Neuro-Cartography:  Mapping the Cereberal Cortex

The majority of higher brain functions take place in the cerebral cortex, the folded outer layer of the Cerebrum.  The cerebral cortex receives sensory information from various sense organs and processes this information from the different organs to give us a clear picture of our environment.  Parts of the cortex that receive this information are called primary sensory areas. Other areas receive impulses from the primary sensory areas and integrate the information coming in from different types of receptors from different parts of the brain.  The critical function of the cortex that we are interested in is the interaction between these sensory areas to produce nuanced output. 

One method in which we can see these cortico-cortical connections is through the use of retrograde tracers which, when injected into an area of the brain of the Macaque monkey, is picked up by the axon terminals in the area.  The tracer then flows backwards through the axon of the cell and into the cell body, in effect tracing the pathway formed by the cell.  Sections are cut through the brain – orthogonal to the sheet of cortex – and neurons labeled by the tracer identified.  Two-dimensional maps of the distribution of labeled neurons are made with a microscope-computer system.  Now we essentially have a series of 2D images representing a 3D object. By themselves, these do not clearly convey the patterns of connections across the sheet of the cortex.  Therefore, we want to reconstruct the sheet of the cortex to allow patterns of connections to be readily visualized.  By taking the images of the individual sections and graphically unfolding the cortex of each section into a line, these lines can be arranged to create a flat map of the cortex for each experiment, with cortical areas and labeled neurons clearly marked. 

I created maps for two cases with injections in both the superior and inferior parts of the rostral temporal cortex.  These showed clearly that the previously defined “medial prefrontal network” is connected to rostral superior temporal cortex, and that the “orbital prefrontal network” is connected to rostral inferior temporal cortex.  Another case with injections in superior and inferior parts of the caudal temporal cortex showed a similar but modified pattern that will require further investigation.

Even with the successful data gathered, this line of research is nowhere near a close, as a complete understanding of the intricate interactions and significance of these areas is yet to be reached.  Future similar experiments will shed further light on the remainder of the cortex and help to define functional systems within the cortex.