Gesture Latency in Chimpanzees (Pan troglodytes): Implication for Left Hemisphere Hand and Vocal Lateralization
Eliza Nelson, Jaime Russell, and William Hopkins
Baldwin-Wallace College, Department of Psychology, Berea, OH
Yerkes National Primate Research Center, Division of Psychobiology, Atlanta, GA
Berry College, Department of Psychology, Mount Berry, GA

Abstract

Studies involving both human and chimpanzee subjects have found a left hemispheric asymmetry in Broca's area. Additionally Broca's area has been shown to be activated for both language and motor tasks in humans. This study proposes that Broca's area has similar functions in non-human primates and may be a neurobiological substrate for linking gestures and vocalizations. Gesture latency was examined in 39 captive chimpanzees. Overall the latency times for right-handed gestures with vocalizations were significantly shorter than latency times for left-handed gestures with vocalizations. These findings suggest lateralization of the left hemisphere for gestures accompanied by vocalizations in chimpanzees.

Introduction

The capacity to produce gestures with vocalizations may have originated in Broca's area, specifically Brodmann's area 44 in the brain. In humans this region which is larger in the left hemisphere is traditionally known for its role in language and recently has been shown to also be activated for motor tasks i.e. gesturing. A study by Cantalupo and Hopkins (2001) showed that chimpanzees have a similar left hemispheric asymmetry in Broca's area to that of humans. It has been established by a number of studies that chimpanzees prefer to gesture with their right hand. Additionally the number of chimpanzees using their right hand for gesturing increased when the gesture was combined with a vocalization.

Methods and Materials

39 chimpanzees (17 females and 22 males) housed at the Yerkes National Primate Research Center in Atlanta, Georgia participated in this study. Each subject received three trials counterbalanced across two conditions. Left Condition: food was offered to the subject's left side to elicit a left- handed gesture with vocalization. Right Condition: food was offered to the subject's right side to elicit a right-handed gesture with vocalization. Each trial was timed to be 60 seconds in duration. When the subject used the appropriate hand to gesture with vocalization time was stopped and recorded. If the subject did not gesture and vocalize simultaneously with the intended hand during the trial they were awarded the max time of 60 seconds.

Results

The mean time for gesture + vocalization is given in seconds for the left and right overall averages respectively as well as for each of the three trials. The overall time it takes for subjects to produce a right-handed gesture + vocalization is significantly less than the time to produce a left-handed gesture + vocalization t(39)=.001 p<.05.

Conclusions and Future Studies

Like previous studies with this group of chimpanzees handedness indices (HI) based on gestures were calculated by subtracting the left-handed response from the right-handed response and dividing by the sum of both responses HI = (R-L)/(R+L). Although HI values from the current study did not correlate with a prior study 4 this may be due to differences in data collection (latency times vs. frequencies). In order to more accurately assess consistency between general measures of handedness and gesturing plus the specific behavior measured in this study it may be of value to score the frequency of gestures accompanied by vocalizations in a given time rather than latencies only. Consistent with expectations based on studies done by Hopkins & Cantero (2003) and Hopkins & Leavens (1998) chimpanzees in the present study produced right-handed gestures + vocalizations significantly faster than left-handed gestures + vocalizations. This finding in gesture latency suggests left hemisphere dominance for certain communicative behaviors in non-human primates specifically gestures accompanied by vocalizations.

Acknowledgements and Funding Attributions

This research was supported by Howard Hughes Medical Institute Grant No. 52003727 and NIH Grants NS42867 and NS36605.

In Plain English

This study looked at the link between manual gestures and vocalizations in chimpanzees. Time was measured in latency which means the duration from when the food was presented to the chimp to when the chimp used a gesture with a vocalization. The chimps were timed with a stopwatch and had sixty seconds to make a response. Overall the chimps gestured and vocalized more quickly with their right hand than with their left. This is evidence that gestures and vocalizations may be linked together in the brain. This shows that chimpanzees may have specialized brain functions which are similiar to humans.