Estimation of Time-to-Contact from Retinal Flow

We investigated four models for estimating time-to-contact (TTC) from retinal flow. Lee's model can deal with sparse flow but fails if the flow contains a rotational component. Koenderink's model, based on div, can deal with rotation but fails if the flow is sparse or if the world does not vary coherently in depth. Two new models were developed by representing retinal flow as the sum of an expansion and a rotation component. The first operates on pairs of points and can deal with sparse flow but fails if the world does not vary coherently in depth. Uniquely, this model provides TTC estimates without prior knowledge of either the focus of expansion (FOE) or focus of rotation (FOR). The second model estimates both the FOE and the FOR and then operates on a point-by-point basis. This model can deal with incoherent depth variations. We compared human performance with these different model properties by requiring subjects to estimate FOE and TTC from random-dot kinematograms. We used kinematograms depicting smooth planes and random 3-D clouds of points, and systematically varied the density of the flow. Performance was not substantially reduced by sparse flow or by incoherent depth, which argues against Koenderink's and the first of our own models. Performance remained good when rotation was added to the flow, which argues against Lee's model. Overall, the data favour a model that first decomposes flow into expansion and rotation components and then estimates TTC on a point-by-point basis.

Application of Scanning Tunneling Microscopy to the Study of Metals: Spectroscopy and Topography

ABSTRACTThe design and performance of a scanning tunneling microscope for use in the study of metal surfaces is described. The system was designed for ultra high vacuum together with standard sample cleaning and characterization techniques. The STM provides both topographic and spectroscopie information. Topographic images of well annealed Au(lll) show very smooth planes with single atomic steps. Corrugation on the (111) planes, which is expected from reconstruction models for this surface, is not seen. Other areas show monatomic steps in the form of an ordered array with a period corresponding to that derived from previous studies. A possible alternative reconstruction mechanism is suggested by these STM data. On the same surface are steeper sloped regions with multiple steps of equal height with wide facets. Spectroscopie first derivative data for Au and Pd show peaks which correspond to surface and bulk electronic states, for both filled and unfilled cases. The energy values of these states are compared directly with the results of other experimental methods. The use of combined topographic and spectroscopie mode for metals is anticipated.