Revisiting the attentional bias in the split brain.

Previous research has revealed a strong right bias in allocation of attention in split brain subjects, suggesting that a pathological attention bias occurs not only after unilateral (usually right-hemispheric) damage but also after functional disconnection of intact right-hemispheric areas involved in allocation of attention from those in the left hemisphere. Here, we investigated the laterality bias in spatial attention, as measured with the greyscales task, in two split-brain subjects (D.D.C. and D.D.V.) who had undergone complete callosotomy. The greyscales task requires participants to judge the darker (or brighter) of two left-right mirror-reversed luminance gradients under conditions of free viewing, and offers an efficient means of quantifying pathological attentional biases in patients with unilateral lesions. As predicted, the results of the two split-brain subjects revealed a pathological rightward bias in allocation of attention, suggesting strong dependence on a single hemisphere (the left) in spatial attention, which is opposite to what one expects from people with intact commissures, and is remarkable in that it occurs in free viewing. In that sense both split-brain patients are behaving as though the brain is indeed split, especially in D.D.C. who had undergone partial resection of the anterior commissure in addition to complete callosotomy, whereas the anterior commissure is still intact in D.D.V. The findings support the view that the commissural pathways play a significant role in integration of attentional processes across cerebral hemispheres.

Can the mind be split? A historical introduction.

The idea that the mind might be composed of distinct conscious entities goes back at least to the mid-19th century, and was at first based on the bilateral symmetry of the brain, with each side seemingly a mirror-image replica of the other. This led to early speculation as to whether section of the forebrain commissures might lead to separate, independent consciousnesses. This was not put to the test until the 1960s, first in commissurotomized cats and monkeys, and then in humans who had undergone commissurotomy for the relief of intractable epilepsy. Initial results did indeed suggest independent consciousness in each separated hemisphere, but later findings have also revealed a degree of mental unity, especially in some perceptual functions and in motor control. Some of these findings might be interpreted in terms of subcortical connections or external cross-cuing, and also address questions about the nature of consciousness in a more concrete way.

How many lateralities?

It is commonly assumed that cerebral asymmetry is unidimensional, but evidence increasingly suggests that different brain circuits are independently lateralized. This might explain why the search for a laterality gene has provided multiple candidates, each with weak linkage. An alternative possibility is that there is a single genetically invariant source of lateralization, perhaps cytoplasmic, and subject to many influences, some genetic, some epigenetic, and some random. This could further explain why laterality is associated with a variety of disorders, such as dyslexia, schizophrenia, stress disorders, and depression.

Humanity and the left hemisphere: The story of half a brain.

Until fairly late in the nineteenth century, it was held that the brain was bilaterally symmetrical. With the discovery of left-brain dominance for language, the so-called "laws of symmetry" were revoked, and asymmetry was then seen as critical to the human condition, with the left hemisphere, in particular, assuming superordinate properties. I trace this idea from the early discoveries of the late nineteenth century through the split-brain studies of the 1960s, and beyond. Although the idea has persisted, the evidence has revealed widespread cerebral asymmetries in nonhuman animals, and even language and its asymmetries are increasingly understood to have evolved gradually, rather than in a single speciation event. The left hemisphere nevertheless seemed to take over a role previously taken by other structures, such as the pineal gland and the hippocampus minor, in a determined effort to place humans on a pedestal above all other species.

Crossing the Rubicon: Behaviorism, Language, and Evolutionary Continuity.

Euan Macphail's work and ideas captured a pivotal time in the late 20th century when behavioral laws were considered to apply equally across vertebrates, implying equal intelligence, but it was also a time when behaviorism was challenged by the view that language was unique to humans, and bestowed a superior mental status. Subsequent work suggests greater continuity between humans and their forebears, challenging the Chomskyan assumption that language evolved in a single step ("the great leap forward") in humans. Language is now understood to be based on an amalgam of cognitive functions, including mental time travel, theory of mind, and what may be more broadly defined as imagination. These functions probably evolved gradually in hominin evolution and are present in varying degrees in non-human species. The blending of language into cognition provides for both interspecies differences in mental function, and continuity between humans and other species. What does seem to be special to humans is the ability to communicate the contents of imagination, although even this is not absolute, and is perhaps less adaptive than we like to think.

Binding identity and orientation in object recognition.

