Grandmother cell

The grandmother cell, also known as the gnostic neuron, is a hypothetical neuron that represents a person's grandmother or, more generally, any complex and specific concept or object.^[1] A grandmother cell activates when a person "sees, hears, or otherwise sensibly discriminates"^[2] his or her grandmother. The term grandmother cell was coined by Jerry Lettvin; the term gnostic neuron comes from Jerzy Konorski.^[3]

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Support for the grandmother cell theory comes from early and replicated studies that found visual neurons in the inferior temporal cortex of the monkey that fired selectively to hands and faces.^[4][5][6][7] However, they didn’t find monkey cells that were selective for other visual objects important for monkeys such as fruit and genitalia. This was explained with the argument that it is more crucial for a monkey to differentiate among faces than among other categories of stimuli such as bananas. Furthermore, faces are more similar to each other in their overall organization and fine detail than any other stimuli that a monkey must discriminate among.^[1]

More support for the theory came from later studies^[8][9] that found that inferior temporal cells can be trained to show great specificity for arbitrary visual objects, and these would seem to fit the requirements of gnostic/grandmother cells.
In addition, there is now good evidence for cells in the human hippocampus that have highly selective responses to gnostic categories^[10][11] including highly selective responses to individual human faces.^[12]

Yet most of the reported face-selective cells do not really fit a very strict criteria of grandmother/gnostic cells in representing a specific percept, that is, a cell narrowly selective for one face and only one face across transformations of size, orientation, and color. Even the most selective face cells usually also discharge, if more weakly, to a variety of individual faces. Furthermore, face-selective cells often vary in their responsiveness to different aspects of faces, suggesting that they form ensembles for the coarse or distributed coding of faces rather than detectors for specific faces. Thus, a specific grandmother may be represented by a specialized ensemble of grandmother or near grandmother cells.^[1]

In 2005, a UCLA and Caltech study found evidence of different grandmother cells that represent people like Bill Clinton or Jennifer Aniston. A neuron for Halle Berry, for example, would respond "to the concept, the abstract entity, of Halle Berry", and would fire not only for images of Halle Berry, but also to the actual name "Halle Berry".^[13] However, there is no suggestion in that study that only the cell being monitored responded to that concept, nor was it suggested that no other actress would cause that cell to respond (although several other presented images of actresses did not cause it to respond).^[14]

The grandmother cell hypothesis is not universally accepted. The opposite of the grandmother cell theory is the distributed representation theory, that states that a specific stimulus is coded by its unique pattern of activity over a group of neurons.

The arguments against the grandmother cell theory include:

1. In principle one would need thousands of cells for each face, as any given face must be recognised from many different angles - profile, 3/4 view, full frontal, from above etc.
2. Rather than becoming more and more specific as visual processing proceeds from retina through the different visual centres of the brain, the image is in fact dissected into even more basic features such as vertical lines, colour, speed etc., distributed in various modules separated by relatively large distances. How all these disparate features are re-integrated to form a seamless whole is known as the binding problem.

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