3/20/2009

1

Spatial Perception

• How do we get to a specific destination?

• How do we return to familiar destinations?

• What systems and cues do we use to track our location?

Demo

Spatial Perception

• Was this task easy?

• What strategy did you use?– Did you create a map of the space?– Did you guess the angle using the imaginary

1st person perspective?– Did you use familiar location? – Did you need to remember the path?

3/20/2009

2

Spatial Perception:Animal study

• Muller & Wehner (1988) used desert ants as their model, in a task similar to last week’s experiment

• Gallistel (1989) reviews spatial cognition, including Muller & Wehner (1988)

What information is available to the ant?

• Desert ants can track changes in their orientation using 2 parameters– Angle

• specialized sensors that detect the polarization of light (on a cloudy day)

• the position of the sun – Distance traveled

• Texture flow• Step counting

• Desert ants do not track position relative to their environment. How do we know this?

Spatial Perception

Internal representation

food

nest

external cues

1

2

3

c

a

blindfoldedwalker

release point

3/20/2009

3

Spatial Perception: Background

Nest (starting point)

Path

Food (destination)

Ant displaced

Alternatives: Does the ant represent its location based on EXTERNAL CUES or some INTERNAL REPRESENTATION (e.g. dead reckoning/path integration)?

?

Possible Outcomes

External CuesInternal Representation

Angle

Erro

r

Erro

r

Angle

Spatial Perception• The Muller & Wehner result: ants use dead reckoning.

However, there is a twist…• Ants do not use perfect dead reckoning, only an

approximation! • Error increases with the angle of the turn…• …but rapidly decreases as the angle approaches 180

degrees

Turn angle =150

Small errorlarge error

3/20/2009

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Mueller and Wehner modeled the ants behavior using the following equation:

The equation assumes three things:• The ant knows where it came from (what’s behind it)• The ant knows where it is heading (what’s in front of it)• These points together suggest a path integrations system

TraveledDistTurnTurnTurnConstAngleOld

HometoAngle

.)180180.

Ant’s Neural Computation ForDead Reckoning

Spatial PerceptionHumans

• How did you do the experiment? – Introspectively, did you use path integration or

external cues?– Did you have the sense that your error was

greater for larger turns?• Did you/how did you integrate external and

internal cues?

Spatial Perception: A New Question

• Can humans also use dead reckoning to navigate?

• Do we make the same systematic errors as ants?

3/20/2009

5

Spatial Perception: Method

1

2

3

c

a

blindfoldedwalker

Same as for ants, except we could not displace humans, so we instead removed environmental cues:

•Blindfold blocks visual cues

•Quietly assisted walker blocks auditory cues

Spatial Perception: Write-ups• Data are posted

online• Graph should have

pointing error on the y-axis and turn angle on the x-axis

• Don’t focus on the large error bars: we have about 50 data points; Wehner had about 1500

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

60 120 135 165 180

Angu

lar

Erro

r(d

egre

es)

Turn Angle(degrees)

Class vs. Ant Data

Class Data

Ant Data

Spatial Perception: Discussion

• What does the sign of the error tell us?

• Is there a trend in the data?

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

60 120 135 165 180

Angu

lar

Erro

r(d

egre

es)

Turn Angle(degrees)

Class vs. Ant Data

Class Data

Ant Data

3/20/2009

6

Possible explanations

The data suggests that humans do not share the same path integrations algorithm as the ants.– Why might this be the case?

Write-ups: Expectations

• QAL- You can use the examples used in class– Keep it simple, use one pair of question

• M – Only include the important points• R - Make sure you include graph and describe

the results in text (YOU MUST DO BOTH)

• Use Scatterplot to make sure x-axis spacing is correct

• I - Compare the human and ant results• I - Give some ORIGINAL ideas in your

inferences section!!!

Important dates

• Spring Break next week -- No class next week

• Spatial QALMRI due following week: March 19th - 24rd

• Midterm 2 -- March 30th