Tracking the implicit acquisition of nonadjacent transitional probabilities by ERPs

The implicit acquisition of complex probabilistic regularities has been found to be crucial in numerous automatized cognitive abilities, including language processing and associative learning. However, the neurocognitive processes supporting the implicit extraction of 2nd order non-adjacent transitional probabilities have not been completely elucidated. Therefore, this study investigated the sensitivity of event-related brain potentials (ERPs) to these probabilistic regularities embedded in a sequence of visual stimuli without providing explicit information on the structure of the stimulus stream. Healthy young adults (N = 32) performed a perceptual-motor RT task that included an alternating sequential regularity between non-adjacent trials while RTs and ERPs were measured time-locked to the onset of the stimulus. RT effects indicated the rapid acquisition of transitional probabilities. The acquisition process was also tracked by the P3 component. Modulations of the P3 amplitude indicated that the more probable short-range relations could be integrated in the internal representations formed on the experienced stimulus environment. Meanwhile, the less probable short-range relations delivered possibly surprising information over the course of the task, by which the internal representations could have been updated. These results suggest that representations on the probabilistic regularities of the ongoing stimulus context are implicitly formed and constantly revised. Overall, this study (1) highlights the role of predictive processes during implicit memory formation, and (2) delineates a potential to gain further insight into the dynamics of implicit acquisition processes

Divided attention does not affect the acquisition and consolidation of transitional probabilities

Statistical learning facilitates the efficient processing and prediction of environmental events and contributes to the acquisition of automatic behaviors. Whereas a minimal level of attention seems to be required for learning to occur, it is still unclear how acquisition and consolidation of statistical knowledge are affected when attention is divided during learning. To test the effect of divided attention on statistical learning and consolidation, ninety-six healthy young adults performed the Alternating Serial Reaction Time task in which they incidentally acquired second-order transitional probabilities. Half of the participants completed the task with a concurrent secondary intentional sequence learning task that was applied to the same stimulus stream. The other half of the participants performed the task without any attention manipulation. Performance was retested after a 12-h post-learning offline period. Half of each group slept during the delay, while the other half had normal daily activity, enabling us to test the effect of delay activity (sleep vs. wake) on the consolidation of statistical knowledge. Divided attention had no effect on statistical learning: The acquisition of second-order transitional probabilities was comparable with and without the secondary task. Consolidation was neither affected by divided attention: Statistical knowledge was similarly retained over the 12-h delay, irrespective of the delay activity. Our findings can contribute to a better understanding of the role of attentional processes in and the robustness of visuomotor statistical learning and consolidation

Perceiving structure in unstructured stimuli: implicitly acquired prior knowledge impacts the processing of unpredictable transitional probabilities

It is unclear how implicit prior knowledge is involved and remains persistent in the extraction of the statistical structure underlying sensory input. Therefore, this study investigated whether the implicit knowledge of second-order transitional probabilities characterizing a stream of visual stimuli impacts the processing of unpredictable transitional probabilities embedded in a similar input stream. Young adults (N = 50) performed a four-choice reaction time (RT) task that consisted of structured and unstructured blocks. In the structured blocks, more probable and less probable short-range nonadjacent transitional probabilities were present. In the unstructured blocks, the unique combinations of the short-range transitional probabilities occurred with equal probability; therefore, they were unpredictable. All task blocks were visually identical at the surface level. While one-half of the participants completed the structured blocks first followed by the unstructured blocks, this was reversed in the other half of them. The change in the structure was not explicitly denoted, and no feedback was provided on the correctness of each response. Participants completing the structured blocks first showed faster RTs to more probable than to less probable short-range transitional probabilities in both the structured and unstructured blocks, indicating the persistent effect of prior knowledge. However, after extended exposure to the unstructured blocks, they updated this prior knowledge. Participants completing the unstructured blocks first showed the RT difference only in the structured blocks, which was not constrained by the preceding exposure to unpredictable stimuli. The results altogether suggest that implicitly acquired prior knowledge of predictable stimuli influences the processing of subsequent unpredictable stimuli. Updating this prior knowledge seems to require a longer stretch of time than its initial acquisition

