Michael C. Corballis

The evolution of foresight: What is mental time travel and is it unique to humans?

In a dynamic world, mechanisms allowing prediction of future situations can provide a selective advantage. We suggest that memory systems differ in the degree of flexibility they offer for anticipatory behavior and put forward a corresponding taxonomy of prospection. The adaptive advantage of any memory system can only lie in what it contributes for future survival. The most flexible is episodic memory, which we suggest is part of a more general faculty of mental time travel that allows us not only to go back in time, but also to foresee, plan, and shape virtually any specific future event. We review comparative studies and find that, in spite of increased research in the area, there is as yet no convincing evidence for mental time travel in nonhuman animals. We submit that mental time travel is not an encapsulated cognitive system, but instead comprises several subsidiary mechanisms. A theater metaphor serves as an analogy for the kind of mechanisms required for effective mental time travel. We propose that future research should consider these mechanisms in addition to direct evidence of future-directed action. We maintain that the emergence of mental time travel in evolution was a crucial step towards our current success.

On the biological basis of human laterality: I. Evidence for a maturational left–right gradient

In this paper, we consider human handedness and cerebral lateralization in a general biological context, and attempt to arrive at some conclusions common to the growth of human laterality and of other structural asymmetries. We suggest that many asymmetries appear to be under the influence of a left-right maturational gradient, which often seems to favor earlier or more rapid development on the left than on the right. If the leading side is damaged or restricted, this gradient may be reversed so that growth occurs with the opposite polarity. A mechanism of this sort appears to underlie the phenomenon of situs inversus viscerum et cordis , and the same principle may help explain the equipotentiality of the two sides of the human brain with respect to the representation of language in the early years of life. However we must also suppose that the leading side normally exerts an inhibitory influence on the lagging side, for otherwise one would expect language ultimately to develop in both halves of the brain. Examples of an inhibitory influence of this kind can also be found in other biological asymmetries; for instance, in the crab Alpheus heterochelis , one claw is normally greatly enlarged relative to the other, but if the larger claw is removed the smaller one is apparently released from its inhibitory influence and grows larger. This last example is particularly interesting because it suggests a mechanism comparable to that proposed by Annett to account for the distribution of handedness in the human population. She argued, in effect, that there is a “right shift” factor among the majority of the population, but that among a minority who lack this factor handedness is determined at random. If it is supposed that cerebral lateralization is also determined at random among this recessive minority, the model can be extended to provide a reasonable fit to the data on the correlation between handedness and cerebral lateralization. However this genetic model (or any other) still fails to account for the near-binomial distribution of handedness among twins and among nontwin siblings. We suggest that right-handedness and leftcerebral dominance for language are manifestations of an underlying gradient which is probably coded in the cytoplasm rather than in the genes. We must leave open the question as to whether departures from this pattern are due to a recessive gene which effectively cancels the asymmetry to environmental influences, or to both genetic and cytoplasmic factors.

Laterality and human evolution.

The question of whether there is a fundamental discontinuity between humans and other primates is discussed in relation to the predominantly human pattern of right-handedness and the left-cerebral representation of language. Both phenomena may go back at least to Homo habilis, 2-3 million years ago. However, a distinctively human mode of cognitive representation may not have emerged until later, beginning with H. erectus and the Acheulean tool culture about 1.5 million years ago and culminating with H. sapiens sapiens and rapid, flexible speech in the last 200,000 years. It is suggested that this mode is characterized by generativity, with multipart representations formed from elementary canonical parts (e.g., phonemes in speech, geons in visual perception). Generativity may be uniquely human and associated with the left-cerebral hemisphere. An alternative, analogue mode of representation, shared with other species, is associated with the right hemisphere in humans.

Decisions about identity and orientation of rotated letters and digits

In three experiments, human observers made timed decisions about alphanumeric characters, displayed singly in different orientations and versions (normal vs. backward). Latency to identify the characters was longer for backward than for normal versions, regardless of angular orientation and even under conditions in which latency was independent of angular orientation. Subjects also took longer to respond to a target orientation (whatever the character) than to respond to a target character (whatever the orientation). The results suggest that the observer first induces a description of a character that is largely independent of orientation but not of version, although the representation of version is too weak at this stage to permit an overt decision about it. Next, the angular orientation of the character is determined. Finally, the observer might “mentally rotate” the representation to the standard upright, for matching against an internally generated template.

Mental time travel and the shaping of the human mind

Episodic memory, enabling conscious recollection of past episodes, can be distinguished from semantic memory, which stores enduring facts about the world. Episodic memory shares a core neural network with the simulation of future episodes, enabling mental time travel into both the past and the future. The notion that there might be something distinctly human about mental time travel has provoked ingenious attempts to demonstrate episodic memory or future simulation in non-human animals, but we argue that they have not yet established a capacity comparable to the human faculty. The evolution of the capacity to simulate possible future events, based on episodic memory, enhanced fitness by enabling action in preparation of different possible scenarios that increased present or future survival and reproduction chances. Human language may have evolved in the first instance for the sharing of past and planned future events, and, indeed, fictional ones, further enhancing fitness in social settings.

