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Explore how your senses detect contrasts and differences in the natural world through this fifth lecture from Colin Blakemore's 1982 Christmas Lectures series at The Royal Institution. Discover why your sensory systems are engaged in a constant battle against information overload and learn about the clever detection and coding tricks that sense organs and nerves have developed to handle their incredible sensitivity across vast ranges of intensity. Understand how most sensory systems prioritize detecting changes and differences rather than absolute values of physical stimuli, with the retina serving as a prime example by transmitting minimal information about uniform light areas while focusing on intensity changes at object edges. Examine how this simple mechanism frees the optic nerve from irrelevant messages but creates curious illusions and misperceptions that artists have exploited to deceive the eye. Learn why your eyes must constantly move not only to explore the visual world but also to prevent your view from fading, and discover what happens when optical instruments hold images stationary on the retina despite eye movement. Investigate how moving objects become easier to see than stationary ones and why some animals are virtually blind when nothing around them moves. Analyze the strategy of detecting difference and change used by most sense organs, including the phenomena of adaptation and fatigue that lead to amusing sensory illusions. Understand the interpretation challenges this creates for the brain, which must rely on accurate memory of preceding signals to decode the real meaning of difference-based messages. Compare this system with senses that must provide absolute rather than relative information, such as those signaling constant pain, gravity direction, or blood chemical composition, because they deal with stimuli that rarely change.
