"Grandmother Cells", or Synchronous Discharges of Neurons?
The ability to recognize faces is especially adaptive from an evolutionary standpoint.
But just because some cells respond specifically to faces as a stimulus does not
necessarily mean that the brain contains group of neurons that process information
about faces exclusively. Quite possibly, these neurons may also contribute to
the processing of other kinds of stimuli as well.
Nevertheless, it is
true that a convergence of visual information can be observed all along the visual
pathways. From the retina to the LGN and the primary visual cortex and on to the
extrastriate areas, the receptive fields of the cells grow increasingly complex
as they integrate increasingly complex information, up to and including images
of faces. This hierarchical processing structure thus does raise the possibility
that there may be a small number of as yet unidentified neurons that may be activated
by the perception of specific, or even unique, objects. The standard way of raising
this question is to ask whether the brain may contain "grandmother cells"a
very small number of cells that have a receptive field so specific that they would
respond to only one precise stimulus: the face of the person's grandmother.
Despite the attractiveness of this hypothesis, there are a lot of data that
argue against it. Researchers have made recordings of cell activity in a great
many different areas of the monkey brain without ever discovering any cells specific
to the thousands of individuals and objects that individuals are likely to encounter
in the course of their lives. The principles on which the nervous system operates
seem to rely more on redundancy and distributed processing of stimuli. Also, the
same cells that seem to be selective for a given characteristic of a stimulus
often respond more weakly to some of its other properties. It would be far too
risky for the nervous system to rely too much on selectivity. If it did, a well
placed blow to the head could damage this small number of "grandmother cells",
and you might never recognize dear old Granny again.
Our growing understanding
of the visual system indicates that it is actually organized into functional modules
that are relatively independent of one another. In addition, the higher information-processing
areas return information to the lower ones, so that information travels in both
directions between the modules, and not just upward.
One possible role
for these "re-entrant circuits" may be to make the higher areas' visual
maps match the more precise ones in the lower areas. Because the visual fields
of the higher areas are generally larger, they are not so precise and detailed
as the ones found in areas V1 and V2. The colour that area V4 attributes to an
area of space located on a table might thus be more specifically attributed to
a book lying on that table, for example.
In the absence of grandmother
cells, visual perception might be based on the parallel processing that takes
place in the visual pathways, from the M and P-IB channels and the blobs to the
various modules of the extrastriate areas. Since the brain appears to engage in
a certain division of labour to analyze the various visual attributes of an object,
perception may arise from the association of the various cortical areas that respond
to the object in question. But then how would the brain combine these various
attributes of the object to produce the unified image that we perceive? On the
basis of the numerous horizontal connections in layer III of the cortex that link
neurons from various modules with one another, some scientists believe that the
interactions among modules that are activated by the same object might be what
makes our perception of this object possible.
Other researchers, such
as Charles Gray and Wolf Singer, think that the key to the problem of how the
various parallel processing modules link up with one another, known as the "binding
problem," may lie in the synchronous discharge of populations of neurons
in very different areas of the brain. Indeed, in 1989, Gray and Singer discovered
that some cells in several different modules of the visual cortex appear to discharge
their action potentials simultaneously when they are activated by parts of the
Thus your mental image of your grandmother may reside not
in a few specialized neurons, but rather in a large population of neurons distributed
across various cortical areas. The functional co-operation of neurons that discharge
synchronously might thus be the "glue" that takes all the distinct elements
analyzed separately by the brain and binds them into a coherent whole.