The evolution and genetics of cerebral asymmetry

Handedness and cerebral asymmetry are commonly assumed to be uniquely human, and even defining characteristics of our species. This is increasingly refuted by the evidence of behavioural asymmetries in non-human species. Although complex manual skill and language are indeed unique to our species and are represented asymmetrically in the brain, some non-human asymmetries appear to be precursors, and others are shared between humans and non-humans. In all behavioural and cerebral asymmetries so far investigated, a minority of individuals reverse or negate the dominant asymmetry, suggesting that such asymmetries are best understood in the context of the overriding bilateral symmetry of the brain and body, and a trade-off between the relative advantages and disadvantages of symmetry and asymmetry. Genetic models of handedness, for example, typically postulate a gene with two alleles, one disposing towards right-handedness and the other imposing no directional influence. There is as yet no convincing evidence as to the location of this putative gene, suggesting that several genes may be involved, or that the gene may be monomorphic with variations due to environmental or epigenetic influences. Nevertheless, it is suggested that, in behavioural, neurological and evolutionary terms, it may be more profitable to examine the degree rather than the direction of asymmetry.

Mental Rotation and the Right Hemisphere

Mental rotation may be considered a prototypical example of a higher-order transformational process that is nonsymbolic and analog as opposed to propositional. It is therefore a paradigm case for testing the view that these properties are fundamentally right-hemispheric. Evidence from brain-imaging, unilateral brain lesions, commissurotomy, and visual-hemifield differences in normals is reviewed. Although there is some support for a right-hemispheric bias, at least for the holistic rotation of relatively simple shapes, it is unlikely that this bias approaches the degree of left-hemispheric dominance for language-related skills. An evolutionary scenario is sketched in which the characteristically symbolic mode of the left hemisphere evolved relatively late and achieved the quality of recursive generativity only in the late stages of hominid evolution. This forced an increasingly right-hemispheric bias onto analog processes like mental rotation. Such processes nevertheless remain important and are integral even to those processes we think of as highly symbolic, such as language and mathematics.

Ear asymmetry in reaction time to musical sounds

Dichotic pairs of musical sounds were presented to 16 right-handed subjects, who were instructed to depress a reaction-time (RT) button when a target sound occurred in either ear. Four blocks of 36 trials were presented. During the first block, RTs to left-ear targets were significantly faster than those to right-ear targets. There were no significant ear differences during the second, third, or fourth blocks. Possible explanations for the limited duration of the left-ear advantage, and its implications for models proposed to explain the basis of RT asymmetries, are discussed.

On the biological basis of human laterality: II. The mechanisms of inheritance

This paper focuses on the inheritance of human handedness and cerebral lateralization within the more general context of structural biological asymmetries. The morphogenesis of asymmetrical structures, such as the heart in vertebrates, depends upon a complex interaction between information coded in the cytoplasm and in the genes, but the polarity of asymmetry seems to depend on the cytoplasmic rather than the genetic code. Indeed it is extremely difficult to find clear-cut examples in which the direction of an asymmetry is under genetic control. As one possible case, there is some evidence that the direction, clockwise or counterclockwise, of rotation of the abdomen in certain mutant strains of Drosophila is controlled by a particular gene locus, although there appears to be some degree of confusion on this point. By contrast, it is much easier to find examples in which the degree but not the direction of asymmetry is under genetic control. For instance, there is a mutant strain of mice in which half of the animals display situs inversus of the viscera. The proportion has remained at one half despite many years of inbreeding, suggesting that the mutant allele effectively cancels the normal situs and allows the asymmetry to be specified in random fashion. Although this account does not deny that the right hemisphere of humans may be the more specialized for certain functions, it does attribute a leading or dominant role to the left hemisphere (at least in most individuals). We suggest that so-called “right-hemisphere” functions are essentially acquired by default, due to the left hemispheres prior involvement with speech and skilled motor acts; we note, for instance, that these right-hemisphere functions include rather elementary perceptual processes. But perhaps the more critical prediction from our account is that the phenomenon of equipotentiality should be unidirectional: the right (lagging) hemisphere should be more disposed to take over left-hemisphere functions following early lesions than is the left (leading) hemisphere to take over right-hemisphere functions. We note preliminary evidence that this may be so.

Cerebral asymmetries: Complementary and independent processes

Most people are right-handed and left-cerebrally dominant for speech, leading historically to the general notion of left-hemispheric dominance, and more recently to genetic models proposing a single lateralizing gene. This hypothetical gene can account for higher incidence of right-handers in those with left cerebral dominance for speech. It remains unclear how this dominance relates to the right-cerebral dominance for some nonverbal functions such as spatial or emotional processing. Here we use functional magnetic resonance imaging with a sample of 155 subjects to measure asymmetrical activation induced by speech production in the frontal lobes, by face processing in the temporal lobes, and by spatial processing in the parietal lobes. Left-frontal, right-temporal, and right-parietal dominance were all intercorrelated, suggesting that right-cerebral biases may be at least in part complementary to the left-hemispheric dominance for language. However, handedness and parietal asymmetry for spatial processing were uncorrelated, implying independent lateralizing processes, one producing a leftward bias most closely associated with handedness, and the other a rightward bias most closely associated with spatial attention.