Any woman will confirm the fact everybody knows, that white clothes make you look larger and black ones make you look slimmer. Neurophysiologists have at last found an explanation for the phenomenon.
Scientists at the State University of New York, led by Jens Kremkow, studied the neural response to light and dark visual signals and concluded that light objects on a dark background excite different types of neuron from dark objects on a light background.
The great Italian physicist Galileo Galilei described an effect he could not account for: when the planets of the Solar System were observed through a telescope, Jupiter looked larger than Venus, while without it the opposite was true. He supposed this happened because of the refraction of light in the moisture covering the eye.
The celebrated German physiologist and physician Hermann Helmholtz came a step closer to the truth. He held that this visual illusion has to do not with the optical system of the eye but with the sensory one, that is, with the perception of the signals.
By the results of the American neurophysiologists' research, the whole of the phenomenon lies in the non-linear response of the visual system. The material and the conclusions of the work were published in 2013 in the journal “Proceedings of the National Academy of Sciences” and on a physics forum.
From the special receptors in the retina of the eye that convert photons into a nerve impulse, the signal travels along the neurons straight to the centre of the visual system, the cortex of the occipital lobe of the brain. The so-called bipolar neurons that carry the impulse are of two types: neurons of the “on” type, which respond to light bodies on a dark background, and of the “off” type, which respond to dark objects on a light background.
The subjects were shown visual stimuli of varying degrees of brightness and illumination, and the electrical activity of the neurons was measured. It turned out that the activity of the off-neurons grew more smoothly than that of the on-neurons, which gave their signals faster and considerably more strongly. Having studied the phenomenon in cats, monkeys and people, the scientists concluded that at one and the same contrast light objects on a dark background led to a greater excitation of the bipolar neurons.
The researchers explain that there is a logic to such a visual asymmetry. The physiologist José-Manuel, one of the authors, says that this feature of our organism lets a person make out extremely faint sources of light. The ability may have arisen in the course of evolution, when a potential danger had to be forestalled, such as the eyes of a predator glinting in the dark.