Visual spatial localization conflict: an fMRI study

Neural correlates of the sound facilitation effect in the modified Simon task in older adults

Introduction

The ability to resolve interference declines with age and is attributed to neurodegeneration and reduced cognitive function and mental alertness in older adults. Our previous study revealed that task-irrelevant but environmentally meaningful sounds improve performance on the modified Simon task in older adults. However, little is known about neural correlates of this sound facilitation effect.

Methods

Twenty right-handed older adults [mean age = 72 (SD = 4), 11 female] participated in the fMRI study. They performed the modified Simon task in which the arrows were presented either in the locations matching the arrow direction (congruent trials) or in the locations mismatching the arrow direction (incongruent trials). A total of 50% of all trials were accompanied by task-irrelevant but environmentally meaningful sounds.

Results

Participants were faster on the trials with concurrent sounds, independently of whether trials were congruent or incongruent. The sound effect was associated with activation in the distributed network of auditory, posterior parietal, frontal, and limbic brain regions. The magnitude of the behavioral facilitation effect due to sound was associated with the changes in activation of the bilateral auditory cortex, cuneal cortex, and occipital fusiform gyrus, precuneus, left superior parietal lobule (SPL) for No Sound vs. Sound trials. These changes were associated with the corresponding changes in reaction time (RT). Older adults with a recent history of falls showed greater activation in the left SPL than those without falls history.

Conclusion

Our findings are consistent with the dedifferentiation hypothesis of cognitive aging. The facilitatory effect of sound could be achieved through recruitment of excessive neural resources, which allows older adults to increase attention and mental alertness during task performance. Considering that the SPL is critical for integration of multisensory information, individuals with slower task responses and those with a history of falls may need to recruit this region more actively than individuals with faster responses and those without a fall history to overcome increased difficulty with interference resolution. Future studies should examine the relationship among activation in the SPL, the effect of sound, and falls history in the individuals who are at heightened risk of falls.

Effect of Second Language Proficiency on Inhibitory Control in the Simon Task: An fMRI Study

How learning a second language (L2) changes our brain has been an important question in neuroscience. Previous neuroimaging studies with different ages and language pairs spoken by bilinguals have consistently shown plastic changes in brain systems supporting executive control. One hypothesis posits that L2 experience-induced neural changes supporting cognitive control, which is responsible for the selection of a target language and minimization of interference from a non-target language. However, it remains poorly understood as to whether such cognitive advantage is reflected as stronger controlled processing or increased automatic inhibition processing. In this study, using functional MRI we scanned 27 Chinese-English late bilinguals while they performed a Simon task. Results showed that bilinguals with higher L2 vocabulary proficiency performed better in the Simon task, and more importantly, higher L2 vocabulary proficiency was associated with weaker activation of brain regions that support more general cognitive control, including the right anterior cingulate cortex, left insula and left superior temporal gyrus. These results suggest that L2 experience may lead to a more automatic and efficient processing in the inhibitory control task. Our finding provides an insight into neural activity changes associated with inhibitory control as a function of L2 proficiency.

The neurocognitive underpinnings of the Simon effect: An integrative review of current research

For as long as half a century the Simon task - in which participants respond to a nonspatial stimulus feature while ignoring its position - has represented a very popular tool to study a variety of cognitive functions, such as attention, cognitive control, and response preparation processes. In particular, the task generates two theoretically interesting effects: the Simon effect proper and the sequential modulations of this effect. In the present study, we review the main theoretical explanations of both kinds of effects and the available neuroscientific studies that investigated the neural underpinnings of the cognitive processes underlying the Simon effect proper and its sequential modulation using electroencephalogram (EEG) and event-related brain potentials (ERP), transcranial magnetic stimulation (TMS), and functional magnetic resonance imaging (fMRI). Then, we relate the neurophysiological findings to the main theoretical accounts and evaluate their validity and empirical plausibility, including general implications related to processing interference and cognitive control. Overall, neurophysiological research supports claims that stimulus location triggers the creation of a spatial code, which activates a spatially compatible response that, in incompatible conditions, interferes with the response based on the task instructions. Integration of stimulus-response features plays a major role in the occurrence of the Simon effect (which is manifested in the selection of the response) and its modulation by sequential congruency effects. Additional neural mechanisms are involved in supporting the correct and inhibiting the incorrect response.

