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Dyslexia and the Mental Lexicon

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On the assumption that there is a part of the human brain that stores information about how we read, speak and understand language, it follows that this facility, or 'lexicon', has the ability to accommodate new words, and to link the new words and their meanings and uses with those already stored. Once stored in the lexicon, it is possible that all stored words are linked to one another, in the manner of hyperlinks on a web-page. Furthermore, it is possible that the links are categorised into 'familiar' links. For instance, the link between the words 'black' and 'cat' may be stronger, or more 'available' than the link between the words 'monkey' and 'jam'. The following entry is more concerned, however, with how such a system is accessed during speech, writing and reading.

Word Recognition

Word recognition has been the subject of a lot of research, and there are some well established findings concerning factors that can affect the speed of recognition and retrieval. Any model describing how words are accessed from the mental lexicon must take these factors into account. Some of these factors include:

  • Common words are recognised faster than uncommon ones - or the frequency effect

  • Strings of letters that do not make words within the language of the lexicon - or the word/non-word effect

  • Words in context are recognised faster than out of context words - or the context effect

  • The effect of distortion of the clarity of the written or spoken word - or the degradation effect


There are two main models that illustrate how words could be accessed from the mental lexicon. These are classed as either 'direct access' models or 'search' models.

The 'Logogen', or 'Direct Access' Model

The 'logogen' (1969, 1979) model invented by John Morton is a typical direct access model. The model states that every word that we know has a feature counter or 'logogen' corresponding to it. It suggests that the visual stimuli feed directly into a 'logogen' system - a set of counters that activates words as features become apparent. Hence, if the first feature is that of a word starting with a vertical line, then all logogens of words starting with I, L, K, N, M, B etc, will gain a point. Each logogen has a threshold level, and when this level is reached the corresponding word is given up as the required response, and all logogens return to zero-count before the system prepares for the next input. Lexical access is direct (hence its classification as a direct access model) and occurs simultaneously for all words. It is also passive as instead of seeking rejection, the logogens wait until they are accepted.

Research into this model has been very complex. Based around the logogen model's threshold level principle, which attempts to account for both frequency effects1 and context effects2, experimental research has concentrated on looking for the ways in which these two variables interact with others. If the threshold level is an accurate description then they should interact in similar, if not identical ways with other variables such as stimulus quality.

Norris (1984) showed that frequency and stimulus quality can interact, but that stimulus quality and context interact more. However, if stimulus quality actually affects the original visual encoding processes, then this has no value when lexical access is considered. The problems with the logogen model when conducting empirical experiments is that there are too many units in which errors can occur to give very similar results. Although it can generally explain all experimental data, it does so by adding more and more units into the system to cope with the explanations, and as such it could be considered vague.

Despite the fact that it can explain most of the basic findings of word recognition, it fails to take into account what happens when a person is presented with non-words or words that they haven't seen before. It can, however, be presumed that each logogen is morpheme3 specific, not word specific. It does not allow for the speed at which we write or read, and it also suggests that each word has another logogen for the auditory features of the word. If this is the case then the issue of space within the mind must be questioned, based upon the universal presumption that the mind works as efficiently as possible in terms of storage. If the brain has a limited amount of storage space, then this model must be considered less of an adequate description than one which has multi-function units.

The 'Search and Find' Model

Forster's (1976, 1979) model demonstrates the other main type of access model, one which utilises the concept of search. A complete perceptual representation of the input is constructed and then compared to a set of access files, one of which is orthographic4. The entries are then searched in order, and when a match is found information from the mental lexicon is accessed and each access file is divided into bins arranged according to frequency, the high-frequency bins being searched first. Context effects are handled by the presumption that as soon as words are accessed, related words are used to create a new list which is searched at the same time as the bin. As soon as a matching word is found in either list the mental lexicon is accessed. Degradation is presumed to delay the start of the search, as the perceptual representation takes longer to complete.

The main criteria for the selection of a model that can be used to helpfully consider dyslexia is that the model is as efficient as possible. Like the logogen model, Forster's model makes no account for the speed in which we read and write and in fact presumes that the process takes longer than the logogen model does. It also suffers from overkill. Like the logogen model it includes extra units in order to deal with the basic findings, and suffers from a lack of refinement. The search model utilises serial processing, which would suggest that a dyslexic suffers from a wide-spread complex problem involving the whole system. As the fundamental assumption here is that dyslexia is caused by a simple problem, it can be discounted.

