Language is unique among mental functions in that only humans truly possess a language system, and it is one of the most fundamental ways in which humanity excels. Language is second nature to humans we do not really have any conscious thought about how to express it, it just happens. However language is a highly complex system and involves many aspects including representation, comprehension and communication via symbolic information. It has now been known for over a century that the regions surrounding the Sylvian fissure of the dominant left hemisphere play a key role in language (Eysenck and Keane 2000).
However new findings from patients with brain lesions and from neuroimaging studies in un-impaired subjects are giving us a deeper insight into the relationship between language and the brain and are showing us that it is far from simplistic. Research is wide ranging and has looked at aspects of both language comprehension and production although I will focus on what has been found about how language is represented and comprehended. Psychologists have investigated the concept of how our brain derives meaning from spoken and written input. To understand this it is firstly necessary to know how words are represented in the brain.
One of the central concepts in word representation is the mental lexicon. This is a mental store of information about words that includes semantic and syntactic information as well as details of word forms (Gazzaniga et al 2002). The majority of psycholinguistic theories agree that the mental lexicon plays a central role in language but there is much debate over how the mental lexicon and conceptual knowledge is organised in the brain. By observing brain damaged patients with language disorders neuropsychologists have been able to infer a great deal about the functional organisation of the mental lexicon.
Patients with Wernicke’s aphasia usually have brain lesions to the posterior parts of the left hemisphere including the posterior part of the STG. They are able to speak fluently but cannot make meaningful sentences and they make speech errors known as semantic paraphasias (Gazzaniga et al 2002). Semantic paraphasias are when patients produce words related to the meaning of the intended word for example using cow instead of horse. Patients with semantic dementia have great difficulty assigning objects to semantic categories although they can still understand and produce the syntactic structure of sentences.
For example when asked to name a picture of a bird they will often name a related category such as ‘animal’. Patients with semantic dementia have damage which is associated to the temporal lobes mostly on the left side of the brain however the superior regions of the temporal lobe that are important for hearing and speech production remain undamaged. This neurological evidence seems to support the idea of the semantic network because brain damaged patients seem to substitute, confuse or lump together related meanings as would be expected from the degrading of a system interconnected by nodes that identify information (Gazzaniga et al 2002).
Around the 1980’s Elizabeth Warrington and her colleagues found that semantic problems, like those suffered by semantic dementia patients, could be localized exclusively to certain semantic categories such as animals or man-made objects. Warrington discovered that there was a remarkable correlation between the site of lesions in brain damaged patients and the type of semantic deficit suffered. Patients whose deficits related to living things tended to have lesions of the inferior and medial temporal cortex, which were often situated anteriorly. These brain areas are situated close to areas that are crucial for visual object perception.
Less in known about the sites of lesions for patients who suffer from man-made related impairments because fewer patients have been observed. However it appears that the left frontal and parietal areas are responsible for this type of semantic deficit, and these areas are close to areas that are important for sensorimotor functions and may therefore be involved in the representation of actions used with tools (Gazzaniga et al 2002). Warrington proposed that these patients’ deficits reflected the types of information stored with various words in the semantic network.
She suggested a modality specific organization of the semantic network where biological categories such as animals rely on physical properties and visual aspects whereas man-made objects are identified by their functional properties. However Damasio et al (1996) provided one of the most convincing pieces of evidence for category specific organisation from a large sample of patients with brain lesions. Damasio found that after examining the position of patients brain lesions she could associate naming deficits with specific brain regions.
She revealed that brain damage to the left temporal pole (TP) correlated with problems retrieving the names of people, lesions to the anterior part of the left inferior temporal (IT) lobe correlated with problems naming animals, and finally lesions to the posterolateral part of the left inferior temporal lobe as well as the temporo-occipito-parietal junction (IT+) was associated with problems in naming of tools. In a corresponding study PET scanning was used to find that in neurologically normal subjects the same brain areas were activated when naming people (TP), animals (IT) and tools (IT+).
