No abstract

Visual short-term memory (VSTM) enables humans to form a stable and coherent representation of the external world. However, the nature and temporal dynamics of the neural representations in VSTM that support this stability are barely understood. Here we combined human intracranial electroencephalography (iEEG) recordings with analyses using deep neural networks and semantic models to probe the representational format and temporal dynamics of information in VSTM. We found clear evidence that VSTM maintenance occurred in two distinct representational formats which originated from different encoding periods. The first format derived from an early encoding period (250 to 770 ms) corresponded to higher-order visual representations. The second format originated from a late encoding period (1,000 to 1,980 ms) and contained abstract semantic representations. These representational formats were overall stable during maintenance, with no consistent transformation across time. Nevertheless, maintenance of both representational formats showed substantial arrhythmic fluctuations, i.e., waxing and waning in irregular intervals. The increases of the maintained representational formats were specific to the phases of hippocampal low-frequency activity. Our results demonstrate that human VSTM simultaneously maintains representations at different levels of processing, from higher-order visual information to abstract semantic representations, which are stably maintained via coupling to hippocampal low-frequency activity.

We theoretically study the field-free molecular orientation by a multi-color laser field with a superposition of the fundamental wave and its harmonics. It is shown that, the molecular orientation will be up to a maximum value pumped by the four-color laser field at the same laser intensity, and the molecular orientation direction and its degree can be controlled by varying the carrier-envelope phase of the four-color laser field. It is also indicated that the molecular orientation induced by a four-color laser field can be tremendously enhanced by applying another in-phase or out-phase four-color laser field at the beginning of the rotational wave packet rephasing or the end of the rotational wave packet dephasing.

A fundamental understanding of interactions between catalysts and gas molecules is essential for the development of efficient heterogeneous catalysts. In this study, ambient pressure X-ray photoelectron spectroscopy (APXPS) and density functional theory (DFT) simulation were employed to investigate the activation of CO2 on Cu surfaces, which acts as a key step in the catalytic reduction of CO2. APXPS results show that CO2 is adsorbed as CO2 δ− on the Cu(111) surface under a pressure of 0.01 mbar at 300 K. Adsorbed CO2 δ− gets partially transformed into carbonate with an increase of pressure to 1 mbar due to the disproportionation reaction between CO2 molecules. Subsequent annealing of the Cu(111) surface in a CO2 atmosphere leads to the dissociation of CO2 δ− and carbonate, and a transformation to a chemisorbed oxygen covered surface occurred at 400 K and elevated temperatures. However, on the Cu(110) surface, the CO2 δ− gradually dissociates to CO and chemisorbed oxygen in the presence of 1 mbar of CO2 at room temperature. The self-deactivation of CO2 adsorption due to the atomic oxygen generated by CO2 dissociation is observed on both Cu(111) and Cu(110) surfaces. Moreover, these experimental results indicate that the Cu(110) surface is more active than the Cu(111) surface in breaking C–O bonds, which is consistent with the results of DFT simulations. Our findings indicate that the activation of CO2 on Cu surfaces is strongly surface orientation- and pressure-dependent, which is an important step to clarify CO2 activation mechanisms on Cu-based catalysts.