iv | NewScientist | 6 April 2013

Many of our uniquely human capabilities arise in the forebrain, which expanded rapidly during the evolution of our mammalian ancestors. It includes the thalamus, a relay station that directs sensory information to the cerebral cortex for higher processing; the hypothalamus, which releases hormones into the bloodstream for distribution to the rest of the body; the amygdala, which deals with emotion; and the hippocampus, which plays a major role in the formation of spatial memories.

Among the most recently evolved parts are the basal ganglia, which regulate the speed and smoothness of intentional movements initiated by the cerebral cortex. Connections in this region are modulated by the neurotransmitter dopamine, provided by the midbrain’s substantia nigra. A deficiency in this source is associated with many of the symptoms of Parkinson’s disease, such as

slowness of movement, tremor and impaired balance. Although

drugs that boost levels of the neurotransmitter in the basal ganglia can help, a

cure for Parkinson’s is still out of reach.

Finally, there is the cerebral cortex – the enveloping hemispheres thought to make us human.

ForebrAIn Here plans are made, words are formed and ideas generated. Home of our creative intelligence, imagination and consciousness, this is where the mind is formed.

Structurally, the cortex is a single sheet of tissue made up of six crinkled layers folded inside the skull; if it were spread flat it would stretch over 1.6 square metres. Information enters and leaves the cortex through about a million neurons, but it has more than 10 billion internal connections, meaning the cortex spends most of its time talking to itself.

each of the cortical hemispheres have four principal lobes (see upper diagram, right). The frontal lobes house the neural circuits for thinking and planning, and are also thought to be responsible for our individual personalities. The occipital and temporal lobes are mainly concerned with the processing of visual and auditory information, respectively. Finally, the parietal lobes are involved in attention and the integration of sensory information.

The body is “mapped” onto the cortex many times, including one map representing the senses and another controlling our movements. These maps tend to preserve the basic structure of the body, so that neurons processing feelings from your feet will be closer to those dealing with sensations from your legs than those crunching data from your nose, for example. but the proportions are distorted, with more brain tissue devoted to the hands and lips than the torso or legs. redrawing the body to represent these

maps results in grotesque figures like Penfield’s homunculus (below left).

The communications bridge between the two cerebral hemispheres is a tract of about a million axons, called the corpus callosum. Cutting this bridge, a procedure sometimes performed to alleviate epileptic seizures, can split the unitary manifestation of “self”. It is as if the body is controlled by two independently thinking brains. one smoker who had the surgery reported that when he reached for a cigarette with his right hand, his left hand would snatch it and throw it away!

As we have seen, different tasks are carried out by different cortical regions. Yet all you have to do is open your eyes to see that these tasks are combined smoothly: depth, shape, colour and motion all merge into a 3D image of the scene. objects are recognised with no awareness of the fragmented nature of the brain’s efforts. Precisely how this is achieved remains a puzzle. It’s called the “problem of binding” and is one of the many questions left to be answered by tomorrow’s neuroscientists.

” Cutting the bridge between the brain’s two hemispheres can split the ‘self’. It is as if the body is controlled by two independently thinking brains”

The midbrain plays a role in many of our physical actions. one of its central structures is the substantia nigra, so-called because it is a rich source of the neurotransmitter dopamine, which turns black in post-mortem tissue. Since dopamine is essential for the control of movement, the substantia nigra is said to “oil the wheels of motion”. Dopamine is also the “reward” neurotransmitter and is necessary for many forms of learning, compulsive behaviour and addiction.

other regions of the midbrain are concerned with hearing, visual information processing, the control of eye movements and the regulation of mood.

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CEREBAL CORTEX

HINDBRAIN REGION

MIDBRAIN REGION

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MEDULLA OBLONGATA

SUBSTANTIA NIGRA

BASAL GANGLIA

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HYPOTHALAMUS

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FOREBRAIN REGION

HIPPOCAMPUS

FRONTAL LOBE PARIETALLOBE

OCCIPITALLOBE

TEMPORAL LOBE

6 April 2013 | NewScientist | v

Our billions of neurons, joined by trillions of neural connections, build the most intricate organ of the human body. Attempts to understand its architecture began with reports of people with brain damage. Localised damage results in highly specific impairments of particular skills – such as language or numeracy – suggesting that our brain is modular, with different locations responsible for different mental functions.

Advanced imaging techniques developed in the late 20th century gave a more nuanced approach by allowing researchers to peer into healthy brains as volunteers carried out different cognitive tasks. The result is a detailed map of where different skills arise in the brain – an important step on the road to understanding our complex mental lives.

Mapping the Mind

as its name suggests, the hindbrain is located at the base of the skull, just above the neck. Comparisons of different organisms suggest it was the first brain structure to have evolved, with its precursor emerging in the earliest vertebrates. in humans it is made up of three structures: the medulla oblongata, pons and cerebellum.

the medulla oblongata is responsible for many of the automatic behaviours that keep us alive, such as breathing, regulating our heart beat and swallowing. Significantly, its axons cross from one side of the brain to the other as they descend to the spinal cord, which explains why each side of the brain controls the opposite side of the body.

a little further up is the pons, which also controls vital functions such as breathing, heart rate, blood pressure and sleep. it also plays an important role in the control of facial expressions and in receiving information about the movements and orientation of the body in space.

the most prominent part of the hindbrain is the cerebellum, which has a very distinctive rippled surface with deep fissures. it is richly supplied with sensory information about the position and movements of the body and can encode and memorise the information needed to carry out complex fine-motor skills and movements.

hindbrain

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