992 Aptitude theory Yesterday today and tomorrow. baum Original work published 1966 Educational Psychologist 27 5 32. Gibson J J 1986 The ecological approach to visual perception Hills Suchman L 1987 Plans and situated actions The problem of hu. dale NJ Erlbaum Original work published 1979 man machine communication Cambridge England Cambridge. Gibson J J Gibson E J 1955a Perceptual learning Differenti University Press. ation or enrichment Psychological Review 62 32 41 Tomasello M Kruger A C Ratner H H in press Cultural learn. Gibson J J Gibson E J 1955b What is learned in perceptual ing Behavioral and Brain Sciences. learning A reply to Professor Postman Psychological Review 62 Turvey M T 1990 Coordination American Psychologist 45 938. 447 450 953, Goodwin C Goodwin M H in press Formulating planes Seeing Turvey M T 1992 Ecological foundations of cognition Invariants of. as situated activity In D Middleton Y Engestrom Eds Cogni perception and action In H L Pick Jr P van den Broek D C. tion and communication at work Beverly Hills CA Sage Knill Eds Cognition Conceptual and methodological issues pp. Greeno J G Moore J L 1993 Situativity and symbols Response 85 117 Washington DC American Psychological Association. to Vera and Simon Cognitive Science 17 49 60 Ullman S 1980 Against direct perception Behavioral and Brain Sci. Greeno J G Moore J L Mather R 1993 A situativity theoretic ences 3 373 415. model of reasoning and problem solving Manuscript submitted for Vera A H Simon H A 1993 Situated action A symbolic inter. publication pretation Cognitive Science 17 7 48, Hall R P 1990 Making mathematics on paper Constructing repre Warren W H Jr Whang S 1987 Visual guidance of walking. sentations of stories about related linearfunctions doctoral disserta through apertures Body scaled information for aifordances Journal. tion Technical Report 90 17 Department of Information and Com ofExperimental Psychology Human Perception and Performance. puter Science University of California Irvine Also available as 13 371 333. Monograph 90 0002 Palo Alto CA Institute for Research on Learn. Hutchins E 1991 Organizing work by adaptation Organization Sci Received August 9 1993.
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CoNTEMPORARY MATHEMATICS 346 Homotopy Theory: Relations with Algebraic Geometry, Group Cohomology, and Algebraic K-Theory An International Conference on Algebraic Topology March 24-28, 2002 Northwestern University Paul Goerss Stewart Priddy Editors American Mathematical Society Providence, Rhode Island
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list of recognised private hospital/diagnostic centres/ ayurveda / naturropathy where the medical facilities could be utilised by schedulae-i(see proviso to rule 8) as on :-23-07-2016 (443)
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Getting Started with Dreamweaver CS6 ... register the product, ... Simply double-click on the document icon (i.e. index.html).
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employment relationship ... ment relationship, and the chapter closes with an explanation of how the law operates in practice. the role of the state in employment relations the state defined As noted in the introduction, the State can be difficult to define because it encompasses more than a single actor, and many different institutions and government departments can influence employment ...
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Therefore, the production of hydrogen by dry reforming appears to be a weak competitor for the steam reforming. 2.2. Production of synthetic fuels from dry reforming of methane The synthesis gas produced by the dry reforming can be used to produce higher value products, especially the sulfur-free diesel (C 6H 14) via Fischer-Tropsch synthesis ...
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Life Cycle Assessment of Hydrogen Production via Natural Gas Steam Reforming Revised February 2001 ? NREL/TP-570-27637 Pamela L. Spath Margaret K. Mann National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393 NREL is a U.S. Department of Energy Laboratory