Structural basis for sugar translocation through maltoporin channels at 3.1 A resolution.

Trimeric maltoporin (LamB protein) facilitates the diffusion of maltodextrins across the outer membrane of Gram-negative bacteria. The crystal structure of maltoporin from Escherichia coli, determined to a resolution of 3.1 angstroms, reveals an 18-stranded, antiparallel beta-barrel that forms the framework of the channel. Three inwardly folded loops contribute to a constriction about halfway through the channel. Six contingent aromatic residues line the channel and form a path from the vestibule to the periplasmic outlet. Soaking of a crystal with maltotriose revealed binding of the sugar to this hydrophobic track across the constriction, which suggests that maltose and linear oligosaccharides may be translocated across the membrane by guided diffusion along this path.

Brain activation modulated by sentence comprehension.

The comprehension of visually presented sentences produces brain activation that increases with the linguistic complexity of the sentence. The volume of neural tissue activated (number of voxels) during sentence comprehension was measured with echo-planar functional magnetic resonance imaging. The modulation of the volume of activation by sentence complexity was observed in a network of four areas: the classical left-hemisphere language areas (the left laterosuperior temporal cortex, or Wernicke's area, and the left inferior frontal gyrus, or Broca's area) and their homologous right-hemisphere areas, although the right areas had much smaller volumes of activation than did the left areas. These findings generally indicate that the amount of neural activity that a given cognitive process engenders is dependent on the computational demand that the task imposes.

Crystallization and preliminary X-ray characterization of maltoporin from Escherichia coli.

Crystals of maltoporin (the bacteriophage lambda receptor of Escherichia coli) that diffract X-rays to 3 A resolution can be grown reproducibly. Maltoporin is an integral membrane protein, which forms a channel in the E. coli outer membrane that specifically facilitates the diffusion of maltose and maltodextrins. The crystals have a rhombic prismatic habit and belong to the orthorhombic space group C222(1) with unit cell dimensions a = 130 A, b = 213 A and c = 216 A. X-ray structure determination is underway.

The capacity theory of comprehension: new frontiers of evidence and arguments.

A capacity theory of comprehension (M.A. Just & P.A. Carpenter, 1992) has provided an integrated account of several central aspects of sentence comprehension, such as the processing of syntactic ambiguity, complex embeddings, syntactic (non) modularity, and individual differences, in terms of the working-memory capacity for language. Some of the evidence supporting the theory is questioned by G.S. Waters and D. Caplan (1996a). This article identifies some of Waters and Caplan's errors about the empirical support in Just and Carpenter (1992), evaluates Waters and Caplan's alternative hypothesis, and presents the results of a new neuroimaging study that supports capacity theory and not Waters and Caplan's separate resources hypothesis.

Two separate verbal processing rates contributing to short-term memory span.

Previous research indicates that verbal memory span, the number of words people can remember and immediately repeat, is related to the fastest rate at which they can pronounce the words. This relation, in turn, has been attributed to a general or global rate of information processing that differs among individuals and changes with age. However, the experiments described in this article showed that the rates of 2 processes (rapid articulation and the retrieval of words from short-term memory) are related to memory span but not to each other. Memory span depends on a profile of processing rates in the brain, not only a global rate. Moreover, there appears to be only a partial overlap between the rate variables that change with age and those that differ among individuals.

Can auditory memory for tone pitch be rehearsed?

This study investigated whether nonverbal auditory memory representations can be affected by rehearsal strategies. The comparison of the pitches of 2 tones separated by a silent, variable delay interval was examined in 2 experiments, both when participants were instructed to rehearse the pitch of the first tone covertly during the intertone interval and when such rehearsal was prevented by 1 of 2 attention-demanding distractor tasks. In both experiments, delayed tone comparison performance was superior when participants were permitted to rehearse, and the type of distractor task (verbal vs. auditory) had no effect on performance under distraction instructions. The results suggest that auditory imagery can be used strategically to slow the rate of decay of auditory information for tone pitch.

The neural bases of sentence comprehension: a fMRI examination of syntactic and lexical processing.

One of the challenges to functional neuroimaging is to understand how the component processes of reading comprehension emerge from the neural activity in a network of brain regions. In this study, functional magnetic resonance imaging (fMRI) was used to examine lexical and syntactic processing in reading comprehension by independently manipulating the cognitive demand on each of the two processes of interest. After establishing a consistency with earlier research showing the involvement of the left perisylvian language areas in both lexical access and syntactic processing, the study produced new findings that are surprising in two ways: (i) the lexical and syntactic factors each impact not just individual areas, but they affect the activation in a network of left-hemisphere areas, suggesting that changing the computational load imposed by a given process produces a cascade of effects in a number of collaborating areas; and (ii) the lexical and syntactic factors usually interact in determining the amount of activation in each affected area, suggesting that comprehension processes that operate on different levels of language may nevertheless draw on a shared infrastructure of cortical resources. The results suggest that many processes in sentence comprehension involve multiple brain regions, and that many brain regions contribute to more than one comprehension process. The implication is that the language network consists of brain areas which each have multiple relative specializations and which engage in extensive interarea collaborations.

Crystallization of monodisperse maltoporin from wild-type and mutant strains of various Enterobacteriaceae.

