“Behaviour is ultimately the product of the brain, the most mysterious organ of them all.” Ian Tattersall (from Becoming Human.Evolution and Human Uniqueness, 1998)
The question of why we are motivated to certain behaviours is perhaps one of the most fundamental in Psychology. Since Pavlov described conditioning in dogs in his famous 1927 paper, scientists have pondered the origins of motivations that drive us to action. For most of the early twentieth century, behaviourists like Watson & Skinner sought to explain behaviour in terms of external physical stimuli, suggesting that learned responses, hedonic reward and reinforcement were motives to elicit a particular behaviour. However, this does not tell the whole story. In the last few decades, the school of cognitive psychology has focused on additional mechanisms of motivation: our desires according to social and cultural factors having an influence on behaviour. Furthermore, recent advances in neuroimaging technology have allowed scientists an insight into the vast complexities and modular nature of specific brain regions. This research has shown that behaviours necessary for survival also have an inherent biological basis.
The biological trigger for inherent behaviours such as eating, drinking and temperature control can be traced to the hypothalamus, an area of the diencephalon. This article will explore the hypothalamic role in such motivated behaviours. It is important to note that a motivated behaviour resulting from internal hypothalamic stimuli is only one aspect of what is a complex and integrated response.
The hypothalamus links the autonomic nervous system to the endocrine system and serves many vital functions. It is the homeostatic ‘control centre’ of the body, maintaining a balanced internal environment by having specific regulatory areas for body temperature, body weight, osmotic balance and blood pressure. It can be categorised as having three main outputs: the autonomic nervous system, the endocrine system and motivated behavioural response. The central role of the hypothalamus in motivated behaviour was proposed as early as 1954 by Eliot Stellar who suggested that “the amount of motivated behaviour is a direct function of the amount of activity in certain excitatory centres of the hypothalamus” (p6). This postulation has inspired a wealth of subsequent research.
Much of this research has been in the field of thermoregulation. The body’s ability to maintain a steady internal environment is of critical importance for survivalas many crucialbiochemical reactions will only function within a narrow temperature range. In 1961, Nakayama et al discovered thermosensitive 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 together or put on extra clothes if you are feeling cold. Similarly, if you are too warm you might remove some clothing or fan yourself to cool down. These motivated behaviours demonstrate that in contrast to a fixed stimulus response, motivated behaviour stimulated by the hypothalamus has a variable relationship between input and output. This interaction with our external environment may be a ‘choice’, however it is clear that the motivation to make these choices has a biological basis.
The mechanics of thermoregulation can be explained by what is sometimes referred to as ‘drive states’. This is essentially a feedback loop that is initiated by an internal stimulus which requires an external response. Kendal (2000) defines drive states as “characterised by tension and discomfort due to a physiological need followed by relief when the need is satisfied”. The process begins with the input. Temperature changes are picked up from peripheral surroundings by thermoreceptive neurons throughout body which sense both warmth and cold separately. An electrical signal (the input) is then sent to the brain. Any divergence from what is known as the ‘set point’ – in this case a temperature of approx 37° – will then be identified as an ‘error signal’ by interoceptive neurons in the periventricular region of the hypothalamus. Armed with these measurements and temperature signals being relayed from the blood, the hypothalamus then launches an appropriate error response. This includes motivating behaviour to make a physical adjustment, e.g. to move around or remove surplus 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 regulation of blood salt and water levels are regulated in a similar way. However, the processes that motivate us to eat is much more complex.
Humans have evolved an intricate physiological system to regulate food intake which encompasses a myriad of organs, hormones and bodily systems. Furthermore, a wealth of experimental research supports the idea that the hypothalamus plays a key role in this energy homeostasis by triggering feeding behaviours. Controlling energy balance is of crucial importance and eating is primarily to maintain fat stores in the event of food shortage. If fat cell reserves in the body are low, they release a hormone called leptin which 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 start to feel hungry which in turns initates 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 similar feedback loop to temperature control. However in practice this is not a reality. The main difficulty in maintaining energy homeostasis is that motivation does not rise solely from internal biological influences. Cultural and social factors also play an important part in motivation about when, what and how often to eat. In western culture, social pressures to be thin can override the need to eat and in extreme cases like anorexia the drive state becomes reversed. The motivation is no longer to eat because they are hungry but is instead not to eat so they do feel hungry. This corruption of the reward system is well documented and is associated with delusions of body image, a concept which is also linked to the hypothalamus and the parietal lobe. Problems can also occur if an individual receives over stimulation to eat. The prevalence of obesity in today’s society is testament to this fact.