Allen R. Braun

Decoupling of the brain's default mode network during deep sleep

The recent discovery of a circuit of brain regions that is highly active in the absence of overt behavior has led to a quest for revealing the possible function of this so-called default-mode network (DMN). A very recent study, finding similarities in awake humans and anesthetized primates, has suggested that DMN activity might not simply reflect ongoing conscious mentation but rather a more general form of network dynamics typical of complex systems. Here, by performing functional MRI in humans, it is shown that a natural, sleep-induced reduction of consciousness is reflected in altered correlation between DMN network components, most notably a reduced involvement of frontal cortex. This suggests that DMN may play an important role in the sustenance of conscious awareness.

Hierarchical and asymmetric temporal sensitivity in human auditory cortices

Lateralization of function in auditory cortex has remained a persistent puzzle. Previous studies using signals with differing spectrotemporal characteristics support a model in which the left hemisphere is more sensitive to temporal and the right more sensitive to spectral stimulus attributes. Here we use single-trial sparse-acquisition fMRI and a stimulus with parametrically varying segmental structure affecting primarily temporal properties. We show that both left and right auditory cortices are remarkably sensitive to temporal structure. Crucially, beyond bilateral sensitivity to timing information, we uncover two functionally significant interactions. First, local spectrotemporal signal structure is differentially processed in the superior temporal gyrus. Second, lateralized responses emerge in the higher-order superior temporal sulcus, where more slowly modulated signals preferentially drive the right hemisphere. The data support a model in which sounds are analyzed on two distinct timescales, 25–50 ms and 200–300 ms.

Neural Substrates of Spontaneous Musical Performance: An fMRI Study of Jazz Improvisation

To investigate the neural substrates that underlie spontaneous musical performance, we examined improvisation in professional jazz pianists using functional MRI. By employing two paradigms that differed widely in musical complexity, we found that improvisation (compared to production of over-learned musical sequences) was consistently characterized by a dissociated pattern of activity in the prefrontal cortex: extensive deactivation of dorsolateral prefrontal and lateral orbital regions with focal activation of the medial prefrontal (frontal polar) cortex. Such a pattern may reflect a combination of psychological processes required for spontaneous improvisation, in which internally motivated, stimulus-independent behaviors unfold in the absence of central processes that typically mediate self-monitoring and conscious volitional control of ongoing performance. Changes in prefrontal activity during improvisation were accompanied by widespread activation of neocortical sensorimotor areas (that mediate the organization and execution of musical performance) as well as deactivation of limbic structures (that regulate motivation and emotional tone). This distributed neural pattern may provide a cognitive context that enables the emergence of spontaneous creative activity.

Language in context: emergent features of word, sentence, and narrative comprehension.

Context exerts a powerful effect on cognitive performance and is clearly important for language processing, where lexical, sentential, and narrative contexts should differentially engage neural systems that support lexical, compositional, and discourse level semantics. Equally important, but thus far unexplored, is the role of context within narrative, as cognitive demands evolve and brain activity changes dynamically as subjects process different narrative segments. In this study, we used fMRI to examine the impact of context, comparing responses to a single, linguistically matched set of texts when these were differentially presented as random word lists, unconnected sentences and coherent narratives. We found emergent, context-dependent patterns of brain activity in each condition. Perisylvian language areas were always active, consistent with their supporting core linguistic computations. Sentence processing was associated with expanded activation of the frontal operculum and temporal poles. The same stimuli presented as narrative evoked robust responses in extrasylvian areas within both hemispheres, including precuneus, medial prefrontal, and dorsal temporo-parieto-occipital cortices. The right hemisphere was increasingly active as contextual complexity increased, maximal at the narrative level. Furthermore, brain activity was dynamically modulated as subjects processed different narrative segments: left hemisphere activity was more prominent at the onset, and right hemisphere more prominent at the resolution of a story, at which point, it may support a coherent representation of the narrative as a whole. These results underscore the importance of studying language in an ecologically valid context, suggesting a neural model for the processing of discourse.

Dopaminergic mechanisms and motor function: Characterization of D-1 and D-2 dopamine receptor interactions

The effect of selective D-1 and D-2 dopamine receptor agonists (SKF 38393 and LY 171555, respectively) on motor function was studied in rats with unilateral, quinolinic acid-induced striatal lesions. Dose-dependent turning ipsilateral to the side of the lesion was elicited by LY 171555, but not by SKF 38393. When the drugs were given together, however, SKF 38393 was able to increase the total number (but not the duration) of turning induced by LY 171555 in a dose-dependent fashion. Furthermore, either alpha-methyl-paratyrosine pretreatment, a DA-depleting agent, or SCH 23390, a D-1 antagonist, inhibited the LY 171555-induced rotation, which would be restored by SKF 38393. These observations suggest that both the D-1 and D-2 dopamine receptor systems participate in the regulation of rotational behaviors in striatally lesioned rats with normosensitive DA receptors.

