The visual perception of identity in humans and other primates is thought to draw upon cortical areas specialized for the analysis of facial structure. A prominent theory of face recognition holds ...that the brain computes and stores average facial structure, which it then uses to efficiently determine individual identity, though the neural mechanisms underlying this process are controversial. Here, we demonstrate that the dynamic suppression of average facial structure plays a prominent role in the responses of neurons in three fMRI-defined face patches of the macaque. Using photorealistic face stimuli that systematically varied in identity level according to a psychophysically based face space, we found that single units in the AF, AM, and ML face patches exhibited robust tuning around average facial structure. This tuning emerged after the initial excitatory response to the face and was expressed as the selective suppression of sustained responses to low-identity faces. The coincidence of this suppression with increased spike timing synchrony across the population suggests a mechanism of active inhibition underlying this effect. Control experiments confirmed that the diminished responses to low-identity faces were not due to short-term adaptation processes. We propose that the brain’s neural suppression of average facial structure facilitates recognition by promoting the extraction of distinctive facial characteristics and suppressing redundant or irrelevant responses across the population.
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•Neurons in face patches showed robust identity tuning around average facial structure•The tuning emerged as delayed suppression to low-identity faces•The coincidence of the suppression and spike synchrony suggests active inhibition•Control experiments confirmed that the tuning was not due to short-term adaptation
Koyano et al. use morphed faces to examine neural tuning to facial identity in three macaque face patches. Neurons respond with a prominent tuning around average facial structure, expressed as a delayed suppression to low-identity faces. The findings suggest a mechanism by which the brain efficiently extracts individuals’ unique facial features.
The external reorganization energy for hole transport in naphthalene, anthracene, tetracene, pentacene, and rubrene is computed using a polarizable force field. The importance of this very small ...contribution to the Hamiltonian of the molecular crystal in the context of charge-transport modeling is discussed. The effect of the force field parameters and the size of the model system on the computed reorganization energy is analyzed. The results are rationalized with a simple model which clarifies the fundamental differences between the external reorganization energy in the solid and liquid phases.
Neurons within fMRI-defined face patches of the macaque brain exhibit shared categorical responses to flashed images but diverge in their responses under more natural viewing conditions. Here we ...investigate functional diversity among neurons in the anterior fundus (AF) face patch, combining whole-brain fMRI with longitudinal single-unit recordings in a local population (<1 mm3). For each cell, we computed a whole-brain correlation map based on its shared time course with voxels throughout the brain during naturalistic movie viewing. Based on this mapping, neighboring neurons showed markedly different affiliation with distant visually responsive areas and fell coarsely into subpopulations. Of these, only one subpopulation (∼16% of neurons) yielded similar correlation maps to the local fMRI signal. The results employ the readout of large-scale fMRI networks and, by indicating multiple functional domains within a single voxel, present a new view of functional diversity within a local neural population.
•We compared responses of macaque face patch cells to fMRI activity across the brain•Single neurons yielded diverse fMRI correlation maps in response to natural videos•Maps generated by single units within <1 mm3 in the AF face patch differed greatly•Clustering neurons based on such maps revealed functional subpopulations within AF
Park et al. compute whole-brain functional maps based on the combined responses of single cells and fMRI to natural videos. Neighboring neurons in a face-selective patch of cortex fall into multiple subpopulations correlated with distinct cortical and subcortical areas.
Molecular crystals cannot be designed in the same manner as macroscopic objects, because they do not assemble according to simple, intuitive rules. Their structures result from the balance of many ...weak interactions, rather than from the strong and predictable bonding patterns found in metal-organic frameworks and covalent organic frameworks. Hence, design strategies that assume a topology or other structural blueprint will often fail. Here we combine computational crystal structure prediction and property prediction to build energy-structure-function maps that describe the possible structures and properties that are available to a candidate molecule. Using these maps, we identify a highly porous solid, which has the lowest density reported for a molecular crystal so far. Both the structure of the crystal and its physical properties, such as methane storage capacity and guest-molecule selectivity, are predicted using the molecular structure as the only input. More generally, energy-structure-function maps could be used to guide the experimental discovery of materials with any target function that can be calculated from predicted crystal structures, such as electronic structure or mechanical properties.
Organic molecules with charge-transfer (CT) excited states are widely used in industry and are especially attractive as candidates for fabrication of energy efficient OLEDs, as they can harvest ...energy from nonradiative triplets by means of thermally activated delayed fluorescence (TADF). It is therefore useful to have computational protocols for accurate estimation of their electronic spectra in order to screen candidate molecules for OLED applications. However, it is difficult to predict the photophysical properties of TADF molecules with LR-TDDFT, as semilocal LR-TDDFT is incapable of accurately modeling CT states. Herein, we study absorption energies, emission energies, zero–zero transition energies, and singlet–triplet gaps of TADF molecules using a restricted open-shell Kohn–Sham (ROKS) approach instead and discover that ROKS calculations with semilocal hybrid functionals are in good agreement with experimentsunlike TDDFT, which significantly underestimates energy gaps. We also propose a cheap computational protocol for studying excited states with large CT character that is found to give good agreement with experimental results without having to perform any excited-state geometry optimizations.
