How Accurately Are Brains Designed?

Abstract

Most of the structure of a brain as we see it by macroscopic and microscopic observation is genetically determined. This conclusion may be reached simply from the observation that newborn, or even embryonic brains already have well-developed structures before the attached sense organs have ever received stimuli. No one seriously believes that we begin life as unstructured beings, as “tabulae rasae ”: clean slates on which only experience will leave marks. Yet, it is not to be doubted that some of the structure of the brain is formed during a learning phase, and probably embodies experience. There have been several claims in recent years of certain microscopic, and even macroscopic features of the brain developing in different ways under the influence of conditions imposed by the environment [5.8, 5.10]. On the whole, however, it is probably safe to say that memory traces have up to now escaped microscopic observation. What we see is inborn structure, or rather, if some of the histologic detail is due to learning, we do not know how to relate it to specific memory traces.

To Martin.

It is surprising how little attention has been paid to the nervous system of arthropods and particularly to that of insects during the last decades which have otherwise produced such a copious neurological bibliography. These animals possess an extraordinarily complex and differentiated nervous system, and a fineness of structure that reaches the limits of the ultramicroscopic.

Cajal and Sanchez [5.5]