t6o8 HANDBOOK OF PHYSIOLOGY ^-.NEUROPHYSIOLOGY III 



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fig. 6. Forms used in experiments on metacontrast (successive interaction of contours). Figures 

 marked a are not perceived if they precede figures marked b within critical intervals, and if their 

 outer contour coincides with the subsequent position of the inner contour of the b figures. [Modified 

 from Werner (534).] 



field. Repeated scanning movements lead to organ- 

 ized neural traces, or 'cell assemblies* [due to some 

 process similar to Rappers' neurobiotaxis (247)]; 

 eventually, the scanning movements as such can 

 drop out, since parts (it the figure will now activate 

 the trace and the corresponding perceptual habit of 

 seeing (or feeling) a given form. Transposition of 

 forms (the ability to recognize them independently 

 of size or of orientation on receptor surfaces) would 

 likewise be acquired in early development. An in- 

 direct argument in favor of these views is sometimes 

 seen in the difficulties of pattern perception under 

 various conditions of •reduced' stimulation, under 



low illuminati with small visual angles or with 



brid periods of time (as on tachistoscopic presenta- 

 tion 



I achisloscopic exposures can impede the pel 

 don 11I patterns to such an extent that one might 

 doubi whether patterns are ever perceived under 

 1.1I conditions so that apprehension ol different 

 parts is stiicilv simultaneous. Apparently some scan- 

 ning is net c--.11 v even for 'good' figures. In fact, ease 



ol perception under reduced conditions, such as 

 tachistoscopy, can be used as still another objective 



index for goodness of a figure. This rank order of 

 difficulty of shapes under abnormal exposure condi- 

 tions has been explored, and the results lilted to a 

 physical diffusion model of shape perception by Bit- 

 terman et al. (46) and Krauskopf el al. I J<|i I. 



That shape perception takes time can be demon- 

 strated even more strikingly by successive tachisto- 

 scopic presentation. If the two patterns shown in figure 

 6, disk and annulus, are exposed each for ta to 20 

 msec., and the annulus follows the disk at a critical 

 interval of about 150 msec, the disk will not lie per- 

 ceived. For the effect to appear, it is necessary that 

 the inner contour of the annulus occupy the same 

 region of the visual field as the outer contour of the 

 disk [f the order of succession is reversed (annulus 

 shown before diski, both patterns are seen. Ap- 

 parently, the outer contour ol the disk is still in the 

 process ol lot in.ition when the surrounding annulus 

 begins to form. The phenomenon has been known for 

 .1 long time under the name of •metacontrast' see 

 Stigler (454) and later Wet tier (534)]; it has been 

 reported to occur when the first pattern is presented 



to one eye, and the second to the other eve, but there 



