Spatial and Feature-Based Attention in a Layered Cortical Microcircuit Model

Directing attention to the spatial location or the distinguishing feature of a visual object modulates neuronal responses in the visual cortex and the stimulus discriminability of subjects. However, the spatial and feature-based modes of attention differently influence visual processing by changing the tuning properties of neurons. Intriguingly, neurons'' tuning curves are modulated similarly across different visual areas under both these modes of attention. Here, we explored the mechanism underlying the effects of these two modes of visual attention on the orientation selectivity of visual cortical neurons. To do this, we developed a layered microcircuit model. This model describes multiple orientation-specific microcircuits sharing their receptive fields and consisting of layers 2/3, 4, 5, and 6. These microcircuits represent a functional grouping of cortical neurons and mutually interact via lateral inhibition and excitatory connections between groups with similar selectivity. The individual microcircuits receive bottom-up visual stimuli and top-down attention in different layers. A crucial assumption of the model is that feature-based attention activates orientation-specific microcircuits for the relevant feature selectively, whereas spatial attention activates all microcircuits homogeneously, irrespective of their orientation selectivity. Consequently, our model simultaneously accounts for the multiplicative scaling of neuronal responses in spatial attention and the additive modulations of orientation tuning curves in feature-based attention, which have been observed widely in various visual cortical areas. Simulations of the model predict contrasting differences between excitatory and inhibitory neurons in the two modes of attentional modulations. Furthermore, the model replicates the modulation of the psychophysical discriminability of visual stimuli in the presence of external noise. Our layered model with a biologically suggested laminar structure describes the basic circuit mechanism underlying the attention-mode specific modulations of neuronal responses and visual perception.     

Plasma membrane of younger and outer cells is the primary specific site for aluminium toxicity in roots

Experiments were carried out to identify the primary site for aluminium (Al) toxicity in roots. Al accumulated in large amounts in the younger and outer cells in roots of pea and was retarded when the ionic strength of the Al solution was high. Cell destruction was extensive in the regions with high Al accumulation. The accumulation of Al in, and potassium (K) leakage from, the root tip were in the order pea>maize>rice, the same order as their sensitivity to Al.The protoplasts from the root tip portion of pea incubated with Al showed a wrinkled and uneven surface. The protoplasts progressively shrank and eventually collapsed. Viability decreased in this process. In the control protoplasts of maize, -glucan formation was uniform on the spherical surfaces, whereas it was spotty in the Al-treated protoplasts; the cell wall material of the latter contained partly 1, 3--glucan which is known to be synthesised by 1, 3--glucan synthase embedded in the plasma membrane. These results suggest that the specific site for Al toxicity is the plasma membrane of younger and outer cells in roots and that Al tolerance depends largely on the integrity of the plasma membrane.     

Effect of root exudate fractions from P-deficient and P-sufficient onion plants on root colonisation by the arbuscular mycorrhizal fungus Gigaspora margarita

 The effect of root exudates from P-deficient onion on root colonisation by an arbuscular mycorrhizal fungus was examined. Onions (Allium cepa L.) were grown in solution culture at phosphorus concentrations of 0 (P0) and 2 (P2) mg P l^–1. Root exudates were collected and fractionated with Amberlite XAD-4 resin to give EtOH and water soluble fractions. Onions inoculated with the arbuscular mycorrhizal fungus Gigaspora margarita Becker & Hall were grown with or without (control) root exudates and exudate fractions in a growth chamber. After 24 days, arbuscular mycorrhiza levels and appressoria formation had increased in plants treated with P0-root exudate or the P0-EtOH fraction when compared to corresponding P2 treatments or control plants. P0 and P2 water-soluble fractions did not significantly affect either aspect of fungal development. These results suggest that hydrophobic compounds found in root exudates from P-deficient onion increase appressorium formation and, therefore, enhance mycorrhiza development. Accepted: 2 June 1998     

Detection of beta-globin mRNA precursor in heterogeneous nuclear ribonucleoprotein associated with U1-RNP by using anti-RNP antibody

Heterogeneous nuclear RNA-ribonucleoprotein (hnRNP) fractions were isolated from Friend erythroleukemia cells and separated by 15-45% sucrose gradient centrifugation. The distribution of small nuclear RNAs (snRNAs) in hnRNP fractions indicated that the snRNAs are associated with hnRNP particles. HnRNP fractions were incubated with normal IgG or anti-U1 RNP IgG, and the resulting immunocomplexes were isolated by binding to a protein A-Sepharose column. HnRNP was found in bound fractions only when anti-U1 RNP IgG was used. By Northern hybridization of RNA extracted from the immunocomplexes with a beta-globin genomic DNA probe, 15S beta-globin mRNA precursors and 10S mature mRNA were detected. These findings suggest the existence of a complex of U1 RNP particles and hnRNP particles containing beta-globin pre-mRNA.     

Direction and viewing area-sensitive influence of EOG artifacts revealed in the EEG topographic pattern analysis

The influence of eye movement-related artifacts on electroencephalography (EEG) signals of human subjects, who were requested to perform a direction or viewing area dependent saccade task, was investigated by using a simultaneous recording with ocular potentials as electro-oculography (EOG). In the past, EOG artifact removals have been studied in tasks with a single fixation point in the screen center, with less attention to the sensitivity of cornea-retinal dipole orientations to the EEG head map. In the present study, we hypothesized the existence of a systematic EOG influence that differs according to coupling conditions of eye-movement directions with viewing areas including different fixation points. The effect was validated in the linear regression analysis by using 12 task conditions combining horizontal/vertical eye-movement direction and three segregated zones of gaze in the screen. In the first place, event-related potential topographic patterns were analyzed to compare the 12 conditions and propagation coefficients of the linear regression analysis were successively calculated in each condition. As a result, the EOG influences were significantly different in a large number of EEG channels, especially in the case of horizontal eye-movements. In the cross validation, the linear regression analysis using the appropriate dataset of the target direction/viewing area combination demonstrated an improved performance compared with the traditional methods using a single fixation at the center. This result may open a potential way to improve artifact correction methods by considering the systematic EOG influence that can be predicted according to the view angle such as using eye-tracker systems.     

