“Behavior is ultimately the product of the brain, the most mysterious organ of all.” Ian Tattersall (from Becoming Human Evolution and Human Uniqueness, 1998)
The question of why we are motivated for certain behaviors is perhaps one of the most fundamental in psychology. Ever since Pavlov described conditioning in dogs in his famous 1927 paper, scientists have pondered the origins of the motivations that drive us to act. For most of the early 20th century, behaviors like Watson and Skinner sought to explain behavior in terms of external physical stimuli, suggesting that learned responses, hedonic reward, and reinforcement were motives for eliciting a particular behavior. However, this does not tell the whole story. In recent decades, the school of cognitive psychology has focused on additional motivational mechanisms: our desires according to social and cultural factors that influence behavior. Additionally, recent advances in neuroimaging technology have enabled scientists to understand the vast complexities and modular nature of specific brain regions. This research has shown that the behaviors necessary for survival also have an inherent biological basis..
The biological trigger for inherent behaviors like eating, drinking, and controlling temperature can be traced back to the hypothalamus, an area of the diencephalon. This article will explore the hypothalamic role in these motivated behaviors. It is important to note that motivated behavior resulting from internal hypothalamic stimuli is only one aspect of what is a complex and integrated response.
The hypothalamus connects the autonomic nervous system with the endocrine system and performs many vital functions. It is the body’s homeostatic ‘control center’, maintaining a balanced internal environment by having specific regulatory areas for body temperature, body weight, osmotic balance, and blood pressure. It can be classified into three main outputs: the autonomic nervous system, the endocrine system, and the motivated behavioral response.. The central role of the hypothalamus in motivated behavior was proposed as early as 1954 by Eliot Stellar, who suggested that “the amount of motivated behavior is a direct function of the amount of activity in certain excitatory centers of the hypothalamus“(p6). This nomination has inspired a great deal of subsequent research.
Much of this research has been done in the field of thermoregulation. The body’s ability to maintain a stable internal environment is critically important for survival, as many crucial biochemical reactions will only work within a narrow temperature range. In 1961, Nakayama et al discovered heat-sensitive neurons in the medial preoptic area of the hypothalamus. Subsequent research showed that stimulation of the hypothalamic region initiated humoral and visceromotor responses such as panting, shivering, sweating, vasodilation, and vasoconstriction. However, somatic motor responses are also initiated by the lateral hypothalamus. It is much more effective to move around, rub your hands, or put on extra clothing if you are cold. Similarly, if you are too hot, you can remove some clothing or fan yourself to cool off. These motivated behaviors demonstrate that, in contrast to a fixed stimulus response, motivated behavior stimulated by the hypothalamus has a variable relationship between input and output. This interaction with our external environment can be a “choice”, however, it is clear that the motivation to make these decisions is biologically based.
The mechanics of thermoregulation can be explained by what are sometimes called “drive states.” It is essentially a feedback loop that is initiated by an internal stimulus that requires an external response. Kendal (2000) defines the states of the unit as “characterized by tension and discomfort due to a physiological need followed by relief when the need is satisfied”. The process begins with the entry. Temperature changes are picked up from the peripheral surroundings by thermoreceptive neurons throughout the body that detect both heat and cold separately.. Then an electrical signal (the input) is sent to the brain. Any divergence from what is known as the ‘set point’, in this case at a temperature of approximately 37 °, will be identified as an ‘error signal’ by interoceptive neurons in the periventricular region of the hypothalamus. Armed with these measurements and temperature signals that are transmitted from the blood, the hypothalamus launches an appropriate error response. This includes motivating behavior to make a physical adjustment, for example moving around or removing excess clothing in an attempt to control your temperature.
This type of feedback system in the body is common. Other systems necessary for survival, such as the regulation of water and salt levels in the blood, are regulated in a similar way. However, the processes that motivate us to eat are much more complex.
Humans have developed an intricate physiological system for regulating food intake that encompasses a myriad of organs, hormones, and body systems. In addition, a large body of experimental research supports the idea that the hypothalamus plays a key role in this energy homeostasis by triggering eating behaviors. Controlling the energy balance is of vital importance and eating is mainly to maintain fat reserves in case of food shortage. If the body’s stores of fat cells are low, they release a hormone called leptin that is detected as an error signal by the periventricular region of the hypothalamus. This then stimulates the lateral hypothalamus to initiate the error response. In this case, we begin to feel hungry, which in turn initiates the somatic motor response by motivating us to eat.
Since the hypothalamus also controls metabolic rate by monitoring blood sugar levels, in theory we seem to have a feedback loop similar to temperature control. However, in practice this is not a reality. The main difficulty in maintaining energy homeostasis is that motivation does not arise solely from internal biological influences. Cultural and social factors also play an important role in motivating when, what, and how often to eat. In Western culture, social pressures to be thin can override the urge to eat, and in extreme cases like anorexia, the drive state is reversed. The motivation is no longer to eat because they are hungry, but not to eat to make them feel hungry. This corruption of the reward system is well documented and is associated with delusions of body image, a concept that is also related to the hypothalamus and the parietal lobe. Problems can also arise if a person is overstimulated to eat. The prevalence of obesity in today’s society is testament to this fact.
