招待講演

A New Look at the Old Riddle : Novelty, Routines and the Evolution of the Bicameral

Elkhonon Goldberg

Key words : hemisphere, novelty, routine, cognitive, evolution,

A new conceptual framework for understanding hemispheric specialization across evolution is proposed, and its multiple theoretical, clinical, and methodological implications are considered. According to the traditional understanding of hemispheric specialization, the “dominant” (usually left) hemisphere is in charge of language, whereas the “subdominant” (usually right) hemisphere is in charge of non-verbal, particularly visuo-spatial functions. While not incorrect, this understanding cannot be regarded as complete, since it does not permit any consideration of evolutionary continuities. Division of the brain into two hemispheres is not unique to humans ; it is a pervasive feature of the central nervous system throughout evolution. A number of morphological, cellular, and biochemical differences between the hemispheres exists, many of which are shared by multiple species. Therefore, it is only logical to assume that functional differences between the two hemispheres also exist and that they are invariant across multiple species. It is proposed that the fundamental functional difference between the two hemispheres is captured by the distinction between cognitive novelty and cognitive routines. According to this view, the left hemisphere is dominant in cognitive processing guided by previously formed, entrenched representations and strategies. In contrast, the right hemisphere is dominant for dealing with novel cognitive challenges, to which none of the previously formed routines or representations are readily applicable. The distinction between cognitive novelty and cognitive routines is universal and applicable to any organism capable of learning, unlike the verbal-nonverbal distinction which is applicable only to humans. Within this framework, language-mediated cognition is understood as a special case of cognition mediated by previously formed cognitive routines, verbal and non-verbal alike.

I. Traditional Views Challenged charge of non-verbal, most notably visuo-spatial, Duality is the cardinal architectural feature of the processes. It would seem that the issue has been brain conserved throughout evolution. Prompted by exhausted, a definitive verdict has been reached, and the seminal findings by Broca and Wernicke1), any current or future understanding of hemispheric hemispheric specialization has been among the most specialization is bound to be guided by this frame- prominent themes in for the last work. century and a half. It is among the most compre- But familiarity often breeds intellectual compla- hensively researched areas of psychology and the cency and several major problems inherent in the consensus in the field has been near-universal and “language-visuospatial” framework for understanding uncontroversial. According to this consensus, the hemispheric specialization have been commonly dominant hemisphere (the left one in most people) is overlooked. In the age when the notion linking in charge of language and the right hemisphere is in language to one cerebral hemisphere and visuospatial processes to the other was originally formulated and Luria Neuroscience Institute and New York University School of Medicine, New York, USA embraced, virtually nothing was known about the

認知神経科学 Vol. 20 No. 3・4 2018 129 structural, let alone biochemical, differences between and complex social cognition, e.g. humans, the two hemispheres. The tacit assumption (one that great apes, dolphins and elephants5). Asymmetries inexplicably pervades many texts to this have also been found in the C3-C1 synapses in the day) was that from a structural standpoint the two hippocampi6). hemispheres were perfect mirror images of each other, On the biochemical level, asymmetries have been in other words, that they were structurally identical. found in the pattern of catecholamine distribution, That two identical structures could give rise to vastly whereby noradrenergic (NE) pathways are somewhat different functions was a logical oxymoron, but the more prolific in the right hemisphere and dopami- field didn’t seem to be bothered by it and it was nergic (DE) pathways in the left hemisphere7). Fur- universally ignored. thermore, at the molecular level, several asymmetries Today we know that the two cerebral hemispheres were found in the cortical expression of multiple are, in fact, very different both in terms of their genes8). morphology, cellular composition, biochemistry, and These biological asymmetries, discovered in the gene expression. second half of the twentieth and beginning of the

