Neural overlap in processing music and speech

Neural overlap in processing music and speech, as measured by the co-activation of brain regions in neuroimaging studies, may suggest that parts of the neural circuitries established for language may have been recycled during evolution for musicality, or vice versa that musicality served as a springboard for language emergence. Such a perspective has important implications for several topics of general interest besides evolutionary origins. For instance, neural overlap is an important premise for the possibility of music training to influence language acquisition and literacy. However, neural overlap in processing music and speech does not entail sharing neural circuitries. Neural separability between music and speech may occur in overlapping brain regions. In this paper, we review the evidence and outline the issues faced in interpreting such neural data, and argue that converging evidence from several methodologies is needed before neural overlap is taken as evidence of sharing.     

Form and content: dissociating syntax and semantics in sentence comprehension

The distinction between syntax (sentence form) and semantics (sentence meaning) is fundamental to our thinking about language. Whether and where this distinction is represented at the neural level is still a matter of considerable debate. In the present fMRI study, we examined the neural correlates of syntactic and semantic functions using an innovative activation paradigm specifically designed to unequivocally disentangle syntactic from lexicosemantic aspects of sentence processing. Our findings strongly indicate that a part of Broca's area (BA 44, pars opercularis) is critically implicated in processing syntactic information, whereas the lower portion of the left inferior frontal gyrus (BA 47, pars orbitalis) is selectively involved in processing the semantic aspects of a sentence.     

Neuroimaging of syntax and syntactic processing

Recent results challenge and refine the prevailing view of the way language is represented in the human brain. Syntactic knowledge and processing mechanisms that implement syntax in use are mapped onto neural tissue in experiments that harness both syntactic concepts and imaging technologies to the study of brain mechanisms in healthy and impaired populations. In the emerging picture, syntax is neurologically segregated, and its component parts are housed in several distinct cerebral loci that extend beyond the traditional ones - Broca's and Wernicke's regions in the left hemisphere. In particular, the new brain map for syntax implicates portions of the right cerebral hemisphere.     

Sexual selection for syntax and kin selection for semantics: problems and prospects

The evolution of human language, and the kind of thought the communication of which requires it, raises considerable explanatory challenges. These systems of representation constitute a radical discontinuity in the natural world. Even species closely related to our own appear incapable of either thought or talk with the recursive structure, generalized systematicity, and task-domain neutrality that characterize human talk and the thought it expresses. W. Tecumseh Fitch’s proposal (2004, in press) that human language is descended from a sexually selected, prosodic proto-language that approximated its syntactic complexity, and later acquired semantics thanks to kin selection for its use as a means of pedagogical transmission, has the promise of meeting these explanatory challenges. However, Fitch’s theory raises two problems of its own: (1) according to Boyd and Richerson (1996, Proc. Br. Acad. 88: 77–93), circumstances in which pedagogy is adaptive are inevitably rare in nature, and (2) it is unlikely that our non-discursive precursors had generally systematic, task-domain neutral thoughts to communicate to their offspring. I propose solutions to these problems. Pedagogy would be favored in a population where complex rituals dominated diverse aspects of life. Prosodic proto-language could emerge as the medium of pedagogic transmission. As this medium was used to teach a greater variety of tasks, it would become increasingly general and domain neutral. The presence and importance of such a system of communication in hominid populations could then drive, via Baldwinian mechanisms, the evolution of a kind of ‘thinking for speaking’ (Slobin 1991, Pragmatics 1: 7–25) characterized by recursive structure, generalized systematicity, and task-domain neutrality.     

Neural development: The semantics of axon guidance

Recent studies of the semaphorin family of axon guidance signals and their receptors have revealed a surprising versatility in the ways that they can be used solve problems in neural development, and provided new opportunities for understanding how guidance information is interpreted beneath the cell surface.     

