Categorical Perception of Expressions in Chimpanzees Pan troglodytes

1
Categorical Perception of Expressions in
Chimpanzees (Pan troglodytes)

• Jennifer M.B. Fugate
• March 15, 2007

2
Classification of Stimuli

• Many animals spontaneously categorize stimuli in
  their environment into groups
• Categorical perception a process by which
  linear physical changes of a stimulus are
  perceived nonlinearly (Harnad, 1987)

GREEN
YELLOW
ORANGE
RED
BLUE
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Categorical Perception of Expression

• This is also true for the perception of facial
  expressions in humans.
• One way this has been tested in the human
  literature is to use stereotypical facial
  expressions and morph them in different
  proportions to see how they are identified.
• Humans perceive at least 5 of the 6 basic
  emotions categorically (Etcoff Magee, 1992
  Calder, et al., 1996 de Gelder, et al., 1997).
• The categorical perception of expression might be
  beneficial in communicating a signalers
  intention (unambiguous signaling).

4
Experiment I

• Do chimpanzees perceive their own expressions
  categorically?

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Chimpanzee Expressions
Relaxed open mouth play Face
Scream Face
Bared Teeth Face
Pant Hoot Face
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Task 1 Categorization of Morphs
1
2
3
4
5
6
6 step morph between bared teeth and play
faces
Bared teeth endpoint
Play endpoint
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Results Task 1
Bared Teeth - Play
n.s.
n.s.
P .015
100.00
P .015
P lt .001
90.00
80.00
70.00
60.00
bared teeth identification
50.00
40.00
30.00
20.00
10.00
0.00
morph 1
morph 2
morph 3
morph 4
morph 5
morph 6
(29 play -
(43 play -
(57 play -
(71 play -
(86 play -
14 play -
86 bt
71 bt
57 bt
43 bt
29 bt
14 bt
Morph
F morph (5,10) 11.99, p .001
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Task 2 Within vs. Between Category
Discrimination
2
4
? 20
? 50
? 80
? 50
1
3
5
3
BETWEEN BOTH CATEGORIES
WITHIN ONE CATEGORY
If boundary between 3-4
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Results Task 2
4 looks more like 5 than 3
sample classified as lower number
Morph triad
t between vs. within (3) 1.842, p .081
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Experiment II

• Can the identification of a visual morph be
  changed depending on the accompanying audio?
• In a nutshell, yes, and the effect is generally
  greatest for morphs along the categorical
  boundaries.

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Conclusions

• Chimpanzees likely perceive the expressions
  studied here categorically.
• The addition of auditory information alters the
  perception of visual expression.

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Acknowledgments

• Harold Gouzoules, Ph.D.
• Lisa Parr, Ph.D. (NIH/NIMH R01-MH068791)
• Yerkes National Primate Research Center
  (RR-00165)
• Center for Behavioral Neuroscience (IBS9876754)