It is well known that the capacity
of the human central nervous system (CNS) to be aroused is an essential aspect
of not only our basic biological functioning but also our mental life, as
expressed by the ability to engage in cognitive and emotional processing of a
vast array of environmental stimuli.  The significance of the phenomenon of
arousal for the functioning of the CNS, and the quest of Donald Pfaff to
understand the mechanisms behind it, are sufficient to make his book Brain Arousal
and Information Theory a compelling read for those interested in uncovering
the never-ending mysteries of the "organ" of the mind. 

In the book, Pfaff argues that
there is a generalized arousal factor whose behavioral effects are quite
compelling (e.g., it accounts for 1/3 of the variance in the behavioral data of
a series of experiments conducted by Pfaff himself with mice as subjects). 
Statistical evidence suggests that this factor has three distinct and
measurable components: sensory alertness, motor activity and emotional
reactivity.  Therefore, Pfaff proposes an operational definition of arousal
that includes all three components, and that conceptualizes increases in
generalized arousal as increments in each one.  Although he treats an organism’s
behavior as the primary physical manifestation of generalized arousal,
recordings of the electrical activity of the cortex (EEG), and evidence of
sympathetic pathways’ activation, are also taken into account as expressions of
generalized arousal.

Generalized arousal is not the only
focus of the author’s quest for a comprehensive account of the central nervous
system’s arousal mechanisms. Pfaff cites specific forms of arousal, which
appear to be defined as those whose sources are more obvious such as sex,
hunger and fear.  He then proposes that an organism’s state of arousal is a
function of generalized arousal, and that specific forms of arousal, along with
constant values, reflect the organism’s stable traits (i.e., temperament).  He
focuses on generalized arousal and specific forms of arousal to demonstrate, by
means of experimental evidence collected over time, that they are capable of
influencing each other (for example: food deprivation can increase emotional
reactivity). Their interconnectivity makes any formal model of arousal, even
the one proposed by the author, a complex and challenging undertaking of
limited success. 

Pfaff relies on information theory
to provide a formal account of neuronal and behavioral arousal-related
responses, which are conceptualized as responses to stimuli that can be
considered high information bearing.  He proposes a theoretical equation for
predicting arousal. Unfortunately, its simplicity does not reflect the
complexity of the subject matter, at least as described by the author, and may
leave readers baffled.  The same argument can be made about the author’s
operational definition of "generalized arousal", which is expressed
by relying on the comparative "more" as in: "more alert to
sensory stimuli of all sorts, and more motorically active, and more reactive
emotionally".  The absence of clearly defined referent points may leave
some readers uncertain as to the measurement of generalized arousal that the
author proposes.   

Notwithstanding some definitional
cloudiness, several sections of Pfaff’s book are quite attention-grabbing for
the specificity of the information they provide. Particularly noteworthy is the
section in which Pfaff describes, with painstaking accuracy, the neuroanatomy
and physiology of the brain’s arousal systems.  Another interesting section highlights
the multiplicity and redundancy of the ascending arousal pathways. Given the
essential nature of the phenomenon of arousal in any organism’s life, it is
easy to agree with Pfaff that multiplicity and redundancy serve to reduce the
probability of failure of an organism’s arousal systems.  In another equally
captivating section of the book, Pfaff focuses on specific nerve cells that
extend from the lower brainstem to the hypothalamus.  These cells are of
particular interest to any account of the mechanisms of arousal because they
are characterized by large-spectrum responses, whose triggering is dependent
upon the informational content of the environment that surrounds them.