The Effect of Illuminant Position on Perceived Curvature

In shaded scenes surface features can appear either concave or convex, depending upon the viewers judment about the direction of the prevailing illuminant. If other curvature cues are added to the image this ambiguity can be removed. However, it is not clear to what extent, if any, illuminant positin exerts an influence on the perceived magnitude of surface curvature. Subjects were presented with pairs of spherical surface patches in a curavture matching task. The patches were defined by shading and texture cues. The percevied curvature of a standard patch was measured as a function of light source position. We found a clear effect of light source position on apparent curvature. Perceived curvature decreased as light source tilt increased and as light source slant decreased. We also found that the strength of this effect is determined partly by a surface's reflectance function and partly by the relative weight of the texture cue. When a specular component was added to the stimuli, the effect of light source orientation was weakened. The weight of the texture cue was manipulated by disrupting the regular distribution of texture elements. We found an inverse relationship between the strength of the effecct and the weight of the texture cue: lowering the texture cue weight resulted in an enhancement of the illuminant position effect.

Test stimulus characteristics determine the perceived speed of the dynamic motion aftereffect

Using a speed-matching task, we measured the speed tuning of the dynamic motion aftereVect (MAE). The results of our Wrst experiment, in which we co-varied dot speed in the adaptation and test stimuli, revealed a speed tuning function. We sought to tease apart what contribution, if any, the test stimulus makes towards the observed speed tuning. This was examined by independently manipulating dot speed in the adaptation and test stimuli, and measuring the eVect this had on the perceived speed of the dynamic MAE. The results revealed that the speed tuning of the dynamic MAE is determined, not by the speed of the adaptation stimulus, but by the local motion characteristics of the dynamic test stimulus. The role of the test stimulus in determining the perceived speed of the dynamic MAE was conWrmed by showing that, if one uses a test stimulus containing two sources of local speed information, observers report seeing a transparent MAE; this is despite the fact that adaptation is induced using a single-speed stimulus. Thus while the adaptation stimulus necessarily determines perceived direction of the dynamic MAE, its perceived speed is determined by the test stimulus. This dissociation of speed and direction supports the notion that the processing of these two visual attributes may be partially independent.

Is the General Public Licence a Rational Choice?

Open source projects are networks of developers, distributors and end-users of non-proprietary created knowledge goods. It has been argued that this form of organization has some advantages over the firm or market coordination. I show that for sufficiently convex and modular projects proprietary licences are not able to sustain sequential knowledge production which, however, can be carried out if the project is run on the open source basis.