Roger N. Shepard, a cognitive scientist, was renowned for his research on mental rotation and visual perception. The historical context of Shepard's quote "Perception is externally guided hallucination" lies in the burgeoning fields of cognitive psychology and neuroscience during the mid-to-late 20th century. Researchers were beginning to unravel the complex processes behind human perception, recognizing that our brains construct our sensory experience based on external stimuli, but do so through internal mechanisms akin to imagination or hallucination.

The quote itself suggests that what we perceive is not a direct absorption of the world around us, but rather a creative reconstruction by our brain. The external world provides cues and a framework, but the brain uses its stored knowledge and expectations to interpret these signals, effectively 'hallucinating' reality based on these limited inputs. 

From today's perspective, Shepard's quote remains profoundly insightful. With advances in technology, particularly in virtual and augmented reality, we see practical applications of this principle—where computer-generated sensory inputs guide our perception to create immersive experiences. Moreover, contemporary discussions about the subjective nature of reality and the role of perception in shaping our understanding of the world continue to draw upon the foundational ideas Shepard articulated.

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