Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

The Animal Self: Neurobehavioral Correlates 147

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

centers. It is the balance of reasoning and emotion that leads to a nor-
mal life. Basic instincts, however, never totally disappear, remaining an
undercurrent even in human behavior. Thus, we see the impulsive crav-
ing for food of an infant replaced by the well-controlled table manners of
an adult, both driven by the need to feed. Nevertheless, under stress or
in brain damage, the primitive impulse may resurface and adult behavior
can revert to that of a baby.

7.6 Examples of Behavior at Different Levels
of Neural Complexity

Following are some examples of behavior in phylogenetic order:

7.6.1 Light-avoidance in planarians

The flatworm planarian displays a light–avoidance behavior called nega-
tive phototaxis. The central nervous system consists of a bi-lobed prim-
itive brain (the head ganglion) and two ventral nerve cords (Fig. 7.7).
A pair of light-sensitive organs at the front end are connected to the
brain. Planarians normally exhibit light avoidance behavior by sending
signal from the eyes to the brain. After decapitation, the brain and “eyes”
regenerate and phototaxis is resumed.^14

7.6.2 Switching light preference in fruit flies

The fruit fly Drosophila prefers darkness in the early larval stage but
prefers light in adulthood. The switch from photophobic to photophilic
behavior is controlled by two pairs of neurons downstream from the pho-
toreceptors. The change enables the adjustment of the animal’s response
strategy to environmental stimuli according to biological needs.^15

7.6.3 Instinctive behavior in insects and higher animals

Social insects are equipped with a rich repertoire of unlearned behav-
iors, especially with respect to intra-species communication. Honeybees