The Simon Effect Based on Allocentric and Egocentric Reference Frame: Common and Specific Neural Correlates

An object’s location can be represented either relative to an observer’s body effectors (egocentric reference frame) or relative to another external object (allocentric reference frame). In non-spatial tasks, an object’s task-irrelevant egocentric position conflicts with the side of a task-relevant manual response, which defines the classical Simon effect. Growing evidence suggests that the Simon effect occurs not only based on conflicting positions within the egocentric but also within the allocentric reference frame. Although neural mechanisms underlying the egocentric Simon effect have been extensively researched, neural mechanisms underlying the allocentric Simon effect and their potential interaction with those underlying its egocentric variant remain to be explored. In this fMRI study, spatial congruency between the task-irrelevant egocentric and allocentric target positions and the task-relevant response hand was orthogonally manipulated. Behaviorally, a significant Simon effect was observed for both reference frames. Neurally, three sub-regions in the frontoparietal network were involved in different aspects of the Simon effect, depending on the source of the task-irrelevant object locations. The right precentral gyrus, extending to the right SMA, was generally activated by Simon conflicts, irrespective of the spatial reference frame involved, and showed no additive activity to Simon conflicts. In contrast, the right postcentral gyrus was specifically involved in Simon conflicts induced by task-irrelevant allocentric, rather than egocentric, representations. Furthermore, a right lateral frontoparietal network showed increased neural activity whenever the egocentric and allocentric target locations were incongruent, indicating its functional role as a mismatch detector that monitors the discrepancy concerning allocentric and egocentric object locations.

Dissociating the neural substrates for inhibition and shifting in domain-general cognitive control

Inhibition and shifting are two key components of domain-general cognitive control. Numerous studies have investigated the neural substrates of both components, but it is still unclear whether the relevant brain regions are specifically involved in one specific component or commonly engaged in both components. Here, we addressed this question by using functional magnetic resonance imaging and a modified saccade paradigm that was effective to disentangle inhibition and shifting in one experiment. The results showed that both the middle frontal gyrus and left parietal lobe were involved in both components but the middle frontal gyrus was more active for the inhibition while the inferior parietal lobe was more active for the shifting processing. The outcome suggests that, although both regions are engaged in inhibition and shifting, each plays a dominant role in one component. These findings provide a further insight into the neural dissociation in inhibition and shifting, as well as a better explanation on the framework of unity and diversity from a neuropsychological viewpoint.

Differential recruitment of executive control regions during phonological competition in monolinguals and bilinguals

Behavioral research suggests that monolinguals and bilinguals differ in how they manage within-language phonological competition when listening to language. The current study explored whether bilingual experience might also change the neural resources recruited to control spoken-word competition. Seventeen Spanish-English bilinguals and eighteen English monolinguals completed an fMRI task in which they searched for a picture representing an aurally presented word (e.g., "candy") from an array of four presented images. On competitor trials, one of the objects in the display shared initial phonological overlap with the target (e.g., candle). While both groups experienced competition and responded more slowly on competitor trials than on unrelated trials, fMRI data suggest that monolinguals, but not bilinguals, activated executive control regions (e.g., anterior cingulate, superior frontal gyrus) during within-language phonological competition. We conclude that differences in how monolinguals and bilinguals manage competition may result from bilinguals' more efficient deployment of neural resources.

Three key regions for supervisory attentional control: evidence from neuroimaging meta-analyses

The supervisory attentional system has been proposed to mediate non-routine, goal-oriented behaviour by guiding the selection and maintenance of the goal-relevant task schema. Here, we aimed to delineate the brain regions that mediate these high-level control processes via neuroimaging meta-analysis. In particular, we investigated the core neural correlates of a wide range of tasks requiring supervisory control for the suppression of a routine action in favour of another, non-routine one. Our sample comprised n=173 experiments employing go/no-go, stop-signal, Stroop or spatial interference tasks. Consistent convergence across all four paradigm classes was restricted to right anterior insula and inferior frontal junction, with anterior midcingulate cortex and pre-supplementary motor area being consistently involved in all but the go/no-go task. Taken together with lesion studies in patients, our findings suggest that the controlled activation and maintenance of adequate task schemata relies, across paradigms, on a right-dominant midcingulo-insular-inferior frontal core network. This also implies that the role of other prefrontal and parietal regions may be less domain-general than previously thought.