Neural syntax: cell assemblies, synapsembles, and readers

A widely discussed hypothesis in neuroscience is that transiently active ensembles of neurons, known as "cell assemblies," underlie numerous operations of the brain, from encoding memories to reasoning. However, the mechanisms responsible for the formation and disbanding of cell assemblies and temporal evolution of cell assembly sequences are not well understood. I introduce and review three interconnected topics, which could facilitate progress in defining cell assemblies, identifying their neuronal organization, and revealing causal relationships between assembly organization and behavior. First, I hypothesize that cell assemblies are best understood in light of their output product, as detected by "reader-actuator" mechanisms. Second, I suggest that the hierarchical organization of cell assemblies may be regarded as a neural syntax. Third, constituents of the neural syntax are linked together by dynamically changing constellations of synaptic weights ("synapsembles"). The existing support for this tripartite framework is reviewed and strategies for experimental testing of its predictions are discussed.     

Neural connectivity and cortical substrates of cognition in hominoids

Cognitive functions and information processing recruit discrete neural systems in the cortex and white matter. We tested the idea that specific regions in the cerebrum are differentially enlarged in humans and that some of the neural reorganizational events that took place during hominoid evolution were species-specific and independent of changes in absolute brain size. We used magnetic resonance images of the living brains of 10 human and 17 ape subjects to obtain volumetric estimates of regions of interest. We parcellated the white matter in the frontal and temporal lobes into two sectors, including the white matter immediately underlying the cortex (gyral white matter) and the rest of white matter (core). We outlined the dorsal, mesial, and orbital subdivisions of the frontal lobe and analyzed the relationship between cortex and gyral white matter within each subdivision. For all regions analyzed, the observed human values are as large as expected, with the exception of the gyral white matter, which is larger than expected in humans. We found that orangutans had a relatively smaller orbital sector than any other great ape species, with no overlap in individual values. We found that the relative size of the dorsal subdivision is larger in chimpanzees than in bonobos, and that the ratio of gyral white matter to cortex stands out in Pan in comparison to Gorilla and Pongo. Individual variability, possible sex differences, and hemispheric asymmetries were present not only in humans, but in apes as well. Differences in the distribution of neural connectivity and cortical sectors were identified among great ape species that share similar absolute brain sizes. Given that these regions are part of neural systems with distinct functional attributes, we suggest that the observed differences may reflect different evolutionary pressures on regulatory mechanisms of complex cognitive functions, including social cognition.     

Neural economics and the biological substrates of valuation

A recent flurry of neuroimaging and decision-making experiments in humans, when combined with single-unit data from orbitofrontal cortex, suggests major additions to current models of reward processing. We review these data and models and use them to develop a specific computational relationship between the value of a predictor and the future rewards or punishments that it promises. The resulting computational model, the predictor-valuation model (PVM), is shown to anticipate a class of single-unit neural responses in orbitofrontal and striatal neurons. The model also suggests how neural responses in the orbitofrontal-striatal circuit may support the conversion of disparate types of future rewards into a kind of internal currency, that is, a common scale used to compare the valuation of future behavioral acts or stimuli.     

The music of your emotions: neural substrates involved in detection of emotional correspondence between auditory and visual music actions

In humans, emotions from music serve important communicative roles. Despite a growing interest in the neural basis of music perception, action and emotion, the majority of previous studies in this area have focused on the auditory aspects of music performances. Here we investigate how the brain processes the emotions elicited by audiovisual music performances. We used event-related functional magnetic resonance imaging, and in Experiment 1 we defined the areas responding to audiovisual (musician's movements with music), visual (musician's movements only), and auditory emotional (music only) displays. Subsequently a region of interest analysis was performed to examine if any of the areas detected in Experiment 1 showed greater activation for emotionally mismatching performances (combining the musician's movements with mismatching emotional sound) than for emotionally matching music performances (combining the musician's movements with matching emotional sound) as presented in Experiment 2 to the same participants. The insula and the left thalamus were found to respond consistently to visual, auditory and audiovisual emotional information and to have increased activation for emotionally mismatching displays in comparison with emotionally matching displays. In contrast, the right thalamus was found to respond to audiovisual emotional displays and to have similar activation for emotionally matching and mismatching displays. These results suggest that the insula and left thalamus have an active role in detecting emotional correspondence between auditory and visual information during music performances, whereas the right thalamus has a different role.     

