Illusory contours and spatial judgment.

We investigated whether, in the human visual system, the mechanisms responsible for relative location judgments are the same when those judgments are made in the context of illusory contours and in the context of mentally joining two points. We asked subjects to align a dot with the oblique contour of an illusory surface or to align a dot with two markers at an oblique orientation. The systematic errors differed in direction for these two conditions. All the systematic errors were orientation dependent. The errors in aligning a dot with an illusory contour seem to be related to the asymmetrical shape of the single objects, which are able to induce an illusory contour, as well as figure-ground segregation.

Change in perceived spatial directions due to context.

We examined the influence of context on exocentric pointing. In a virtual three-dimensional set-up, we asked our subjects to aim a pointer toward a target in two conditions. The target and the pointer were visible alone, or they were visible with planes through each of them. The planes consisted of a regular grid of horizontal and vertical lines. The presence of the planes had a significant influence on the indicated direction. These changes in indicated direction depended systematically on the orientation of the planes relative to the subject and on the angle between the planes. When the orientation of the (perpendicular) planes varied from asymmetrical to symmetrical to the frontoparallel plane, the indicated direction varied over a range of 15 degrees--from a slightly larger slant to a smaller slant--as compared with the condition without the contextual planes. When the dihedral angle between the two planes varied from 90 degrees to 40 degrees, the indicated direction varied over a range of less than 5 degrees: A smaller angle led to a slightly larger slant. The standard deviations in the indicated directions (about 3 degrees) did not change systematically. The additional structure provided by the planes did not lead to more consistent pointing. The systematic changes in the indicated direction contradict all theories that assume that the perceived distance between any two given points is independent of whatever else is present in the visual field--that is, they contradict all theories of visual space that assume that its geometry is independent of its contents (e.g., Gilinsky, 1951; Luneburg, 1947; Wagner, 1985).

Exocentric pointing in three-dimensional space.

A study is reported of an exocentric pointing task in all three dimensions, in near space, with only two visible luminous objects--a pointer and a target. The task of the subject was to aim a pointer at a target. The results clearly show that visual space is not isotropic, since every set direction appeared to consist of two independent components--one in the projection onto a frontoparallel plane (tilt), the other in depth (slant). The tilt component shows a general trend across subjects, an oblique effect, and can be judged monocularly. The slant component is symmetrical in the mid-sagittal plane, requires the use of binocular information, and shows considerable differences between subjects. These differences seem to depend on the amount of binocular information used by each subject. There was a remarkably high level of consistency in the exocentric pointing, despite the absence of environmental cues. The within-subject consistency in the settings of the pointer corresponds to a consistency of about 1 min of arc in disparity of its tip, even though the pointer and target are separated by more than 5 deg.

Chlorosomes of green sulfur bacteria: Pigment composition and energy transfer.

The pigment composition and energy transfer pathways in isolated chlorosomes ofChlorobium phaeovibrioides andChlorobium vibrioforme were studied by means of high performance liquid chromatography (HPLC) and picosecond absorbance difference spectroscopy. Analysis of pigment extracts of the chlorosomes revealed that they contain small amounts of bacteriochlorophyll (BChl)a esterified with phytol, whereas the BChlsc, d ande are predominantly esterified with farnesol. The chlorosomal BChla content inC. phaeovibrioides andC. vibrioforme was found to be 1.5% and 0.9%, respectively. The time resolved absorbance difference spectra showed a bleaching shifted to longer wavelengths as compared to the Qy absorption maxima and in chlorosomes ofC. vibrioforme also an absorbance increase at shorter wavelengths was observed. These spectral features were ascribed to excitation of oligomers of BChle and BChlc/d, respectively. 'One-color' and 'two-color' pump-probe kinetics ofC. phaeovibrioides showed rapid energy transfer to long-wavelength absorbing BChle oligomers, followed by trapping of excitations by BChla with a time constant of about 60 ps. Time resolved anisotropy measurements inC. vibrioforme showed randomization of excitations among BChla molecules with a time constant of about 20 ps, indicating that BChla in the baseplate is organized in clusters. One-color and two-color pump-probe measurements inC. vibrioforme showed rapid energy transfer from short-wavelength to long-wavelength absorbing oligomers with a time constant of about 11 ps. Trapping of excitations by BChla in this species could not be resolved unambiguously due to annihilation processes in the BChla clusters, but may occur with time constants of 15, 70 and 200 ps.