Vergence control across saccades in dyslexic adults.

Many aspects of vision have been investigated in developmental dyslexia. Some research suggests deficits in vergence control (e.g. Buzzelli, 1991, Optom. Vision Sci. 68, 842-846), although ability to control vergence across saccades has not yet been investigated. We have explored this question indirectly using Enright's (1996 Vision Res. 36, 307-312.) sequential stereopsis task. The task requires observers to set two adjacent targets (whose textures cannot be resolved simultaneously if either is fixated) to appear equi-distant. Enright has argued that sequential stereopsis stereoacuity thresholds offer an indication of vergence control across saccades. We report two experiments using a total of 17 dyslexic and 18 control adults. Performance was measured on a sequential stereopsis task and an ordinary 'simultaneous' stereopsis task. No significant differences between groups were found. However, whereas practice of the sequential task lowered control group thresholds on the simultaneous task, for the dyslexic group it significantly raised thresholds, suggesting that visual fatigue is especially important in investigations of visual functions in dyslexia. Although the small samples used limit conclusions at this stage, the main sequential stereopsis results suggest that, if Enright is correct, dyslexic adults can show normal vergence control across saccades.

The proterozoic ophiolite problem, continental emergence, and the venus connection.

To account for the lack of preservation of ophiolites (fragments of oceanic crust and mantle) in old orogenic belts (age 1000 to 2500 million years), a hypothesis proposes that the magmatic oceanic crust formed during sea-floor spreading was thicker during the cited time interval. This thickening led to reduced contrast between the elevation of continental and oceanic regions and to greater average flooding of the continents. The resultant distribution of elevation may have resembled modern Venus more than modern Earth.

Ancient suture zones within continents.

Ancient suture belts within continents are deformed regions which contain the remnants of former ocean basins. They form when two continents or island arcs that earlier were separated by an ocean basin converge and collide during plate tectonic activity. These belts provide the only record we have of deep oceanic crust and of ancient sea-floor processes for the first 94 percent of the earth's history, that is, prior to the oldest preserved crust in the oceans. Ten criteria for the recognition and interpretation of these ancient belts are discussed. A comprehensive program for the study of these belts should have great scientific and economic benefit for the United States and would be relatively cheap compared to other large national scientific efforts.

Formes frustes of Marfan's syndrome presenting with severe aortic regurgitation. Clinicogenetic study of 18 families.

Eighteen patients who presented with severe aortic regurgitation and dilatation of the ascending aorta were found to be formes frustes of Marfan's syndrome and formed the basis for this clinicogenetic study. All had aortic valve replacement and reconstruction of part of the ascending aorta. The diagnosis was confirmed by histological examination of the aortic tissue. There were 126 first degree relatives; 85 were living and 67 (78-8%) of these were examined. Limited information was available about 32 of the 41 relatives who had died. No relative had the classical clinical features of Marfan's syndrome but stigmata of the disease were found in 25 (37-3%) of the 67 first degree relatives examined. In 21, the abnormality was confined to the cardiovascular system, the skeleton, or the eye, but in 4, abnormalities involved 2 systems. Cardiovascular abnormalities affecting the aortic valve or aortic wall were present in 6 (9-0%) of the 67 first degree relatives examined. One or more of the skeletal indices measured (height-span difference, metacarpal index, phalangeal index) was abnormal in 18 (26-9%) and ocular abnormalities were found in 5 of 51 (9-8%) examined. There were no relatives with dislocation of the lens or iridodonesis. Using strict diagnostic criteria, a minimum of 37-3 per cent of the first degree relatives examined were affected; this involved 12 of the 18 families studied. There was nothing in our data to suggest that the formes frustes of the disease had a different mode of inheritance from the classical syndrome.