Learning when Not to Generalize - What does the Thatcher Illusion mean?

A Role for Gain Control

Compare the two faces – no big difference? Now start the Quicktime movie, or move the mouse over the picture below.

What does the Thatcher illusion tell us about the brain? It tells us that for faces in unfamiliar orientations, the activation of the representation of a face is triggered by the presence of component elements, such as eyes, regardless of the element's orientation with regard to the rest of the elements, making the representation invariant to such orientation. For faces in familiar orientations, in contrast, the correct relative orientation of the elements is necessary for recognition of the face as a normal face. In other words, invariance is lost through exposure to faces (in upright orientation). This is the same process that occurs with children's representations of letters, which start off invariant to several transformations, including mirror reflections, and lose the invariance through years of exposure (Chris Wetzel and Alex Backer, unpublished observations and 2004 Caltech SURF report).

How are such invariances lost? A first possibility is through the strengthening of inhibition, with unencountered stimuli triggering inhibition of the neuronal representation, in the example above, of Thatcher's face. Yet it is hard to imagine how inhibition would build up in response to stimuli never encountered. Instead, I believe it more likely that neurons increase their firing threshold over the course of experience through a process of gain control that follows the gradual strengthening of the synapses and/or branchpoints which are commonly activated in conjunction with the perception of the represented object (Thatcher's face in the example). This process makes the activation of the neuron more likely for configurations previously encountered, but less likely for configurations not encountered.

Original Reference for the Thatcher Illusion: Thompson P (1980) Margaret Thatcher: a new illusion. Perception 9:483–484

Quicktime movie: Michael Bach, http://www.michaelbach.de/ot/fcs_thompson-thatcher/index.html .