Contour Symmetry Research Articles

SPATIAL FLOWER PARAMETERS AND INSECT SPATIAL VISION

ABSTRACTThe present article reviews recent and older literature on the spatial parameters that flowers display, as well as on the capacities of anthophilous insects to perceive and use these parameters for optimizing their foraging success. Although co‐evolution of plants and pollinators has frequently been discussed with respect to floral colours and insect colour vision, it has rarely been assessed with respect to insect spatial vision and spatial floral cues, such as shape, pattern, size, contrast, symmetry, spatial frequency, contour density and orientation of contours. This review is an attempt to fill this gap. From experimental findings and observations on both flowers and insects, we arrive at the conclusion that all of the spatial and spatio‐temporal parameters that flowers offer are relevant to the foraging task and are tuned to the insect's visual capacities and visually guided behaviour. We try, in addition, to indicate that temporal cues are closely related to spatial cues, and must therefore be included when flower–pollinator interactions are examined. We include results that show that colour vision and spatial vision have diverged over the course of evolution, particularly regarding the processing of spatio‐temporal information, but that colour vision plays a role in the processing of spatial cues that are independent of temporal parameters. By presenting this review we hope to contribute to closer collaboration among scientists working in the vast fields of botany, ecology, evolution, ethology and sensory physiology.

Large-magnitude Middle Ordovician volcanic ash falls in North America and Europe: dimensions, emplacement and post-emplacement characteristics

Middle Ordovician K-bentonites represent some of the largest known fallout ash deposits in the Phanerozoic Era. They cover minimally 2.2 × 10 6 km 2 in eastern North America and 6.9 × 10 5 km 2 in northwestern Europe, and represents the coeval accumulation of plinian and co-ignimbrite ash on both Laurentia and Baltica during the closure of the Iapetus Ocean. The three most widespread beds are the Deicke and Millbrig K-bentonites in North America and the Kinnekulle K-bentonite in northwestern Europe. The vents were located near the Laurentian margin of Iapetus on an arc or microplate undergoing collision with Laurentia. The volume of ash preserved in the stratigraphie record converted to dense rock equivalent (DRE) of silicic magma is minimally estimated to be 943 km 3 for the Deicke, 1509 km 3 for the Millbrig and 972 km 3 for the Kinnekulle. The Millbrig and Kinnekulle beds are coeval and possibly equivalent, yielding a combined DRE volume of nearly 2500 km 3. Some unknown but probably large amount of additional ash fell into oceanic regions of the Iapetus, but these areas became subducted and the ash is not preserved in the geologic record. The symmetry of the thickness contours is suggestive that one or more ash clouds interacting with equatorial stratospheric and tropospheric wind patterns dispersed pyroclastic material to both the northwest and southeast in terms of Ordovician paleogeography. Based on grain size measurements and thickness/area 1 2 plots we conclude the three beds were each formed from co-ignimbrite or possibly phreatoplinian eruption columns. Analyses of melt inclusions in primary quartz crystals indicate the parental magma contained approximately 4% dissolved water at the time of the eruption. This water provided the explosive energy during the initial gas thrust phase. The implied fragmentation pressure on the magma would have reduced much of the ejecta to small particles, forming a deposit composed largely of single crystals and glassy dust. Conversion of the ash to K-bentonite resulted in a mass loss of approximately 35%, mostly in the form of Si with lesser amounts of Na and K.

Symmetry-based recognition of vehicle rears

Symmetry is an effective feature for locating vehicle rears. Three symmetry criteria are presented: contour symmetry, gray level symmetry and horizontal line symmetry. The application of these criteria to an image yields three histograms which are analyzed to find the vehicle centerline. The mean error in the vehicle centerline location is generally less than five percent. The contours of the vehicle are followed outward from the centerline to find the vehicle width. The calculated width averages 87 percent of the true width. A nonlinear filter for edge extraction and an automatic thresholding scheme are also presented. The complete algorithm requires approximately 20 seconds on a 25 MHz 80386 based computer for a 512 by 512 image.

Eccentricity of the scotopic fovea about a photopic point as revealed by acuity contours

Both photopic and scotopic (Vernier offset) acuity contours were determined along the horizontal and vertical meridians for 6° on either side of visual field center. The photopic contours evidenced bisymmetry about a photopic fixation point coincident with the center of the target. Scotopic contours, on the other hand, were not symmetrical about this same fixation point. A half maximum acuity criterion was employed to obtain a measure of contour symmetry along a specific meridian. The authors conclude that the physiologic foveal scotoma, as mapped by retinal acuity, is decentered with respect to a photopic point of fixation.