Behavioral and neural interaction between spatial inhibition of return and the Simon effect

It has been well documented that the anatomically independent attention networks in the human brain interact functionally to achieve goal-directed behaviors. By combining spatial inhibition of return (IOR) which implicates the orienting network with some executive function tasks (e.g., the Stroop and the flanker tasks) which implicate the executive network, researchers consistently found that the interference effects are significantly reduced at cued compared to uncued locations, indicating the functional interaction between the two attention networks. However, a unique, but consistent effect is observed when spatial IOR is combined with the Simon effect: the Simon effect is significantly larger at the cued than uncued locations. To investigate the neural substrates underlying this phenomenon, we orthogonally combined the spatial IOR with the Simon effect in the present event-related fMRI study. Our behavioral data replicated previous results by showing larger Simon effect at the cued location. At the neural level, we found shared spatial representation system between spatial IOR and the Simon effect in bilateral posterior parietal cortex (PPC); spatial IOR specifically activated bilateral superior parietal cortex while the Simon effect specifically activated bilateral middle frontal cortex. Moreover, left precentral gyrus was involved in the neural interaction between spatial IOR and the Simon effect by showing significantly higher neural activity in the "Cued_Congruent" condition. Taken together, our results suggest that due to the shared spatial representation system in the PPC, responses were significantly facilitated when spatial IOR and the Simon effect relied on the same spatial representations, i.e., in the "Cued_Congruent" condition. Correspondingly, the sensorimotor system was significantly involved in the "Cued_Congruent" condition to fasten the responses, which indirectly resulted in the enhanced Simon effect at the cued location.

Meta-analytic evidence for a superordinate cognitive control network subserving diverse executive functions

Classic cognitive theory conceptualizes executive functions as involving multiple specific domains, including initiation, inhibition, working memory, flexibility, planning, and vigilance. Lesion and neuroimaging experiments over the past two decades have suggested that both common and unique processes contribute to executive functions during higher cognition. It has been suggested that a superordinate fronto-cingulo-parietal network supporting cognitive control may also underlie a range of distinct executive functions. To test this hypothesis in the largest sample to date, we used quantitative meta-analytic methods to analyze 193 functional neuroimaging studies of 2,832 healthy individuals, ages 18-60, in which performance on executive function measures was contrasted with an active control condition. A common pattern of activation was observed in the prefrontal, dorsal anterior cingulate, and parietal cortices across executive function domains, supporting the idea that executive functions are supported by a superordinate cognitive control network. However, domain-specific analyses showed some variation in the recruitment of anterior prefrontal cortex, anterior and midcingulate regions, and unique subcortical regions such as the basal ganglia and cerebellum. These results are consistent with the existence of a superordinate cognitive control network in the brain, involving dorsolateral prefrontal, anterior cingulate, and parietal cortices, that supports a broad range of executive functions.

A psychobiological perspective on working memory performance at 8 months of age

Fifty 8-month-old infants participated in a study of the interrelations among cognition, temperament, and electrophysiology. Better performance on a working memory task (assessed using a looking version of the A-not-B task) was associated with increases in frontal-parietal EEG coherence from baseline to task, as well as elevated levels of frontal-occipital coherence during both baseline and task. Enhanced performance was also associated with decreased heart period (increased heart rate) from baseline to task. Infants with better working memory performance had parents who rated them high on activity level and distress to limitations. When considered collectively, EEG coherence and heart period contributed unique variance in the prediction of high and low performance groups. Implications for the study of infant cognition are discussed.

Examining a supramodal network for conflict processing: a systematic review and novel functional magnetic resonance imaging data for related visual and auditory stroop tasks

