Rock and Brosgole, using a stimulus array which could be tilted in depth, have shown that the crucial factor in grouping by proximity is nearness in perceived space rather than simple retinal proximity.
Hochberg and his co-workers (Hardy, Silver- stein) have devised a technique to set the factors of proximity and similarity against each other and thereby assess their relative strengths in different situations.
Beck, in an import series of experiments, has forced a reanalysis of the factors involved in similarity grouping. Beck found that figures having lines in the same orientation rather than figures having generally the same shape tended to be seen together. In his study line orientation took precedence over physical similarity in determining perceptual grouping.
The results corroborated in subsequent experiments in which by him threshold measures and ratings of clarity were used to index the effectiveness of perceptual segregation. In the light of his findings, Beck suggested that the processes in grouping are based upon spontaneous pre- attentive responses to relatively simple properties such as brightness, size, and line direction.
Olson and Attneave found that arrays in which elements differed in terms of slope (line segments of one orientation opposed to others of a different orientation) yielded more “immediate” grouping times than arrays in which the same slopes were combined to form different angles.
They also discovered that grouping was dependent upon the orientation of the entire stimulus array; arrangements with horizontal and vertical elements gave better grouping than those with diagonal elements.
These findings led them to propose that grouping may depend upon the relation of elements to an internal reference system of x-y axis. More recently Uttal, has provided some interesting evidence favouring the Law of Pragnanz in perceptual grouping.
His research showed that regular and similar dot patterns are segregated more accurately from dotted visual noise than less regular patterns; moreover, the organization of a set of dotted lines into regular linear arrays enhances the accuracy of segregation over irregular patterns.