The following figure is presented here as an example of the confusion found in the vision literature today. It is due primarily to the lack of a global forum, or Secretariat, on the overall visual process.

Simple example of the confusion in terminology found in the field of Vision
[from Section 17.1.1]

Researchers exploring the retina have frequently adopted single letter abbreviations to describe the morphological type of ganglion cells, M for midget and P for parasol.

Researchers exploring the mid-brain have frequently followed the same procedure when discussing the signal pathways associated with the optic nerve. They describe those nerves terminating in the LGN by P if terminating in the parvocellular region and M if terminating in the magnocellular region.

There is an obvious problem with the above conventions when an attempt is made to trace the paths between the two locations. This is illustrated by the crossover.

The problem is further complicated when the additional pathway associated with the ganglion cells carrying signals from the foveola is addressed. In this pathway, the signals are monopolar like the luminance signals carried by the magnocellular pathway. However, the signals only contain signal content from one spectral channel. Following the logic of assigning a name to this path based on its terminus would suggest using the letter P for pretectum. This is clearly not helpful. Here the pathway is labeled relative to its initial source for clarity. From a global review of the literature, it is logical to define the pathways in terms of the signal content they carry. These designations are shown on the right. The luminance and chrominance channels carry matrixed signals while the spectral channel carries un-matrixed spectral signals from individual photoreceptors.

This discussion is continued in Chapter 17.