Laterality In Nonhuman Species

Behavioral and biological laterality is also ubiquitous in many nonhuman species, with many instances of asymmetry being at least analogous to asymmetries found in humans. At least some may also be homologous, in the sense of sharing common structures and developmental origins. Here, I review some of the most well-established laterality effects in other species and note their relationship to human laterality.

Motor asymmetries have been discovered for a number of species, with individuals sometimes showing very strong left-right preferences. However, population-level biases have been much rarer and none has matched the magnitude of right-handedness in the human population. Furthermore, preferences seem to depend on variables such as age and sex and on specific task demands. Despite these caveats, in several species of primates there tends to be a left-hand preference for reaching and maintaining postural control but a right-hand preference for manipulation and other high-level skilled activities. In addition to individual variation in paw preference, individual members of many species of mice and rats show a preference to turn in one direction or the other. Although an individual may exhibit a strong directional rotation bias, approximately equal numbers prefer each side. That is, there does not tend to be a population-level asymmetry. Individual rotation biases in rats have been related to asymmetries in distribution of the neurotransmitter dopamine, although the specific direction of the relationship differs for different populations.

There are also certain parallels between left hemisphere language dominance in humans and asymmetries in other species for the production and perception of vocalizations. In Japanese macaques, for example, the left hemisphere is dominant for the discrimination of species-specific vocalizations that are relevant for communication but not for the discrimination of other vocalizations. Also, in chimpanzees that have been trained to use certain visual symbols to communicate, there is evidence of left hemisphere dominance for processing those symbols but not for processing other, nonmeaningful symbols. There is even evidence that the ultrasonic calls emitted by rat pups are processed preferentially by the left hemisphere of their mother and it is well-known that there is left-brain dominance for the control of song in some species of song birds.

A number of asymmetries have been reported with respect to processing the identity and spatial characteristics of visual stimuli. In language-trained chimpanzees, for example, there is a right hemisphere advantage for processing the location of a line within a geometric figure and for identifying complex visual patterns that are not relevant for communication. In addition, rhesus monkeys have been reported to have right hemisphere superiority for recognizing monkey faces. In rats, there is evidence that the right hemisphere may be more involved than the left hemisphere in spatial exploration, although the asymmetry emerges only in rats that have been handled during the course of their early development. Pigeons and newly hatched chicks exhibit left hemisphere dominance for visual pattern discrimination. In chicks, this population-level bias occurs because light strikes only the right eye during a critical period of incubation during which the visual system is developing rapidly. Finding effects of such variables as handling and light stimulation suggests that functional hemispheric asymmetries are likely to be shaped by the complex interaction of both biological and environmental factors.

Research with rats and chicks has also demonstrated asymmetry for emotional behaviors. For example, in both handled rats and chicks the right hemisphere tends to produce emotional activity, whereas the left hemisphere tends to inhibit emotional activity. In addition to providing interesting instances of lateral-ity, effects such as these also illustrate the importance of reciprocal activity between the left and right sides of the brain.

There are also indications that some of the biological asymmetries found in the human brain characterize the brains of certain primates, although the nonhuman asymmetries are smaller and less frequent than those of humans. For example, the brains of both humans and apes show the kind of counterclockwise torque described earlier and in chimpanzees as well as humans the Sylvian fissure tends to be longer on the left side than on the right side.

As noted previously, it is difficult to know which laterality effects in other species are truly homologous to the effects found in humans. Nevertheless, the presence of so many asymmetries in other species provides a useful range of animal models that can be used to learn about the development of laterality across the life span of an individual and across evolutionary time. Among other things, laterality in other species indicates that the emergence of language is not a prerequisite for the emergence of other behavioral and biological asymmetries.

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