Maintaining and shifting attention within left or right hemifield.

Positron emission tomography (PET) was used to examine two questions: (i) which structures of the intact human brain change their activity with the direction of attention to left or right visual field; and (ii) how does activity in these structures, and in parietal cortex in particular, depend on the frequency of attentional shifts? Subjects were required to discriminate the orientation of peripheral gratings. The two main experimental variables were the attended hemifield (left or right) and the proportion of trials requiring a shift within that hemifield (20% or 80%). A detection control condition was also included. Behaviourally, subjects were less accurate and significantly slower when a trial required a shift than when it did not. Ventral and lateral occipital areas showed significantly higher blood flow levels contralateral to the direction of attention. Replicating previous work, there was also a significant main effect of the direction of attention in left lateral prefrontal cortex: blood flow levels were higher during leftward attention in comparison both to baseline and to rightward attention. This left frontal effect reached significance in single subjects in whom several activation sites could be distinguished within left middle and inferior frontal gyrus. Right and left parietal cortex were activated during both left- and right-field attention conditions, with a tendency for higher activity levels when attention was directed contralaterally. Contrary to the experimental hypothesis, however, parietal regions were not activated differentially by high versus low numbers of attentional shifts. The current experiment confirms that left frontal convexity is sensitive to manipulations of the direction of visuospatial attention. The results do not indicate a specific role of parietal cortex in attentional shifting.

Redefining the functional organization of working memory processes within human lateral prefrontal cortex.

It is widely held that the frontal cortex plays a critical part in certain aspects of spatial and non-spatial working memory. One unresolved issue is whether there are functionally distinct subdivisions of the lateral frontal cortex that subserve different aspects of working memory. The present study used positron emission tomography (PET) to demonstrate that working memory processes within the human mid-dorsolateral and mid-ventrolateral frontal regions are organized according to the type of processing required rather than according to the nature (i.e. spatial or non-spatial), of the information being processed, as has been widely assumed. Two spatial working memory tasks were used which varied in the extent to which they required different executive processes. During a 'spatial span' task that required the subject to hold a sequence of five previously remembered locations in working memory a significant change in blood-flow was observed in the right mid-ventrolateral frontal cortex, but not in the anatomically and cytoarchitectonically distinct mid-dorsolateral frontal-lobe region. By contrast, during a '2-back' task that required the subject to continually update and manipulate an ongoing sequence of locations within working memory, significant blood flow increases were observed in both mid-ventrolateral and mid-dorsolateral frontal regions. When the two working memory tasks were compared directly, the one that emphasized manipulation of information within working memory yielded significantly greater activity in the right mid-dorsolateral frontal cortex only. This dissociation provides unambiguous evidence that the mid-dorsolateral and mid-ventrolateral frontal cortical areas make distinct functional contributions to spatial working memory and corresponds with a fractionation of working memory processes in psychological terms.

Visualization by magnetic resonance imaging of focal cartilage lesions in the excised mini-pig knee.

This study showed that magnetic resonance imaging can be used to visualize partial thickness cartilage lesions, 0.7 x 10 mm in area and 0.5 mm in depth, surgically induced in the femur (femoropatellar compartment) of a mini-pig knee joint. Formalin-fixed joints, intact as well as disarticulated, were studied by high resolution imaging using a 2.35 T, 31 cm horizontal-bore superconducting magnet. The two-dimensional and three-dimensional spatial resolutions achievable were as follows: 0.12 x 0.23 mm (two-dimensional) and 0.35 x 0.35 x 0.35 mm (three-dimensional) for the intact joint, and 0.08 x 0.08 mm (two-dimensional) and 0.14 x 0.14 x 0.27 mm (three-dimensional) for the disarticulated joint. These results demonstrate that magnetic resonance imaging, together with edge detection and volume rendering, can be used to visualize focal cartilage lesions.

A combined analysis and magnetic resonance imaging technique for computerised automatic measurement of cartilage thickness in the distal interphalangeal joint.

It is well known that magnetic resonance imaging (MRI) contrast can be controlled, albeit sometimes at the expense of image resolution and signal-to-noise ratio, and most studies of articular joints have used a single MRI protocol, which is optimised for subjective image analysis. Inevitably that single protocol frequently compromises the detection of one or another of the boundaries between which any measurement must be made. This paper describes an alternative approach in which the criteria for computerised edge detection necessary for fully automated measurement of cartilage thickness are used to define the MRI acquisition parameters. This necessitates the combined use of two MRI sequences, one optimised for the cartilage-bone boundary, and the other for cartilage-synovial fluid. This provides a highly effective combination and its efficacy is demonstrated for the distal interphalangeal joint of a range of asymptomatic adults.

Magnetic resonance imaging in a model of atherosclerosis: use of a collar around the rabbit carotid artery.

Transverse cardiac-cycle gated high resolution magnetic resonance images have been obtained from the neck of the New Zealand white rabbit both in normal animals and from those in which a collar had been earlier positioned around one carotid artery. The study included animals fed on normal and on high cholesterol diets with the surgical modification having been demonstrated previously to cause a rapid and reproducible lesion resembling early atherosclerosis. The aim of the work was to investigate the attainable spatial resolution and sensitivity at a field strength of 2 T using a large radiofrequency transmitter system and a surface coil receiver with which spin-echo images have been obtained. Visualization was enhanced using a three-dimensional interpolation technique. An image resolution of 200 microns was readily obtained but was shown to be insufficient for delineating pathological features within the artery wall such as intimal layer thickening. The results have been compared with histopathological findings which confirmed that any morphological changes were within the pixel resolution of the image. Extensions to the methodology are proposed which should be able to detect atherosclerotic changes with a resolution of 50 microns within a feasible imaging time. In addition, the MRI study of how the surgical intervention alters the artery shape and curvature was carried out and the MRI demonstrated that collar implantation in general does not occlude the artery and causes only a slight and gradual degree of curvature to the vessel.