Neural processing of imminent collision in humans

Detecting a looming object and its imminent collision is imperative to survival. For most humans, it is a fundamental aspect of daily activities such as driving, road crossing and participating in sport, yet little is known about how the brain both detects and responds to such stimuli. Here we use functional magnetic resonance imaging to assess neural response to looming stimuli in comparison with receding stimuli and motion-controlled static stimuli. We demonstrate for the first time that, in the human, the superior colliculus and the pulvinar nucleus of the thalamus respond to looming in addition to cortical regions associated with motor preparation. We also implicate the anterior insula in making timing computations for collision events.     

Neural processing as causal inference

Perception is about making sense, that is, understanding what events in the outside world caused the sensory observations. Consistent with this intuition, many aspects of human behavior confronting noise and ambiguity are well explained by principles of causal inference. Extending these insights, recent studies have applied the same powerful set of tools to perceptual processing at the neural level. According to these approaches, microscopic neural structures solve elementary probabilistic tasks and can be combined to construct hierarchical predictive models of the sensory input. This framework suggests that variability in neural responses reflects the inherent uncertainty associated with sensory interpretations and that sensory neurons are active predictors rather than passive filters of their inputs. Causal inference can account parsimoniously and quantitatively for non-linear dynamical properties in single synapses, single neurons and sensory receptive fields.     

Prerequisites for terminal processing of thylakoidal Tat substrates

In bacteria and chloroplasts, the Tat (twin arginine translocation) system is capable of translocating folded passenger proteins across the cytoplasmic and thylakoidal membranes, respectively. Transport depends on signal peptides that are characterized by a twin pair of arginine residues. The signal peptides are generally removed after transport by specific processing peptidases, namely the leader peptidase and the thylakoidal processing peptidase. To gain insight into the prerequisites for such signal peptide removal, we mutagenized the vicinity of thylakoidal processing peptidase cleavage sites in several thylakoidal Tat substrates. Analysis of these mutants in thylakoid transport experiments showed that the amino acid composition of both the C-terminal segment of the signal peptide and the N-terminal part of the mature protein plays an important role in the maturation process. Efficient removal of the signal peptide requires the presence of charged or polar residues within at least one of those regions, whereas increased hydrophobicity impairs the process. The relative extent of this effect varies to some degree depending on the nature of the precursor protein. Unprocessed transport intermediates with fully translocated passenger proteins are found in membrane complexes of high molecular mass, which presumably represent Tat complexes, as well as free in the lipid bilayer. This seems to indicate that the Tat substrates can be laterally released from the complexes prior to processing and that membrane transport and terminal processing of Tat substrates are independent processes.     

Neural processing of gustatory information in insular circuits

The insular cortex is the primary cortical site devoted to taste processing. A large body of evidence is available for how insular neurons respond to gustatory stimulation in both anesthetized and behaving animals. Most of the reports describe broadly tuned neurons that are involved in processing the chemosensory, physiological and psychological aspects of gustatory experience. However little is known about how these neural responses map onto insular circuits. Particularly mysterious is the functional role of the three subdivisions of the insular cortex: the granular, the dysgranular and the agranular insular cortices. In this article we review data on the organization of the local and long-distance circuits in the three subdivisions. The functional significance of these results is discussed in light of the latest electrophysiological data. A view of the insular cortex as a functionally integrated system devoted to processing gustatory, multimodal, cognitive and affective information is proposed.     

Neural substrates of memory: from synapse to system

One of the fundamental challenges of modern neuroscience is to understand how memories are acquired, stored, and retrieved by the brain. In the broadest terms, attempts to dissect memory can be broken down into four experimental disciplines: (1) identification of molecular components, (2) ex vivo and in vivo cellular analysis of neuronal function, (3) theoretical modeling approaches of neural systems, and (4) organismal-level behavioral analyses. Our objective here is to offer a conceptually unifying perspective and to discuss this perspective in relation to an experiment analysis of memory in Drosophila.     

