Who is that? Brain networks and mechanisms for identifying individuals

Social animals can identify conspecifics by many forms of sensory input. However, whether the neuronal computations that support this ability to identify individuals rely on modality-independent convergence or involve ongoing synergistic interactions along the multiple sensory streams remains controversial. Direct neuronal measurements at relevant brain sites could address such questions, but this requires better bridging the work in humans and animal models. Here, we overview recent studies in nonhuman primates on voice and face identity-sensitive pathways and evaluate the correspondences to relevant findings in humans. This synthesis provides insights into converging sensory streams in the primate anterior temporal lobe (ATL) for identity processing. Furthermore, we advance a model and suggest how alternative neuronal mechanisms could be tested

Behavioral, electrophysiological and histopathological consequences of systemic manganese administration in MEMRI

Manganese (Mn2+)-enhanced magnetic resonance imaging (MEMRI) offers the possibility to generate longitudinal maps of brain activity in unrestrained and behaving animals. However, Mn2+ is a metabolic toxin and a competitive inhibitor for Ca2+, and therefore, a yet unsolved question in MEMRI studies is whether the concentrations of metal ion used may alter brain physiology. In the present work we have investigated the behavioral, electrophysiological and histopathological consequences of MnCl2 administration at concentrations and dosage protocols regularly used in MEMRI. Three groups of animals were sc injected with saline, 0.1 and 0.5 mmol/kg MnCl2, respectively. In vivo electrophysiological recordings in the hippocampal formation revealed a mild but detectable decrease in both excitatory postsynaptic potentials (EPSP) and population spike (PS) amplitude under the highest MnCl2 dose. The EPSP to PS ratio was preserved at control levels, indicating that neuronal excitability was not affected. Experiments of pair pulse facilitation demonstrated a dose dependent increase in the potentiation of the second pulse, suggesting presynaptic Ca2+ competition as the mechanism for the decreased neuronal response. Tetanization of the perforant path induced a long-term potentiation of synaptic transmission that was comparable in all groups, regardless of treatment. Accordingly, the choice accuracy tested on a hippocampal-dependent learning task was not affected. However, the response latency in the same task was largely increased in the group receiving 0.5 mmol/kg of MnCl2. Immunohistological examination of the hippocampus at the end of the experiments revealed no sign of neuronal toxicity or glial reaction. Although we show that MEMRI at 0.1 mmol/Kg MnCl2 may be safely applied to the study of cognitive networks, a detailed assessment of toxicity is strongly recommended for each particular study and Mn2+ administration protocol

Functionalized azamacrocyclic compounds as Ca2+ sensitive contrast agents for MR imaging

The ability to non-invasively observe changes in Ca2+ concentration is important for neuroscience. We have therefore developed a series of gadolinium chelate complexes based on DO3A (Scheme 1), which is hypothesized to change relaxivity in magnetic resonance experiments dynamically with Ca2+ concentration. Different lengths of the phosphonate side chains are expected to lead to different binding constants of the phosphonate - gadolinium bonds. The latter property can be exploited for fine-tuning the sensitivity of the agent to calcium ion concentration

Cortical Microcircuit Dynamics Mediating Binocular Rivalry: The Role of Adaptation in Inhibition

Perceptual bistability arises when two conflicting interpretations of an ambiguous stimulus or images in binocular rivalry (BR) compete for perceptual dominance. From a computational point of view, competition models based on cross-inhibition and adaptation have shown that noise is a crucial force for rivalry, and operates in balance with adaptation. In particular, noise-driven transitions and adaptation-driven oscillations define two dynamical regimes and the system explains the observed alternations in perception when it operates near their boundary. In order to gain insights into the microcircuit dynamics mediating spontaneous perceptual alternations, we used a reduced recurrent attractor-based biophysically realistic spiking network, well known for working memory, attention, and decision making, where a spike-frequency adaptation mechanism is implemented to account for perceptual bistability. We thus derived a consistently reduced four-variable population rate model using mean-field techniques, and we tested it on BR data collected from human subjects. Our model accounts for experimental data parameters such as mean time dominance, coefficient of variation, and gamma distribution fit. In addition, we show that our model operates near the bifurcation that separates the noise-driven transitions regime from the adaptation-driven oscillations regime, and agrees with Levelt’s second revised and fourth propositions. These results demonstrate for the first time that a consistent reduction of a biophysically realistic spiking network of leaky integrate-and-fire neurons with spike-frequency adaptation could account for BR. Moreover, we demonstrate that BR can be explained only through the dynamics of competing neuronal pools, without taking into account the adaptation of inhibitory interneurons. However, the adaptation of interneurons affects the optimal parametric space of the system by decreasing the overall adaptation necessary for the bifurcation to occur, and introduces oscillations in the spontaneous state

Azamacrocyclic Ca2+ Sensitive Contrast Agents for MR Imaging

As calcium plays an important role in regulating a great variety of neuronal processes, many efforts are already made to generate gadolinium complexes that can act as a calcium-sensors in MRI.1 We developed a series of the DO3A-based macrocyclic and bismacrocyclic gadolinium chelates, bearing phosphonate groups as an additional coordination sites. These complexes are hypothesized to change the MRI contrast dynamically with Ca2+ concentration. Different lengths of the phosphonate side chains are exploited for fine-tuning the sensitivity of the agent to calcium ion concentration

