An unexpected specialization for horizontal disparity in primate primary visual cortex

The horizontal separation of the eyes means that objects nearer or farther than the fixation point project to different locations on the two retinae, differing principally in their horizontal coordinates (horizontal binocular disparity). Disparity-selective neurons have generally been studied with disparities applied in only one direction (often horizontal), which cannot determine whether the encoding is specialized for processing disparities along the horizontal axis. It is therefore unclear if disparity selectivity represents a specialization for naturally occurring disparities. I used random dot stereograms to study disparity-selective neurons from the primary visual cortex (V1) of awake fixating monkeys. Many combinations of vertical and horizontal disparity were used, characterizing the surface of responses as a function of two-dimensional disparity. Here I report that the response surface usually showed elongation along the horizontal disparity axis, despite the isotropic stimulus. Thus these neurons modulated their firing rate over a wider range of horizontal disparity than vertical disparity. This demonstrates that disparity-selective cells are specialized for processing horizontal disparity, and that existing models of disparity selectivity require substantial revision.     

John MacCulloch's ‘Millstone Survey’ and its consequences

During the Napoleonic Wars there was a shortage of suitable millstones for the British Ordnance gunpowder mills. John MacCulloch spent five summers searching Western Britain for a source of non-siliceous limestones to be used as gunpowder millstones. His search was authorized by the Board of Ordnance, which also paid all his expenses.

By 1812 MacCulloch had found suitable limestones in Sutherland, Skye, and at Glen Tilt, but the Board of Ordnance were unable to exploit any of the sources until 1815 when unsuccessful quarries were opened on Skye. After the defeat of Napoleon at Waterloo the British millstone project was abandoned. The military objective of the millstone survey was not achieved, but MacCulloch learned much about the geology of the older rocks of Britain and he built a reputation as an expert geologist through the papers he submitted to the Geological Society of London.     

Microdeletion encompassing MAPT at chromosome 17q21.3 is associated with developmental delay and learning disability

Recently, the application of array-based comparative genomic hybridization (array CGH) has improved rates of detection of chromosomal imbalances in individuals with mental retardation and dysmorphic features. Here, we describe three individuals with learning disability and a heterozygous deletion at chromosome 17q21.3, detected in each case by array CGH. FISH analysis demonstrated that the deletions occurred as de novo events in each individual and were between 500 kb and 650 kb in size. A recently described 900-kb inversion that suppresses recombination between ancestral H1 and H2 haplotypes encompasses the deletion. We show that, in each trio, the parent of origin of the deleted chromosome 17 carries at least one H2 chromosome. This region of 17q21.3 shows complex genomic architecture with well-described low-copy repeats (LCRs). The orientation of LCRs flanking the deleted segment in inversion heterozygotes is likely to facilitate the generation of this microdeletion by means of non-allelic homologous recombination.     

Rainfall and drought in equatorial east Africa during the past 1,100 years

Knowledge of natural long-term rainfall variability is essential for water-resource and land-use management in sub-humid regions of the world. In tropical Africa, data relevant to determining this variability are scarce because of the lack of long instrumental climate records and the limited potential of standard high-resolution proxy records such as tree rings and ice cores. Here we present a decade-scale reconstruction of rainfall and drought in equatorial east Africa over the past 1,100 years, based on lake-level and salinity fluctuations of Lake Naivasha (Kenya) inferred from three different palaeolimnological proxies: sediment stratigraphy and the species compositions of fossil diatom and midge assemblages. Our data indicate that, over the past millennium, equatorial east Africa has alternated between contrasting climate conditions, with significantly drier climate than today during the 'Medieval Warm Period' (approximately AD 1000-1270) and a relatively wet climate during the 'Little Ice Age' (approximately AD 1270-1850) which was interrupted by three prolonged dry episodes. We also find strong chronological links between the reconstructed history of natural long-term rainfall variation and the pre-colonial cultural history of east Africa, highlighting the importance of a detailed knowledge of natural long-term rainfall fluctuations for sustainable socio-economic development.     

Vertical disparities and perception of three-dimensional shape

The information about depth and three-dimensional shape available from the horizontal component of the stereo disparity field requires interpretation in conjunction with information about egocentric viewing distance (D). A novel computational approach for estimating D was proposed by Mayhew and Longuet-Higgins, who demonstrated that the horizontal gradient of vertical disparities uniquely specifies the viewing distance. We have now used random dot stereograms in a shape judgement task to show that changes in vertical disparities have no effect on perceived three-dimensional shape. Changes in ocular convergence do alter perceived shape, suggesting substantial changes in the subjects' scaling of horizontal disparities. We conclude that vertical disparities are not used to scale disparities for viewing distance, and that extraretinal signals must be considered when analysing human three-dimensional shape perception.