As the human understanding surpasses that of the ape, and that of the ape surw passes that of the fishes, so in almost as extreme a degree the vertebrate brain surpasses the nervous organs of the invertebrates. A reason for this difference, one thinks, is to be found in history, and in a very elementary biological fact.
All animals are dependent on either plants or animals for food, and from its beginnings the evolution of the animal kingdom has in the main presented a pageant of predator and prey-eat or be eaten!
H. W. Smith, From Fish to Philosopher ( 1 953)
CENTRAL THEME
Mammals can survive only if they maintain relative constancy of various bodily processes, including oxy
gen and carbon dioxide content in blood, body water levels, salt and energy balance, and body temperature.
Complex brain and body systems sustain these con�
stancies, and the overall concept used to describe this ability is homeostasis. Some people prefer the word heterostasis, since the level of regulation sometimes changes as a function of environmental conditions as well as internal bodily cycles (e.g., 24-hour rhythms).
Homeostasis is sustained by a diversity of mechanisms ranging from rapid physiological changes, such as re�
flexive modification of breathing rate as a function of oxygen need (which is actually signaled to the brain by carbon dioxide buildup in the bloodstream), to in
stinctual behavioral tendencies, such as an animal's urge to seek resources needed for longer�term survival, including food, water, warmth, and micronutrients such as vitamins and minerals. Bodily needs instigate distinct forms of bodily arousal and psychological feelings of distress such as hunger, thirst, and coldness. Animals have exquisite sensory systems to identify the most important items they need-for example, the ability to taste sweetness identifies foods laden with sugar, and saltiness identifies sources of sodium. Jt is commonly believed that color vision evolved, in part, to help pri
mates identify the ripest fruit. The needed items that cannot be identified by taste-for instance, many
vi-tam ins and other micronutrients-can usually be regu
lated via learned selection of foodstuffs, based on their postingestive consequences. Most interesting from the affective point of view are the intrinsic brain mechanisms that mediate pleasure and displeasure to provide an intrinsic guide for food selection. The plea�
sures of sensation arise from the interactions of many sensory systems with poorly understood hedonic mechanisms of the brain. Sensations generate pleasure or displeasure in direct relation to their influence on the homeostatic equilibrium of the body. For instance, if one is depleted of energy resources, foods taste better than when the body is already replete with enetgy. Al
though it is difficult to study internally experienced pleasures and displeasures in animals, for an accurate description of certain brain functions it will be essen·
tial to conceptualize how such processes are elaborated by specific neural circuits. A variety of distinct pleasures may arise from essentially the same types of neuro
chemical systems, such as the release of endogenous opioids. Likewise, regulation requires some type of set� point mechanisms, resembling the thermostats that control furnaces in our homes, which help specify deviations from physiologi.cal equilibrium and thereby promote SEEKING behaviors. To highlight genetal principles of how regulatory processes operate, in the present chapter I will focus largely on the physiologi
cal, behavioral, and psychological mechanisms that help sustain body energy balance. In more than a poetic sense, energy is delight.
164
Feeling States Related to Homeostasis
por· those who have never been extremely hungry or thirsty, it is hard to imagine the distress that such bodily imbalances create within the brain. Consider this arche�
typal situation: the Black Hole of Calcutta, the horror of 146 survivors of the British garrison that lost Fort William to a Bengali attack on a summer evening in the middle of the 18th century. The prisoners were crammed into a room 1 8 feet square, and only a few survived that sultry Indian night. The description, by the officer in command, tells of the agony of thirst and the manner in which powerful motivations funnel mental energies to one single goal-survival:
We had been but a few minutes confined before every one fell into a perspiration so profuse, you can form no idea of it. This brought on a raging thirst.