The electronic and geometric structure of a prototypical polymer/fullerene interface used in photovoltaic cells (P3HT/PCBM) is investigated theoretically using a combination of classical and quantum ...simulation methods. It is shown that the electronic structure of P3HT in contact with PCBM is significantly altered compared to bulk P3HT. Due to the additional free volume of the interface, P3HT chains close to PCBM are more disordered, and consequently, they are characterized by an increased band gap. Excitons and holes are therefore repelled by the interface. This provides a possible explanation of the low recombination efficiency and supports the direct formation of “quasi-free” charge-separated species at the interface.
1 Center for the Neural Basis of Cognition, Mellon Institute, and 2 Department of Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania
Submitted 29 September 2006;
accepted in final form ...21 February 2007
In tasks requiring judgments about visual stimuli, humans exhibit repetition priming, responding with increased speed when a stimulus is repeated. Repetition priming might depend on repetition suppression, a phenomenon first observed in monkey inferotemporal cortex (IT) whereby, when a stimulus is repeated, the strength of the neuronal visual response is reduced. If the reduction resulted in sharpening of the cortical representation of the stimulus, and did not just scale it down, then speeded processing might result. To explore the relation between repetition priming and repetition suppression, we monitored neuronal activity in IT while monkeys performed a symmetry decision task. We found 1 ) that monkeys exhibit repetition priming, 2 ) that IT neurons simultaneously exhibit repetition suppression, 3 ) that repetition priming and repetition suppression do not vary in a significantly correlated fashion across trials, and 4 ) that repetition suppression scales down the representation of the stimulus without sharpening it. We conclude that repetition suppression accompanies repetition priming but is unlikely to be its cause.
Present address and address for reprint requests and other correspondence: D. McMahon, Laboratory of Neuropsychology, National Institutes of Health, 49 Convent Drive, Bethesda, MD 20892 (E-mail: dbtm{at}mit.edu )
Humans are able to efficiently learn and remember complex visual patterns after only a few seconds of exposure 1. At a cellular level, such learning is thought to involve changes in synaptic ...efficacy, which have been linked to the precise timing of action potentials relative to synaptic inputs 2–4. Previous experiments have tapped into the timing of neural spiking events by using repeated asynchronous presentation of visual stimuli to induce changes in both the tuning properties of visual neurons and the perception of simple stimulus attributes 5, 6. Here we used a similar approach to investigate potential mechanisms underlying the perceptual learning of face identity, a high-level stimulus property based on the spatial configuration of local features. Periods of stimulus pairing induced a systematic bias in face-identity perception in a manner consistent with the predictions of spike timing-dependent plasticity. The perceptual shifts induced for face identity were tolerant to a 2-fold change in stimulus size, suggesting that they reflected neuronal changes in nonretinotopic areas, and were more than twice as strong as the perceptual shifts induced for low-level visual features. These results support the idea that spike timing-dependent plasticity can rapidly adjust the neural encoding of high-level stimulus attributes 7–11.
► Perception of face identity is susceptible to stimulus timing-dependent plasticity ► The impact of stimulus timing on face perception shows scale invariance ► Plasticity had a stronger effect on face perception than on orientation perception
Face perception in both humans and monkeys is thought to depend on neurons clustered in discrete, specialized brain regions. Because primates are frequently called upon to recognize and remember new ...individuals, the neuronal representation of faces in the brain might be expected to change over time. The functional properties of neurons in behaving animals are typically assessed over time periods ranging from minutes to hours, which amounts to a snapshot compared to a lifespan of a neuron. It therefore remains unclear how neuronal properties observed on a given day predict that same neuron's activity months or years later. Here we show that the macaque inferotemporal cortex contains face-selective cells that show virtually no change in their patterns of visual responses over time periods as long as one year. Using chronically implanted microwire electrodes guided by functional MRI targeting, we obtained distinct profiles of selectivity for face and nonface stimuli that served as fingerprints for individual neurons in the anterior fundus (AF) face patch within the superior temporal sulcus. Longitudinal tracking over a series of daily recording sessions revealed that face-selective neurons maintain consistent visual response profiles across months-long time spans despite the influence of ongoing daily experience. We propose that neurons in the AF face patch are specialized for aspects of face perception that demand stability as opposed to plasticity.
Organic molecules tend to close pack to form dense structures when they are crystallised from organic solvents. Porous molecular crystals defy this rule: they contain open space, which is typically ...stabilised by inclusion of solvent in the interconnected pores during crystallisation. The design and discovery of such structures is often challenging and time consuming, in part because it is difficult to predict solvent effects on crystal form stability. Here, we combine crystal structure prediction (CSP) with a robotic crystallisation screen to accelerate the discovery of stable hydrogen-bonded frameworks. We exemplify this strategy by finding new phases of two well-studied molecules in a computationally targeted way. Specifically, we find a new 'hidden' porous polymorph of trimesic acid, δ-
TMA
, that has a guest-free hexagonal pore structure, as well as three new solvent-stabilized diamondoid frameworks of adamantane-1,3,5,7-tetracarboxylic acid (
ADTA
). Beyond porous solids, this hybrid computational-experimental approach could be applied to a wide range of materials problems, such as organic electronics and drug formulation.
New crystal forms of two well-studied organic molecules are identified in a computationally targeted way, by combining structure prediction with a robotic crystallisation screen, including a 'hidden' porous polymorph of trimesic acid.
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