Catabolism of GABA, succinic semialdehyde or gamma-hydroxybutyrate through the GABA shunt impair mitochondrial substrate-level phosphorylation

GABA is catabolized in the mitochondrial matrix through the GABA shunt, encompassing transamination to succinic semialdehyde followed by oxidation to succinate by the concerted actions of GABA transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH), respectively. Gamma-hydroxybutyrate (GHB) is a neurotransmitter and a psychoactive drug that could enter the citric acid cycle through transhydrogenation with alpha-ketoglutarate to succinic semialdehyde and d-hydroxyglutarate, a reaction catalyzed by hydroxyacid-oxoacid transhydrogenase (HOT). Here, we tested the hypothesis that the elevation in matrix succinate concentration caused by exogenous addition of GABA, succinic semialdehyde or GHB shifts the equilibrium of the reversible reaction catalyzed by succinate-CoA ligase towards ATP (or GTP) hydrolysis, effectively negating substrate-level phosphorylation (SLP). Mitochondrial SLP was addressed by interrogating the directionality of the adenine nucleotide translocase during anoxia in isolated mouse brain and liver mitochondria. GABA eliminated SLP, and this was rescued by the GABA-T inhibitors vigabatrin and aminooxyacetic acid. Succinic semialdehyde was an extremely efficient substrate energizing mitochondria during normoxia but mimicked GABA in abolishing SLP in anoxia, in a manner refractory to vigabatrin and aminooxyacetic acid. GHB could moderately energize liver but not brain mitochondria consistent with the scarcity of HOT expression in the latter. In line with these results, GHB abolished SLP in liver but not brain mitochondria during anoxia and this was unaffected by either vigabatrin or aminooxyacetic acid. It is concluded that when mitochondria catabolize GABA or succinic semialdehyde or GHB through the GABA shunt, their ability to perform SLP is impaired

Long term stabilization of reaction center protein photochemistry by carbon nanotubes

The long term stability and the redox interaction between single walled carbon nanotubes (SWNTs) and photosynthetic reaction center proteins (RCs) purified from purple bacterium Rhodobacter sphaeroides R-26 in the SWNT/RC complex has been investigated. The binding of SWNT to RC results in an accumulation of positive (the oxidized primary electron donor, P+) and negative (semiquinone forms, Q(A)(-) and Q(B)(-), the reduced primary and secondary quinones, respectively) charges followed by slow reorganization of the protein structure after excitation. The photochemical activity of the SWNT/RC complexes remains stable for several weeks even in dried form. In the absence of SWNT the secondary quinone activity decays quickly as a function of time after drying the RC onto a glass surface. Polarography measurements substantiate the idea that there is an electronic interaction between the RCs and SWNTs after light excitation, which was suggested earlier by optical measurements. The special electronic properties of the SWNT/protein complexes open the possibility for several applications, e. g., in microelectronics, analytics, or energy conversion and storage. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Equilibrium concentration of singlet oxygen in photoreaction of reaction center/carbon nanotube bionanocomposites

The reaction center protein is the unit in the photosynthetic organisms in which the primary events of the photoelectric energy conversion take place during photosynthesis. When the photochemistry following the excitation of the reaction center by light is oversaturated there is a large probability to form reactive oxygen species (ROS, e.g., singlet oxygen (O-1(2))). Because the ROS components decrease the overall yield of the photochemical energy conversion there is a considerable effort in many laboratories to find conditions to reduce these harmful compounds. The aim of our work is to create a system by using carbon nanotubes (CNTs) and RCs for efficient light-energy conversion. The role of the O-1(2) that destroys the RC structure is discussed. 1,3-Diphenylisobenzofuran was used to detect the concentration of the O-1(2). Although, O-1(2) can be sensitized by CNTs in certain conditions, in our experiments the main sources of the O-1(2) are the RCs. The concentration of the O-1(2) in the equilibrium is determined by the forward sensitization and the backward deactivation processes of the components and functions of the CNT/RC composite. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Sensing hydrogen peroxide by carbon nanotube/horseradish peroxidase bio-nanocomposite