Neural substrates of sensory-guided locomotor decisions in the rat superior colliculus

Deciding in which direction to move is a ubiquitous feature of animal behavior, but the neural substrates of locomotor choices are not well understood. The superior colliculus (SC) is a midbrain structure known to be important for controlling the direction of gaze, particularly when guided by visual or auditory cues, but which may play a more general role in behavior involving spatial orienting. To test this idea, we recorded and manipulated activity in the SC of freely moving rats performing an odor-guided spatial choice task. In this context, not only did a substantial majority of SC neurons encode choice direction during goal-directed locomotion, but many also predicted the upcoming choice and maintained selectivity for it after movement completion. Unilateral inactivation of SC activity profoundly altered spatial choices. These results indicate that the SC processes information necessary for spatial locomotion, suggesting a broad role for this structure in sensory-guided orienting and navigation.     

Neural processing at the speed of smell

Olfaction is typically described as behaviorally slow, suggesting neural processes on the order of hundreds of milliseconds to seconds as candidate mechanisms in the creation of olfactory percepts. Whereas a recent study challenged this view in suggesting that a single sniff was sufficient for optimal olfactory discrimination, a study by Abraham et al. in this issue of Neuron sets out to negate the challenge by demonstrating increased processing time for discrimination of similar versus dissimilar stimuli. Here we reconcile both studies, which in our view together support the notion of a speed-accuracy tradeoff in olfactory discriminations that are made within about 200 ms. These findings are discussed in light of the challenges related to defining olfactory perceptual similarity in nonhuman animals.     

Effect of phosphorylation and single nucleotide polymorphisms on caspase substrates processing

Posttranslational modifications that involve either reversible covalent modification of proteins or irreversible proteolysis are central to the regulation of key cellular mechanisms, including apoptosis, cell-cycle regulation and signal transduction. There is mounting evidence suggesting cross-talk between proteases and kinases. For instance: caspases, a class of proteases involved in programmed cell death—apoptosis, cleave a large set of various types of proteins. Simultaneously, kinases restrict caspase activity by phosphorylating their protein substrates in the vicinity of cleavage site. In addition, the caspase cleavage pattern in target proteins may be modified as a result of single nucleotide polymorphisms (SNPs) in the coding gene. This may either create a novel cleavage site, or increase/decrease the cleavage efficiency of a substrate. Such point mutations are often associated with the onset of disease. In this study, we predicted how phosphorylation and SNPs affect known human caspase proteolytic events collected in the CASBAH and Degrabase databases by applying Random Forest caspases’ substrates prediction method, as implemented in the CaspDB, and the molecular dynamics free energy simulations approach. Our analysis confirms several experimental observations. Phosphorylation could have both positive or negative regulatory effects depending on its position with respect to the caspase cleavage site. For instance, we demonstrate that phosphorylation at P1′ is the most detrimental for proteolytic efficiency of caspases. Phosphorylation at the P2 and P2′ positions also negatively affect the cleavage events. In addition, we uncovered SNPs in 11 caspase substrates capable of completely abolishing the cleavage site due to polymorphism at the P1 position. The findings presented here may be useful for determining the link between aberrant proteolysis and disease.     

Neural processing of auditory looming in the human brain

Acoustic intensity change, along with interaural, spectral, and reverberation information, is an important cue for the perception of auditory motion. Approaching sound sources produce increases in intensity, and receding sound sources produce corresponding decreases. Human listeners typically overestimate increasing compared to equivalent decreasing sound intensity and underestimate the time to contact of approaching sound sources. These characteristics could provide a selective advantage by increasing the margin of safety for response to looming objects. Here, we used dynamic intensity and functional magnetic resonance imaging to examine the neural underpinnings of the perceptual priority for rising intensity. We found that, consistent with activation by horizontal and vertical auditory apparent motion paradigms, rising and falling intensity activated the right temporal plane more than constant intensity. Rising compared to falling intensity activated a distributed neural network subserving space recognition, auditory motion perception, and attention and comprising the superior temporal sulci and the middle temporal gyri, the right temporoparietal junction, the right motor and premotor cortices, the left cerebellar cortex, and a circumscribed region in the midbrain. This anisotropic processing of acoustic intensity change may reflect the salience of rising intensity produced by looming sources in natural environments.