Even though the fetus is now developing areas that will become specific sections of the brain, not until the end of week 5 and into week 6 (usually around forty to forty-three days) does the first electrical brain activity begin to occur. This activity, however, is not coherent activity of the kind that underlies human consciousness, or even the coherent activity seen in a shrimp's nervous system. Just as neural activity is present in clinically brain-dead patients, early neural activity consists of unorganized neuron firing of a primitive kind. Neuronal activity by itself does not represent integrated behavior.
During weeks 8 to 10, the cerebrum begins its development in earnest. Neurons proliferate and begin their migration throughout the brain. The anterior commissure, which is the first interhemispheric connection (a small one), also develops. Reflexes appear for the first time during this period.
The frontal and temporal poles of the brain are apparent during weeks 12 to 16, and the frontal pole (which becomes the neocortex) grows disproportionately fast when compared with the rest of the cortex. The surface of the cortex appears flat through the third month, but by the end of the fourth month indentations, or sulci, appear. (These develop into the familiar folds of the cerebrum.) The different lobes of the brain also become apparent, and neurons continue to proliferate and migrate throughout the cortex. By week 13 the fetus has begun to move. Around this time the corpus callosum, the massive collection of fibers (the axons of neurons) that allow for communication between the hemispheres, begins to develop, forming the infrastructure for the major part of the cross talk between the two sides of the brain. Yet the fetus is not a sentient, self-aware organism at this point; it is more like a sea slug, a writhing, reflex-bound hunk of sensory-motor processes that does not respond to anything in a directed, purposeful way. Laying down the infrastructure for a mature brain and possessing a mature brain are two very different states of being.
Synapses-the points where two neurons, the basic building blocks of the nervous system, come together to interact-form in large numbers during the seventeenth and following weeks, allowing for communication between individual neurons. Synaptic activity underlies all brain functions. Synaptic growth does not skyrocket until around postconception day 200 (week 28). Nonetheless, at around week 23 the fetus can survive outside the womb, with medical support; also around this time the fetus can respond to aversive stimuli. Major synaptic growth continues until the third or fourth postnatal month. Sulci continue to develop as the cortex starts folding to create a larger surface area and to accommodate the growing neurons and their supporting glial cells. During this period, neurons begin to myelinate (a process of insulation that speeds their electrical communication). By the thirty-second week, the fetal brain is in control of breathing and body temperature.