Several gross morphological features are distinctly twenty-first century, provide an ample basis for the asymmetric. The right frontal pole is somewhat functional differences between the two hemispheres, thicker than, and protrudes over, the left frontal pole. thus resolving the logical conundrum mentioned In contrast, the left occipital pole is somewhat thicker before. They do, however, present a different, quite and protrudes over the right occipital pole. In fundamental challenge to the prevailing understanding combination, these two asymmetries are referred to as of hemispheric specialization. The above-mentioned Yakovlevian torque, named after its discoverer, the biological asymmetries between the two hemispheres

Russian-American neuroanatomist Paul Yakovlev are not unique to humans. Most, or possibly even all from Harvard University2). Furthermore, ventro- of them, are shared by humans with a range of other lateral prefrontal regions were found to be larger in the species, both primate and non-primate. The right hemisphere, whereas orbitofrontal and anterior existence of structural, cellular, and biochemical cingulate regions larger in the left hemisphere3). hemispheric differences, which have been conserved Additionally, planum temporale (an aspect of the tem- across multiple species and are similar in kind, forces poral lobe linked in humans to phonological process- a logical conclusion that functional differences ing) is larger in the left than in the right hemisphere ; between the two hemispheres should also be present and so is the frontal operculum, an area in the posterior and that these differences should also be similar in portion of the linked to speech produc- kind. But whatever these differences are, they cannot 4) tion . be captured by the distinction between verbal and non- Notable asymmetries are present also at a cellular verbal functions, since language is unique to humans, level. Spindle cells (also known as von Economo which renders this distinction devoid of any meaning cells) are more prolific in the right than left hemis- outside of our species. Therefore, while the phere. These cells, found in the anterior cingulate privileged relationship between language and the (in cortex as well as in multiple regions of prefrontal most people) left hemisphere is undisputable, it cannot neocortex, are found only in the species with large be regarded as the cornerstone of hemispheric

130 Japanese Journal of specialization but must instead be regarded as a special the subject’s cognitive repertoire. Unlike the “verbal- case of some more fundamental, yet to be revealed nonverbal” distinction, the “novelty-routinization” principle, one that would be meaningful across distinction is applicable to any species capable of multiple species. The challenge then is to identify learning ; and unlike the “analytic-gestalt” distinction this fundamental principle. it can be operationalized with relative ease, even in a

The sense that the “verbal-nonverbal” distinction rodent. fails to capture the whole complexity of hemispheric Curiously, the fundamental importance of the specialization has been simmering in neuropsychology novelty-routinization distinction was rediscovered in for quite some time and alternative dichotomies have the fields of machine learning and artificial intelli- been proposed. The “analytic-gestalt” and “simul- gence (AI), and soon thereafter imported into cognitive taneous-sequential” dichotomies are among those neuroscience, under the name of “exploration- most commonly invoked, but they are difficult to exploitation”10). The introduction of this distinction operationalize even in humans, let alone in members was prompted by the fact that it proved to be very of other species, and they failed to lead to any useful difficult to reconcile in AI devices (particularly in scientific breakthroughs. those based on neural nets) two basic functions : the So what is the fundamental functional difference ability to form new representations, and the ability to between the two hemispheres ? In order to be useful, preserve the previously formed ones and protect them it has to satisfy at least two requirements. It has to be from degradation. As the neural net “acquired” new conceptually meaningful across species, thus permitting knowledge, the previously acquired knowledge was an evolutionary approach to hemispheric speciali za- often degraded. Consequently, it was proposed by tion. It should also be possible to operatio nalize (and several leading neural net modelers that in order to thus falsify) it in multiple species in a sufficiently reconcile these two capabilities, they need to be straightforward and unequivocal way. spatially separated in the net11). It is very likely that evolution resolved this conundrum in the biological II. Cognitive Novelty and Cognitive Routines in neural net, the brain, by endowing it with two hemi- the spheres differentially adept at “exploration” vs. A number of years ago, I proposed that the “exploitation” and that this solution proved so success- fundamental functional differences between the two ful that it has been conserved across multiple species hemispheres rest on the distinction between cognitive in the course of millions of years of evolution (or, novelty and cognitive routines9). This approach came alternatively, independently arrived at in diverse species to be known as the “novelty-routinization” theory of via the mechanism of convergent evolution). hemispheric specialization. According to this theory, Multiple sources of evidence support the “novelty- the left hemisphere is particularly adept at cognitive routinization” theory of hemispheric specialization. processes driven by well-established routines and At the biochemical level, it has been demonstrated that representations, whereas the right hemisphere is the stimulation of the noradrenergic (NE) system, particularly adept at dealing with novel cognitive known to be more prevalent in the right hemisphere, challenges, which cannot be resolved with reliance on triggers exploratory behavior12). In contrast, stimula- the strategies and representations already available in tion of the dopaminergic (DA) system, known to be

