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Appreciating symmetry: How the ordinary becomes special in your brain

Photo : Jean-Pierre Dalbéra. CC by SA 2.0

Humans have always been fascinated by symmetry. Many celebrated works of art are appreciated for their symmetry, such as the Vitruvian Man by Leonardo Da Vinci, or Somnathpur temple above. Given the importance of symmetry in our lives, does the brain have a special way of processing symmetric objects? In a recent publication in Psychological Science, RT Pramod & SP Arun from the Centre for Neuroscience, IISc show that the response to symmetric objects at the level of single brain cells, or neurons, is not very remarkable. However, simple neural computations make symmetric objects stand out very clearly compared to other objects

The researchers based their investigation on a well-known rule of object summation. Single neurons in the monkey brain respond to some shapes but not others. For example, a neuron might respond strongly to a triangle but not a circle shape. However, the neural response to both triangle and circle shown side by side is the sum of the responses to the triangle and circle shown separately. In this study, the researchers wondered whether the same rule of simple summation applied for symmetric objects too. “Since symmetry is a property of the whole object but not of its parts, we reasoned that symmetric objects might be special in how their parts interact, compared to asymmetric objects”, says Prof. Arun.

To test this hypothesis, the researchers showed monkeys images of symmetric and asymmetric objects, while recording the activity of single neurons in the inferior temporal cortex, a region of the brain important for object perception. To their surprise, they found that the neural responses to symmetric and asymmetric objects were equally well explained as a sum of their parts. In other words, symmetric objects were not special in the way they were processed at the level of individual neurons. However, they found that symmetric objects were more ‘distinct’ than asymmetric objects, in the sense that symmetric objects evoked different patterns of neural firing compared to asymmetric objects.

The researchers then looked for an analogous effect in perception by performing experiments with human subjects. In one experiment, they asked human volunteers to identify the oddball item in an array of otherwise identical items. They found that the volunteers were able to find symmetric objects faster than asymmetric objects. The volunteers were then asked to perform a separate task in which they had to see individual objects and evaluate whether it was symmetric or asymmetric. The time they took to determine if an object was symmetric was closely related to its distinctiveness, confirming their hypothesis.

Based on their findings, the researchers proposed that even though single neurons respond to symmetric objects much like any other object, it is precisely the simple summation of responses at the single neuron level that causes symmetric objects to stand out among their asymmetric peers. Prof. Arun uses an elegant analogy with paint mixing to explain this concept. “Just as mixing very different paints results in similar colors, mixing different parts to create an asymmetric object makes it less distinct. In contrast, mixing identical parts to create a symmetric object maintains its distinctiveness”, he explains. Thus, simple computations in our brain can lead to profound advantages in perceiving symmetry, encapsulating the adage ‘great beauty lies in simplicity.’