Hess applied focal electrical stimulation to map the diencephalon and linked each site to changes in circulation, respiration, body temperature, and feeding behavior. He showed that the hypothalamus is the command center of the autonomic nervous system, switching between sympathetic and parasympathetic activity. This clarified brain pathways for stress-induced blood-pressure rises and hunger–satiety signals, benefiting internal medicine and neurology. The findings underpin modern devices like pacemakers and principles of hormone therapy, forming a bridge between basic and clinical medicine.

Using a stereotaxic technique, Hess delivered weak direct currents (2–3 μA) to hypothalamic and thalamic loci in unrestrained cats while simultaneously recording heart rate, blood pressure, respiration, pupil size, food intake, and body temperature. Reconstruction of stimulation sites revealed that the medial hypothalamus elicited parasympathetic-dominant responses (hypotension, gastric motility) whereas the lateral hypothalamus produced sympathetic activation (hypertension, mydriasis). He also localized feeding/drinking and sleep–wake centers, providing physiological evidence for functional localization. These findings deepened understanding of autonomic dysregulation, obesity, and fever mechanisms and stimulated growth of neuroendocrinology.

Hess’s in vivo stimulation studies were groundbreaking in quantifying discharge thresholds and response latencies of hypothalamic nuclei using movable micro-electrodes and chronaxy analysis. Strength–duration curves allowed metric comparison of end-organ responses, leading to a hierarchical description of integrative roles for medial/lateral hypothalamus, mammillary bodies, and suprachiasmatic nuclei. By contrasting focal chemo-stimulation (acetylcholine, adrenaline injections) with electrical pulses, he demonstrated pharmacological modulation of sympathetic–parasympathetic outputs. These findings foreshadowed later HRP retrograde tracing and optogenetic circuit analyses.