. . . Water! water! became the general cry . . . . [A little was brought by the guards] . . . . There ensued such violent struggles and frequent contests to get it. . . . These supplies, like sprinkling water on fire, only seemed to feed the flame. Ohl my, how shall I give you a just conception of what I felt at the cries and cravings of those in the remoter parts of the prison, who could not entertain a probable hope of obtaining a drop, yet could not divest themselves of e
X
pectations, however unavailing, calling on me by the tender considerations of affection and friendship.The confusion now became general and horrid . . . . Many, forcing their way from the further part of the room, pressed down those in their passage who had less strength, and trampled them to death . . . . My thirst now grew insupportable . . . . I kept my mouth moist from time to time by sucking the perspiration out of my shirt-sleeves, and catching the drops as they fell like heavy rain from my head and face; you can hardly imagine how unhappy I was if any of them escaped my mouth. . . I was observed by one of my companions on the right in the expedient of allaying my thirst by sucking my shirt�sleeve. He took the hint, and robbed me from time to time of a considerable part of my store, though, after I de�
tected him, I had the address to begin on that sleeve first when I thought my reservoirs were sufficiently replenished, and our mouths and noses often met in contact. This man was one of the few who escaped death, and he has since paid me the compliment of assuring nie he believed he owed his life to the many comfortable draughts he had from my sleeves. No Bristol water could be more soft or pleasant than what arose from perspiration . . . . At six in the morn
ing the door was opened, when only three and
twenty out of the hundred and forty-six still breathed.1 Clearly, a broad range of subjective feelings are associated with intense regulatory imbalances and the many specific sensations that accompany the satisfac�
tion of bodily needs-the hungers, thirsts, cravings, and
the various sensory delights that arise from interacting with needed resources. These types of feelings have posed troublesome issues for investigators devoted to a scientific analysis of motivations. It has been diffi�
cult to conceptualize mechanistically how such feelings might actually participate in the causal control of ani�
mal behavior, but here we recognize that they are the brain's value-coding devices, which can be studied through the analysis of various behavioral indicators, ranging from food choices to facial gestures.
Although the preceding topics may seem somewhat remote from our main goal of clarifying emotional pro
cesses, they do highlight the types of neuropsychological analyses of animal feelings that will be required in order for us to make sense of energy-balance regulation. We will eventually need to understand the affective nature of hunger, as well as the more nebulous feeling of meta
bolic well�being. At the end of this chapter, I will also tackle the important issue of gustatory pleasures. In gen
eral, I trust the reader appreciates that we will have to understand the various regulatory mechanisms of the brain and body in order to really understand the nature of the many affective experiences that sun·mmd our eat�
ing habits and other regulator�y behaviors. To achieve such knowledge, we must be willing to use measures of behavioral events as indices of mental events. The logic is the same as the use of cloud chambers to track the movements of subatomic particles in physics.
Many other related feelings are impossible to study in animal models. For instance, the problems people develop with their self-image when they are anorexic or obese cannot be addressed in animal models, prob
ably because they emerge from the higher reaches of the human brain, which can generate thoughts beyond the imagination of other creatures. However, the sim�
pier feelings that are more directly related to primitive homeostatic processes shared by all animals can be in
directly (i.e., inferentially) studied in animals by care�
fully observing their behaviors. Let us consider one example that has received virtually no experimental attention.
One of the most powerful and rapidly acting regula
tory urges is our continual need for oxygen. Of course, breathing regulation is very different from the other motivations, since the needed resource, under most cir
cumstances, is readily available. Most other motivations, such as the need for food or sex, can be fulfilled only by active exploratory and search behaviors generated by the SEEKING system. Breathing does not require such as�
sistance. When breathing proceeds normally, the controls remain totally at a subconscious leveL Obviously, evo
lution has automated and eliminated choice in the most important aspects of homeostasis. Indeed, our bodies do not actually detect oxygen need directly. Our brain only monitors correlated variables such as the buildup of car
bon dioxide in the body and changes in the acid-base balance of the bloodstream.