However many of Damasio’s patients were still able to activate various conceptual properties relevant to the word in question even if they could not name it. For example if shown a picture of a hedgehog they would reply, ‘it is very spiky and sharp; it doesn’t move very quickly and often gets hit by cars’. Consequently Damasio and her colleagues concluded that the lesioned brain areas found to correlate were involved in word retrieval and did not necessarily reflect the organisation of a conceptual network in the brain but more likely reflected the arrangement at the word or lexical level.
They believed that their results showed that the brain had three levels of representation for word knowledge a conceptual level that contained semantic features of words, the lexical level where word forms that match the concepts are represented, and finally a phonological level where he sound information that corresponds to the word is represented (Damasio et al 1996). Damasio suggested from her results that the brain’s conceptual networks involved neuronal structures both in the left and right hemispheres.
She believed these conceptual networks were connected to lexical networks in left temporal lobe and may contain particular information for persons, animals, and tools. (Gazzaniga et al 2002). Once it is understood how words are represented the next stage is to look at how we understand linguistic input. Language can either be written or spoken and it is believed that there are certain differences in which spoken and written inputs are analysed and comprehended.
Spoken language is thought to be very different from written language as people have to distinguish the relevant speech sounds from other general background noise. Patients with brain lesions and PET and fMRI studies have shown us that that the superior temporal cortex plays a role in spoken word processing as it is important in sound perception Patients with bilateral lesions restricted to the superior parts of the temporal lobe have what is known as pure word deafness (Eysenck and Keane 2000)..
Patients with pure word deafness have difficulties in recognising only speech sounds and their problems are mostly restricted to auditory or phonemic deficits. The areas that surround Heschl’s gyri and extend into the superior temporal gyrus (STG) are auditory association areas. PET studies have shown us that these areas in both hemispheres are activated by speech and also non-speech sounds. This suggests that this area is not necessarily important for specialized linguistic processes as it is also activated by non-speech sounds e. g. ones (Gazzaniga et al 2002). However this is quite surprising as it has long been thought that Wernicke’s area which consists of the left temporal-parietal region including the STG was vital for word comprehension. Although in Karl Wernicke’s original patients from whom this belief was conceived damage was not restricted to the STG so it has therefore been concluded that the STG alone is not responsible for word comprehension. Binder et al (2000 citied in Gazzaniga et al 2002) conducted an fMRI study comparing different types of non-speech and speech sounds.
Areas that were found to be more sensitive to speech sounds were located ventrolaterally in or around the superior temporal sulcus. However because these areas were equally activated for words, pseudowords, and reserved speech Binder et al believed that these areas are probably not concerned with the lexical semantic aspects of word processing and the areas involved in processing lexical-semantic information were lateralized much more to the left side and situated mostly ventral to the superior temporal areas.
Written input requires readers to recognise a visual pattern and regardless of the writing system they must be able to analyse certain primitive features like the shape of the symbols. The actual identification of the orthographic units is believed to take place in the occipital-temporal regions of the left hemisphere. McCarthy et al (1996 cited in Gazzaniga 2002) used fMRI to contrast brain activation in response to letters with activation in response to faces and visual textures in neurologically normal subjects.
They found that regions of the occipital-temporal lobe were activated in response to unpronounceable letter strings. In a second study by the same group they found a large negative polarity potential around 200 msec in occipital-temporal regions in response to the visual presentation of letter strings. Importantly it was also found that this area did not respond to any other visual stimuli such as faces which suggests that this area is specialised for visual comprehension. Patients with lesions to this area offer further support for its role in visual language comprehension.
Lesions to this area cause pure alexia where patients are unable to read words although they are still able to understand spoken language and their other aspects of language remain normal (Gazzaniga et al 2002). It is clear that the subject of language is very extensive and much research has been conducted by neuropsychologists to try and understand the fundamental aspects of language. Only the evidence relating to language representation and certain aspects of comprehension have been discussed here much additional evidence has been found in the area of language production.
It seems clear that neurological and neuroimaging methods have given us invaluable insight into the area of language. However there are problems with both evidence from imaging techniques and brain damaged people. It is therefore most useful to converge neurological and neuroimaging evidence to create a more accurate picture as a single method cannot bring about a complete and reliable understanding of such a complex system as language.