Global factors that determine the maximum disparity for seeing cyclopean surface shape

A disparity gradient limit explains why the maximum amplitude of sinusoidal disparity gratings increases with decreasing disparity spatial frequency. It also explains why the largest disparity for binocular fusion (diplopia threshold) varies directly with stimulus element separation. Does a disparity gradient limit also apply to the detection of cyclopean shape? A previous study addressed this question and concluded that it does not. We examined this question by measuring the largest disparity amplitude (dmax) at which observers could judge the shape of cyclopean disparity gratings. We used trapezoidal, triangular, sinusoidal, and square wave gratings in order to dissociate the effects of disparity gradient and disparity spatial frequency. Gabor micropatterns were used to minimize potential scale-dependent interactions with luminance processing. Our results support a disparity gradient limit for cyclopean shape perception, with additional factors being involved at high disparity spatial frequencies. Combining the gradient limit hypothesis with lowpass disparity filtering describes the pattern of dmax for both smooth and discontinuous surface shapes.     

Interactions between chromatic- and luminance-contrast-sensitive stereopsis mechanisms

It is well known that chromatic information can assist in solving the stereo correspondence problem. It has also been suggested that there are two independent first-order stereopsis mechanisms, one sensitive to chromatic contrast and the other sensitive to luminance contrast (Vision Research 37 (1997) 1271). Could the effect of chromatic information on stereo correspondence be subserved by interactions between these mechanisms? To address this question, disparity thresholds (1/stereoacuity) were measured using 0.5 cpd Gabor patches. The stimuli possessed different relative amounts of chromatic and luminance contrast which could be correlated or anti-correlated between the eyes. Stereoscopic performance with these compound stimuli was compared to that with purely isoluminant and isochromatic stimuli at different contrasts. It was found that anti-correlated chromatic contrast severely disrupted stereopsis with achromatic stimuli and that anti-correlated luminance contrast severely disrupted stereopsis with chromatic stimuli. Less dramatic, but still significant, was the improvement in stereoacuity obtained using correlated colour and luminance contrast. These data are consistent with there being positive and negative interactions between chromatic and achromatic stereopsis mechanisms that take place after the initial encoding of disparity information, but before the extraction of stereoscopic depth. These interactions can be modelled satisfactorily assuming probability summation of depth sign information between independent mechanisms.     

Texture-orientation mechanisms pool colour and luminance contrast

Do texture-sensitive mechanisms operate separately on, or pool, luminance and colour contrast information? We addressed this question by measuring threshold-versus-amplitude functions for orientation-modulated (OM) gratings comprised of gabor elements defined by either colour or luminance contrast. In both the uncrossed (all elements in test and mask defined by either colour or luminance contrast) and crossed (equal mixtures of luminance and colour contrast in both test and mask) conditions, evidence of sub-threshold facilitation between test and mask was obtained. The sub-threshold facilitation in the crossed condition could not be accounted for by luminance artifacts in the ostensibly isoluminant gabors. The results are consistent with a single visual mechanism sensitive to OM textures that pools information from both the luminance and chromatic post-receptoral mechanisms.     

Does a kidney-sharing alliance have to sacrifice cold ischemic time for better HLA matching?

BACKGROUND: The Scotland-Northern Ireland Kidney Allocation Alliance was created in August 1998. The purpose was to optimize the transplant service through increased regional exchange, higher quality matched kidneys, and better organ distribution. METHODS: An analysis was performed on prospectively collected data regarding retrieval and transplant activity. The degree of HLA matching, the cold ischemic time (CIT), the balance of exchange, and graft survival were analyzed for a 2-year period after the introduction of the new alliance and compared with the last year before alliance. RESULTS: There was a 17.7% increase in the number of transplants performed. In the 2-year period, 78% of kidneys were exported from the retrieving center compared with 55% in the prealliance year, (P<0.05, chi2). The proportion of 000 mismatched transplants and other favorable matches increased from 9.5 to 21% and from 52.5 to 61%, respectively. There was no significant difference between the CIT for the three study periods, nor between the CIT for locally used kidneys versus those exchanged within the Alliance (P>0.05, Student's t test). The largest center was a net importer of kidneys, whereas small and medium-sized centers balanced their exchange within the 2-year period. The 1-year transplant survival rate improved from 81.5% in the prealliance year to 88.4% at the end of the second year. CONCLUSIONS: The introduction of a regional kidney allocation alliance has improved the degree of HLA matching and increased the exchange of organs, without a significant increase in the CIT and any detrimental effect on graft survival.     

Tuberculous meningitis in pregnancy

Four women with tuberculous meningitis in association with pregnancy are described to illustrate both the serious nature and varied presentation of this condition. Mortality or neurological morbidity may occur more frequently during pregnancy, but in general correlate closely with delay in diagnosis and initiation of appropriate therapy. Treatment often needs to be started empirically, and neurosurgical intervention may prove life saving.     

Use of nonviral vectors for cystic fibrosis gene therapy

Over the last decade, three groups within the United Kingdom (Edinburgh, Oxford, and Imperial College, London) have undertaken key studies in the development of clinical gene therapy for cystic fibrosis. In 2001, catalyzed by the Cystic Fibrosis Trust, these groups came together to form the United Kingdom Cystic Fibrosis Gene Therapy Consortium. The Consortium has removed duplication and competition, developed core facilities playing to the respective strengths of the centers, and introduced the joint strategy described in this article. This is driven by a clinical trial program, with a product pipeline and the necessary development of novel preclinical and human assays. The program is milestone-related, has a structure that lies between the pharmaceutical industry and academia, and has as its endpoint negotiations with industry to undertake a phase III clinical trial of the identified product.