認知神経科学 Vol. 20 No. 3・4 2018 131 more prevalent in the left hemisphere, triggers stereo- activation shift as the function of the task ceasing to be typic behavior13). Consistent with these findings, novel and becoming increasingly familiar, has been clinical observations suggest that pharmaco logical demonstrated for multiple cognitive tasks, both visuo- stimulation of the DE system often leads to stereotypic spatial and verbal, both perceptual, motor, and higher- and perseverative behavior in patients suffering from order . Furthermore, this spatio- Parkinson’s disease14). Perseverative behavior has temporal dynamics of learning has been demonstrated also been reported in drug addicts with presumably with multiple neuroimaging methodologies : fMRI, overstimulated DA system15). Further more, the two PET, and EEG18). signature manifestations of Tourette syndrome, tics On the time scale of years to decades, the right-to- and exploratory behaviors, are likely to be differen- left spatio-temporal dynamics of learning has been tially caused by dysregulation in the left vs, right demonstrated in several studies of facial recognition. 16) fronto-striatal systems . Likewise, focal lateralized According to these studies, while obscure unfamiliar prefrontal lesions result differentially in excessively faces are better recognized by the right hemisphere, context-independent (perseverative) response selection heavily familiarized faces of famous individuals are 19) (perseveration) vs. excessive context-dependent (field better recognized by the left hemisphere . In a dependent) response selection17). similar vein, while music is processed more efficiently According to classic neuropsychology, various by the right hemisphere in musically naïve individuals, cognitive functions are linked to particular neural sub- it is processed more efficiently by the left hemisphere strates, ̶brain regions or brain networks (“functional in trained musicians20). systems”), ̶in an inviolate and stable way. In con- Neurodevelopmental evidence points in the same trast, the novelty-routinization theory of hemispheric direction. In patients with intractable infantile specialization predicts that the neural substrates under- seizure disorder, early-life hemispherectomies affect lying a cognitive task change with increasing task subsequent cognitive development in very different mastery, and that this change is unidirectional: from ways. Early left hemispherectomy interferes with the the right hemisphere (often right frontal cortical subsequent development of language but not of visuo- regions) to the left hemisphere (often left posterior spatial functions. In contrast, early right hemisphere- cortical regions). This spatio-temporal cortical ctomy interferes with the subsequent cognitive dynamics of learning appears to be universal and can development of both language and visuo-spatial be demonstrated with diverse cognitive tasks and on functions21). This suggests that the right hemisphere multiple time scales. plays a role at early stages of language acquisition.

On the time scale of minutes to hours, the right-to- The right hemisphere appears to be more physio- left spatio-temporal dynamics of learning has been logically active than the left hemisphere during the demonstrated in several functional neuroimaging first three years of life, after which the hemispheric studies with cognitive activation tasks in healthy activation ratio gradually changes22). This finding is subjects. These studies have consistently revealed a complemented by the observation that in aging the shift of neocortical and hippocampal activation, from right hemisphere becomes subject to atrophy earlier the right hemisphere to the left hemisphere in the than the left hemisphere, presumably because typically course of task familiarization. Such a right-to-left with age our exposure to novelty decreases and