We tested whether an object's orientation is inherently bound to its identity in a holistic view-based representation at the early stages of visual identification, or whether identity and orientation are represented separately. Observers saw brief and masked stimulus sequences containing two rotated objects. They had to detect if a previously cued object was present in the sequence and report its orientation. In Experiments 1 and 2, the objects were presented sequentially in the same spatial location for 70 ms each, whereas in Experiments 3 and 4 they were presented simultaneously in different spatial locations for 70 ms and 140 ms, respectively. Across all experiments, observers reported the correct orientation for approximately 70% of the positively identified objects, and were at chance in reporting the orientation when they had not recognized the object. This finding suggests that orientation information is accessed after an object has been identified. In addition, when the two objects were presented sequentially in the same spatial location, orientation errors were not random-observers tended to report the orientation of the alternative object in the sequence, indicating misbindings between the identities and orientations of objects that share spatial location. This susceptibility to binding errors was not observed when the objects were in different spatial locations. These results suggest that identity and orientation may be prone to misbinding, and that spatial location may serve to protect their joint integrity.

Evolution of cerebral asymmetry.

The human brain is often characterized in terms of a duality, with the left and right brains serving complementary functions, and even individuals are sometimes classified as either "left-brained" or "right-brained." Recent evidence from brain imaging shows that hemispheric asymmetry is multidimensional, comprised of independent lateralized circuits. Cerebral asymmetries, which include handedness, probably arise in phylogenesis through the fissioning of ancestral systems that divided and lateralized with increasing demand for specialization. They also vary between individuals, with some showing absent or reversed asymmetries. It is unlikely that this variation is controlled by a single gene, as sometimes assumed, but depends rather on complex interplay among several, perhaps many, genes. Hemispheric asymmetry has often been regarded as a unique mark of being human, but it has also become evident that behavioral and cerebral asymmetries are not confined to humans, and are widespread among animal species. They nevertheless exist against a fundamental background of bilateral symmetry, suggesting a tradeoff between the two. Individual differences in asymmetry, moreover, are themselves adaptive, contributing to the cognitive and behavioral specializations necessary for societies to operate efficiently.

Mental time travel, language, and evolution.

Tulving (1985) was probably the first to use the term "mental time travel" to describe the human capacity to imagine personal events from the past, as well as to envisage possible future ones. He and others have also claimed that this capacity is unique to humans. One likely reason for this is that mental travel into the past, or episodic memory, is part of what has been termed declarative memory-or memory that can be declared. This implies a critical role for language itself, also generally regarded as unique to humans. I argue here that language probably did evolve to enable explicit reference to the nonpresent, a property known as displacement. This need not imply, though, that mental time travel is itself unique to humans. Behavioral evidence from a number of different species, including birds, mammals, and apes, increasingly implies the ability to recall specific events from the past, and even to imagine possible future ones. Brain recordings from both rodents and humans also show the hippocampus to be active in both replay of past events and the "play" of events that did not actually occur, including imagined future ones, suggesting an evolutionary continuity. The wider hippocampal/entorhinal system, with likely involvement of other cortical areas, has a generative capacity, underlying spatiotemporal imagination in both humans and nonhuman animals, which may underlie the generativity of language itself. As a communication system, though, language evolved late as a means of sharing imaginative explorations, or what Dor (2015) termed "the instruction of imagination."

Language, Memory, and Mental Time Travel: An Evolutionary Perspective.

Language could not exist without memory, in all its forms: working memory for sequential production and understanding, implicit memory for grammatical rules, semantic memory for knowledge, and episodic memory for communicating personal experience. Episodic memory is part of a more general capacity for mental travel both forward and backward in time, and extending even into fantasy and stories. I argue that the generativity of mental time travel underlies the generativity of language itself, and could be the basis of what Chomsky calls I-language, or universal grammar (UG), a capacity for recursive thought independent of communicative language itself. Whereas Chomsky proposed that I-language evolved in a single step well after the emergence of Homo sapiens, I suggest that generative imagination, extended in space and time, has a long evolutionary history, and that it was the capacity to share internal thoughts, rather than the nature of the thoughts themselves, that more clearly distinguishes humans from other species.

When averaging goes wrong: The case for mixture model estimation in psychological science.

Recent failed attempts to replicate numerous findings in psychology have raised concerns about methodological practices in the behavioral sciences. More caution appears to be required when evaluating single studies, while systematic replications and meta-analyses are being encouraged. Here, we provide an additional element to this ongoing discussion, by proposing that typical assumptions of meta-analyses be substantiated. Specifically, we argue that when effects come from more than one underlying distributions, meta-analytic averages extracted from a series of studies can be deceptive, with potentially detrimental consequences. The underlying distribution properties, we propose, should be modeled, based on the variability in a given population of effect sizes. We describe how to test for the plurality of distribution modes adequately, how to use the resulting probabilistic assessments to refine evaluations of a body of evidence, and discuss why current models are insufficient in addressing these concerns. We also consider the advantages and limitations of this method, and demonstrate how systematic testing could lead to stronger inferences. Additional material with details regarding all the examples, algorithm, and code is provided online to facilitate replication and to allow broader use across the field of psychology. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Space, time, and language.

