Mark Bodner

Cross-modal and cross-temporal association in neurons of frontal cortex.

The prefrontal cortex is essential for the temporal integration of sensory information in behavioural and linguistic sequences. Such information is commonly encoded in more than one sense modality, notably sight and sound. Connections from sensory cortices to the prefrontal cortex support its integrative function. Here we present the first evidence that prefrontal cortex cells associate visual and auditory stimuli across time. We gave monkeys the task of remembering a tone of a certain pitch for 10 s and then choosing the colour associated with it. In this task, prefrontal cortex cells responded selectively to tones, and most of them also responded to colours according to the task rule. Thus, their reaction to a tone was correlated with their subsequent reaction to the associated colour. This correlation faltered in trials ending in behavioural error. We conclude that prefrontal cortex neurons are part of integrative networks that represent behaviourally meaningful cross-modal associations. The orderly and timely activation of neurons in such networks is crucial for the temporal transfer of information in the structuring of behaviour, reasoning and language.

Auditory memory cells in dorsolateral prefrontal cortex

The activity of single neurons was recorded extracellularly from dorsolateral prefrontal cortex (DPC) of monkeys during the performance of a cross-modal audio-visual short-term memory task. Cells in DPC show sustained elevated firing levels (higher than spontaneous discharge) during the retention of the auditory stimulus. In some cells this elevated firing was significantly different depending on the particular auditory memorandum of each trial. These results support the notion that DPC participates in auditory short-term memory and the integration of auditory and visual information for prospective action.

VARIABILITY IN NEURONAL ACTIVITY IN PRIMATE CORTEX DURING WORKING MEMORY TASKS

Persistent elevated neuronal activity has been identified as the neuronal correlate of working memory. It is generally assumed in the literature and in computational and theoretical models of working memory that memory-cell activity is stable and replicable; however, this assumption may be an artifact of the averaging of data collected across trials, and needs experimental verification. In this study, we introduce a classification scheme to characterize the firing frequency trends of cells recorded from the cortex of monkeys during performance of working memory tasks. We examine the frequency statistics and variability of firing during baseline and memory periods. We also study the behavior of cells on individual trials and across trials, and explore the stability of cellular firing during the memory period. We find that cells from different firing-trend classes possess markedly different statistics. We also find that individual cells show substantial variability in their firing behavior across trials, and that firing frequency also varies markedly over the course of a single trial. Finally, the average frequency distribution is wider, the magnitude of the frequency increases from baseline to memory smaller, and the magnitude of frequency decreases larger than is generally assumed. These results may serve as a guide in the evaluation of current theories of the cortical mechanisms of working memory.

Dimensional reduction of type IIB supergravity and exceptional quaternionic manifolds

The authors describe the SU(3) invariant reduction of the type IIB supergravity from ten to four dimensions, and show that the low-energy scalars parametrise the quaternionic sigma model G2,2/SO(4). They identify the scalar Lagrangian with the one obtained from a similar SU(3) truncation of the type IIA theory, followed by a dimensional reduction to three dimensions. This is an explicit realisation of the c map, that in general relates the low-energy Lagrangians of the type-II superstrings when compactified on the same (2, 2) superconformal theory.

Near-infrared spectroscopy (NIRS) in cognitive neuroscience of the primate brain

We describe the use of near-infrared spectroscopy (NIRS) as a suitable means of assessing hemodynamic changes in the cerebral cortex of awake and behaving monkeys. NIRS can be applied to animals performing cognitive tasks in conjunction with electrophysiological methods, thus offering the possibility of investigating cortical neurovascular coupling in cognition. Because it imposes fewer constraints on behavior than fMRI, NIRS appears more practical than fMRI for certain studies of cognitive neuroscience on the primate cortex. In the present study, NIRS and field potential signals were simultaneously recorded from the association cortex (posterior parietal and prefrontal) of monkeys performing two delay tasks, one spatial and the other non-spatial. Working memory was accompanied by an increase in oxygenated hemoglobin mirrored by a decrease in deoxygenated hemoglobin. Both the trends and the amplitudes of these changes differed by task and by area. Field potential records revealed slow negative potentials that preceded the task trials and persisted during their memory period. The negativity during that period was greater in prefrontal than in parietal cortex. Between tasks, the potential differences were less pronounced than the hemodynamic differences. The present feasibility study lays the groundwork for future correlative studies of cognitive function and neurovascular coupling in the primate.

Working memory cells' behavior may be explained by cross-regional networks with synaptic facilitation

Neurons in the cortex exhibit a number of patterns that correlate with working memory. Specifically, averaged across trials of working memory tasks, neurons exhibit different firing rate patterns during the delay of those tasks. These patterns include: 1) persistent fixed-frequency elevated rates above baseline, 2) elevated rates that decay throughout the tasks memory period, 3) rates that accelerate throughout the delay, and 4) patterns of inhibited firing (below baseline) analogous to each of the preceding excitatory patterns. Persistent elevated rate patterns are believed to be the neural correlate of working memory retention and preparation for execution of behavioral/motor responses as required in working memory tasks. Models have proposed that such activity corresponds to stable attractors in cortical neural networks with fixed synaptic weights. However, the variability in patterned behavior and the firing statistics of real neurons across the entire range of those behaviors across and within trials of working memory tasks are typical not reproduced. Here we examine the effect of dynamic synapses and network architectures with multiple cortical areas on the states and dynamics of working memory networks. The analysis indicates that the multiple pattern types exhibited by cells in working memory networks are inherent in networks with dynamic synapses, and that the variability and firing statistics in such networks with distributed architectures agree with that observed in the cortex.

