The effect of infantile stimulation upon hypothalamic CRF levels following adrenalectomy in the adult rat

Variation in lateralization: Selected samples do not a population make

The two major points of Denenberg's article are (1) that animals have lateralized brains, and (2) that the pattern of cerebral lateralization is consistent across species (i.e., “the left hemisphere will be primarily involved in communicative functions,” the right hemisphere with processing “spatial and affective information.” In addition, there is an unstated assumption that the pattern of lateralization is consistent within species. The evidence reviewed by Denenberg leaves little doubt that nonhuman animals have asymmetrically organized brains. However, there are problems with the suggestion that there is a consistent pattern of cerebral lateralization within or across different populations of species.

On asymmetries exhibiting a near-equiprobable distribution of directions

In his target article as well as in other writings, Denenberg presents a view of lateralization with which I fundamentally disagree: namely, that an affirmation of lateralization in a population is to be based primarily, if not exclusively, on observing a nonequiprobable distribution of asymmetric forms in that population.

An asymmetric view of brain laterality

The enigma of hemispheric specialization of the human brain continues to attract the attention of BBS readers. Although the lateralization of language is obviously specific to man, some scientists find the idea of human uniqueness unacceptable. Corballis and Morgan (1978) presented hemispheric dominance in man as a special case of a left-right maturational gradient, examples of which can be found throughout the animal kingdom. According to Denenberg, brain laterality can be induced in animals by nonlateralized environmental factors such as handling. Since nonlateralized influences can only unmask latent asymmetries, Denenberg's position is essentially similar to the views espoused by Corballis and Morgan (1978) and can, therefore, be criticized on the same grounds.

Hemispheric laterality in animals and the effects of early experience

A review of research with chicks, songbirds, rodents, and nonhuman primates indicates that the brain is lateralized for a number of behavioral functions. These findings can be understood in terms of three hypothetical brain processes derived from a brain model based on general systems theory: hemispheric activation, interhemispheric inhibition, and interhemispheric coupling.

Left-hemisphere activation occurs in songbirds and nonhuman primates in response to salient auditory or visual input, or when a communicative output is required. The right hemisphere is activated in rats when spatial performance is required, and in chicks when they are placed in an emotion-provoking situation. In rats and chicks interhemispheric activation and inhibition occur when there is an affective component in the environment (novelty, aversive conditioning) or when an emotional response is emitted (copulation, attack, killing). An interhemispheric coupling (correlation) found in rats and rabbits implies that the hemispheres are two major components in a control system with a negative feedback loop. Early-experience variables in rats can induce laterality in a symmetric brain or facilitate its development in an already biased brain.

It is predicted that functional lateralization, when present, will be similar across species: the left hemisphere will tend to be involved in communicative functions while the right hemisphere will respond to spatial and affective information; both hemispheres will often interact via activation-inhibition mechanisms when affective or emotional processes are involved. Homologous brain areas and their connecting callosal fibers must be intact at birth and must remain intact throughout development for lateralization to reach its maximum level. Injury to any portion of this unit will result in hemispheric redundancy rather than specialization. One major function of early experience is to provide stimulation during development, which acts to enhance the growth and development of the corpus callosum, thereby giving rise to a more specialized brain.

Effects of handling during infancy on the sympathetic-adrenal medullary system of rats

We examined the effects of daily handling and maternal separation (5 min per day) on the responsiveness of the sympathetic-adrenal medullary system of Sprague-Dawley rats before weaning and in adulthood. Plasma levels of norepinephrine (derived primarily from sympathetic nerves) and epinephrine (released from the adrenal medulla) were elevated in handled pups compared to unhandled controls at 14 and 18 days of age but not at 6 and 10 days of age. When tested in adulthood, previously handled and control rats did not differ with respect to basal or stress-induced increments in plasma levels of norepinephrine and epinephrine. These results indicate that brief daily episodes of handling and maternal separation are attended by an increase in sympathetic-adrenal medullary tone in 14-18-day-old rats. However, the enhanced response of the sympathetic-adrenal medullary system of separated rats may not persist into adulthood.

On the evolution and growth of lateralization

This is a welcome attempt to seek common biological principles underlying laterality in animals and humans. I do not think it is wholly convincing, but this is partly because many of the results from nonhuman species are not yet firmly established or understood. I suspect that the author's characterization of lateralization, with the left hemisphere supposedly specialized for communicative functions and the right for spatial and affective functions, will require modification. For instance, it now seems fairly clear that the left hemisphere in humans plays a general role in the production and perception of sequences not restricted to communicative acts (Craig 1980; Kimura 1979), and indeed some of the examples of left-hemispheric specialization listed in Table 2 are not obviously communicative. Even so, there are some fairly striking parallels between humans and nonhumans with respect to the pattern of lateralization, and one suspects that common principles are operating. At the same time, in the enthusiastic search for functional asymmetries, we should not overlook the striking degree of bilateral symmetry that characterizes the brains of all animals, including humans.

Cerebral predominance in the monkey?

The issues raised by Dr. Denenberg are complex, not least because the apparent communalities of hemispheric specialization among birds, rodents, monkeys, and human beings are also associated with negative instances (e.g., for parrots, Nottebohm 1976; for Macaco, other than fuscata, Petersen et al. 1978). To extend the available evidence I would like to refer to the preliminary findings of Garcha et al. (1980) on the monkey.

Hemispheric laterality and an evolutionary perspective

Denenberg rightly stresses the importance of studying ethologically meaningful species-specific behavior in animals, and makes the interesting distinction between lateralization at an individual and at a population level. However, in the case of man, I believe Denenberg is wrong in arguing that lateralization in the individual increases with maturation. The overall evidence nowadays tends very much to the contrary. Moreover, with respect to a population, why should it become lateralized? If there is indeed an advantage for the individual in hemispheric specialization, why should the direction of such specialization be so consistent across a majority of individuals, whether human or, as Denenberg points out, other members of the phylum? Is there an evolutionary advantage in most animals' sharing the same direction, or is it a necessary consequence of some other preexisting, more fundamental anatomical, biochemical, or physical property of the organism and its constituents? If the former, why are not all members of the species, rather than just a majority, lateralized in the same direction? (Or, to put it another way, what is the evolutionary advantage to the species or individual of dimorphism, of retaining a minority who polarize in the opposite direction?) If the latter - i.e., if lateralization is a necessary consequence of some prior state - then there should not be any dimorphism, exceptions, or minority members, unless they are somehow disadvantaged in consequence. Indeed, there is some evidence of a cognitive deficit in sinistrals, though it is disputed (see Bradshaw 1980 for review), and others have even suggested that the species as a whole may benefit in some way from such an uneven dimorphism (Levy 1974), but what evidence is there for such propositions with respect to rats, apes, monkeys, or chicks? This is an issue that should be addressed in any general model that includes laterality in animals. [See Corhallis & Morgan: “On the Biological Basis of Human Laterality” BBS 1(2) 1978.]