132 Japanese Journal of Cognitive Neuroscience reliance on established cognitive routines increases, suggests that the cortical representations of over- with the left hemisphere being the main beneficiary of learned skilled movements also populate mostly the the activity-mediated protective effects of neuro plasti- left hemisphere. city23). While the right hemisphere has a privileged role in III. Novelty and Routines in Evolution dealing with cognitive novelty, the left hemisphere has One of the main appeals of the novelty-routinization a privileged role as the repository of long-term know- distinction is that, unlike the distinction between ledge and established patterns. This role of the left language and visuo-spatial functions, it is applicable hemisphere incorporates language dominance but is across diverse species and permits placing hemispheric much broader than that : the left hemisphere appears specialization in an evolutionary context. This was 27) to be critical for long-term storage of both verbal and first proposed by Goldberg and Costa and further non-verbal information. Indeed, left-lateralized elaborated upon and expanded by MacNeilage, damage to the ventral brain stem (VTA and surround- Rogers, and Vallortigara28). What is the evidence that ing areas) may result in severe for processing the novel and the familiar is differentially 24) both verbal and non-verbal information . Likewise, linked to the two hemispheres throughout evolution? left hemispherectomy in adult patients with intractable In fact, such evidence abounds across multiple species. temporal-lobe seizure disorder may result in massive Differential linkage of the two hemispheres to cog- retrograde amnesia for both verbal and visuo-spatial nitive novelty vs. cognitive routines has been demon- information, whereas right hemi sphere ctomy will strated in several mammalian species. When baboons result in neither25). were required to discriminate complex geometric Most notably, associative agnosias, both visual, forms varying in the degree of novelty vs. familiarity, auditory and tactile, can be caused by damage to the it turned out that in the beginning of the experiment posterior aspects of the left but not right hemisphere26). performance was faster for the stimuli presented to the Associative agnosias include visual object agnosia, baboon’s left visual field (i.e. the right hemisphere) auditory associative agnosia, and pure astereognosis. and toward the end of the experiment performance was In all of them the ability to recognize unique exemplars better for the stimuli presented to the right visual field as members of generic object categories is impaired in (i.e. the left hemisphere)29). The performance pattern the absence of more basic sensory impairment. in baboons is a virtually exact replica of the perfor- Degradation of the cortical representation capturing mance pattern demonstrated in humans with similar the essential perceptual attributes, which are shared by experimental paradigms using tachistoscopic stimulus the members of a particular class of objects, is presentations. In several dolphin species (in which presumed to be the mechanism of associative agnosias. the visual pathways are also crossed) exploratory The fact that this class of agnosias can be caused by behavior involves predominantly the left eye (i.e. the unilateral damage to the left but not right hemisphere right hemisphere)30). suggests that the affected cortical representations The differential affiliation of novelty vs. routines populate mostly the left hemisphere. In a similar with the right vs. left halves of the brain is not limited vein, ideational and ideomotor apraxias can be caused to the mammalian species. It is present also in the by damage to the left but not right hemisphere. This avians, whose brain represents a very different type of

認知神経科学 Vol. 20 No. 3・4 2018 133 neural architecture, one dominated by nuclei rather cognitive functions to specific neural structures, or than cortex. These basic architectural differences constellations of neural structures, in a permanent and notwithstanding, the avian brain is also lateralized in inviolate way. In contrast, the novelty-routinization ways functionally similar to those documented in the theory implies a dynamic change of the underlying mammalian species. In a zebra finch exposure to a neural substrates in the process of cognitive skill familiar, previously learned song activates the left acquisition. It dictates that when an individual hemisphere and exposure to an unfamiliar song encounters a novel cognitive challenge, there is a activates the right hemisphere31). The more a visual predominant involvement of the right hemisphere. or auditory pattern is rewarded, the more it activates With an increasing skill mastery, the let hemispheric the left hemisphere32). contribution becomes progressively more prominent