Forster's model has a phonological file. This means that words can be accessed simply by their phonological features and could explain why the majority of errors a dyslexic makes are phonological, rather than simple spelling slips. (For example, recognising 'fizzicks' as 'physics' without acknowledgement of the mistake). The problem is the way in which it does this is over-complicated. It presumes that the dyslexic searches the orphographic file and then the phonological file without taking twice the amount of time to do so in comparison to recognising a word spelled correctly.

Spelling slips in general are not considered in either model, although this is not a major oversight as these models are both concerned primarily with recognition and not production. However, it is logical to suggest that recognition and production are dealt with by the same area, again due to restraints on space.

Instead, it has been proposed that there is a learned set of rules, known as the grapheme5-phoneme6 correspondence rules that exist and the mental lexicon can only be accessed phonologically. As spoken language comes before written language, both species-wise and in child development, this assumption seems logical. One of the theories presented with this process in mind is Rumelhart and McClelland's (1981-2) Interactive Activation model which belongs to a whole subset of theories within psycho-linguistics known as 'connectionism'. These theories present processing that occurs through the actions of many simple inter-connected units. Some of these units are dedicated to the recognition of letters and presenting to the units next along the line the phonemes these correspond to.

A Third Possible Model - The Interactive Activation Model

Rumelhart and McClelland's (1981-2) Interactive Activation model was originally presented to account for word context effects on letter identification ie, to account for the fact that it is easier to tell which letter is which if it occurs within a word.

The model is made of lots of processing units arranged in three levels:

  • An input level where the units look for visual features.

  • A level where units look at the correspondence between these features and gives the next level a set of letters.

  • An output level which has units which correspond to each word.

Within a level each unit is connected to every unit in the level above and below. These connections fall into two types, excitatory or inhibitory. Excitatory or positive connections make the units at the end of them (the ones in the higher level) more active, whereas inhibitory or negative connections make the end units less active. As well as these connections between levels there are also intra-level connections, which are inhibitory and connect each unit within a level to each other. This has the effect of quickly causing this pathway to have preferential access to the next level and causing all the other units to defer. If this were not the case it would be possible that there would be too many access points into the next level, and thus into the mental lexicon, causing too much ambiguity in the choice of word meaning.

When a unit becomes activated it sends a signal down each of its connections simultaneously. If the connection is inhibitory then it will decrease the level of activation of the unit it is connected to, or if it is excitatory it will increase its activation level. Eventually, after the initial strong flow of energy, the system relaxes until systematically only one word is left.

In comparison to the other two models discussed, this model still needs a mental lexicon. However it becomes more interactive with the process of reading, and this is the only type of model which seriously proposes parallel processing as an integral part of reading. Due to the speed at which this process occurs, this is likely to be the case. This model builds up a features list, similarly to the other two models, but the list is produced in parallel to the comprehension of the word. If the input was phonological then there is another first level that deals with it, sending messages on to the same second level for access of the mental lexicon. In a way this second level acts as a kind of grapheme-to-phoneme conversion processor.

In short it seems that the Interactive Activation model, although at first looking the most complicated, does the most thorough job when attempting to explain the process of written language comprehension and writing. The dyslexic could suffer from errors at any point along this system.

The Lexicon

Regardless of which model, if any, is actually an accurate approximation to the way in which we read and write, all state that there is a mental lexicon. Therefore, putting these models aside for a moment, in summary it is a logical assumption that the goal of reading or listening is to divine meaning from the marks seen (reading) or the waves heard (listening).

The fact that a dyslexic person can produce speech as well as any person of similar age does not necessarily have an effect on this lexicon. Child language development may suggest that there are two lexicons, one for production and one for comprehension.

1The more frequently a word is used the more quickly it is recognised.2The words around the target word aid recognition if they are within a sentence for example.3Morpheme - the smallest part of a sentence (a word) or the smallest element of a word.4Orthographic/orthography - a method of representing the sounds of a language by written or printed symbols.5Grapheme, in linguistics, the smallest unit of writing distinguishing one word from another - a grapheme is a single letter.6A Phoneme is the smallest sound that conveys meaning in a language. The word 'bog', for example, consists of three phonemes.

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