Hermann Grid Illusion

‘What does the Hermann Grid tell us about visual processing?’

The Hermann Grid Illusion

The optical illusion known as the Hermann Grid (see Figure 1.1), and credited to Hermann (1870), is composed of pale grey dots that appear at the intersections of white horizontal and vertical lines forming a grid on a dark (preferably black) background. These dots disappear when an attempt is made to look directly at the intersections. Bach (2008) suggested that this phenomenon demonstrated a valuable principle of perception, that what is seen does not always exist, and that this is important because ‘our perceptions depend up how our visual system responds to environmental stimuli and how our brain then interprets this information’.

Baumgartner’s Explanation

The disappearance of the effect by curving the gridlines demonstrates the inapplicability of lateral inhibition to explain the overall effect as Geier et al. (2008) established that the net inhibition in their modified Hermann Grid was identical to that of the original, so the centre-and-surround antagonism would be expected to be virtually identical, i.e. the relationship between stimulus and receptive fields would have had to be unchanged to support the original hypothesis.

A further problem revolves around Baumgartner’s claim that the assumed relationship between the stimulus and receptive fields will not change if the net antagonism remains the same. As shown by Schiller & Carvey (2005), and contrary to the ‘lateral inhibition’ theory, the illusory effect is not increased by enlarging the centre-and-surround antagonism of on-centre retinal ganglion cells. If the illusion was due to the classical theory, the effect would be greater in figure 3.7 as opposed to figure 3.6, due to the increased surround antagonism for receptive fields at the gridline

In this, it is suggested that the illusory effect is brought about by the response and activity of S1-type simple cells in the area V1 in the primary visual cortex. For the Hermann Grid illusion to be effective, the alignment of orientation-specific elements is considered important (Schiller & Carvey, 2005), suggesting in turn that orientation-selective neurons have a role in the visual processing of the illusion, so further supporting the conclusion that the effect arises in the cortex, not the retinal