Una imagen PET amiloide estática del primer minuto (FMF) se correlaciona con [18F]FDG PET en pacientes con afasia progresiva primaria / Static first-minute-frame (FMF) PET imaging after 18F-labeled amyloid tracer injection is correlated to [18F]FDG PET in patients with primary progressive aphasia

Objetivo Estudiar la correlación entre una imagen PET estática del primer minuto (FMF) adquirida con radiotrazadores de amiloide marcados con flúor-18 y la PET cerebral con [18F]FDG en pacientes con afasia primaria progresiva (APP). Material y métodos La cohorte de estudio incluyó a 17 pacientes diagnosticados de APP con la siguiente distribución: 9APP variante no fluente, 4APP variante logopénica, 1APP variante semántica, 3APP inclasificables. Se extrajeron los SUVR regionales de las FMF y sus correspondientes imágenes PET con [18F]FDG y se calcularon los coeficientes de correlación de Pearson. Resultados Los SUVR de ambas imágenes mostraron patrones similares de alteración cerebral regional. Los análisis de correlación intrapaciente dieron como resultado un coeficiente medio de r=0,94 ±0,06. Los coeficientes de correlación regional entre pacientes de la cohorte del estudio fueron superiores a 0,81. Las subcohortes específicas según el radiotrazador y la variante de APP no mostraron diferencias en la similitud de las imágenes. Conclusiones La FMF estática podría ser una alternativa válida a la PET dinámica de amiloide en fase inicial propuesta en la literatura, así como un biomarcador de neurodegeneración para el diagnóstico y la clasificación de la APP en los estudios de PET amiloide (AU)

Objective To study the correlation between a static PET image of the first-minute-frame (FMF) acquired with 18F-labeled amyloid-binding radiotracers and brain [18F]FDG PET in patients with primary progressive aphasia (PPA). Material and methods The study cohort includes 17 patients diagnosed with PPA with the following distribution: 9nonfluent variant PPA, 4logopenic variant PPA, 1semantic variant PPA, 3unclassifiable PPA. Regional SUVRs are extracted from FMFs and their corresponding [18F]FDG PET images and Pearson's correlation coefficients are calculated. Results SUVRs of both images show similar patterns of regional cerebral alterations. Intrapatient correlation analyses result in a mean coefficient of r=.94 ±.06. Regional interpatient correlation coefficients of the study cohort are greater than 0.81. Radiotracer-specific and variant-specific subcohorts show no difference in the similarity between the images. Conclusions The static FMF could be a valid alternative to dynamic early-phase amyloid PET proposed in the literature, and a neurodegeneration biomarker for the diagnosis and classification of PPA in amyloid PET studies (AU)

Inefficient visual search for second-order motion.

Visual search rate was used to assess attentional resources required for detection of opposing motions defined either by luminance or by modulations of texture contrast, flicker, or size. Though luminance-based targets were detected quickly, search through second-order motion was slow. Control experiments ruled out stimuli visibility, complexity, eccentricity sensitivity, and attributes of the carrier as possible accounts. Results suggest separate processing of the two types of stimuli: Luminance-based motion is detected by spatiotemporal filters, whereas second-order motion is likely processed by a capacity-limited, later stage. Rate-reducing effects of increased contrast and speed mirrored previous research suggesting that effortful feature tracking may be the mechanism.

Functional MRI reveals spatially specific attentional modulation in human primary visual cortex.

Selective visual attention can strongly influence perceptual processing, even for apparently low-level visual stimuli. Although it is largely accepted that attention modulates neural activity in extrastriate visual cortex, the extent to which attention operates in the first cortical stage, striate visual cortex (area V1), remains controversial. Here, functional MRI was used at high field strength (3 T) to study humans during attentionally demanding visual discriminations. Similar, robust attentional modulations were observed in both striate and extrastriate cortical areas. Functional mapping of cortical retinotopy demonstrates that attentional modulations were spatially specific, enhancing responses to attended stimuli and suppressing responses when attention was directed elsewhere. The spatial pattern of modulation reveals a complex attentional window that is consistent with object-based attention but is inconsistent with a simple attentional spotlight. These data suggest that neural processing in V1 is not governed simply by sensory stimulation, but, like extrastriate regions, V1 can be strongly and specifically influenced by attention.

Position-based motion perception for color and texture stimuli: effects of contrast and speed.

Motion can be perceived either through low-level, motion-energy detection or through tracking the change in position of features. Previously we have shown that, while luminance-based motion likely is detected with velocity-sensitive motion-energy units, patterns defined by texture or binocular disparity ('second-order' stimuli) were tracked by a position-sensitive mechanism (Seiffert & Cavanagh (1998) Vision Research, 38, 3569-3582). Here, we use the same technique, measuring motion amplitude thresholds of oscillating gratings over a range of temporal frequencies and find that the motion of low-contrast equiluminant red/green gratings is also detected with position tracking. In addition, we find that as contrast or speed increases these results change: high-contrast or high-speed equiluminant color or texture-based motion is detected by velocity-sensitive mechanisms. These results help resolve the dispute over the processes which detect the motion of non-luminance based stimuli. Both systems are available, but their relative efficiency changes as a function of contrast and speed. A position-tracking process is more sensitive at low contrasts and low speeds whereas a motion-energy system is more sensitive at high contrasts and high speeds.

Wakes and spokes: new motion-induced brightness illusions.

