Take a look at this picture:
Can you see the C hidden in the logo? It took me some time, but maybe it’s easier when looking at this picture to the left.
To the right you see triangles who are in fact implied. The triangle is only suggested because of the way the ‘Pac-Man’ shapes are positioned; there appears to be a light-grey triangle on top of three black circles. Researchers from the Radboud University use visual illusions to demonstrate to what extent the brain interprets visual signals. They were surprised to discover that active interpretation occurs early on in signal processing. In other words, we see not only with our eyes, but with our brain, too. The primary visual brain cortex is normally regarded as the area where eye signals are merely processed, but that has now been refuted by the new results.
From the press release:
How does the brain do that? That was the question Peter Kok and Floris de Lange, from the Donders Institute at Radboud University in Nijmegen, asked themselves. Using fMRI, they discovered that the triangle — although non-existent — activates the primary visual brain cortex. This is the first area in the cortex to deal with a signal from the eyes.
The primary visual brain cortex is normally regarded as the area where eye signals are merely processed, but that has now been refuted by the results Kok and De Lange obtained.
Active interpretation
Recent theories assume that the brain does not simply process or filter external information, but actively interprets it. In the example described above, the brain decides it is more likely that a triangle would be on top of black circles than that three such circles, each with a bite taken out, would by coincidence point in a particular direction. After all, when we look around, we see triangles and circles more often than Pac-Man shapes.
Furthermore, objects very often lie on top of other things; just think of the books and piles of paper on your desk. The imaginary triangle is a feasible explanation for the bites taken out of the circles; the brain ‘understands’ they are ‘merely’ partly covered black circles.
The unexpected requires more processing Kok and De Lange also noticed that whenever the Pac-Man shapes do not form a triangle, more brain activity is required. In the above image on the right, we see that the three Pac-Man shapes ‘underneath’ the triangle cause little brain activity (coloured blue), but the separate Pac-Man on the right causes more activity. This also fits in with the theory that perception is a question of interpretation: if something is easy to explain, less brain activity is needed to process that information, compared to when something is unexpected or difficult to account for — as in the adjacent diagram.
Abstract of the research:
An essential part of visual perception is the grouping of local elements (such as edges and lines) into coherent shapes. Previous studies have shown that this grouping process modulates neural activity in the primary visual cortex (V1) that is signaling the local elements [ 1–4 ]. However, the nature of this modulation is controversial. Some studies find that shape perception reduces neural activity in V1 [ 2, 5, 6 ], while others report increased V1 activity during shape perception [ 1, 3, 4, 7–10 ]. Neurocomputational theories that cast perception as a generative process [ 11–13 ] propose that feedback connections carry predictions (i.e., the generative model), while feedforward connections signal the mismatch between top-down predictions and bottom-up inputs. Within this framework, the effect of feedback on early visual cortex may be either enhancing or suppressive, depending on whether the feedback signal is met by congruent bottom-up input. Here, we tested this hypothesis by quantifying the spatial profile of neural activity in V1 during the perception of illusory shapes using population receptive field mapping. We find that shape perception concurrently increases neural activity in regions of V1 that have a receptive field on the shape but do not receive bottom-up input and suppresses activity in regions of V1 that receive bottom-up input that is predicted by the shape. These effects were not modulated by task requirements. Together, these findings suggest that shape perception changes lower-order sensory representations in a highly specific and automatic manner, in line with theories that cast perception in terms of hierarchical generative models.