The universality of the novelty-routinization division while reliance on the right hemisphere decreases. of labor between the two sides of the nervous system Instead of a static association between structure and extends even further. Evidence exists that a similar function, we must consider spatio-temporal vectors of division of labor exists between the two halves of the learning. Adopting the spatio-temporal vector as the invertebrate (uncrossed) nervous system. In a honey- conceptual framework for understanding brain- bee, recall of a recently encountered olfactory stimulus behavior relations is ripe with multiple and profound requires the integrity of the right antenna, whereas implications. recall of an olfactory stimulus learned in a more dis- This conceptual framework implies a much greater 33) tant past requires the integrity of the left antenna . inter- and intrapersonal differences in the neural The conclusion is inescapable that the differential substrates of cognition than those postulated in affiliation of dealing with cognitive novelty vs. reli- traditional neuropsychology. Indeed, two individuals ance on previously formed representations with the may have vastly different degrees of mastery of the two halves of the brain is a fundamental and pervasive “same” cognitive skills, which will be reflected in dif- feature throughout evolution across taxonomic catego- ferent degrees of their reliance on the right vs. left ries and neural architectures. While it is unclear hemispheres. And even in the same individual such whether this universality represents a conservation of a relative reliance will change as a function of learning very early (and very successful) attribute of the central over time. These considerations should radically nervous system, or whether it is a product of convergent affect the way findings are interpreted in clinical evolution, the neuroanatomical separation of the neural neuropsychology. substrates of novelty vs routines is both pervasive and The implications of this framework for cognitive fundamental in evolution. neuroscience research, particularly for functional neuroimaging, are no less profound. Functional

IV. Novelty, Routines, and Spatio-Temporal neuroimaging has become one of the main research Dynamics of Learning tools of neuropsychology and cognitive neuroscience. Linking cognitive novelty and cognitive routines to It has qualitatively changed the very nature of the way different neural substrates has major implications for the relationship between the brain and cognition is the understanding of how cognition is mapped into the investigated. Two broad research methodologies brain. Traditional neuropsychology has linked specific have been developed, both relying on functional

134 Japanese Journal of Cognitive Neuroscience neuro̶imaging, particularly on functional MRI sampled from the same underlying neural network. (fMRI) : those involving cognitive activation and Such an assumption is often implicit in the design of those involv ing resting states. We will consider the functional neuroimaging protocols, and it reflects the impli cations of the novelty-routinization theory for the traditional premise of classical neuropsychology of a way data are analyzed and interpreted within each of fixed relationship between the cognitive task and the these methodologies. underlying neural substrates. But as the foregoing Cognitive activation fMRI. The ability to scan discussion has demonstrated, unless the subject had the brain of the subject while he/she is engaged in a been very extensively pre-trained on the task before cognitive task in real time has radically changed the scanning, this assumption is false. Far from being very nature of cognitive neuroscience. In the past, all fixed and static, the underlying neural network and/or our knowledge regarding the neural substrates of relative degree of involvement of its components cognitive processes rested solely on the inference from changes in the course of task familiarization and learn- the effects of brain lesions on cognition ; in other ing. Under these circumstances, averaging across the words the knowledge of normal brain-behavior whole experimental sequence is almost certain to lead relations was indirect, since it rested exclusively on to spurious findings and interpretations. Imagine a the studies of brain pathology. The advent of simple analogy. When he was a little boy, Johnny functional neuroimaging made direct investigation of weighed 50 kg ; as an adult he weighed 100 kg ; and normal brain-behavior relations possible. as he aged his weight came down to 60 kg. So you In cognitive activation, the subject is engaged in a add the three values, divide the sum by 3 and conclude specific cognitive task while in the scanner. Whereas that Johnny’s weight is 70 kg. This sounds perfectly several specific methodologies exist, in the most plausible, yet in reality is meaningless and misleading. commonly used methodology the activation pattern A case ca be made that this is what happens with the (the BOLD signal measuring the excess level of interpretation of numerous functional neuroimaging oxygenated blood) under the critical task condition is studies bases on the cognitive activation paradigm. compared to the activation pattern under a baseline To capture the dynamic nature of most cognitive condition (either a separate, so-called “subtraction” processes, a different approach to data processing is cognitive task ; or contrast conditions built into the required. According to this approach, the experi- task itself, so-called “factorial” or “parametric” mental sequence should be divided into segments and designs ; or some other methodology) for multiple averaging should be conducted separately for each 34) locations throughout the brain . Typically, the segment. Then instead of a static data set, a spatio- activation patterns are averaged across all the data temporal vector can be generated, which will capture points throughout the whole duration of experiment. the dynamics of neural activation in the course of the One would think that this approach is perfectly cognitive task performance with much greater understandable, since it maximizes the number of data precision and accuracy. The research methodology points, and by so doing increases the signal-to-noise based on such spatio-temporal vectors has the discriminability of the task-specific activation pattern. potential for offering more powerful insights into the However, averaging across the whole sequence is only nature of normal cognition, as well as for providing a legitimate if one can assume that all the data points are more refined approach to characterizing impaired

