A large proportion of the studies done on the brain’s language functions since the 19th century have involved establishing correlations between a particular language deficit and the locations of lesions in the brains of autopsy subjects. But a single lesion can sometimes cause damage to several brain structures at once, which makes interpreting such findings difficult.

Modern brain imaging techniques have made it possible to study the activation of the brain areas associated with language in healthy subjects while they perform specified language activities. These studies have confirmed the importance of Broca’s and Wernicke’s areas for language while also identifying them as part of a wider network of interconnected areas of the brain that contribute to language. This concept has now replaced the historical notion of language “centres”.

In bilingual people, the earlier in life the second language was acquired, the more similar the areas of the brain involved in understanding and producing the two languages. In contrast, brain-imaging studies have shown that when people learn a second language later in life, the areas of the cortex involved in understanding the two languages are not always the same. Interestingly, when bilingual people lose the use of one of their languages as the result of a brain injury, the language that they retain is not always necessarily their mother tongue.

Indeed, bilingualism is a complex phenomenon, and much about its functional bases remains unknown. For example, because Italian uses phonemes and syntax that are much closer to French than to Chinese, will the brain of someone who is bilingual in French and Italian operate differently from that of someone who is bilingual in French and Chinese? Among people who are bilingual in French and Chinese, are their differences between the brains of those whose mother tongue is Chinese and those whose mother tongue is French? How does the frequency with which a person uses a language affect the corresponding structures in their brains? Clearly, the number of factors that may influence the language-processing areas of the brains of bilingual persons is quite considerable.

A general problem encountered in any attempt to determine the locations of brain functions is that every brain is unique. Just as every normal human hand has five fingers, but everyone’s hands are different, all human brains have the same major structures, but the size and shape of these structures can vary from one individual to another—by as much as several millimetres. Average measurements can be used, of course, in studying the brain, but the fact remains that the same type of lesion will not always cause exactly the same type of deficit in several different individuals.

With functional brain maps standardized for the sizes of various brains, we obtain a reference that is useful but does not really correspond to the brain of any one particular individual.

Often, when you have a word on the tip of your tongue, you can remember what letter it starts with, or what sound it ends with, or how many syllables it has, though you have not yet recalled the word itself. This shows that accessing a word when you are preparing to speak is not an all-or-nothing process: you can retrieve its various characteristics independently of one another.

In general, women’s overall reading abilities are better than men’s, and this gender difference often makes itself apparent when children are still in primary school. Researchers are still investigating how much of this difference is inborn and how much is acquired, but part of the answer would seem to be that girls have a greater taste for reading. Thus at least part of the reason that girls are better readers might simply be that they spend more time reading, while boys often spend more time playing sports. Some experts believe that increasing the time that boys spend reading and writing, and offering them content that interests them, could reduce this gap between boys and girls substantially.

Girls also seem to be better at spelling. The explanation here might be that females use both hemispheres of the brain in processing sounds, while males tend to use mainly the left side. If girls are therefore better at isolating the various sounds in a word, it would make sense that they would also be better at decoding it and spelling it.

In addition to semantic memory, which lets us retain the various meanings of words, we must use other specialized forms of memory in order to speak. For example, to pronounce any given phoneme of a language that you know how to speak, you must place your tongue and mouth in a particular position. They assume this position unconsciously, but obviously you must have stored it in memory somewhere in your brain.

In some languages, such as Spanish, the relationship between spelling and pronunciation is fairly straightforward, so it is fairly easy to retrieve the pronunciation of a word when you read it. But in other languages, the exact same string of letters may be pronounced very different ways in different words—for instance, the “ough” in “thought”, “tough”, “through” and “though”, in English, or the “ars” in “jars”, “mars”, and “gars”, in French. These arbitrary variations must be memorized as such, with no logical rules to help.

The brain hemisphere in which the main language abilities reside has often been referred to as the “dominant” hemisphere for language. But since we now know that the other hemisphere also contributes to language, it would be more accurate to describe the two hemispheres as sharing responsibility for the many aspects of language, rather than one hemisphere’s somehow exercising dominance over the other.

Anthropologists have been able to investigate handedness in ancient cultures by examining their tools. For example, by examining the marks left on a flint ax by the blows struck to make it, researchers can tell whether the person who did this work was right-handed or left-handed. Researchers have also examined ancient art to see what proportions of people are depicted using their right hand and what proportion using their left.

Researchers can generally estimate how right-handed or left-handed someone is by asking him or her a simple set of questions, such as “What hand do you write with?”, “What hand do you use to throw a ball?”, and “What hand do you use to brush your teeth?”

Even though language has a sort of “music” to it, from a neurological standpoint, music and language are distinct functions, because the sounds of music and the sounds of language are processed in different parts of the brain. Here are two famous cases demonstrating that language functions and musical functions are independent.

The first case is the French composer Maurice Ravel. After suffering an injury to the left side of his brain, he became aphasic. As a result, he could no longer transcribe melodies, but he could still recognize them, which showed that his ability to perceive music (as opposed to writing or performing it) had been preserved.

The other example is Ernesto “Che” Guevara. Though Che was a brilliant speaker, he suffered from congenital amusia, which made him completely unable to perceive music. Some nasty wits might say that the only reason Che made revolution was to take out his frustration at not being able to tell a salsa from a tango. But of course, they’d be wrong.

Because the sign language used by the deaf involves so many visual and spatial tasks, you might expect it to be controlled by the right hemisphere. But in fact, the proportion of people who are left-lateralized for language is just as high among deaf people who use sign language as it is among people with normal hearing.