Minimum perturbation theory of deep perceptual learning.

Perceptual learning (PL) involves long-lasting improvement in perceptual tasks following extensive training and is accompanied by modified neuronal responses in sensory cortical areas in the brain. Understanding the dynamics of PL and the resultant synaptic changes is important for causally connecting PL to the observed neural plasticity. This is theoretically challenging because learning-related changes are distributed across many stages of the sensory hierarchy. In this paper, we modeled the sensory hierarchy as a deep nonlinear neural network and studied PL of fine discrimination, a common and well-studied paradigm of PL. Using tools from statistical physics, we developed a mean-field theory of the network in the limit of a large number of neurons and large number of examples. Our theory suggests that, in this thermodynamic limit, the input-output function of the network can be exactly mapped to that of a deep linear network, allowing us to characterize the space of solutions for the task. Surprisingly, we found that modifying synaptic weights in the first layer of the hierarchy is both sufficient and necessary for PL. To address the degeneracy of the space of solutions, we postulate that PL dynamics are constrained by a normative minimum perturbation (MP) principle, which favors weight matrices with minimal changes relative to their prelearning values. Interestingly, MP plasticity induces changes to weights and neural representations in all layers of the network, except for the readout weight vector. While weight changes in higher layers are not necessary for learning, they help reduce overall perturbation to the network. In addition, such plasticity can be learned simply through slow learning. We further elucidate the properties of MP changes and compare them against experimental findings. Overall, our statistical mechanics theory of PL provides mechanistic and normative understanding of several important empirical findings of PL.

Evolution Process and Controlled Synthesis of Humins with 5-Hydroxymethylfurfural (HMF) as Model Molecule.

Elucidation of the chemical structure and formation mechanism of humins is a requisite to further improve the efficiency of acid-catalyzed biomass conversion. Through a low-temperature approach, the key intermediates resulting in the formation of 5-hydroxymethylfurfural (HMF)-derived humins were captured, revealing multiple elementary reactions such as etherification, esterification, aldol condensation, and acetalization. Through humin characterization, it was found out that the aldol condensation moiety between aldehyde group and levulinic acid is critical to justify the characteristic IR peaks (1620 and 1710 cm-1 ) and aromatic fragments from pyrolysis GC-MS. Based on the investigations by means of HPLC-MS/MS, IR, pyrolysis GC-MS, and SEM, the structural models of humins at different temperatures were proposed, which are comprised of the elementary reaction types confirmed by the key intermediates. Humin structures with varying content of aldol condensation could be controllably synthesized under different reaction conditions (temperature and time), demonstrating the evolution process of HMF-derived humins.

Family of closed-form solutions for two-dimensional correlated diffusion processes.

Diffusion processes with boundaries are models of transport phenomena with wide applicability across many fields. These processes are described by their probability density functions (PDFs), which often obey Fokker-Planck equations (FPEs). While obtaining analytical solutions is often possible in the absence of boundaries, obtaining closed-form solutions to the FPE is more challenging once absorbing boundaries are present. As a result, analyses of these processes have largely relied on approximations or direct simulations. In this paper, we studied two-dimensional, time-homogeneous, spatially correlated diffusion with linear, axis-aligned, absorbing boundaries. Our main result is the explicit construction of a full family of closed-form solutions for their PDFs using the method of images. We found that such solutions can be built if and only if the correlation coefficient ρ between the two diffusing processes takes one of a numerable set of values. Using a geometric argument, we derived the complete set of ρ's where such solutions can be found. Solvable ρ's are given by ρ=-cos(π/k), where k∈Z^{+}∪{+∞}. Solutions were validated in simulations. Qualitative behaviors of the process appear to vary smoothly over ρ, allowing extrapolation from our solutions to cases with unsolvable ρ's.

A valence-free definition of sociality as any violation of inter-individual independence.

Well over 90% of studies in biomedical sciences are performed on single animals. While knowledge of the genetics, development and physiology of single individuals accrues, an understanding of the biological mechanisms by which individuals interact has barely budged. Yet many of society's greatest problems derive from an inability of humans to get along with each other. Studies in social neuroscience are primarily observational and rarely employ subjects who physically interact. Thus, social interaction represents a largely unexplored frontier of biology. The neuroscience that underlies social behaviour and interactions can and should be studied using the scientific method. However, a workable and objective definitional framework of sociality is needed for scientific progress in this field. Here we propose a definition that uses a test of independence from the presence of others. The null hypothesis is that a behaviour is independent from the influence of others. Rejection of this null hypothesis means that the actions of an individual depend on the actions of one or more other individuals. This definition has the advantages of not being contaminated by moral judgements or biases in favour of pro-social behaviour, and of being applicable to a wide range of physiological processes. The definition of a social behaviour proposed here says nothing regarding the valence of the behaviour with respect to others. Thus, a behaviour that is influenced by the presence of others may benefit, harm, or have no effect on others. It is hoped that this definitional framework for sociality will facilitate our understanding of the origins and mechanisms of social behaviour among animals including humans as well as offer efficacious approaches to social disorders such as autism.

Anxiolytic Treatment Impairs Helping Behavior in Rats.

Despite decades of research with humans, the biological mechanisms that motivate an individual to help others remain poorly understood. In order to investigate the roots of pro-sociality in mammals, we established the helping behavior test, a paradigm in which rats are faced with a conspecific trapped in a restrainer that can only be opened from the outside. Over the course of repeated test sessions, rats exposed to a trapped cagemate learn to open the door to the restrainer, thereby helping the trapped rat to escape (Ben-Ami Bartal et al., 2011). The discovery of this natural behavior provides a unique opportunity to probe the motivation of rodent helping behavior, leading to a deeper understanding of biological influences on human pro-sociality. To determine if an affective response motivates door-opening, rats receiving midazolam, a benzodiazepine anxiolytic, were tested in the helping behavior test. Midazolam-treated rats showed less helping behavior than saline-treated rats or rats receiving no injection. Yet, midazolam-treated rats opened a restrainer containing chocolate, highlighting the socially specific effects of the anxiolytic. To determine if midazolam interferes with helping through a sympatholytic effect, the peripherally restricted beta-adrenergic receptor antagonist nadolol was administered; nadolol did not interfere with helping. The corticosterone response of rats exposed to a trapped cagemate was measured and compared to the rats' subsequent helping behavior. Rats with the greatest corticosterone responses showed the least helping behavior and those with the smallest responses showed the most consistent helping at the shortest latency. These results are discussed in terms of their implications for the interaction between stress and pro-social behavior. Finally, we observed that door-opening appeared to be reinforcing. A novel analytical tool was designed to interrogate the pattern of door-opening for signs that a rat's behavior on one session influenced his behavior on the next session. Results suggest that helping a trapped rat has a greater motivational value than does chocolate. In sum, this series of experiments clearly demonstrates the fundamental role of affect in motivating pro-social behavior in rodents and the need for a helper to resonate with the affect of a victim.