Motivated Behaviour And The Role Of The Hypothalamus

The behaviours of eating and sexual activity in humans are regarded in terms of motivated behaviour such that they appear to be purposeful and directed toward a goal. The hypothalamus is a brain structure located deep within the brain that, although small in size less than 1% of the brains total mass, is largely responsible for and controls a wide variety of human behaviour including motivation, eating, sexual activity and emotion. However in trying to understand this role it is important to first investigate the fundamental reasons and causes behind motivated behaviour. One apparent reason for behaviour is that we act out of free will however this is not a viable cause of behaviour as sometimes the central nervous system produces behaviour, which cannot be due to free will. Hebb (1955) and Heron (1957) argued that in order for the brain to maintain a minimum level of interaction people are motivated to engage with stimuli present within the environment. This is known as environmental stimulation and was examined through a series of experiments focusing on sensory deprivation. Hebb (1955) set up a cubicle in which subjects were deprived of nearly all sensory input such as sound, sight and touch for 24 hours. The results showed that for 4-8 hours the subjects were content just to "enjoy the peace and quiet" (Kolb & Whishaw, 2001) as they were instructed to do. However beyond this time set subjects became increasingly distressed and developed a significant need for stimulation of any kind. Hebb (1955) concluded that since all the basic needs of the subjects were met and they were still distressed, the brain must therefore have an intrinsic need for stimulation, which could explain the causes of behaviour.

However the brains inherent need for stimulation is not the only reason for behaviour. There is also an internal, energising factor that drives behaviour and is known as instincts or drives. The 'Flush Model' represents the notion that behaviour is caused by drives. It states that there is a store of action-specific energy that is released and flows out in the form of behaviour. It follows therefore that the greater the store of energy the longer the behaviour will persist and if there is no store of energy than there is no behaviour. Drive theories appear to make intuitive sense yet research has failed to establish any link between this apparent reason for behaviour and brain activity. Instead what was discovered was a correlation between behavioural change and changes in cellular activity as well as hormones. For example while studying sexual drive in rats Davidson (1980) discovered that the frequency of sexual activity toward receptive female rats is correlated with levels of male hormones known as androgens. This is also true for human males and suggests that the concept of sexual drive is not necessary to explain sexual activity. Instead the focus is now on the actions of hormones on neural circuits, which in turn affects behaviour. This is where the role of the hypothalamus is evident. Malsbury (1971) supported the findings of Davidson (1980) when he discovered that electrical stimulation of the medial preoptic area (MPA), which is the area in the brain activated by androgens, elicited copulatory behaviour in males. Evidently then the activity of neurons has control over how we act, think, feel and ultimately behave.

The hypothalamus is a hugely important brain structure when it comes to behaviour and consequently any damage, even tiny amounts, can cause devastating and sometimes fatal disruptions to human behaviour and bodily functions. On each side the hypothalamus is divided into three sections the lateral, the medial and the periventricular. The lateral zone is comprised of nuclei and tracts that connect it to the brain stem and various parts of the limbic system. The main tract is the medial forebrain bundle (MFB), which is formed partly from dopamine-containing fibres. Dopamine is linked to the experience of reward and thus the MFB is important in motivated behaviours such as eating and sexual activity. One function of the hypothalamus is to regulate internal bodily levels, such as temperature, in response to the external world. This is known as homeostasis and involves "the maintenance of the body's internal environment within a narrow physiological range" (Bear, Connors, & Paradiso, 2001, p. 500). Another more principal function of the hypothalamus is to control the pituitary gland. The pituitary has two lobes - the posterior pituitary and the anterior pituitary and the hypothalamus controls bodily functions and behaviour through these two lobes in very different ways. Active neurons in the hypothalamus cause the release of peptides, which are stored in the posterior pituitary and these then travel through the blood stream to targets in the body where they have their intended effects. For example, oxytocin, a type of peptide, controls uterine contractions. On the other hand anterior pituitary hormones are controlled by the hypothalamus through 'releasing hormones'. These releasing hormones act to either increase or decrease the release of anterior pituitary hormones, which in turn allows the brain to take control of other parts of the body.

The hypothalamic mechanism of homeostasis plays a key role in generating motivated behaviour. Any deviations from the optimal range of a regulated parameter are detected and corrected by neurons located in the periventricular zone of the hypothalamus. This was first demonstrated by studies in which electrical stimulation of the hypothalamus in various animals resulted in the animals engaging in complex behaviours such as displaying fear, digging, eating and reproductive behaviour. The type of complex behaviour elicited depended on which area of the hypothalamus was stimulated. All of the behaviours had common features, which made them distinguishable from normal behaviours, such that they were well integrated, smooth and goal directed. Two further results were found - that all the behaviours were related to the survival of the species and that the animals found the stimulation pleasant. Grossman (1960) further advanced this research by applying different chemicals into the lateral hypothalamus via a chronically implanted cannula. He found that drugs that were similar to acetylcholine elicited drinking only whereas drugs similar to nonadrenaline educed eating only. In addition drugs that blocked acetylcholine and nonadrenaline receptors caused drinking and eating to cease. These results provided more concrete evidence that "within the anatomic boundaries of the lateral hypothalamus are chemically coded functions for eating (nonadrenergic) and drinking (cholinergic)" (Kimble & Colman, 1995).

In addition to neural circuits motivated behaviour is a result