The aim of this study is to verify the conditions under which a series of visual stimuli (line segments) will be subjectively perceived as visual lines or surfaces employing four experiments. Two experiments were conducted with the method of subjective evaluation of the line segments, and the other two with the Osgood semantic differential. We analysed five variables (thickness, type, orientation, and colour) potentially responsible for the lines’ categorisation. The four experiments gave similar results: higher importance of the variables thickness and type; general lower significance of the variable colour; and general insignificance of the variable orientation. Interestingly, for the variable type, straight lines are evaluated as surfaces more frequently than curved lines and perceived as geometrical, flat, hard, static, rough, sharp, bound, sour, frigid, masculine, cold and passive. Curved lines are prevalently evaluated as lines, and categorised as organic, rounded, soft, dynamic, fluffy, blunt, free, sweet, sensual, feminine, warm and active. These results highlight the specificity of perceptual characteristics for the considered variables and confirm the relevance of the characteristics of variables such as thickness and type.

This study analyses the categorical ambiguity between visual appearances that belong in different categories, such as Oriental Calligraphy and certain Abstract Paintings selected from the works of Klee. In particular, the aim of our research was to identify whether there exist abstract features at the basis of purely visual configurations that determine the way in which they are categorized. Specifically, the intention was to determine whether, and to what extent, two artistic forms that display documented shared graphic and conceptual characteristics differ, or do not differ, visually, and whether certain features exist that identify them as a specific type of graphic work. The assumption that both categories had shared characteristics that made them graphically and conceptually similar and that resulted in categorical ambiguity, was confirmed. Moreover, the presence of some constituent features specific to one or the other was also confirmed. The results show that a Calligraphic Image and an Abstract Painting by Klee can be ‘exchanged’, but the tendency to confuse Calligraphy with an Abstract Painting is greater than that of confusing an Abstract Painting with Calligraphy.