Drawing Accuracy, Quality and Expertise
Carson, Linda Christine
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Drawing from a still-life is a complex visuomotor task. Nevertheless, experts depict three-dimensional subjects convincingly with two-dimensional images. Drawing research has previously been limited by its general dependence on qualitative assessment of drawings by human critics and on retrospective self-report of expertise by drawers. Accuracy measures cannot hope to encompass all the properties of “goodness” in a drawing but this thesis will show that they are consistent with the expertise of the drawers and with the quality ratings of human critics, they are robust enough to support analysis of ecologically valid drawing tasks from complex three-dimensional stimuli, and they are sensitive enough to study global and local properties of drawings. Drawing expertise may depend to some extent on more accurate internal models of 3D space. To explore this possibility we had adults with a range of drawing experience draw a still life. We measured the angles at intersecting edges in the drawings to calculate each person's mean percentage magnitude error across angles in the still life. This gave a continuous objective measure of drawing accuracy which correlated well with years of art experience. Participants also made perceptual judgements of still lifes, both from direct observation and from an imagined side view. A conventional mental rotation task failed to differentiate drawing expertise. However, those who drew angles more accurately were also significantly better judges of slant, i.e., the pitch of edges in the still life. Those with the most drawing experience were significantly better judges of spatial extent, i.e., which landmarks were leftmost, rightmost, nearest, farthest etc.. The ability to visualize in three dimensions the orientation and relationships of components of a still life is related to drawing accuracy and expertise. In our second study, we set out to extend our understanding of drawing accuracy and to develop measures that would support more complex research questions about both drawing and visual perception. We developed and applied novel objective geometric measures of accuracy to analyze a perspective drawing task. We measured the deformation of shapes in drawings relative to the ground truth of a reference photograph and validated these measures by showing that they discriminate appropriately between experts and novices. On all measures—orientation, proportionality, scale and position of shapes—experts outperformed novices. However, error is not uniform across the image. Participants were better at capturing the proportions and positions of objects (the “positive space”) than of the spaces between those objects (the “negative space”) and worse at orienting those objects than shapes in the negative space, but scale error did not differ significantly between positive and negative space. We have demonstrated that objective geometric measures of drawing accuracy are consistent with expertise and that they can be applied to new levels of analysis, not merely to support the conventional wisdom of art educators but to develop new, evidence-based means of training this fundamental skill. Most or all prior research into drawing was based on human ratings of drawing quality, but we cannot take for granted that the “goodness” of a drawing is related to its accuracy. In order to determine whether our objective measures of accuracy are consistent with drawing quality, we invited more than one hundred participants to grade the quality of all of the drawings we had collected and measured. We showed participants photographs of the still lifes on which the drawings were based and asked them to grade the quality of each drawing on a scale from 1 (“Poor”) to 10 (“Excellent”). People's quality ratings were consistent with one another. People without drawing experience rated drawings slightly more highly than the drawing experts did, but the ratings of both groups correlated well. As we predicted, the more drawing experience the artist had, the more highly rated the drawing was, and the more accurate the drawing was, the more highly rated it was. Furthermore, scaling error (but not proportionality, orientation or position) also predicted drawing quality. In perspective drawing, accuracy—as measured by angle error or polygon error—is related to drawing quality. If drawing practice strengthens an artist's model of 3D space, we would expect the three-dimensionality of drawings to be disrupted by damage to the dorsal stream or the connection between the dorsal and ventral streams. A former illustrator and animator, DM, who had suffered a right hemisphere stroke and presented with spatial neglect, performed modified versions of the angle judgement, spatial judgement and indirect drawing tasks of our second study. Despite his previous experience, he showed weaknesses in his mental model of 3D space, weaknesses that were not evident in his drawings before the stroke. Taken together, the thesis has developed and validated two objective measures of drawing accuracy that both capture expert/novice differences well and provide superior measures when contrasted with self-reported expertise. The performance of a single patient with neglect highlights the potential involvement of the dorsal stream in drawing. The novel quantitative measures developed here allow for testable hypotheses concerning the cognitive and neural mechanisms that support the complex skill of drawing to be objectively measured.