Existence and stability of local excitations in homogeneous neural fields

Summary Dynamics of excitation patterns is studied in one-dimensional homogeneous lateral-inhibition type neural fields. The existence of a local excitation pattern solution as well as its waveform stability is proved by the use of the Schauder fixed-point theorem and a generalized version of the Perron-Frobenius theorem of positive matrices to the function space. The dynamics of the field is in general multi-stable so that the field can keep short-term memory.     

Neural theory of association and concept-formation

The present paper looks for possible neural mechanism underlying such high-level brain functioning as association and concept-formation. Primitive neural models of association and concept-formation are presented, which will elucidate the distributed and multiply superposed manner of retaining knowledge in the brain. The models are subject to two rules of self-organization of synaptic weights, orthogonal and covariance learning. The convergence of self-organization is proved, and the characteristics of these learning rules are shown. The performances, especially the noise immunity, of the association net and concept-formation net are analyzed.This research was supported in part by a grant by the Sloan Foundation to the Center for Systems Neuroscience, University of Massachusetts     

Purification of CBS 819.72 α-amylase by aqueous two-phase systems: Modelling using Response Surface Methodology

The partition and purification of α-amylase from a culture supernatant of Aspergillus oryzae CBS 819.72 was made in aqueous two-phase system (ATPS). According to bibliography and preliminary studies, the factors polyethylene glycol (PEG) molecular weight (M_PEG) and concentration (C_PEG), buffer type (BU) and concentration (C_BU), temperature (T), salt nature (SALT) and concentration (C_SALT), bioligand (BL) and concentration (C_BL) and pH were investigated using a Plackett–Burman design to identify the factors affecting separation. Taking into consideration a simultaneous increase in enzyme recovery (R_Y) and purification factor (P_F), the best performance of the system was obtained at 4 °C and pH 6 using PEG 8000 g/mol, citrate buffer, KCl and sucrose. Experimental Box–Behnken design together with the Response Surface Methodology (RSM) have been used to find optimum C_PEG, C_Citrate and C_SALT. Quadratic models were predicted for P_F and R_Y in the top phase and a better compromise between these two parameters can be found by superimposing the contour plots of P_F and R_Y for 8% citrate. A region in the experimental space can be defined where the purification factor is always higher than 3 with yields exceeding 65%.     

Inoculum standardization for biodegradability tests

Summary To obtain standardized inoculum with constant characteristics for biodegradability test, Actizym powder was used instead of sludge. A predigestion and stabilization step were introduced to prepare “activated Actizym”, before inoculation and addition of test chemical. The inoculum obtained is similar to activated sludge in its microbiological and physical aspects. The reproducibility of the method was prooved and several controls were made using chemicals with different biodegradation rates.