By Madison Knaub (Claremont McKenna College) [Edited by Lars Schmitz, as part of BIOL 167 “Sensory Evolution”, an upper division class at the W.M. Keck Science Department. Written for educational purposes only].
How it all began: From Galileo to the big leagues
When looking at the planets with the naked eye it often seems like Venus is larger than Jupiter. Of course, anyone who has taken rudimentary astronomy knows that Jupiter is in fact 11.7 times larger than Venus (for a cool visual representation of planet size click here). Galileo was one of the first people to observe, and document, this visual spatial discrepancy. He believed that this strange visual phenomenon was due to a light-formed optical illusion that caused Venus to appear larger.This visual conundrum was later termed the “irradiation illusion” by German physicist Hermann von Helmholtz, who first began to question the science behind Galileo’s observation. But while many scientists and researchers have confirmed that there is indeed a glitch in the visual system causing visual processes to inaccurately perceive illuminated object size (Ts’o et al. 1990) no one ever really understood why. That is, until now. In 2014, a team of scientists around Jens Kremkow and Jose-Manuel Alonso at the State University of New York found neurophysiological processes within the visual system that are responsible for this mysterious visual conundrum.
Some background (because who really remembers what they learned in intro BIO or NEURO?)
Light is processed within the eye through two separate mechanisms. These mechanisms are referred to as ON and OFF channels. The ON cells function in response to positive contrast, which is a light stimulus on a dark background. Conversely, the OFF cells function in response to negative contrast which is dark stimulus on a lighter background, and so far it had been assumed that the neuronal response to positive and negative contrasts was about equal. (For a more in-depth look at ON and OFF channels check out this cool slideshow by Peter H. Schiller at MIT!) So when you look at the night sky what you are seeing is the positive contrast of the bright stars against the dark sky. How is it that the spatial resolution of dark objects (Jupiter) appears better than that of bright objects (Venus), which appear as a big, poorly defined light spot?Cats, brains, and neurons. Oh my!
In order to study the neuronal mechanism responsible for the “irradiation illusion” the scientists at the State University of New York looked at how ON/OFF channels responded to different variations of positive and negative contrast. In the first experiment they performed, neuronal activity was recorded in anesthetized cats as the animals were exposed to different variations of light stimuli, dark stimuli, and different backgrounds to increase or decrease contrast. Researchers focused on neuronal activity in parts of the brain responsible for vision, the primary visual cortex and the lateral geniculate nucleus in the thalamus. (For a video showing how vision works with the brain click here).
When the cats were exposed to positive contrast, light stimuli on a dark background, their ON channels were activated and the researchers were able to measure the strength on the neuronal response within the brain. Here comes the interesting part. When the researchers exposed the cats to negative contrast, dark stimuli on a light background, they noticed that strength of response for the OFF channels was linearly related to the luminance decrements, for both white and gray backgrounds. A different pattern was observed for the ON channels. Especially when seen against a dark background, the ON channels saturated much more quickly with luminance decrements, or in other words even the faintest bright signal on a dark background elicits a strong neuronal response (Figure 3). It seems that functions resulting from bright signals on dark backgrounds (ON functions) are highly compressed compared to OFF functions, as seen in the much more linear, gradual response increase. Hence, bright signals on dark background produce somewhat of a ”neuronal blur”.While to us this may seem like nothing special, these findings contradict many preexisting assumptions held about visual processes. For quite some time it had been believed that ON and OFF channels in the brain are roughly equal in presence and function. But, these new finding imply that OFF functions are actually predominate in the visual process, while the ON functions are much more compressed.
How does this tie in with the irradiation illusion? With a lesser amount of ON functions, the brain has a weakened ability to process and resolve the spatial resolution for light stimuli and positive contrast. So when looking at objects like Venus and Jupiter the brain is unable to accurately perceive the planet’s actual size. Venus is very bright compared to Jupiter and stands out against a dark background. The compressed signal from negative contrasts, combined with neuronal blur causes the brain to perceive light stimuli, like Venus, as larger in appearance than its actual physical size.
What it all means: evolution and beyond
These findings have interesting implications for our understanding of visual evolution. The researchers concluded that the nonlinear response of ON channels may have originated as early as the photoreceptor (Kremkow et al. 2014). This implies that the neural nonlinearity of ON channels evolved with an eyes ability to detect light. So why would an eye evolve to have greater spatial resolution for dark stimuli and poorer spatial resolution for light stimuli? One possible answer put forward in the paper was that this evolutionary puzzle was due to a greater prevalence of dark stimuli in nature (Kremkow et al. 2014). But, of course, there are many possible answers to this question of “why” eyes are better at resolving dark images. Perhaps this visual process evolved in response to the light conditions of predator favored environments. Since many predatory species favor dark conditions for hunting, better spatial resolution of dark images would give prey an advantage at detecting hidden predators like the black jaguar shown below. It would be interesting to do further research on known predators and prey species to see the differences in their spatial resolution for dark and light stimuli. As my sensory evolution Professor, Lars Schmitz, would say, “I smell a paper.”In conclusion, these findings on the discrepancies between neuronal functioning in the ON and OFF channels finally provide some much needed details for the understanding of Galileo’s strange, starry observation. So the next time you step outside on a clear night take a look at Jupiter and Venus. Which looks bigger? If Venus catches your eye, you now know that this spatial discrepancy is not the work or a visual illusion but rather your brain and visual processes creating your very own visual conundrum.
Kremkow, J., Jin, J., Komban, S.J., Wang, Y., Lashgari, R., Jansen, M., Li, X., Zaidi, Q, Alonso, J.-M. 2014. Neuronal nonlinearity explains greater visual spatial resolution for darks than lights. PNAS, published online. (DOI: 10.1073/pnas.1310442111).
Ts’o, D., Frostig, R., Lieke, E., Grinvald, A. 1990. Functional organization of primate visual cortex revealed by high resolution optical imaging. Science, 249: 417-420. (DOI: 10.1126/science.2165630)