Increasing the temporal g(r)ain: double-pulse resolution is affected by the size of the attention focus

Spatial cueing of transient attention has recently been shown to reduce temporal sensitivity. We investigated how the size of the sustained attentional focus influences double-pulse resolution (DPR) thresholds mapped across the visual field in a sample of 95 healthy subjects using a 9-fold interleaved adaptive algorithm (YAAP). Peripheral DPR thresholds increased for measurements between 2.5 degrees and 20 degrees eccentricity. Additionally, central DPR thresholds increased at a similar rate when measured with increasingly larger stimulus displays for peripheral measurements. This latter effect suggests that temporal resolution decreases with a larger sustained attention focus and cannot be explained by retinal characteristics only.     

Non-invasive assessment of the Ionescu-Shiley pericardial xenograft heart valve

This study was undertaken to describe the auscultatory, phonocardiographic, M-mode and two-dimensional echocardiographic features of the Ionescu-Shiley prosthesis in a group of unselected subjects. Twenty-seven valves were studied - 15 aortic, 10 mitral and 2 tricuspid. An opening sound was noted by phonocardiography and auscultation from the majority of valves in the three positions. A mid-systolic murmur was heard from 14 aortic valves and two had regurgitant murmurs. Three mitral valves had apical diastolic murmurs and two of these had mitral regurgitation. Two-dimensional echocardiography provided important spatial information but M-mode allowed superior resolution of valve leaflets. Calcification was seen in one four year old aortic valve and dehiscence of the sewing ring in another leading to severe aortic regurgitation. These data indicate that an opening sound is a frequent normal feature of an Ionescu-Shiley prosthetic valve and a mid systolic murmur is normal for aortic valves. In the three valve positions, the normal M-mode pattern resembles a native aortic valve. Abnormal features which can be recognized include calcification of valve cusps and dehiscence of the sewing ring.     

Preparation and pharmacokinetics of labeled derivatives of apamin

Apamin, the specific, centrally acting peptide neurotoxin from bee venom, was radiolabeled by different methods in order to study its in vitro and in vivo fate.

Iodination of the histidyl residue resulted in a derivative of diminished toxicity whose label was partially lost within 1 week. Furthermore, it was adsorbed on to Sephadex G-25 gel in a non-specific manner.

Acetylation with ³H-acetic anhydride yielded a slightly less toxic and, according to its behaviour on carboxymethyl cellulose, less basic derivative as compared with the starting material. Acetyl apamin was degraded in vitro by homogenates from liver, kidney, and brain, but not by serum, yielding ³H-acetic acid. In vivo it was degraded to tritiated water.

Reductive alkylation with formaldehyde and sodium borohydride resulted in methylated apamin possessing the basicity and about half the toxicity of the starting peptide. It was also degraded in vitro and in vivo, although to a lesser degree than acetyl apamin. Reductive alkylation appears to be the most favourable approach towards biologically active, labeled apamin.

The radiological or metabolic instability of the different apamin derivatives prevented a quantitative pharmacokinetic approach towards the in vivo fate of intact apamin. However, it is already evident that its acetyl or N-methyl derivatives are enriched not in the pharmacodynamic target organ, i.e. the central nervous system, but in the kidney.     

An intron within the 16S ribosomal RNA gene of the archaeon Pyrobaculum aerophilum.

The 16S rRNA genes of Pyrobaculum aerophilum and Pyrobaculum islandicum were amplified by the polymerase chain reaction, and the resulting products were sequenced directly. The two organisms are closely related by this measure (over 98% similar). However, they differ in that the (lone) 16S rRNA gene of Pyrobaculum aerophilum contains a 713-bp intron not seen in the corresponding gene of Pyrobaculum islandicum. To our knowledge, this is the only intron so far reported in the small subunit rRNA gene of a prokaryote. Upon excision the intron is circularized. A secondary structure model of the intron-containing rRNA suggests a splicing mechanism of the same type as that invoked for the tRNA introns of the Archaea and Eucarya and 23S rRNAs of the Archaea. The intron contains an open reading frame whose protein translation shows no certain homology with any known protein sequence.     

Analysis of pathways mediating preserved vision after striate cortex lesions

This study investigated the neural substrates of preserved visual functioning in a patient with homonymous hemianopsia and Riddoch syndrome after a posterior cerebral artery stroke affecting the primary visual cortex (area V1). The limited visual abilities of this patient included above-chance verbal reports of movement and color change as well as discrimination of movement direction in the hemianopic field. Functional magnetic resonance imaging showed that motion and color-change stimuli presented to the hemianopic field produced activation in several extrastriate areas of the lesioned hemisphere that were defined using retinotopic mapping. Magnetoencephalographic recordings indicated that evoked activity occurred earlier in the higher-tier visual areas V4/V8 and V5 than in the lower-tier areas V2/V3 adjacent to the lesion. In addition, the functional magnetic resonance imaging analysis showed an increased functional connectivity between areas V4/V8 and V5 of the lesioned hemisphere in comparison with the same areas in the intact hemisphere during the presentation of color changes. These results suggest that visual perception after the V1 lesion in Riddoch syndrome is mediated by subcortical pathways that bypass V1 and project first to higher-tier visual areas V5 and V4/V8 and subsequently to lower-tier areas V2/V3.     

A mirror in the mind: a case of visual allaesthesia in homonymous hemianopia

We report the case of a 61-year-old female patient with intracranial bleeding in left parietal and parieto-temporal regions and a history of epilepsy and migraine. MR images showed lesions of the optic radiation, but primary visual cortical areas were intact. Perimetric testing revealed an incomplete right hemianopia. The patient claimed that visual percepts from her intact field were projected as "mirror images" into the hemianopic field. The illusory images were weak and sometimes difficult to detect, but focusing spatial attention on the "mirror" image increased its saliency. Drawings the patient made of her pseudo-hallucinations revealed that the illusions were lateral transpositions instead of mirror images of real objects. The illusions were tilted in clockwise direction and were never colored, although color discrimination was unimpaired in the patient's left hemifield. We quantified the characteristics of the pseudohallucinations in several experiments: The patient was asked to adjust the position, rotation angle, and size of a white test card in her blind field so that it corresponded with the illusory projection of a card of standard size and position that was presented in the intact field. The test card was compressed in horizontal size by 20% and positioned 17 degrees visual angle to the right of the standard, shifted 1.5 degrees upward or downward, and rotated in clockwise direction by 22.6 degrees on average. Large objects in the intact field were projected incompletely into the blind area. Our patient's symptoms are similar to those reported in earlier case studies of visual allaesthesia. We hypothesize that the "mirror image" is induced by sparse input from contralesional V1 via the corpus callosum upstream of the lesion site and a lack of inhibition or hyperexcitability of ipsilesional early visual areas after deafferentation. The rotation of the illusions may be induced by the parietal lesions causing faulty co-ordinate computations, e.g., an inability to integrate visual and otholitic input.