Sentence reading: A functional mri study at 4 tesla

In this study, changes in blood oxygenation and volume were monitored while monolingual right-handed subjects read English sentences. Our results confirm the role of the left peri-sylvian cortex in language processing. Interestingly, individual subject analyses reveal a pattern of activation characterized by several small, limited patches rather than a few large, anatomically well-circumscribed centers. Between-subject analyses confirm a lateralized pattern of activation and reveal active classical language areas including Brocas area, Wernickes area, and the angular gyms. In addition they point to areas only more recently considered as language-relevant including the anterior portion of the superior temporal sulcus. This area has not been reliably observed in imaging studies of isolated word processing. This raises the hypothesis that activation in this area is dependent on processes specific to sentence reading.

Auditory lexical decision, categorical perception, and FM direction discrimination differentially engage left and right auditory cortex

Recent neuroimaging and neuropsychological data suggest that speech perception is supported in bilaterally auditory areas. We evaluate this issue building on well-known behavioral effects. While undergoing positron emission tomography (PET), subjects performed standard auditory tasks: direction discrimination of frequency-modulated (FM) tones, categorical perception (CP) of consonant-vowel (CV) syllables, and word/non-word judgments (lexical decision, LD). Compared to rest, the three conditions led to bilateral activation of the auditory cortices. However, lateralization patterns differed as a function of stimulus type: the LD task generated stronger responses in the left, the FM task a stronger response in the right hemisphere. Contrasts between either words or syllables versus FM were associated with significantly greater activity bilaterally in superior temporal gyrus (STG) ventro-lateral to Heschls gyrus. These activations extended into the superior temporal sulcus (STS) and the middle temporal gyrus (MTG) and were greater in the left. The same areas were more active in the LD than the CP task. In contrast, the FM task was associated with significantly greater activity in the right lateral-posterior STG and lateral MTG. The findings argue for a view in which speech perception is mediated bilaterally in the auditory cortices and that the well-documented lateralization is likely associated with processes subsequent to the auditory analysis of speech.

The Functional Neuroanatomy of Tourette's Syndrome: An FDG-PET Study. I. Regional Changes in Cerebral Glucose Metabolism Differentiating Patients and Controls

Regional metabolic rates for glucose estimated using [18F]fluorodeoxyglucose positron-emission tomography were compared in 16 drug-free patients with Tourettes syndrome (TS) and 16 age- and sex-matched normal volunteers. Tourettes syndrome patients were characterized by decreased normalized metabolic rates in paralimbic and ventral prefrontal cortices, particularly in orbitofrontal, inferior insular, and parahippocampal regions. Similar decreases were observed in subcortical regions, including the ventral striatum (nucleus accumbens/ventromedial caudate) and in the midbrain. These changes were more robust and occurred with greater frequency in the left hemisphere. They were associated with concomitant bilateral increases in metabolic activity the supplementary motor, lateral premotor, and Rolandic cortices. Effects of prior exposure to neuroactive drugs did not account for these findings. These results suggest that an altered relationship between limbic-related regions of the cortex and striatum and cortical regions involved in the initiation of movement may play a role in the pathogenesis of this illness.

Activation of Broca’s area during the production of spoken and signed language: a combined cytoarchitectonic mapping and PET analysis

Brocas area in the inferior frontal gyrus consists of two cytoarchitectonically defined regions-Brodmann areas (BA) 44 and 45. Combining probabilistic maps of these two areas with functional neuroimaging data obtained using PET, it is shown that BA45, not BA44, is activated by both speech and signing during the production of language narratives in bilingual subjects fluent from early childhood in both American Sign Language (ASL) and English when the generation of complex movements and sounds is taken into account. It is BA44, not BA45, that is activated by the generation of complex articulatory movements of oral/laryngeal or limb musculature. The same patterns of activation are found for oral language production in a group of English speaking monolingual subjects. These findings implicate BA45 as the part of Brocas area that is fundamental to the modality-independent aspects of language generation.

Species-specific calls activate homologs of Broca's and Wernicke's areas in the macaque

The origin of brain mechanisms that support human language—whether these originated de novo in humans or evolved from a neural substrate that existed in a common ancestor—remains a controversial issue. Although the answer is not provided by the fossil record, it is possible to make inferences by studying living species of nonhuman primates. Here we identified neural systems associated with perceiving species-specific vocalizations in rhesus macaques using H215O positron emission tomography (PET). These vocalizations evoke distinct patterns of brain activity in homologs of the human perisylvian language areas. Rather than resulting from differences in elementary acoustic properties, this activity seems to reflect higher order auditory processing. Although parallel evolution within independent primate species is feasible, this finding suggests the possibility that the last common ancestor of macaques and humans, which lived 25–30 million years ago, possessed key neural mechanisms that were plausible candidates for exaptation during the evolution of language.