Sensory Physiology

Johannes Müller (1801–1858), the “Father of Physiology,” produced the classic systematic handbook (Handbuch der Physiologie des Menschen, 1833–1840) that set forth what was then known about human physiology and offered observations and hypotheses for further research. Among the formulations that Müller provided in the Handbuch was the law of specific nerve energies, which stated that the mind is not directly aware of objects as such but can only be aware of the stimulation in the brain conveyed by sensory nerves. The perceived qualities of stimulation depend upon the sense organ stimulated, the nerve that carries the excitation from the sense organ, and the part of the brain that receives the stimulation. Müller’s pupil, Hermann von Helmholtz (1821–1894), extended the law of specific nerve energies by theorizing that qualities of stimuli within a sensory modality are encoded in the same way that they are encoded among modalities. That is, distinguishing red from green, or a low pitch from a high one, depended upon specialized receptors in the eye or ear, distinct nerve connections within the visual or auditory system, and specific locations within the visual or auditory areas of the brain that receive the stimulation. The testing of the theory depended upon an individual’s report of the sensory experience (“I see red”), the nature of the stimulus to which the individual responded (a specific wavelength of the energy spectrum), and knowledge of the physiological organization
of the sensory systems. Relating the experience to the stimulus was a matter of experimental research that could be carried out with intact human beings; detecting the activity of nerves and the location of the brain to which stimulation was transmitted was possible then only with in vitro preparations of animals. Relating subjective, psychological experience to specific external stimulation was one step in suggesting how psychology might become a science.