An increase in systemic carbon dioxide elevates the
166 BASIC EMOTIONAL AND MOTIVATIONAL PROCESSES
rate and depth of breathing, without being accompanied by major psychological distress. What causes respira
tory distress is impairment of our rhythmic breathing
for instance, by airway obstruction, suffocation, or strangulation. When such events occur, a very power
ful emotional state arises-a paniclike condition that reflects the existence of a primitive brain stem response system, called the "suffocation alarm" reflex. When this happens, one's mind is rapidly filled with a precipitous anxiety, and one begins to flail about in anguish. In
deed, human "panic attacks" may emerge, in part, from activation of this powerful emotional response system. 2 This example exquisitely indicates how rapidly feeling states and behaviors can change in response to a regu
latory crises. Although the spontaneous activation of such an emotional state leads to a large number of cog
nitive evaluations, there is no reason to believe that the suffocation-alarm response itself is normally activated by any higher appraisal mechanism. This highlights the normal flow of motivational events in the brain-emo
tions and regulatory feelings have stronger effects on cognitions than the other way around.
Obviously, several types of cognitive and emotional arousal occur in any strong motivational situation.
Consider the simple cases of excessive bladder or rec
tal distention. The concern these sensations often cause derives from the type of social inhibitions that do not seem to worry other animals. Still such feelings of dis
tention can become incredibly insistent, filling our minds with nothing but the urge for relief. The feelings are so insistent that it is difficult to sustain other thoughts in one's mind. Unfortunately, we know little about the neural systems that subserve such feelings, but it is possible that they are organized qu_ite low in the neuroaxis, perhaps at the brain stem level. If we could specify the exact neural systems that create such feelings, we would probably understand more about consciousness than can presently be found in most of the learned texts on the topic (see Chapter 16).
Regulatory urges rarely become as intense as in the examples described earlier because we can anticipate their coming and their consequences. To some extent, our cognitive abilities allow us to anticipate such events and relieve ourselves of potential embarrassments. The higher cognitive processes can also promote, at least in humans, many other subtle emotional concerns that are not directly related to aroused needs. For instance, dur
ing one's search for resources, anxiety can easily arise from contemplations such as "Will I find what I need?
Will there be enough? And what if someone gets there before me?" Frustration and anger may be engendered if a competitor is successful in snatching away a val
ued resource. If the needed resources are rare or con
sistently lost in competitive encounters, one may de
velop such chronic behavioral tendencies as hoarding or learned affective habits such as greediness. Indeed, these tendencies also appear to be part of the instinc
tive potential of the mammalian nervous system, even
though they are typically expressed only if resources are perceived to be scarce for prolonged periods.3
Conversely, it should be noted that virtually aU emotional states affect the intensity of our motivations.
Most animals, even humans, are unlikely to eat much or exhibit any inclination for play or sex when they are very scared or angry. One long�term emotion that is especially incompatible with normal appetite is sepa�
ration distress. When young animals are socially iso
lated, they typically lose weight even if they have free access to lots of food. When the young are reunited with their kin, and a mood of apparent contentment is rees�
tablished, appetite returns. From this vantage, it is not surprising that one's appetite is best in the presence of social companionship and social facilitation of feeding is such a robust phenomenon in nature.4
Because of the lack of relevant brain data, we will rarely address the cross-modal influences among emo�
tiona! and motivational systems, but the available be
havioral data indicate that such interactions are perva
sive-which makes it difficult to study regulatory influences, such as satiety factors, without also consid
ering a host of other affective changes in the organism.
As a heuristic exercise into the nature of regulatory feelings associated with bodily need states, I will pro�
vide a fairly detailed brain overview of one major moti
vational system-the one that mediates body energy regulation. I will also seek to illuminate the nature of the affective feelings that accompany energy need states-the distress engendered by hunger, as well as the pleasures arising from consummatory acts.
Regulatory Behaviors and the SEEKING System
As we saw in Chapter 8, the SEEKING system can motivate animals to pursue a diversity of distinct re�
wards in the environment. The nervous system does most of this automatically, with no obvious delibera
tion. Many bodily needs access the SEEKING system and thereby arouse appetitive search tendencies that motivate animals to approach and learn about available resources. It would have been wasteful for evolution to have constructed separate search and approach systems for each bodily need. The most efficient course was for each need-detection system to control two distinct func
tions: a generalized, nonspecific form of appetitive arousal and various need-specific resource-detection systems. In addition, learning would increase the effi
ciency with which the SEEKING system could guide animals to appropriate goal objects. This is, in fact, what transpires in the mammalian brain.
