11 - Neonatal Behavior

Cross references:   

11 - Perinatal Behavior

Before I complete my discussion of the role of the frontal cortex, I want to describe what is known as "perinatal" behavior.  "Perinatal" means before and just after birth.  This will be to continue to dispute the book's statement on K&W:358 that the "the frontal lobe of each hemisphere is responsible for planning and initiating sequences of behavior".  

Behavior begins really early, in the womb, long before the fetus has a functioning cortex, and this behavior is apparently necessary for normal development.  If the activity of an arm or leg is restricted while the fetus is developing in the womb, that arm or leg fails to develop fully, and the infant is born with a stunted arm or leg.  

Your book has some good pictures on K&W:238-253 showing the brain's development.  Real time ultra sound imaging permits noninvasive study of the fetus while it is still in the womb.  General movements of the trunk have already begun by the tenth week, post-conception.  Isolated movements of the arms and legs, rotation of the head, and bringing the thumb near the mouth are visible by the twelth week.  Stretching, yawning and sucking are established before the sixteenth week.  Although none of these movements are coordinated with any of the others, I still think it is fair to refer to them as behaviors.  

About thirty-two weeks, post-conception, the previously uncoordinated movements begin to coalesce into behavior states.  By thirty-eight weeks, these fetal behavior states are identical to three of the five behavior states exhibited by the full-term infant born at forty-one weeks, post-conception.  

In addition to REM and NREM sleep [K&W:463,465], the near-term fetus also has periods of activity, while still in the womb, which are indistinguishable from the agitated waking behavior of the infant after it is born.  The two behavior states of the newly-born infant not displayed by the near-term fetus are crying and quiet wakefulness, and quiet wakefulness is displayed by the infant only while it is feeding.  The as-yet-unborn near-term fetus also responds to both sound and strong light applied to the mother's abdomen.  

Learning also begins before birth.  The newly born child will orient towards its mother's voice, which it has been hearing for many months, in preference to the voice of anyone else.  The unborn fetus also exhibits habituation [K&W:177-179,185].  When increased heart rate is accepted as an indication of arousal, the arousal of the fetus by sound directed toward the mother's abdomen habituates within ten minutes, and the arousal response remains habituated for more than 24 hours.  

All of this behavior is predominantly, perhaps even solely, subcortical, since the newborn's cortex is as yet unmyelinated [K&W:253] and its cortical neurons have only just begun to sprout dendrites [K&W:249].  

Thus, it seems clear that, while the frontal lobes, which continue to develop until we are thirty, may be necessary for planning behavior, they are most certainly not necessary for initiating it.  

As a footnote, I would like to point out that crawling, which would seem to require only the mesencephalic locomotor region and therefore would be strictly subcortical, does not usually begin until about the eighth month after birth, and it is not replaced by walking-with-assistance for several more months.  On the other hand, hungry children will start reaching toward the spoon with which they are being fed at six months after birth, and I know of no clear subcortical precedence for this.  So there may not be any definite point at which behavior switches from subcortical to cortical.  In fact, I would like to suggest that subcortical behavior does not ever cease.  Rather, it continues throughout our entire lives, but it becomes increasingly shaped and modified by the cortex.  In the example above of a child reaching for the feeding spoon, the reaching is hypothesized to be a mesencephalic forelimb stepping motion which has been modified or shaped by the cortex into a reaching motion.  

Since the subcortical brain had already shown itself to be capable of initiating behavior before the evolution of the cortex, I suspect that it retains this ability after the cortex has evolved.  In fact, my working hypothesis is that all behavior is subcortically initiated.  At the very least, for the cortex to participate in either the initiation or the planning of any behavior, it must receive input from two subcortical structures: the reticular formation and the thalamus.