(not including the diffuse room illumination). Owing to computational constraints, each fractal was limited to64364 pixels (18 318 mm) in size and 64 frames in du-ration. Thus each stimulus subtended 2. 58 arc deg em-bedded in a 43. 9 (H) by 36. 4 (V) deg dark surround andlasted 2. 133 s. Each 64-frame stimulus sequence wascreated by ﬁrst producing a series of random white noiseimages. Gray-scale values are computed digitally withuse of 15-bit ﬂoating-point numbers and are displayedwith 8 bits (256-level gray scale). Each image in the se-ries was Fourier transformed, and the amplitudes of allspatial frequencies were equalized to ensure that thenoise was uniformly white. The resulting frequency dis-tributions were then ﬁltered so that the amplitude spec-trum varied over time following the power law relation-ship 1/ f taand over space (isotrophically over both Cartesian coordinates) following the power law relation-ship 1/ f sb@s5(x21y2)1/2#. Ten spatial exponents ( b Fig. 2. Appearance map of spatiotemporal fractal space. The appearance map is based on the responses of four observers. To help render the chart more readable to a diverse audience, the real-world descriptions provided by the subjects (which were highly individu-alistic and culturally based) have been replaced where possible with stimulus-based descriptions provided by the other subjects. 2406 J. Opt. Soc. Am. A/Vol. 18, No. 10/October 2001 Billock et al.