Patterned firing of parietal cells in a haptic working memory task

Cells in the somatosensory cortex of the monkey are known to exhibit sustained elevations of firing frequency during the short‐term mnemonic retention of tactile information in a haptic delay task. In this study, we examine the possibility that those firing elevations are accompanied by changes in firing pattern. Patterns are identified by the application of a pattern‐searching algorithm to the interspike intervals of spike trains. By sequential use of sets of pattern templates with a range of temporal resolutions, we find patterned activity in the majority of the cells investigated. In general, the degree of patterning significantly increases during active memory. Surrogate analysis suggests that the observed patterns may not be simple linear stochastic functions of instantaneous or average firing frequency. Therefore, during the active retention of a memorandum, the activity of a ‘memory cell’ may be characterized not only by changes in frequency but also by changes in pattern.

Differential roles of delay-period neural activity in the monkey dorsolateral prefrontal cortex in visual–haptic crossmodal working memory

Significance Neural activity was recorded from the dorsolateral prefrontal cortex (DLPFC) when a monkey performed visuo–haptic crossmodal and haptic–haptic unimodal delayed matching-to-sample (cue) tasks. Results indicate that neural networks in the DLPFC function sequentially in the crossmodal task from visual stimulus encoding and crossmodal information transferring between visual and tactile stimuli to the behavioral action. Our findings may clarify the neural mechanisms by which the cerebral cortex stores information in working memory, a cognitive function of prime importance in the coordination of behavior, speech, and reasoning. Previous studies have shown that neurons of monkey dorsolateral prefrontal cortex (DLPFC) integrate information across modalities and maintain it throughout the delay period of working-memory (WM) tasks. However, the mechanisms of this temporal integration in the DLPFC are still poorly understood. In the present study, to further elucidate the role of the DLPFC in crossmodal WM, we trained monkeys to perform visuo–haptic (VH) crossmodal and haptic–haptic (HH) unimodal WM tasks. The neuronal activity recorded in the DLPFC in the delay period of both tasks indicates that the early-delay differential activity probably is related to the encoding of sample information with different strengths depending on task modality, that the late-delay differential activity reflects the associated (modality-independent) action component of haptic choice in both tasks (that is, the anticipation of the behavioral choice and/or active recall and maintenance of sample information for subsequent action), and that the sustained whole-delay differential activity likely bridges and integrates the sensory and action components. In addition, the VH late-delay differential activity was significantly diminished when the haptic choice was not required. Taken together, the results show that, in addition to the whole-delay differential activity, DLPFC neurons also show early- and late-delay differential activities. These previously unidentified findings indicate that DLPFC is capable of (i) holding the coded sample information (e.g., visual or tactile information) in the early-delay activity, (ii) retrieving the abstract information (orientations) of the sample (whether the sample has been haptic or visual) and holding it in the late-delay activity, and (iii) preparing for behavioral choice acting on that abstract information.

Prefrontal Cortex and Somatosensory Cortex in Tactile Crossmodal Association: An Independent Component Analysis of ERP Recordings

Our previous studies on scalp-recorded event-related potentials (ERPs) showed that somatosensory N140 evoked by a tactile vibration in working memory tasks was enhanced when human subjects expected a coming visual stimulus that had been paired with the tactile stimulus. The results suggested that such enhancement represented the cortical activities involved in tactile-visual crossmodal association. In the present study, we further hypothesized that the enhancement represented the neural activities in somatosensory and frontal cortices in the crossmodal association. By applying independent component analysis (ICA) to the ERP data, we found independent components (ICs) located in the medial prefrontal cortex (around the anterior cingulate cortex, ACC) and the primary somatosensory cortex (SI). The activity represented by the IC in SI cortex showed enhancement in expectation of the visual stimulus. Such differential activity thus suggested the participation of SI cortex in the task-related crossmodal association. Further, the coherence analysis and the Granger causality spectral analysis of the ICs showed that SI cortex appeared to cooperate with ACC in attention and perception of the tactile stimulus in crossmodal association. The results of our study support with new evidence an important idea in cortical neurophysiology: higher cognitive operations develop from the modality-specific sensory cortices (in the present study, SI cortex) that are involved in sensation and perception of various stimuli.

Calabi-Yau supermoduli space, field strength duality and mirror manifolds

Abstract Calabi-Yau moduli space have a super extension due to their relation to type II superstring compactifications. Supermoduli can be defined as N = 2 target space vector-supermultiplets containing both the moduli and their vector field bosonic superpartners. A change of basis in the cohomology vector space corresponds to (modular) duality transformations of target space vector field strengths. Target space two-forms will take values in the cohomology spaces of Calabi-Yau manifolds, and are therefore related to the Kahler class and complex structure deformations, which may be relevant for the description of mirror symmetries of (2, 2) superconformal field theories.