H2O2 is a product of reactions catalysed by several oxidase enzymes and it is essential in environmental and pharmaceutical analyses. The most commonly used enzyme in understanding the biological behaviour of catalysed oxidation of H2O2 is horseradish peroxidase (HRP). In our experiments HRP was bound to carboxyl-functionalized multiwalled carbon nanotubes (MWNT-COOH) by N-hydroxysuccinimide (NHS) and 1-[3-dimethylaminopropyl]-3-ethyl-carbodiimide (EDC) crosslinkers. The activity of this bio-nanocomposite and the limit of detection (LOD) for H2O2 were determined by measuring the fluorescence of tetraguaiacol (which chemical is the product of guaiacol oxidation after addition of H2O2 to the reaction mixture) as a function of time. The hydrogen peroxide biosensor we developed exhibited a detection limit of 1.2 mu MH(2)O(2)s(-1) which resolution was better than the one measured in solution by about a factor of eight (it was 10 mu MH(2)O(2)s(-1) in solution). An attempt has been made to measure the concentration of H2O2 in an electrochemical cell with HRP immobilized on the surface of an electrode made of indium tin oxide (ITO, a transparent conductive oxide) and MWNT. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Carbon nanotubes quench singlet oxygen generated by photosynthetic reaction centers

Photosensitizers may convert molecular oxygen into reactive oxygen species (ROS) including, e.g., singlet oxygen (O-1(2)), superoxide anion (O-2(-center dot)), and hydroxyl radicals ((OH)-O-center dot), chemicals with extremely high cyto- and potential genotoxicity. Photodynamic ROS reactions are determinative in medical photodynamic therapy (cancer treatment with externally added photosensitizers) and in reactions damaging the photosynthetic apparatus of plants (via internal pigments). The primary events of photosynthesis take place in the chlorophyll containing reaction center protein complex (RC), where the energy of light is converted into chemical potential. O-1(2) is formed by both bacterial bacteriochlorophylls and plant RC triplet chlorophylls in high light and if the quenching of O-1(2) is impaired. In plant physiology, reducing the formation of the ROS and thus lessening photooxidative membrane damage (including the RC protein) and increasing the efficiency of the photochemical energy conversion is of special interest. Carbon nanotubes, in artificial systems, are also known to react with singlet oxygen. To investigate the possibility of O-1(2) quenching by carbon nanotubes in a biological system, we studied the effect of carbon nanotubes on O-1(2) photogenerated by photosynthetic RCs purified from purple bacteria. 1,3-Diphenylisobenzofuran (DPBF), a dye responding to oxidation by O-1(2) with absorption decrease at 420nm was used to measure O-1(2) concentrations. O-1(2) was produced either from a photosensitizer (methylene blue) or from triplet photosynthetic RCs and the antioxidant capacity of carbon nanotubes was assessed. Less O-1(2) was detected by DPBF in the presence of carbon nanotubes, suggesting that these are potential quenchers of this ROS. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Photosynthetic reaction centre/carbon nanotube bundle composites

Since their discovery, carbon nanotubes (CNTs) have attracted intense attention to broad range of potential applications. In contrast to the 1D isolated single-walled carbon nanotubes (SWCNT), 2D films or bundles made of thousands of tubes have been introduced as more advantageous building blocks for new types of applications in mechanically flexible and stretchable, optically transparent electronic systems. In our experiments, we combined photosynthetic reaction centre proteins, the light energy converter units in living cells, purified from purple bacteria, with multiwalled carbon nanotube (MWCNT) bundles. The change in the conductivity of the bare MWCNT bundles and the RC/MWCNT composite after light excitation was measured and compared. We found that the electrical conductivity under light excitation depends on the intrinsic conductivity of individual tubes within the bundles and on structural characteristics, like geometry (diameter, length, spatial arrangement, interconnects, etc.) and the electronic coupling with the RCs. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei