Roger Shepard (1929-2022) was a groundbreaking cognitive psychologist whose experiments on mental rotation provided some of the most compelling evidence that the mind constructs and manipulates internal representations that preserve the structural properties of the external world. His work helped establish the legitimacy of studying mental imagery — a topic banished during the behaviorist era — and provided foundational methods and results for spatial cognition research.
Mental Rotation
In the classic Shepard and Metzler (1971) experiment, participants judged whether pairs of three-dimensional block figures were the same shape or mirror images. The key finding was that reaction time increased linearly with the angular disparity between the two figures, suggesting that participants mentally rotated one figure to match the other at a rate of approximately 60 degrees per second. This linear relationship between angle and time implied that mental rotation is an analog process — it passes through intermediate orientations, just as physical rotation does — rather than a discrete symbolic comparison.
Shepard's other major contribution was the universal law of generalization: the probability of generalizing a learned response from one stimulus to another decreases exponentially with the psychological distance between them. This law has been confirmed across species, sensory modalities, and types of stimuli. Shepard argued it reflects an optimal adaptation to a world in which the relevance of past experience to current situations decreases with dissimilarity. This work bridged cognitive psychology and evolutionary psychology, suggesting that fundamental cognitive principles reflect adaptation to environmental structure.