認知神経科学 Vol. 20 No. 3・4 2018 135 cognition across a wide range of neurological and Therefore, any meaningful interpretation of DMN neuropsychiatric disorders. data hinges on the ability to disentangle these two very Resting state fMRI. For a long time the commonly different sources of neural activity. To this end, embraced assumption had been that in order to gener- several approaches can me adopted : (a) Subjects are ate any meaningful and interpretable fMRI data set, pre-exposed to the scanning environment before the activation with a specific cognitive task was necessary. scanning ; (b) Scanning is conducted twice and only As the argument went, without such a task random those components of activation which are present in activity would be recorded, rendering any meaningful both recordings is assumed to represent internally interpretation impossible. These assumptions were generated processes ; (c) The early portion of the challenged and overturned a few decades ago with the scanned sequence is removed from the analysis. discovery of the Default Mode Network (DMN)35). Far from being random, this network, or rather a class V. Conclusions of networks, is characterized by a distinct architecture. Hemispheric specialization has been among the

DMN represents activity of the brain when it is not central, most heavily researched topics in brain- engaged in any specific, externally imposed task. behavior relations. For many years, understanding of The assumption is that in such states the brain is hemispheric specialization has been guided by the engaged in non-random internally generated activities notion linking language to the left hemisphere and

“of its own choosing,” such as rumination, introspec- visuo-spatial processes to the right hemisphere. tion, etc. This has generated considerable interest in While research methodology employed by cognitive DMN as a window into the neural mechanisms of the neuroscientists and neuropsychologists has progressed inner life of the brain. from low-tech paper-and-pencil techniques to However, the assumption that DMN reflects exclu- functional neuroimaging and genetics, no comparable sively, or even predominantly, the inner mental life of conceptual breakthroughs have taken place in laterality an individual free from external intrusions, can be eas- research and the underlying theoretical assumptions ily challenged. The absence of a specific, externally for understanding hemispheric specialization remained imposed task does not, in and of itself, guarantee the unchanged to a degree bordering on fossilized. As a absence of external influences. Quite the contrary, it result, while the body of research findings regarding can be argued that the mere novelty of the scanning hemispheric specialization has continued to grow over environment, into which most subjects are immersed the last few decades, its heuristic impact remained without any prior exposure, will exert a significant limited and it failed to generate significant new impact on their neural activities. Contrary to the insights. Here I attempted to outline a radically commonly made assumption that DMN is an accurate different conceptual framework for understanding reflection of the internally generated cognition, DMN hemispheric specialization across evolution and is likely to be a composite of two very different considered its multiple theoretical, clinical, and processes : response to the novelty of the scanning methodological implications. Hopefully, this new environment and internally generated processes. framework will generate original and interesting Placed in this context, only the latter component will research resulting in important insights into the mecha- be regarded as representing the true DMN. nisms of both normal and disordered cognition.

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