Neon colour spreading appears to have made its scientific debut in Italian psychologist Dario Varin’s 1971 monograph on ‘Chromatic Contrast and Diffusion Phenomena’—the main image on this page is a facsimile of the front cover of that book. The book was written in Varin’s native language and did not receive a translation at the time, and so made little impact in the rest of the world. Four years later the neon colour spreading phenomenon was rediscovered and named in van Tuijl (1975), where its ethereal, glowing quality was noted.

Other examples of neon colour spreading have been noted in different colours, which a version of the Ehrenstein Figure below beautifully illustrates.
​Figure 1 – Neon colour spreading as seen in an Ehrenstein Figure

The physiological mechanisms underlying neon colour spreading are not entirely understood. The figures exhibit apparent contours and experiential filling-in. Peterhans et al. (1986) suggest that the illusory completed contour – i.e. the transparent blue boundary - can be explained by the action of end-stopped neurons in the visual cortex. These cells correspond to elongated receptive fields on the retina and can fire selectively for both length and orientation of stimulus. Activity in spatially separated, end-stopped cells may trigger a gating mechanism, allowing for communication between neurons at previously inactive synapses. Bressan et al. (1997) tentatively suggest that the colour spreading, or filling-in, can be explained as a kind of diffusion of neural activity which occurs until a boundary caused by an illusory contour is detected. Friedman et al. (2003) discuss the possibility that visual perception is based on a neural image operating as a 2-dimensional array, in which colour signals ‘diffuse’ in all directions until meeting a contour. However, none of these explanations are close to being a complete theory—and indeed are still somewhat controversial.

The term ‘filling-in’ has two usages in vision science. Filling-in of the uncontroversial sort has been documented in laboratory conditions – if a subject is presented with the stimulus of a green disc surrounded by a red annulus, and the green region is retinally fixed (moves with the eye), the subject may report that they see only a red disc (Krauskopf 1963). This is because retinally fixed stimuli will fade from the visual field due to neural adaptation (see the entries for Troxler Effect and Negative Afterimages). In this case, the green disc has been filled in by the surrounding red, in a process similar to the filling-in of the blind spot which is inherent in the anatomy of the human eye. We may call this kind of documented phenomena, 'experiential filling-in'.

A more controversial kind of ‘filling-in’ is neural filling-in. This is posited as one explanation for the experiential filling-in described above. In neural filling-in the brain actively generates information which is triggered by an absence of information. The account of Friedman et al. (2003) above is one such theory.

Philosopher and cognitive scientist Daniel Dennett has attacked the idea of neural filling-in (Dennett 1992), arguing that it presupposes a passive conscious observer embodied within the brain who is receiving information from the visual pathway as though viewing an image on a cinema screen (the homunculus fallacy). Dennett considers this picture to be false and to be inherent in any suggestion that there is a neural-perceptual isomorphism, which means a kind of (abstract) form-preserving map between the pattern of neural activity and the organized perceptual experience. Dennett (1991) makes the case that the neural substrate or basis supporting visual conscious experience is in fact distributed and transient in the brain. In short, experiential filling-in need not require neural filling-in.

However, Dennett's idea is highly controversial. See Myin and de Nul (2009) for a survey of experimental research suggesting that visual perceptions must indeed match neural activity isomorphically, and that neural filling-in does in fact take place.
Figure 2 – Another example of Neon colour spreading produced by the visual perception lab, ‘blelb’.