Cognitive control over conflicting information has been studied extensively using tasks such as the color-word Stroop, flanker, and spatial conflict task. Neuroimaging studies typically identify a fronto-parietal network engaged in conflict processing, but numerous additional regions are also reported. Ascribing putative functional roles to these regions is problematic because some may have less to do with conflict processing per se, but could be engaged in specific processes related to the chosen stimulus modality, stimulus feature, or type of conflict task. In addition, some studies contrast activation on incongruent and congruent trials, even though a neutral baseline is needed to separate the effect of inhibition from that of facilitation. In the first part of this article, we report a systematic review of 34 neuroimaging publications, which reveals that conflict-related activity is reliably reported in the anterior cingulate cortex and bilaterally in the lateral prefrontal cortex, the anterior insula, and the parietal lobe. In the second part, we further explore these candidate "conflict" regions through a novel functional magnetic resonance imaging experiment, in which the same group of subjects perform related visual and auditory Stroop tasks. By carefully controlling for the same task (Stroop), the same to-be-ignored stimulus dimension (word meaning), and by separating out inhibitory processes from those of facilitation, we attempt to minimize the potential differences between the two tasks. The results provide converging evidence that the regions identified by the systematic review are reliably engaged in conflict processing. Despite carefully matching the Stroop tasks, some regions of differential activity remained, particularly in the parietal cortex. We discuss some of the task-specific processes which might account for this finding.

At the edge of consciousness: automatic motor activation and voluntary control

Conventionally, voluntary conscious acts and automatic behavior have been considered to be mediated by separate processes-and by separate brain structures. In this review, the authors consider the evidence that this might not be the case. First, they draw together disparate lines of evidence showing that visual stimuli cause automatic and unconscious motor activation. They briefly discuss the visual grasp reflex (automatic orienting of gaze to a salient visual stimulus), subliminal priming, and object affordances in healthy individuals. They also consider cases where inhibition of such reflexive behavior may be disrupted following brain lesions, as in patients demonstrating alien limb syndrome and utilization behavior. The authors argue that automatic motor activation forms an intrinsic part of all behavior, rather than being categorically different from voluntary actions. A crucial issue is how such automatic mechanisms are controlled so that the most appropriate responses are made and unwanted responses inhibited. The authors discuss some of the brain areas involved, including the supplementary motor area and the parietal cortex. Last, they review evidence that some control may actually be achieved by automatically triggered inhibition as well as modulation of unconscious processes by attention and task goals.

Human intraparietal sulcus (IPS) and competition between exogenous and endogenous saccade plans

How are stimulus-driven reflexes generated, and what controls their competition with voluntary action? The saccadic reflex to look towards an abrupt visual onset (prosaccade) has been associated with the retinotectal and magnocellular pathways, which rapidly convey signals to the superior colliculus and cortical eye fields. Such stimulus-driven reflexes need to be suppressed when making an eye movement in the opposite direction (antisaccade), resulting in a cost in saccade latency. We compared the latencies of pro- and anti-saccades elicited by conventional luminance stimuli with those evoked by stimuli visible only to short-wave-sensitive cones (S cones) embedded in dynamic luminance noise. Critically, the retinotectal and magnocellular pathways are functionally blind to such stimuli. Compared to luminance stimuli, antisaccade latency costs were significantly reduced for 'S-cone' stimuli. This behavioural interaction is consistent with reduced competition between reflexive and endogenous saccade plans when S-cone stimuli are employed, while other processes involved in making an antisaccade, such as changing preparatory set or generating an endogenous saccade, are predicted to be equivalent for each kind of stimulus. Using fMRI, we found that activity in the right intraparietal sulcus (IPS) mirrored the behavioural interaction in saccade latencies. Thus, the right IPS appears to index the degree of competition between exogenous and endogenous saccade plans, showing the activity pattern predicted for an area involved in suppressing the saccade reflex. Furthermore, signals recorded from the superior colliculus showed the reverse pattern of responses, consistent with a direct inhibitory influence of IPS on SC.

Interference resolution: insights from a meta-analysis of neuroimaging tasks

A quantitative meta-analysis was performed on 47 neuroimaging studies involving tasks purported to require the resolution of interference. The tasks included the Stroop, flanker, go/no-go, stimulus-response compatibility, Simon, and stop signal tasks. Peak density-based analyses of these combined tasks reveal that the anterior cingulate cortex, dorsolateral prefrontal cortex, inferior frontal gyrus, posterior parietal cortex, and anterior insula may be important sites for the detection and/or resolution of interference. Individual task analyses reveal differential patterns of activation among the tasks. We propose that the drawing of distinctions among the processing stages at which interference may be resolved may explain regional activation differences. Our analyses suggest that resolution processes acting upon stimulus encoding, response selection, and response execution may recruit different neural regions.

