A Model of Object Permanence Psych 419/719 March 6, 2001

What is the Concept of Object Permanence? The realization that an object still exists even if it is out of sight. Exploring when children seem to be able to use this concept tells us about their general cognitive development.

Early Usage.. Infants at a very early age (3.5 months) seem to understand object permanence. Measured by gaze: infants look longer at “impossible” events; ones that violate the principle of object permanence –Impossible Event: Put a brick on toy train tracks. Hide tracks and brick behind screen. Train goes along tracks and reappears at other end. Seems to go though the brick.

Reaching Tasks Even though children at age 3.5 months seem to be able to direct gaze based on object permanence, can’t always retrieve hidden objects. Not until about 8 months old can they retrieve hidden objects.

It’s Not Just Visual Input.. Children younger than 8 months can retrieve objects that are in the dark, but not occluded by something. Implies that they don’t have to see the object to know where it is.. But they’re thrown by it being hidden by something.

One Standard Account Means-ends abilities: The ability to get at object X by manipulating object Y –Say, object X is under a blanket, or a pail, or behind something. –Need to move that thing in order to get at object X. Claim: Children learn about object permanence around 3.5 months, but don’t learn means-ends abilities until 8 months.

Visually... Concept of Object Permanence Means-ends Abilities 3.5 Months 8 Months “Knowledge”

The A ~ B Error When children can retrieve object, they still can make errors: –Hide object under pail to the left of child. Child retrieves it. Repeat several times. –Then, hide object under pail to the right of the child. –Sometimes, child reaches to the left, out of habit –But often their gaze is to the right

An Account of this Error Failure to inhibit an overlearned response. Problems: –Seems influenced by factors irrelevant to inhibition, like cover on location A –A lot of these may just be random errors –Why would gaze and reaching have different overlearned responses?

Characteristics of these Accounts Tend to view knowledge as all-or-nothing: you either have a skill or you don’t. When studying a given skill, then, evidence of success is evidence you have the skill So, failures must be attributed to something else that is outside of the skill you’re studying Implies you can ignore it...

The Competence / Performance Distinction Competence is defined as “having the skill” Performance is defined as your actual ability to do the task requiring the skill. These are theoretical constructs. They can be useful: –Throwing out noise from an experiment, like coughs, slips of the tongue, etc.

The Danger of Taking These Constructs Too Seriously We always want to separate the true signal from noise. But: sometimes the noise isn’t really noise, but is diagnostic of your true “knowledge”. … Then we end up coming up with a lot of ancillary constructs to explain data (like “means-ends” or “inhibition”) that may be unnecessary

A Different Theory Knowledge is graded, not all-or-none Ability to apply knowledge is a function of the task. –Easy tasks can work with weak knowledge –Harder tasks require stronger knowledge or representations

This Theory’s Account Young children have weaker representations of occluded objects These are strong enough to direct gaze –Gaze is an easy task Before 8 months of age, not strong enough to guide reaching

Occluded versus Darkness When an object is occluded, the child has visual evidence that it isn’t there. This works against weak representations of the object. When the object is in the dark, the child has no visual evidence that the object isn’t there. Recall: masking of stimuli from IAC model of word perception

Visually Months 8 Months “Knowledge” Enough to direct gaze Enough to direct reach in dark Enough to direct occluded reach

Evaluating The Theories “Principles” Knowledge is all-or- nothing Failure to reach is attributed to failure of means-ends ability “Graded Knowledge” Representations build up during development Failure to reach is a result of difficulty of task, not means-ends ability

An Empirical Test Previous experiments: confound between occluded-ness and means-ends. –Need to remove thing that hides object If failure to reach for occluded object is indeed a failure of means-ends ability, we should see no difference in performance for visible and occluded objects if both tasks require means-ends manipulation.

Their Experiments Devise a scenario where child needs to manipulate object X in order to get object Y –Whether Y is visible or not Expt 1: Child must pull blanket to get toy Expt 2: Child pushes button to cause toy to slide towards him.

Their Results Even with means-ends requirements equated, children 7 months old better at retrieving object that is visible than hidden –See Figure 3 (Expt 1), and Figure 5 (Expt 2) Conclusion: means-ends differences can’t account for why children have a harder time retrieving hidden objects.

A Model of the Graded-Knowledge Theory A visual representation encodes what is seen A context layer: –activated by its own state, –and what is currently seen –Predicts what will be seen next context Visual Rep Encoding Weights Predictive Weights Recurrent Weights

The Visual Layer Seven units coding object position Seven more coding position of screen. If screen obscures object, object unit turns off.

What Training Prediction Does To predict accurately, network must know: –The current position of the obstruction –Direction of the obstruction, or whether it has stopped moving –Where the object is in relation to the obstruction Must learn to store the location of the object! Must learn to infer it will still be there

Evaluating the Network Plot sensitivity to occluded objects: Activity on prediction that ball will be there, minus activity on prediction when ball was not present Plot as a function of duration of occlusion (Fig 9) Training Sensitivity t3 t5 t7

What This Means The network learns, simply through predicting what it will “see” next, that objects that become hidden will be visible when the occlusion is removed. The ability of the network to do this develops gradually.

Simulating Reaching Train network to perform two tasks: –Predict next scene –Reach for object Delay training on reaching task Also, use lower learning rate context Visual Rep Encoding Weights Reach Units

Results Training Sensitivity Look 3 Performance on Looking task (same as before) Reach 3 Performance on reaching for occluded object Reach Vis Performance on reaching for visible object

What this Shows If feedback or capacity for reaching is reduced relative to that of gazing, then gaze develops more quickly than reaching This means that there is a point in development where infants will: –Correctly gaze at occluded objects –Correctly reach for visible objects –Not correctly reach for occluded objects

Interesting Caveat In the simulations, error was not propagated back from reach units to context units Hence, representations that develop in context units not sensitive to reaching task What might happen if they were?

For Next Class (Next Tues) Read PDP1, Chapter 5, “Feature discovery by competitive learning” Optional reading: handout Remember: Homework 3 due next Tues Remember: Project proposals due March 15