Mechanisms for Allocating Auditory Attention: An Auditory Saliency Map

SummaryOur nervous system is confronted with a barrage of sensory stimuli, but neural resources are limited and not all stimuli can be processed to the same extent. Mechanisms exist to bias attention toward the particularly salient events, thereby providing a weighted representation of our environment [1]. Our understanding of these mechanisms is still limited, but theoretical models can replicate such a weighting of sensory inputs and provide a basis for understanding the underlying principles [2, 3]. Here, we describe such a model for the auditory system—an auditory saliency map. We experimentally validate the model on natural acoustical scenarios, demonstrating that it reproduces human judgments of auditory saliency and predicts the detectability of salient sounds embedded in noisy backgrounds. In addition, it also predicts the natural orienting behavior of naive macaque monkeys to the same salient stimuli. The structure of the suggested model is identical to that of successfully used visual saliency maps. Hence, we conclude that saliency is determined either by implementing similar mechanisms in different unisensory pathways or by the same mechanism in multisensory areas. In any case, our results demonstrate that different primate sensory systems rely on common principles for extracting relevant sensory events

Functional Imaging Reveals Numerous Fields in the Monkey Auditory Cortex

Anatomical studies propose that the primate auditory cortex contains more fields than have actually been functionally confirmed or described. Spatially resolved functional magnetic resonance imaging (fMRI) with carefully designed acoustical stimulation could be ideally suited to extend our understanding of the processing within these fields. However, after numerous experiments in humans, many auditory fields remain poorly characterized. Imaging the macaque monkey is of particular interest as these species have a richer set of anatomical and neurophysiological data to clarify the source of the imaged activity. We functionally mapped the auditory cortex of behaving and of anesthetized macaque monkeys with high resolution fMRI. By optimizing our imaging and stimulation procedures, we obtained robust activity throughout auditory cortex using tonal and band-passed noise sounds. Then, by varying the frequency content of the sounds, spatially specific activity patterns were observed over this region. As a result, the activity patterns could be assigned to many auditory cortical fields, including those whose functional properties were previously undescribed. The results provide an extensive functional tessellation of the macaque auditory cortex and suggest that 11 fields contain neurons tuned for the frequency of sounds. This study provides functional support for a model where three fields in primary auditory cortex are surrounded by eight neighboring “belt” fields in non-primary auditory cortex. The findings can now guide neurophysiological recordings in the monkey to expand our understanding of the processing within these fields. Additionally, this work will improve fMRI investigations of the human auditory cortex

Auditory and visual modulation of temporal lobe neurons in voice-sensitive and association cortices

Perrodin C, Kayser C, Logothetis NK, Petkov CI. Auditory and visual modulation of temporal lobe neurons in voice-sensitive and association cortices. Journal of Neuroscience. 2014;34(7):2524-37

Smart MRI Agents Sensing Extracellular Calcium Fluctuations

Functional Magnetic Resonance Imaging (fMRI) is currently the main tool used for the study of function and dysfunction of the human brain. The current mainstay of fMRI, the so-called Blood-Oxygen-Level-Dependent (BOLD) contrast, capitalizes on the detection of changes in cerebral blood flow, volume and oxygenation, but cannot directly report neural activity, as it suffers from poor spatiotemporal resolution and specificity compared to the actual neural events. An alternative methodology could be that relying on the responsive, ‘smart’ contrast agents whose relaxivity depends on the concentration of substances directly related to neuronal activity. Ca2+ is an excellent marker closely linked to brain activation and is preferred target for various imaging methods. We report two Gd3+ chelates linked to a modified EGTA moiety that have a relaxivity response to extracellular Ca2+ fluctuations in the brain. The proton relaxivity of both Gd3+ complexes is sensitive to the variation of Ca2+ concentration. They are selective to Ca2+ with respect to the main competitor cation Mg2+. Upon interaction with Ca2+, the complexes exhibit high and reversible relaxivity changes; the relaxivity response of one complex upon addition of Ca2+ exceeds 80. Moreover, the relaxivity changes remain remarkable (>50) even in the medium mimicking the brain extracellular fluid, exhibiting a ~10 relaxivity change in the physiologically relevant Ca2+ concentration range (changes induced during the neural activity). These agents have great potential to be applied as functional MR markers and be used for the visualization of the neural processes. They can substantially increase the specificity and spatial resolution of the MR-detected signals and open new perspectives in fMRI

Multisensory integration of dynamic voices and faces in the monkey brain

[...] Primates are social animals whose communication is based on their conspecifics' vocalizations and facial expressions. Although a lot of work to date has studied the unimodal representation of vocal or facial information, little is known about the way the nervous system supports the processing of communication signals from different sensory modalities to combine them into a coherent audiovisual percept. [... click on "Download fulltext" for full abstract