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
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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
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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
