The scintillating grid is a simultaneous lightness contrast illusion of a similar type to the Hermann grid, although it was discovered over a century later (Lingelbach et al. 1995). One important difference is that the scintillating grid figure comprises white dots at the intersection of grey gridlines on a black ground, whereas there are no such dots on the Hermann grid, and so any dots which might appear within the Hermann grid are purely artefacts of the visual system. In short, the dots in the scintillating grid are really there - they are just sometimes experienced as being black when they are in fact white. Philosophers of perception often distinguish between three kinds of perceptual experience: (i) veridical (accurate) perception of the world; (ii) illusion—nonveridical (innaccurate) perception of the world; (iii) hallucination—failure to perceive the world (Macpherson 2013). In the case of the scintillating grid, the nonveridical perception of the white dots as black seem to make it an unequivocal example of an illusion. This means that some of the philosophical interest surrounding the Hermann grid – specifically, the question of whether objects which exist only as artefacts of the visual system should be considered hallucinatory – does not apply to the scintillating grid. However, it is worth noting that on some philosophical views there is no difference in kind between hallucination and illusion (e.g. Brewer 2008).
The classical explanation of the physiology behind the scintillating grid illusion is due to Baumgartner (1960), which he put forward to explain the Hermann grid. Baumgartner believed that the effect is due to inhibitory processes in the retinal ganglion cells, the neurons that transmit signals from the eye to the brain. To each cell there corresponds a small region of the retina called the receptive field, where photoreceptive rods and cones can trigger an electrical response in that cell. The receptive fields of adjacent ganglion cells may overlap. Kuffler (1953) used microelectrodes placed in the retinae of cats to measure the response of individual neurons which were stimulated by pinpoints of light, and showed that the receptive field can be broken down into a central disc and a surrounding annulus. Kuffler was also able to demonstrate that retinal ganglion cells come in two distinct kinds – either ON-centre or OFF-centre (see Fig. 1). For a given neuron, it is the difference between the stimulation of the centre and that of the surround that determines the strength of neural response (i.e. how rapidly it fires).
Following Baumgartner's reasoning, the ON-centre ganglion cells whose receptive fields are centred on the grid crossings have 4 inhibiting light (grey) areas in their surround, whereas those whose fields centre on ‘streets’ have only 2 inhibiting light (grey) areas (see Fig. 2). The on-centre neurons centred at grid crossings will fire less and so these locations on the grid appear darker. The disappearance of the grey patches whenever we try to focus on them is explained by the fact that the ganglion cells in the centre of the retina (the fovea) which mediate high-acuity vision have very small receptive fields, so the range of their stimulus lies entirely inside the intersection point. Furthermore, the fact that the illusion occurs with inverted colours is neatly explained by the existence of the second kind of retinal ganglion cell, the OFF-centre neuron.
For all the attractions of Baumgartner-type accounts of the scintillating grid, recent decades have proven it unsatisfactory. Schiller and Tehovnik (2015) cite three main flaws: firstly, the illusion persists when we increase the size of the figure despite the fact that receptive fields are of a fixed size; secondly, the illusory effect can be greatly diminished or even removed entirely by skewing or otherwise distorting the grid in such a way that difference in stimulus between centre and surround is unchanged (see Fig. 3); thirdly, the actual arrangement of retinal ganglion cells and their corresponding receptive fields is not as simple as Baumgartner supposed – midget and parasol ganglion cells exist in different ratios throughout the retina, the latter having much larger centre-surround receptive fields than the former. This complicated arrangement of excitatory centres and inhibitory surrounds, operating across various distances on the 2-D retinal image, means that Baumgartner’s localized retinal processes cannot explain the scintillating grid effect grid effect (Schiller and Carvey 2005). Modern theories of vision have tended to explain lightness contrast phenomena by appealing to cortical processes – e.g. the existence of multiple spatial filters which operate at different spatial frequencies (corresponding to receptive field size) to measure non-local contrast (Blakeslee and McCourt 2012).