Color in physiology

As humans, we detect and process external stimuli through sensory perception.¹ Vision, scent, touch, sound, and taste, often in combination and/or simultaneously, help us interpret and adequately react to our environments. Sensory and perceptual alterations in humans occur on a wide spectrum,² ranging from physiological phenomena in response to everyday environmental³ or hormonal⁴ influences, to the aura symptoms in migraine patients, to severe, prolonged alterations that qualify as hallucinations.⁵

Color perception is a fundamental aspect of the human experience. Colors, manifestations of different wavelengths of light, influence human emotions, behaviors and physiological responses.⁶ From the vibrant hues of a sunset to the subtle shades of a painting, the way we perceive color plays a huge role in shaping our understanding of the world around us. In recent years, scientific inquiry into the details of the mechanisms behind visual perception have deepened our comprehension of how our visual system processes and interprets the rich array of colors in our environment. Recent insights into visual perception and signal interpretation include for example the role of pacsin proteins in vision, a novel function of these regulators of the actin cytoskeleton and cellular transport processes.⁷ Also, in the context of visual perception and the role of visual stimuli in maintaining healthy circadian rhythms, the influence of an impaired eyesight⁸ and the role of the eye as not only a light- but also temperature-sensitive organ^{9, 10} have been described.

Aside from human perception and interpretation of light of different wavelenghths as color, colored light has become an indispensable methodological tool for physiologists to precisely manipulate light to activate or inhibit intracerebral neural circuits to selectively stimulate or suppress neuronal activity with high temporal and spatial precision, to e.g. analyze complex neural networks and elucidate causal relationships between neuronal activity and behavior.^11-13

However, color is also a multifaceted communicative tool across various domains, encompassing fields from biology to design. In nature, colors often act as a visual language, signaling vital information, for example survival and reproduction. From the vibrant hues of flowers attracting pollinators to the warning colors of venomous creatures, organisms utilize color to convey messages efficiently. Beyond biology, humans harness color as a potent means of expression and communication. Color plays, for example, a pivotal role in branding, where shades become synonymous with companies and products and influence consumer perception and behavior. Color choices evoke emotions, convey cultural significance, and guide attention—a process to which physiological research has significantly contributed. Take, for example, color vision deficiency, commonly known as color blindness, a condition characterized by the inability to perceive certain colors accurately. On November 15, 1875, approximately 8 km west of Linköping, a signaling error between a station master and a steam engine driver resulted in the fatal Lagerlunda train accident. Ophthalmologist Frithiof Holmgren was the first to suggest that the engineer of the northbound express, Andersson, or his oiler, Larsson, had been color blind, contributing to the accident. Concurrently, Holmgren pioneered the first screening test for color vision deficiency,¹⁴ revolutionizing the assessment of this condition. Although neither Andersson nor Larsson survived to be tested, the implementation of mandatory color-vision screening for railroad staff remains to this day a cornerstone of public transport safety. In academic contexts, we harness the power of color as a communicative tool when we teach, write or present our results. Ensuring compatibility with color vision deficiencies by avoiding problematic color combinations such as red-green can promote inclusivity in data interpretation.

When presenting research results, the strategic utilization of color enhances the clarity and impact of our communication. The careful selection of colors helps to for example distinguish between different data categories, facilitates easy comprehension for the audience, and draws attention to important trends or patterns. Additionally, the adherence to color conventions, such as heat maps of mRNA expression levels¹⁵ or visualization of gradual physiological responses such as shivering,¹⁶ can improve consistency and comprehension across different presentations. Visualizing multimodal datasets in biomedical research poses challenges due to the disparate nature of the data sources, requiring innovative techniques to integrate and present information cohesively.¹⁷ Incorporating diverse data types such as images, genomic sequences, and clinical records demands sophisticated visualization tools capable of capturing complex relationships and patterns across modalities, in which color coding and colored visualization can aid greatly. Standardization and color coding facilitate clarity and consistency in multiplex analyses,^{18, 19} streamlining data interpretation and enhancing reproducibility across biomedical research, thereby fostering efficient collaboration and advancing scientific understanding.

In general, by advancing our understanding of how color impacts human physiology, we not only deepen our knowledge of the human mind and body but also pave the way for innovative applications in fields ranging from healthcare to design.

The authors declare no conflict of interest.