Thus, resource depletions within the body can lead to a generalized arousal of seeking behaviors regard
less of the specific regulatory imbalances that exist, and specific need states that sensitize distinct consumma
tory reflex tendencies (e.g., licking, biting, chewing, and
swallowing) and key support mechanisms, such as sen
sory, perceptual, and memory fields relevant for the specific needs. By the interplay .o
�
these p_roces�es, a generalized search system can efftctently gmde ammals to relevant environmental goal objects. In other words, the nonspecific SEEKING system, under the guidance of various regulatory imbalances, external incentive cues, and past learning, helps take thirsty animals to water, cold animals to warmth, hungry animals to food, and sexually aroused animals toward opportunities for orgasmic gratification (Figure 9.1).Existing evidence suggests that the SEEKING sys
tem is under the control of internal homeostatic recep
tor systems that detect various bodily imbalances. This is suggested by the fact that many imbalances can modify the rate at which animals self-stimulate lateral hypothalamic (LH) electrode sites.5 For instance, hun
ger reduces the current threshold needed to sustain LH self-stimulation while also increasing the rate at which animals behave. Similar effects can be evoked by thirst, cold, and various sex hormones, even though these have not been studied as thoroughly as the effects of food deprivation. The exact manner in which the various interoreceptive systems interface with the foraging sys
tem remains to be worked out in detail, but there are many candidate neurons with the right anatomical and physiological characteristics throughout the LH and adjacent zones of the medial hypothalamus.
The axons of many medial hypothalamic neurons make synaptic contact with the ventral tegmental area (VTA). Their dendrites extend radially across the ascend
ing and descending axons of the SEEKING system, pro
viding feedback loops that can regulate the vigor of
for-ENERGY IS DELIGHT 1 67
aging.6 Many of these neurons are sensitive to circulat
ing nutrients such as amino acids, fatty acids, and blood sugar, or glucose.? Other nearby interoreceptive neurons, known as osmoreceptors, are sensitive to the osmotic concentrations of solutes in the blood. Various others, known as thermoreceptors, are sensitive to bodily (core) and peripheral temperature fluctuations. 8 There are also specialized neurons that are sensitive to the various honnones that control sexual tendencies (e.g., steroid
receptive neurons for testosterone, dihydrotestosterone, estrogen, and progesterone). 9 There are bound to be other detectors (e.g., sodium and perhaps other micronutrient detectors), but our knowledge about their locations and properties remains more rudimentary.
It is important to note that electrode locations that readily yield self-stimulation in rats typically yield predatory aggression in cats.10 Obviously, this is a rea
sonable species-typical SEEKING behavior for a car
nivorous animal that subsists at the top of the food chain. Outward behavior can sometimes mislead us about the functions of an underlying brain syStem. The failure to recognize this appears to have been another instance in which the variety of behaviors evoked by LH stimulation has deceived investigators about the generalized emotive functions of a brain circuit.
As we will see in the next chapter, a great deal of evi
dence suggests that predatory aggression is a result of arousal of the SEEKING system, as opposed to acti
vation of a distinct emotional system. As discussed in the previous chapter, a comparable mistake was the idea that activation of the LH self-stimulation system evoked consummatory pleasure or reward responses in animals. 1 1
HOMEOSTATIC INPUTS INTO A GENERALIZED SEEKING SYSTEM
PATHWAYS
NON-SPECIFIC SYSTEMS
BRAIN-STEM CONSUMMATORY REFLEX CONTROL AREAS
MEDIAL HYPOTHALAMIC HOMEOSTATIC DETECTORS
Figure 9 . 1 . A conceptual schematic of how specific regulatory detector systems in the
Figure 9 . 1 . A conceptual schematic of how specific regulatory detector systems in the