Cognition is heavily grounded in space. As animals that move in space, we travel both physically and mentally in space and time, reliving past events, imagining future ones, and even constructing imaginary scenarios that play out in stories. Mental exploration of space is extraordinarily flexible, allowing us to zoom, adopt different vantage points, mentally rotate, and attach objects and sense impressions to create events, whether remembered, planned, or simply invented. The properties of spatiotemporal cognition depend on a hippocampal-entorhinal circuit of place cells, grid cells and border cells, with combinations of grid-cell modules generating a vast number of potential spatial remappings. The generativity of language, often considered one of its defining properties, may therefore derive not from the nature of language itself, but rather from the generativity of spatiotemporal scenarios, with language having evolved as a means of sharing them. Much our understanding of the hippocampal-entorhinal circuit is derived from neurophysiological recording in the rat brain, implying that the spatiotemporal cognition underpinning language has a long evolutionary history.

Mirror-Image Equivalence and Interhemispheric Mirror-Image Reversal.

Mirror-image confusions are common, especially in children and in some cases of neurological impairment. They can be a special impediment in activities such as reading and writing directional scripts, where mirror-image patterns (such as b and d) must be distinguished. Treating mirror images as equivalent, though, can also be adaptive in the natural world, which carries no systematic left-right bias and where the same object or event can appear in opposite viewpoints. Mirror-image equivalence and confusion are natural consequences of a bilaterally symmetrical brain. In the course of learning, mirror-image equivalence may be established through a process of symmetrization, achieved through homotopic interhemispheric exchange in the formation of memory circuits. Such circuits would not distinguish between mirror images. Learning to discriminate mirror-image discriminations may depend either on existing brain asymmetries, or on extensive learning overriding the symmetrization process. The balance between mirror-image equivalence and mirror-image discrimination may nevertheless be precarious, with spontaneous confusions or reversals, such as mirror writing, sometimes appearing naturally or as a manifestation of conditions like dyslexia.

The Many Sides of Hemispheric Asymmetry: A Selective Review and Outlook.

Hemispheric asymmetry is commonly viewed as a dual system, unique to humans, with the two sides of the human brain in complementary roles. To the contrary, modern research shows that cerebral and behavioral asymmetries are widespread in the animal kingdom, and that the concept of duality is an oversimplification. The brain has many networks serving different functions; these are differentially lateralized, and involve many genes. Unlike the asymmetries of the internal organs, brain asymmetry is variable, with a significant minority of the population showing reversed asymmetries or the absence of asymmetry. This variability may underlie the divisions of labor and the specializations that sustain social life. (JINS, 2017, 23, 710-718).

Do 'literate' pigeons (Columba livia) show mirror-word generalization?

Many children pass through a mirror stage in reading, where they write individual letters or digits in mirror and find it difficult to correctly utilize letters that are mirror images of one another (e.g., b and d). This phenomenon is thought to reflect the fact that the brain does not naturally discriminate left from right. Indeed, it has been argued that reading acquisition involves the inhibition of this default process. In the current study, we tested the ability of literate pigeons, which had learned to discriminate between 30 and 62 words from 7832 nonwords, to discriminate between words and their mirror counterparts. Subjects were sensitive to the left-right orientation of the individual letters, but not the order of letters within a word. This finding may reflect the fact that, in the absence of human-unique top-down processes, the inhibition of mirror generalization may be limited.

The Evolution of Lateralized Brain Circuits.

In the vast clade of animals known as the bilateria, cerebral and behavioral asymmetries emerge against the backdrop of bilateral symmetry, with a functional trade-off between the two. Asymmetries can lead to more efficient processing and packaging of internal structures, but at the expense of efficient adaptation to a natural world without systematic left-right bias. Asymmetries may arise through the fissioning of ancestral structures that are largely symmetrical, creating new circuits. In humans these may include asymmetrical adaptations to language and manufacture, and as one or other hemisphere gains dominance for functions that were previously represented bilaterally. This is best illustrated in the evolution of such functions as language and tool manufacture in humans, which may derive from the mirror-neuron system in primates, but similar principles probably apply to the many other asymmetries now evident in a wide range of animals. Asymmetries arise in largely independent manner with multi-genetic sources, rather than as a single over-riding principle.

Language Evolution: A Changing Perspective.

From ancient times, religion and philosophy have regarded language as a faculty bestowed uniquely and suddenly on our own species, primarily as a mode of thought with communication as a byproduct. This view persists among some scientists and linguists and is counter to the theory of evolution, which implies that the evolution of complex structures is incremental. I argue here that language derives from mental processes with gradual evolutionary trajectories, including the generative capacities to travel mentally in time and space and into the minds of others. What may be distinctive in humans is the means to communicate these mental experiences along with knowledge gained from them.