Reduced Cognitive Control of a Visually Bistable Image in Schizophrenia

Schizophrenia is associated with the inability to control and coordinate thoughts, actions, and perceptions. In conventional assessments of cognitive control, multiple sensory features of stimuli are concomitantly manipulated, introducing a confounding role of bottom-up perceptual information. To overcome this difficulty, we used an ambiguous visual stimulus (Necker cube), which allowed measurement of cognitive control with constant sensory input. Subjects (20 patients, 20 controls) were asked to control their perception of a transparent Necker cube by keeping a designated plane at the front or back of the stimulus, the position of which is perceptually bistable. Patients were highly deficient at controlling their perception of the cube. When a visual feature (the luminance contrast between a designated cube plane and the other planes) was systematically manipulated, an interaction was found whereby schizophrenia patients no longer under-performed on the highest contrast condition. These results show patients’ impairment of controlling perception in the absence of visual modulation and suggest the potential utility of perceptually based approaches to cognitive remediation in schizophrenia. (JINS, 2011, 551–556)

Creep of Anomalous Ni3Ga

ABSTRACTNi3Ga is among a number of LI2 ordered intermetallic alloys whose yield stress increases with temperature. In this work we have examined the creep strength of [123] and [001] oriented Ni3Ga specimens in the temperature regime of the yield stress anomaly and confirmed that the creep strength shows the normal rapid decrease with temperature. Inverse creep occurs in the [001] specimens where slip is on the {111} planes only. [123] specimens exhibit steady-state creep and slip line and TEM observations have shown slip on the cube plane and dislocations of both <110> and <100> Burgers vectors are present. We have carried out creep tests of prestrained [123] samples and demonstrated that the storage of primary <110> {111} screw dislocations, locked in the Kear-Wilsdorf configuration, has no effect on creep.

Modeling of the Dislocation Dynamics in Ni3Al and the Flow Stress Anomaly

ABSTRACTNi3Al single crystals are known to exhibit a flow stress anomaly between 200 and 800K. The purpose of our work is to examine such an anomaly by means of a simulation of the dislocation dynamics at a mesoscopic scale. The simulation basic rules are: i) the dislocation glide in {111} octahedral planes, ii) the conditions at which screw lines are locked and unlocked by the formation of Kear-Wilsdorf locks, iii) the mobility of jogs in the {100} cube plane. Our results suggest that two different temperature regimes occur in the domain of the anomaly. At low temperatures, the plastic flow is governed by kink bow-out, itself a function of the kink length. At high temperatures, the plastic flow is governed by the unlocking of the weakest Kear-Wilsdorf locks in the microstructure. These outcomes of the simulation are discussed in relation with the existing theoretical models of the flow stress anomaly.

An Analysis of The Slip of Screw Dislocations in Ll2 Alloys

ABSTRACTThe annihilation of dislocations by cross-slip is studied by numerical simulation of infinitely long dissociated screw dislocations, allowed to move in an elastically anisotropie crystal. The external load is along [123] and cross-slip is permitted both on the octahedral and on the cube plane. The latter, together with cube slip, is thermally activated. Anisotropie elasticity modifies the properties of cross-slip significantly. Under the conditions of the simulations, the processes of APB jumps (APBJs) and repeated APB jumps (RAPBJs) can be largely promoted by interactions with other dislocations, while it is much less likely to occur at an isolated dislocation submitted to the same applied stress. The encounter of dislocations of opposite signs produces dipoles which may or may not tend to annihilate by cross-slip. APB tubes may form upon annihilation under certain circumstances again largely controlled by elastic anisotropy.

The Mechanics of Cross-Slip in L12 Alloys

Relationships between the microstructural organization in Ll2 alloys and the flow stress anomaly are discussed. Attention is paid to the origin of friction on the cube plane, to the stability of Kear-Wilsdorf (KW) locks and to the fine structure of kinks. The difficulty of dislocation multiplication and the easiness of their annihilation are outlined. Directions for a future model of the flow stress anomaly based on a close analysis of the microstructure are given.

The Dissociation of Sessile Superdislocations at a Coherent Twin Eoundary in Ordered CU3AU During Plastic Deformation

The glide behaviour of superdislocations at a coherent twin boundary in ordered Cu3Au was examined in a transmission electron microscope for the case when the superdislocations are sessile in the boundary. Possible schemes for dissociation of a superdislocation in the boundary were analysed geometrically. The leading superpartial of each superdislocation dissociated into a superpartial in the matrix and a residual Shockley partial in the boundary of glissile type. The trailing superpartial remained undissociated in the boundary. The superpartial in the matrix glided on a cube plane, and a ribbon of APB connected to the boundary was left in its trail. The cube slip occurs as a result of (i) a maximal resolved shear stress for the observed slip system and (ii) the geometric criteria for slip applied to all possible slip systems in the matrix. The Schmid factors for the slip systems in the matrix could be calculated by assuming a uniform tensile axis in the foil. The tensile axis was deduced from the observed slip systems in the twin.

The Deformation Microstructure in NI3Si Polycrystals Strained Over the Range of Temperature of Flow Stress Anomaly

ABSTRACTIn order to correlate the flow stress anomaly of Ni3Si with dislocation properties, a weakbeam study ofpolycrystalline samples deformed between ambient and the peak temperature was carried out. Samples with two extreme Ni/Si ratios were tested.The most frequently activated slip system changes progressively from octahedral to cubic with increasing temperature. The transformation of superdislocations into Kear-Wilsdorf configurations gives rise to screw dislocations that are rectilinear only after deformation at room temperature. The effect of temperature is to gradually promote bending of Kear-Wisdorf configurations in the cube plane, from a few nanometers at 230°C to several tenths of micrometers at intermediate temperature. Cube slip begins to be massively activated a little below the peak temperature. It is suggested that the flow stress anomaly is controlled by progressive exhaustion of octahedral slip by thermally-activated expansion of superdislocations on the cube cross-slip plane.

A Tem Study of the Relationship Between Dislocation Structure and the Anomalous Temperature Dependence of the Flow Stress in Ordered Ni3 (Al,1%Ta).

ABSTRACTThe anomalous increase in the yield strength of single crystals of Ni3AI(1%Ta) as a function of temperature has been correlated with the post-deformation substructure using weak beam TEM techniques. At low temperatures (77K), there is evidence for abundant cross-slip between (111) and (111). With increasing temperature, this conventional cross-slip process is gradually replaced by the formation of KW locks--straight screw segments which have crossslipped and completely dissociated on (010). This change in the mode of cross slip corresponds with the onset of the yield strength anomaly. At still higher temperatures (544K and 715K), the KW locks become mobile and bow out on (010). The density of dislocations on the cube plane also increases sharply at higher temperatures, even though deformation occurs principally by glide on the primary (111) based on slip trace analysis. These observations are inconsistent with the widely accepted cross slip pinning model and suggest that deformation occurs primarily by the motion of the non-screw components. A dislocation model is introduced which attempts to account for the observed dislocation configurations while remaining consistent with the yield strength and work hardening behavior of these alloys.