The brain game
Could differences in brain structure explain behavioural differences between the sexes?
There is a popular perception that there are ‘male’ and ‘female’ ways of thinking. There is some evidence, for example, that female brains have more connections, that women tend to use more areas of their brain when thinking, and that the two hemispheres of the female brain have more cross-connections. It has been suggested that the greater use of several areas at the same time underlies women’s supposed superiority at ‘multitasking’.
Overall, while sex differences in brain structure and function do seem to exist, we cannot be sure of their full extent nor of their implications. Almost certainly the differences are small, so we have to be careful not to make too much of them. They also naturally vary between individuals, and indeed any sex differences are part of that natural variation.
Fear and loathing in the amygdala
Recent research has focused on several structures where sex differences may be important. One is the amygdala, a small structure at the base of the brain that is important in emotional responses, particularly responses to fear-inducing stimuli.
In humans, interestingly, emotionally challenging images tend to activate the amygdala in different hemispheres in males and females. This may be important, because the right and left hemispheres differ in their roles – the right hemisphere focuses more on the ‘big picture’ of a trauma, while the left handles specific details. Men and women do seem to show corresponding differences in how they recall traumatic events.
Also, there is some interest in using drugs such as the beta-blocker propanolol to inhibit memory formation of traumatic events and prevent post-traumatic stress disorder. Propanolol has a more powerful effect on the right hemisphere, however.
Stress and the hypothalamus
Another intriguing sex difference has been seen in the hypothalamus. This part of the brain has a number of roles, particularly in regulating the body’s homeostatic systems, and it is an important link between the brain and hormone systems. The hypothalamus is also the area that controls sexual behaviours, so differences between the sexes would be expected.
The link between the hypothalamus and the body’s hormone systems (including sex hormones) suggests the potential for cross-talk between the brain and body. A wide range of challenges to the body (types of stress) impact on the hypothalamus and its linked hormones – so if there are sex differences in the hypothalamus, they could have far-reaching implications on body physiology and health.
This could be relevant to mental health, as women are more likely to suffer stress-related disorders such as anxiety or depression (though men are more likely to commit suicide).
However, there are many social factors that also affect males and females differently, so it is unlikely that gender-specific differences in hormones and the brain hold all the answers.
Maps and the hippocampus
The hippocampus, a brain structure involved in memory formation and the storing of ‘spatial maps’ (internal representations of the external world), is larger in women than men.
This could relate to the different ways in which men and women navigate – men tend to mentally visualise spatial relationships between places while women go from landmark to landmark.
In rats interesting sex differences have been seen in the way they learn and how they are affected by stress. Male rats, for example, sometimes learn better than females in stressful circumstances, while female rats seem to be better at coping with long-term stress (cells in a female rat’s hippocampus are less sensitive to toxins after stress).
These findings hint that the two sexes may have significant differences in the way they learn, which could have implications for education. But it is a large leap from animal studies to people.
While there are no clear-cut differences in the sensitivity of men and women to pain, some studies suggest the sexes respond differently.
Pain originates when particular neurons sense tissue damage or an unpleasant stimulus (such as heat). Messages are sent to the spinal cord and then to the brain, several areas of which analyse the incoming sensory signals. So pain is a phenomenon of the brain, even if the sensation is projected onto, say, a stubbed toe.
Pain has a strong subjective element to it – there is no objective way to measure how much pain someone is in, and the impact of an unpleasant stimulus can be quite different depending on circumstances. Under controlled circumstances, women appear to have lower pain thresholds – a stimulus is perceived as painful at lower levels of stimulation.
Brain imaging shows that processing of pain signals differs between males and females. Pain sensitivity may also be linked to sex hormone levels – in women, pain is often said to vary through the menstrual cycle. However, some of the studies in this area are not very reliable, and those that are suggest ‘the time of the month’ makes little difference.
Responses to painkillers also seem to depend on sex. Differences in male and female body composition and metabolism may affect how the drugs work. But the situation is complicated – different painkillers act in different ways, and studies on animals do not always reveal the same effects as in people. Overall, women appear to be more sensitive to the action of some (but not all) classes of painkillers.Lead image:
Patrick Hales, UCL/Wellcome Images CC BY NC ND
- Common drug eases memories of fear
- Eternal sunshine of propranolol
- Sex-related impairment of memory for emotional events with beta-adrenergic blockade (2003)
- The hypothalamus gland
- Sex differences in the dendritic arbor of hypothalamic ventromedial nucleus neurons (2009) [PDF]
- Sex comparison on long-lasting behavioral and physiological disturbances induced by single shock experience in rats (2012)
- Sex-related hemispheric lateralization of amygdala function in emotionally influenced memory: an fMRI investigation (2004)
- Scientific American: Psychotropic drugs affect men and women differently
- Sex differences in opioid analgesia and addiction: interactions among opioid receptors and estrogen receptors (2013)