Maltoporin has been purified by affinity chromatography on starch gel columns. This single-step procedure affords the rapid purification of active protein from wild-type and mutants of E. coli, and from other Gram-negative bacteria. The monodisperse protein was crystallized under various conditions. Several preparations have yielded crystals amendable to X-ray analysis, notably a single cysteine substitution, S57C.

fMRI investigation of sentence comprehension by eye and by ear: modality fingerprints on cognitive processes.

The neural substrate underlying reading vs. listening comprehension of sentences was compared using fMRI. One way in which this issue was addressed was by comparing the patterns of activation particularly in cortical association areas that classically are implicated in language processing. The precise locations of the activation differed between the two modalities. In the left inferior frontal gyrus (Broca's area), the activation associated with listening was more anterior and inferior than the activation associated with reading, suggesting more semantic processing during listening comprehension. In the left posterior superior and middle temporal region (roughly, Wernicke's area), the activation for listening was closer to primary auditory cortex (more anterior and somewhat more lateral) than the activation for reading. In several regions, the activation was much more left lateralized for reading than for listening. In addition to differences in the location of the activation, there were also differences in the total amount of activation in the two modalities in several regions. A second way in which the modality comparison was addressed was by examining how the neural systems responded to comprehension workload in the two modalities by systematically varying the structural complexity of the sentences to be processed. Here, the distribution of the workload increase associated with the processing of additional structural complexity was very similar across the two input modalities. The results suggest a number of subtle differences in the cognitive processing underlying listening vs. reading comprehension.

Reversible oriented surface immobilization of functional proteins on oxide surfaces.

Reversible and oriented immobilization of proteins in a functionally active form on solid surfaces is a prerequisite for the investigation of molecular interactions by surface-sensitive techniques. We demonstrate a method generally applicable for the attachment of proteins to oxide surfaces. A nitrilotriacetic acid group serving as a chelator for transition metal ions was covalently bound to the surface via silane chemistry. Reversible binding of the green fluorescent protein, modified with a hexahistidine extension, was monitored in situ using total internal reflection fluorescence. The association constant and kinetic parameters of the binding process were determined. The reversible, directed immobilization of proteins on surfaces as described here opens new ways for structural investigation of proteins and receptor-ligand interactions.

Graded functional activation in the visuospatial system with the amount of task demand.

Two studies examined how the amount and type of computational demand are related to fMRI-measured activation in three bilateral cortical regions involved in the Shepard-Metzler (1971) mental-rotation paradigm. The amount of demand for the computation of visuospatial coordinates was manipulated by presenting mental rotation problems with increasing angular disparity (0, 40, 80, or 120 degrees). Activation in both the left and right intraparietal sulcal regions increased linearly with angular disparity in two separate studies. Activation also occurred in the fusiform gyrus and inferior temporal regions, regions that are primarily associated with the processes of object and object-part identification. By contrast, the demand for object recognition and rotation processes was relatively low, and the demand for executing saccades was high in a control condition that required making a systematic visual scan of two grids. The grid-scanning condition resulted in relatively less activation in the parietal and inferior temporal regions but considerable activation in frontal areas that are associated with planning and executing saccades, including the precentral gyrus and sulcus into the posterior middle frontal region. These data suggest that the amount of activation in the various cortical regions that support visuospatial processing is related to the amount, as well as to the type, of computational demand.

Interdependence of nonoverlapping cortical systems in dual cognitive tasks.

One of the classic questions about human thinking concerns the limited ability to perform two cognitive tasks concurrently, such as a novice driver's difficulty in simultaneously driving and conversing. Limitations on the concurrent performance of two unrelated tasks challenge the tacitly assumed independence of two brain systems that seemingly have little overlap. The current study used fMRI (functional magnetic resonance imaging) to measure cortical activation during the concurrent performance of two high-level cognitive tasks that involve different sensory modalities and activate largely nonoverlapping areas of sensory and association cortex. One task was auditory sentence comprehension, and the other was the mental rotation of visually depicted 3-D objects. If the neural systems underlying the two tasks functioned independently, then in the dual task the brain activation in the main areas supporting the cognitive processing should be approximately the conjunction of the activation for each of the two tasks performed alone. We found instead that in the dual task, the activation in association areas (primarily temporal and parietal areas of cortex) was substantially less than the sum of the activation when the two tasks were performed alone, suggesting some mutual constraint among association areas. A similar result was obtained for sensory areas as well.

Time course of fMRI-activation in language and spatial networks during sentence comprehension.

Functional neuroimaging previously has been considered to provide inadequate temporal resolution to study changes of brain states as a function of cognitive computations; however, we have obtained evidence of differential amounts of brain activity related to high-level cognition (sentence processing) within 1.5 s of stimulus onset. The study used an event-related paradigm with high-speed echoplanar functional magnetic resonance imaging (fMRI) to trace the time course of the brain activation in the temporal and parietal regions as participants comprehended single sentences describing a spatial configuration. Within the first set of images, on average 1 s from when the participant begins to read a sentence, there was significant activation in a key cortical area involved in language comprehension (the left posterior temporal gyrus) and visuospatial processing (the left and right parietal regions). In all three areas, the amount of activation during sentence comprehension was higher for negative sentences than for their affirmative counterparts, which are linguistically less complex. The effect of negation indicates that the activation in these areas is modulated by the difficulty of the linguistic processing. These results suggest a relatively rapid coactivation in both linguistic and spatial cortical regions to support the integration of information from multiple processing streams.