Under certain conditions, high-contrast moving figures induce adjacent illusory regions, 'wakes' and 'spokes', which have contrast polarity opposite the inducing figures. In this paper we document properties of these novel phenomena. When the illusions are induced by a moving bar, spokes appear on the side of the bar closer to fixation and connect the bar to the fixation point, regardless of the momentary position of the bar or whether it is moving to the left or to the right. Although spokes often extend up to the fixation point, they never extend beyond it. This is not due to blocking of the spoke's spread by the fixation point, because in another experiment spokes extend directly through an intervening figure. Whereas spokes emanate from the end of a horizontally moving bar closest to fixation, wakes emanate from the end farthest from fixation. In contrast to spokes, wakes do not show a towards-fixation bias. Instead, the wake's end trails the position of the bar, like a ship's wake. The higher the bar velocity, the more the end of the wake appears to trail it, suggesting that wakes are caused by a process which spreads from the edge of moving figures. Wakes and spokes, as distinct illusions, should provide significant constraints on theories of human motion and brightness perception processes.

Selective attention: a reevaluation of the implications of negative priming.

The notion that inhibitory processes play a critical role in selective attention has gained wide support. Much of this support derives from studies of negative priming. The authors note that the attribution of negative priming to an inhibitory mechanism of attention draws its support from a common assumption underlying priming procedures, together with the procedure that has been used to measure negative priming. The results from a series of experiments demonstrate that selection between 2 competing prime items is not required to observe negative priming. This result is demonstrated across several experiments in which participants named 1 of 2 items in a second display following presentation of a single-item prime. The implications of these results for existing theories of negative priming are discussed, and a theoretical framework for interpreting negative priming and several related phenomena is forwarded.

Position displacement, not velocity, is the cue to motion detection of second-order stimuli.

Motion detection can be achieved either with mechanisms sensitive to a target's velocity, or sensitive to change in a target's position. Using a procedure to dissociate these two provided by Nakayama and Tyler (Vis Res 1981;21:427-433), we explored detection of first-order (luminance-based) and various second-order (texture-based and stereo-based) motion. In the first experiment, observers viewed annular gratings oscillating in rotational motion at various rates. For each oscillation temporal frequency, we determined the minimum displacement of the pattern for which observers could reliably see motion. For first-order motion, these motion detection thresholds decreased with increasing temporal frequency, and thus were determined by a minimum velocity. In contrast, motion detection thresholds for second-order motion remained roughly constant across temporal frequency, and thus were determined by a minimum displacement. In Experiment 2, luminance-based gratings of different contrasts were tested to show that the velocity-dependence was not an artifact of pattern visibility. In the remaining experiments, results similar to Experiment 1 were obtained with a central presentation of a linear grating, instead of an annular grating (Experiment 3), and with a motion discrimination (phase discrimination) rather than motion detection task (Experiment 4). We conclude that, within the ranges tested here, second-order motion is more readily detected with a mechanism which tracks the change of position of features over time.

Phosphor persistence of oscilloscopic displays: a comparison of four phosphors.

The period for which phosphor decay remains visible after stimulus offset was assessed for four phosphors commonly used in psychophysical experiments: P4, P15, P31, and P46. Stimuli were displayed behind closed shutters which opened at various intervals after stimulus offset. Thus, the observers' responses were based solely on the visibility of phosphor persistence. We varied viewing conditions (dark-adapted vs. veiling light), type of task (detection vs. identification), and intensity of the stimuli. No detectable persistence was ever produced by the P15 phosphor. In contrast, the P31 phosphor remained visible for several hundred ms. even with a veiling light. The P4 and P46 phosphors produced persistence of intermediate durations. It is concluded that P15 is the phosphor of choice for visual experiments.

Production of the siderophore enterobactin: use of four different fermentation systems and identification of the compound by HPLC.

The article describes four different fermentation procedures for Escherichia coli AN311, a producer of enterobactin. A regular rotary shaker culture with a biphasic system consisting of an agar layer (as a reservoir for feeding processes) and a layer of liquid medium, 2.4 L and 10 L batch cultures, and a novel dialysis membrane fermentor were used. With the use of this latter fermentor type, the production of enterobactin could be increased by a factor of about 9.5, while growth increased by a factor of 12 compared to the other systems. For the rapid and reliable quantification of the concentration and purity of enterobactin an analytical and preparative high-performance liquid chromatography (HPLC) method was established. The degradation compounds of this siderophore were detected by diodearray and bioassays. A comparison of total catechol production as well as the distribution between enterobactin and its degradation compounds is given.

Deletions or duplications in the BtuB protein affect its level in the outer membrane of Escherichia coli.

The Escherichia coli btuB product is an outer membrane protein that mediates the TonB-coupled active transport of cobalamins and the uptake of the E colicins and bacteriophage BF23. The roles of various segments of the BtuB protein in its function or cellular localization were investigated by analysis of several genetic constructs. Hybrid proteins in which various lengths from the amino terminus of BtuB were linked to alkaline phosphatase (btuB::phoA genes) were all secreted across the cytoplasmic membrane. The BtuB-PhoA proteins that carried up to 327 amino acids of BtuB appeared to reside in the periplasmic space, whereas hybrid proteins containing at least 399 amino acids of BtuB were associated with the outer membrane. Eleven in-frame internal deletion mutations that spanned more than half of the mature sequence were prepared by combining appropriate restriction fragments from btuB variants with 6-bp linker insertions. None of the deleted proteins was able to complement any BtuB functions, and only three of them were detectable in the outer membrane, suggesting that most of the deletions affected sequences needed for stable association with the outer membrane. Duplications covering the same portions of BtuB were prepared in the same manner. All of these partial duplication variants complemented all BtuB functions, although some gave substantially reduced levels of activity. These proteins were found in the outer membrane, although some were subject to proteolytic cleavage within or near the duplicated segment. These results indicate that the insertion of BtuB into the outer membrane requires the presence of several regions of teh BtuB protein and that the presence of extra or redundant segments of the protein can be tolerated during its insertion and function.