Abstract # 80:

Scheduled for Thursday, August 18, 2005 07:00 PM-09:00 PM: (Cambridge/Oxford Room) Poster Presentation


E. Reynolds, H. Freeman, J. L. Russell and W. D. Hopkins
Div. of Psychobiology, Yerkes Reg. Prim. Res. Ctr., Atlanta, GA 30322, USA
     Human facial expressions of emotion are more exaggerated on the left half of the face while the right side of the mouth opens wider during speech. These facial biases are traditionally used to infer hemispheric specialization. Several primate species also exhibit a leftward bias for facial expressions of emotion. In this study, the asymmetry of three categories of expressions was measured in 48 chimpanzees including hoots, food barks, and idiosyncratic attention-getting sounds used to request food (bilabial fricative and voiced open-mouth grunt). Use of the bilabial fricative and open-mouth grunt appears to be under voluntary control unlike the facial expressions of emotion previously investigated in primates. Measurements of pixel area of the right and left halves of the mouth were made on digital photographs of the expressions and facial asymmetry indexes (FAIs) were calculated ((R-L)/(R+L)). The FAIs were averaged for three images/expression/subject and one sample t-tests were carried out on these values in each of the expression categories. Both the hoot and the food bark exhibited a left side bias (hoot: t(14) = -3.16, P < 0.01; food bark: t(16) = -3.36, P < 0.01). The volitional attention-getting sounds were biased to the right (t(37) = -2.68, P < 0.05). There were no effects of age or sex on the direction of oro-facial asymmetries. These results suggest that, in chimpanzees, the left and right hemispheres are differentially involved in the production of voluntary and involuntary vocal signals. Supported by NIH grants NS-36605 and NS-42867.