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H Bülthoff

Publications and source records attributed to H Bülthoff.

7 recordsLinked to original sources
PubMed1991

Shape from specularities: computation and psychophysics.

Images of artificial and natural scenes typically contain many 'specularities' generated by mirror-like reflection from glossy surfaces. Until fairly recently computational models of visual processes have tended to regard specularities as obscuring underlying scene structure. Mathematical modelling shows that, on the contrary, they are rich in local geometric information. Recent psychophysical findings support the notion that the brain can apply that information. Our results concern the inference of 3D structure from 2D shaded images of glossy surfaces. Stereoscopically viewed highlights or 'specularities' are found to serve as cues for 3D local surface-geometry.

Cues↗
PubMed1991

Perceived depth scales with disparity gradient.

Perceived difference in depth between two adjacent stimuli decreases with increasing disparity gradient even if the disparity stays constant, ie when the stimuli approach each other along paths within fronto-parallel planes. This depth scaling effect is more pronounced with line stimuli than with two isolated points or two small symbols and is insignificant for easily discriminable symbols. The decrease in perceived depth is more pronounced for horizontal orientation than for oblique or vertical orientation. The ratio of perceived depth difference to displayed disparity difference also decreases when the distance between the stimuli increases at a constant gradient in depth. This is to say that we are more correct in our depth estimates for steep gradients in depth when the euclidean distance between the stimuli is short.

Attention↗
PubMed1990

Does the brain know the physics of specular reflection?

Images of artificial and natural scenes typically contain many highlights generated by mirror-like reflection from glossy surfaces. Until recently, computational models of visual processes have tended to regard highlights as obscuring the structure of the underlying scene. The truth is that, on the contrary, highlights are rich in local geometric information. Here we report that the three-dimensional appearance of a highlight on a computer-simulated stereoscopic curved surface affects observers' judgment of surface gloss. We also show that the 3-D appearance of a highlight affects the perception of surface curvature--that is, it can force an ambiguous convex-concave figure to change state. We thus conclude that human visual analysis seems to employ a physical model of the interaction of light with curved surfaces, a model firmly based on ray optics and differential geometry.

Brain↗
PubMed1989

A parallel algorithm for real-time computation of optical flow.

The precise management of two-dimensional field of velocities from time-varying two-dimensional images is impossible in general. It is, however, possible to compute suitable 'optical flows' that are qualitatively similar to the velocity field in most cases. We describe a simple, parallel algorithm that computes an optical flow from sequences of real images, which is consistent with human psychophysics and suggests plausible physiological models. In particular, our algorithm runs on a Connection Machine supercomputer in close-to-real time. It shows several of the same 'illusions' that are perceived by humans. A natural physiological implementation of the model is consistent with data from cortical areas V1 and MT.

Algorithms↗
PubMed1988

Independent spatial waves of biochemical differentiation along the surface of chicken brain as revealed by the sequential expression of acetylcholinesterase.

AChE-positive cells suddenly amass in a superficial layer of the neuroepithelium; this layer finally covers, in a sheat-like manner, the entire surface of the embryonic chicken brain. This feature is functionally not understood; however, it appears shortly after the neurons become postmitotic, and the lateral extensions of this layer can easily be traced using histochemistry on serial brain sections. The layer can therefore be exploited to delineate spatially the waves of onset of biochemical tissue differentiation. We have studied whole brains between stages 11 and 30 and provide the first complete spatial schemes of brain differentiation based on computer-reconstructed, two- and three-dimensional maps. The brain does not differentiate in one smooth coherent wave, but instead five separate primary AChE-activation zones are detected: the first originating at stage 11 ("rhombencephalic wave"), the second at the same time ("midbrain wave"), the third at stage 15 ("tectal wave"). A fourth zone develops later, at stage 18, from the bottom part of the telencephalon to the top. Retinal development also starts at stage 18. In a given area, it appears that AChE-development shortly precedes that of the formation of major fiber tracts. AChE might therefore represent a prerequisite for fiber growth and pathfinding.

Acetylcholinesterase↗
PubMed1987

GABA-antagonist inverts movement and object detection in flies.

Movement detection is one of the most elementary visual computations performed by vertebrates as well as invertebrates. However, comparatively little is known about the biophysical mechanisms underlying this computation. It has been proposed on both physiological and theoretical grounds that inhibition plays a crucial role in the directional selectivity of elementary movement detectors (EMDs). For the first time, we have studied electrophysiological and behavioral changes induced in flies after application of picrotoxinin, an antagonist of GABA. The results show that inhibitory interactions play an important role in movement detection in flies. Furthermore, our behavioral results suggest that the computation of object position is based primarily on movement detection.

Action Potentials↗