Phosphorus deficiency enhances aluminum tolerance of rice (Oryza sativa) by changing the physicochemical characteristics of root plasma membranes and cell walls

The negative charge at the root surface is mainly derived from the phosphate group of phospholipids in plasma membranes (PMs) and the carboxyl group of pectins in cell walls, which are usually neutralized by calcium (Ca) ions contributing to maintain the root integrity. The major toxic effect of aluminum (Al) in plants is the inhibition of root elongation due to Al binding tightly to these negative sites in exchange for Ca. Because phospholipid and pectin concentrations decrease in roots of some plant species under phosphorus (P)-limiting conditions, we hypothesized that rice (Oryza sativa L.) seedlings grown under P-limiting conditions would demonstrate enhanced Al tolerance because of their fewer sites on their roots. For pretreatment, rice seedlings were grown in a culture solution with (+P) or without (−P) P. Thereafter, the seedlings were transferred to a solution with or without Al, and the lipid, pectin, hemicellulose, and mineral concentrations as well as Al tolerance were then determined. Furthermore, the low-Ca tolerance of P-pretreated seedlings was investigated under different pH conditions. The concentrations of phospholipids and pectins in the roots of rice receiving −P pretreatment were lower than those receiving +P pretreatment. As expected, seedlings receiving the −P pretreatment showed enhanced Al tolerance, accompanied by the decrease in Al accumulation in their roots and shoots. This low P-induced enhanced Al tolerance was not explained by enhanced antioxidant activities or organic acid secretion from roots but by the decrease in phospholipid and pectin concentrations in the roots. In addition, low-Ca tolerance of the roots was enhanced by the −P pretreatment under low pH conditions. This low P-induced enhancement of low-Ca tolerance may be related to the lower Ca requirement to maintain PM and cell wall structures in roots of rice with fewer phospholipids and pectins.     

Arsenic Exposure Affects Plasma Insulin-Like Growth Factor 1 (IGF-1) in Children in Rural Bangladesh

Background

Exposure to inorganic arsenic (As) through drinking water during pregnancy is associated with lower birth size and child growth. The aim of the study was to assess the effects of As exposure on child growth parameters to evaluate causal associations.

Methodology/Findings

Children born in a longitudinal mother-child cohort in rural Bangladesh were studied at 4.5 years (n=640) as well as at birth (n=134). Exposure to arsenic was assessed by concurrent and prenatal (maternal) urinary concentrations of arsenic metabolites (U-As). Associations with plasma concentrations of insulin-like growth factor 1 (IGF-1), calcium (Ca), vitamin D (Vit-D), bone-specific alkaline phosphatase (B-ALP), intact parathyroid hormone (iPTH), and phosphate (PO₄) were evaluated by linear regression analysis, adjusted for socioeconomic factor, parity and child sex. Child U-As (per 10 µg/L) was significantly inversely associated with concurrent plasma IGF-1 (β=-0.27; 95% confidence interval: -0.50, -0.0042) at 4.5 years. The effect was more obvious in girls (β=-0.29; -0.59, 0.021) than in boys, and particularly in girls with adequate height (β=-0.491; -0.97, -0.02) or weight (β=-0.47; 0.97, 0.01). Maternal U-As was inversely associated with child IGF-1 at birth (r=-0.254, P=0.003), but not at 4.5 years. There was a tendency of positive association between U-As and plasma PO₄ in stunted boys (β=0.27; 0.089, 0.46). When stratified by % monomethylarsonic acid (MMA, arsenic metabolite) (median split at 9.7%), a much stronger inverse association between U-As and IGF-1 in the girls (β=-0.41; -0.77, -0.03) was obtained above the median split.

Conclusion

The results suggest that As-related growth impairment in children is mediated, at least partly, through suppressed IGF-1 levels.     

Pharmacological inhibition of HSP90 activity negatively modulates myogenic differentiation and cell survival in C2C12 cells

Heat-shock protein90 (HSP90) plays an essential role in maintaining stability and activity of its clients. HSP90 is involved in cell differentiation and survival in a variety of cell types. To elucidate the possible role of HSP90 in myogenic differentiation and cell survival, we examined the time course of changes in the expression of myogenic regulatory factors, intracellular signaling molecules, and anti-/pro-apoptotic factors when C2C12 cells were cultured in differentiation condition in the presence of a HSP90-specific inhibitor, geldanamycin. Furthermore, we examined the effects of geldanamycin on muscle regeneration in vivo. Our results showed that geldanamycin inhibited myogenic differentiation with decreased expression of MyoD, myogenin and reduced phosphorylation levels of Akt1. Geldanamycin had little effect on the phosphorylation levels of p38MAPK and ERK1/2 but reduced the phosphorylation levels of JNK. Along with myogenic differentiation, geldanamycin increased apoptotic nuclei with decreased expression of Bcl-2. The skeletal muscles forced to regenerate in the presence of geldanamycin were of poor repair with small regenerating myofibers and increased connective tissues. Together, our findings suggest that HSP90 may modulate myogenic differentiation and may be involved in cell survival.