Controlling emotional expression: behavioral and neural correlates of nonimitative emotional responses

Emotional facial expressions can engender similar expressions in others. However, adaptive social and motivational behavior can require individuals to suppress, conceal, or override prepotent imitative responses. We predicted, in line with a theory of "emotion contagion," that when viewing a facial expression, expressing a different emotion would manifest as behavioral conflict and interference. We employed facial electromyography (EMG) and functional magnetic resonance imaging (fMRI) to investigate brain activity related to this emotion expression interference (EEI) effect, where the expressed response was either concordant or discordant with the observed emotion. The Simon task was included as a nonemotional comparison for the fMRI study. Facilitation and interference effects were observed in the latency of facial EMG responses. Neuroimaging revealed activation of distributed brain regions including anterior right inferior frontal gyrus (brain area [BA] 47), supplementary motor area (facial area), posterior superior temporal sulcus (STS), and right anterior insula during emotion expression-associated interference. In contrast, nonemotional response conflict (Simon task) engaged a distinct frontostriatal network. Individual differences in empathy and emotion regulatory tendency predicted the magnitude of EEI-evoked regional activity with BA 47 and STS. Our findings point to these regions as providing a putative neural substrate underpinning a crucial adaptive aspect of social/emotional behavior.

Development of cognitive control and executive functions from 4 to 13 years: evidence from manipulations of memory, inhibition, and task switching

Predictions concerning development, interrelations, and possible independence of working memory, inhibition, and cognitive flexibility were tested in 325 participants (roughly 30 per age from 4 to 13 years and young adults; 50% female). All were tested on the same computerized battery, designed to manipulate memory and inhibition independently and together, in steady state (single-task blocks) and during task-switching, and to be appropriate over the lifespan and for neuroimaging (fMRI). This is one of the first studies, in children or adults, to explore: (a) how memory requirements interact with spatial compatibility and (b) spatial incompatibility effects both with stimulus-specific rules (Simon task) and with higher-level, conceptual rules. Even the youngest children could hold information in mind, inhibit a dominant response, and combine those as long as the inhibition required was steady-state and the rules remained constant. Cognitive flexibility (switching between rules), even with memory demands minimized, showed a longer developmental progression, with 13-year-olds still not at adult levels. Effects elicited only in Mixed blocks with adults were found in young children even in single-task blocks; while young children could exercise inhibition in steady state it exacted a cost not seen in adults, who (unlike young children) seemed to re-set their default response when inhibition of the same tendency was required throughout a block. The costs associated with manipulations of inhibition were greater in young children while the costs associated with increasing memory demands were greater in adults. Effects seen only in RT in adults were seen primarily in accuracy in young children. Adults slowed down on difficult trials to preserve accuracy; but the youngest children were impulsive; their RT remained more constant but at an accuracy cost on difficult trials. Contrary to our predictions of independence between memory and inhibition, when matched for difficulty RT correlations between these were as high as 0.8, although accuracy correlations were less than half that. Spatial incompatibility effects and global and local switch costs were evident in children and adults, differing only in size. Other effects (e.g., asymmetric switch costs and the interaction of switching rules and switching response-sites) differed fundamentally over age.

Differential prefrontal cortex activation during inhibitory control in adolescents with and without childhood attention-deficit/hyperactivity disorder

The authors examined inhibitory control processes in 8 adolescents diagnosed with attention-deficit/ hyperactivity disorder (ADHD) during childhood and in 8 adolescent control participants using functional MRI with the Stimulus and Response Conflict Tasks (K. W. Nassauer & J. M. Halperin, 2003). No group differences in performance were evident on measures of interference control and/or response competition created by location and direction stimuli. However, the ADHD group demonstrated significantly greater activation of the left ventrolateral prefrontal cortex during interference control as well as greater activation of the left anterior cingulate cortex, right ventrolateral prefrontal cortex, and left basal ganglia during the dual task of interference control and response competition. The magnitude of the prefrontal and basal ganglia activation was positively correlated with severity of ADHD. Response competition alone did not yield group differences in activation.

Effect of bilingualism on cognitive control in the Simon task: evidence from MEG

The present study used magneto-encephalography (MEG) to determine the neural correlates of the bilingual advantage previously reported for behavioral measures in conflict tasks. Bilingual Cantonese-English, bilingual French-English, and monolingual English speakers, performed the Simon task in the MEG. Reaction times were faster for congruent than for incongruent trials, and the Cantonese group was faster than the other two groups, which did not differ from each other. Analyses of the MEG data using synthetic aperture magnetometry (SAM) and partial last squares (PLS) showed that the same pattern of activity, involving signal changes in left and medial prefrontal areas, characterized all three groups. Correlations between activated regions and reaction times, however, showed that the two bilingual groups demonstrated faster reaction times with greater activity in superior and middle temporal, cingulate, and superior and inferior frontal regions, largely in the left hemisphere. The monolinguals demonstrated faster reaction times with activation in middle frontal regions. The interpretation is that the management of two language systems led to systematic changes in frontal executive functions.

Physiological basis and image processing in functional magnetic resonance imaging: neuronal and motor activity in brain

Functional magnetic resonance imaging (fMRI) is recently developing as imaging modality used for mapping hemodynamics of neuronal and motor event related tissue blood oxygen level dependence (BOLD) in terms of brain activation. Image processing is performed by segmentation and registration methods. Segmentation algorithms provide brain surface-based analysis, automated anatomical labeling of cortical fields in magnetic resonance data sets based on oxygen metabolic state. Registration algorithms provide geometric features using two or more imaging modalities to assure clinically useful neuronal and motor information of brain activation. This review article summarizes the physiological basis of fMRI signal, its origin, contrast enhancement, physical factors, anatomical labeling by segmentation, registration approaches with examples of visual and motor activity in brain. Latest developments are reviewed for clinical applications of fMRI along with other different neurophysiological and imaging modalities.

The human mind: building bridges between neuroscience and psychiatry

This essay proposes the existence of four unique behavioral characteristics that distinguish Homo sapiens from its nearest evolutionary kin, the great apes. These are inventiveness, capacity for language, curiosity, and self-reflection or self-analysis. Some would counterargue that none of these features are "unique" to humans. Examples would be given of animal "intelligence" involving at times surprising problem-solving abilities. Even "conversations" between humans and parrots might be cited. However, this is to misunderstand what the focus is in the article. This is not a thesis on the continuity or discontinuities of evolution. Every well-trained biologist learns about evolution and understands that the most complex human traits have some hint of related traits in nearest relatives. Rather, the emphasis of this article has been to select and emphasize certain distinctive aspects of human behavior that are the most important features distinguishing human "uniqueness." Uniqueness may be quantified by the number and complexity of processes within the four attributes presented in this article. While this may be an interesting mathematical exercise, the author would rather take it for granted that the outcome would be that humans are significantly more complex in each of the four features referred to than the great apes. For those in the mental health field, one would hope that this overview of the most highly evolved systems in human brain may provide a useful framework where creative therapeutic processes can be applied to the ultimate beneficiary, the client or patient. This article proposes that psychosis can be considered the impossibility of conflict resolution among the special neuronal assemblies that separately mediate feelings of attachment versus abandonment, security versus anxiety, calmness versus anger, fulfillment versus helplessness, and satisfaction of sexual needs. Given this model, the role of the therapist is to consider whether the client is in touch with the various equilibrium points in their current life situation or crisis. A questionnaire designed to help access information on these internal states may work better than an oral interview because it forces the client to reflect. How much conflict does the client feel in sorting out personal priorities? Does the client have any practice at resolving internal conflicts? If unpracticed, is the client likely to act out in destructive ways to self or others to resolve the pressure of internal conflicts? I am sure that we can all think of current well-publicized destructive acts in which one has to wonder precisely what model of mind the psychotherapists were using to assist the client. At a higher level, each human has the potential to be creative. Whether expressed as curiosity about the universe and nature or inventiveness, as in improving the quality of one's life or by the use of gifted language as in art and music, the therapist needs to determine how to assist the client into these kinds of self-defined fulfillments, which one hopes will reduce the other tensions of conflict resolution. There is no doubt that the journey for both the client and therapist will be challenging. Ultimately, the journey is the reward for both the client and therapist. In closing, the following quotation seems appropriate. Sydney Brenner (2002), a pioneer in molecular biology, reviewed a current book on the Science, Politics, and Ethics of the Human Genome Project (Sulston and Ferry 2002). He states, "What I found interesting in the account is that Sulston doesn't tell us anything about the genomes he has sequenced. What did he find there that excited him? What did he learn about genes, about life, about evolution, about worlds to come? It is the play of Hamlet without Hamlet" (p. 794).