A deficit of spatial remapping in constructional apraxia after right-hemisphere stroke

This Article is provided by the Brunel Open Access Publising Fund - Copyright @ 2010 Oxford University PressConstructional apraxia refers to the inability of patients to copy accurately drawings or three-dimensional constructions. It is a common disorder after right parietal stroke, often persisting after initial problems such as visuospatial neglect have resolved. However, there has been very little experimental investigation regarding mechanisms that might contribute to the syndrome. Here, we examined whether a key deficit might be failure to integrate visual information correctly from one fixation to the next. Specifically, we tested whether this deficit might concern remapping of spatial locations across saccades. Right-hemisphere stroke patients with constructional apraxia were compared to patients without constructional problems and neurologically healthy controls. Participants judged whether a pattern shifted position (spatial task) or changed in pattern (non-spatial task) across two saccades, compared to a control condition with an equivalent delay but without intervening eye movements. Patients with constructional apraxia were found to be significantly impaired in position judgements with intervening saccades, particularly when the first saccade of the sequence was to the right. The importance of these remapping deficits in constructional apraxia was confirmed through a highly significant correlation between saccade task performance and constructional impairment on standard neuropsychological tasks. A second study revealed that even single saccades to the right can impair constructional apraxia patients’ perception of location shifts. These data are consistent with the view that rightward eye movements result in loss of remembered spatial information from previous fixations, presumably due to constructional apraxia patients’ damage to the right-hemisphere regions involved in remapping locations across saccades. These findings provide the first evidence for a deficit in remapping visual information across saccades underlying right-hemisphere constructional apraxia.European Commission Marie Curie Intra-European Fellowship (011457 to C.R.) and a Wellcome Trust Senior Fellowship (to M.H.)

The two rotor components of yeast mitochondrial ATP synthase are mechanically coupled by subunit δ

The mitochondrial ATP synthase is made of a membrane-integrated F(0) component that forms a proton-permeable pore through the inner membrane and a globular peripheral F(1) domain where ATP is synthesized. The catalytic mechanism is thought to involve the rotation of a 10-12 c subunit ring in the F(0) together with the γ subunit of F(1). An important and not yet resolved question is to define precisely how the γ subunit is connected with the c-ring. In this study, using a doxycycline-regulatable expression system, we provide direct evidence that the rest of the enzyme can assemble without the δ subunit of F(1), and we show that δ-less mitochondria are uncoupled because of an F(0)-mediated proton leak. Based on these observations, and taking into account high-resolution structural models, we propose that subunit δ plays a key role in the mechanical coupling of the c-ring to subunit γ

A Method for Approximating Univariate Convex Functions using Only Function Value Evaluations

In this paper, piecewise linear upper and lower bounds for univariate convex functions are derived that are only based on function value information. These upper and lower bounds can be used to approximate univariate convex functions. Furthermore, new Sandwich algo- rithms are proposed, that iteratively add new input data points in a systematic way, until a desired accuracy of the approximation is obtained. We show that our new algorithms that use only function-value evaluations converge quadratically under certain conditions on the derivatives. Under other conditions, linear convergence can be shown. Some numeri- cal examples, including a Strategic investment model, that illustrate the usefulness of the algorithm, are given.