This discussion is based on the double dissociations and the methodologies that help us to understand and evaluate how the functional organisations of the human brain work. A number of different methodologies and alternative views will be evaluated and compared in support of the brains functions. To begin with, an outline of Fodor (1983) and the basic assumptions of modularity will be explained. There are five basic assumptions which are; 1) Informational encapsulation; this module carries out its own form of processing, ignoring other processes going on in the rest of the cognitive system 2) Domain specific; the module can only accept information from one domain 3) Neurologically specific; the brain lesions will selectively impair certain modules while leaving the others intact and operating at normal. Pre- injury levels of efficiency (Shallice, 1984) occurs if the assumptions of informational encapsulation and domain specificity are combined with an assumption of neurological specificity 4) Mandatory; is unstoppable and is beyond voluntary control 5) Innate, this is part of our genetic makeover. The evidence for the basic assumptions of modularity are supported with evidence of associations, the associations which will be most significant to this discussion is double associations. There are also two opposing views involved in the human brain and the behaviours which are carried out first is the ‘localisationist’ view which assumes regions of the brain are specialised for different cognitive functions. The ‘Aggregate field’ view assumes mental operations are not localised but the product of diffuse brain activity.
Associations can be varied between certain tasks, for example, a patient may be impaired on task ‘a’ and also on a second task ‘b’. This supports the assumption that cognitive processes are necessary for both tasks to be carried out. Single associations are when performance on task ‘a’ is impaired yet performance on task ‘b’ is normal, meaning that different processes are involved in carrying these tasks out. The most significant association is the double association which is where a person makes an association where they are impaired on task ‘a’ and perform normally on task ‘b’ whilst on the other hand another person in contrast performs normally on task ‘a’ but has impaired performance on task ‘b’. Double dissociations occur due to breakdowns in the cognitive system; some tasks can be performed better then others. An explanation of this could be due to no overlapping in the cognitive processes, for example ‘four sets of cognitive processes are mediated by four adjacent areas of the brain, if a brain injury damages one area of then the rest will be damaged as well’ (Brain, 1964). The performance on these tasks of a given person is reflected on four factors; 1) the contribution attributable to the true effect of the hypothesized disruption of one or more processing components (modules). 2) Normal individual variation in performance. 3) The effects of compensatory operations and 4) effects that result from disruptions to processing mechanisms other then the hypothesised component. This model is suggested by Caramazza (1984).
An example of a double dissociation is shown in two case studies observed and studied by Riddoch and Humphrey (1987b) the patients studied are known by the initials, ‘JB’ and ‘HJA’. JB has a left hemisphere brain injury and this type of injury left him unable to read or write, although he has intact oral spelling. JB also suffers with amnesia, and although his vision did not seem to be affected he has difficulty naming seen objects. Semantic errors were made when trying to identify these objects and JB used gestures to refer to the uses of the objects but could not identify them as a whole. On the other hand, HJA the other studied patient could not use these gestures to mime the uses of the objects. In other tasks such as naming line drawings as real objects or meaningless objects JB could retrieve stored knowledge of object recognition units while HJA could not do this. Riddoch and Humphrey (1987b) thus describe JB’s impairment as involving semantic access from vision. Findings from these observations showed that this problem was ‘bidirectional’; JB was equally poor at accessing knowledge of an objects appearance from semantics. His ability to draw objects from memory was poor, and he commented that “I know what it (the object he was asked to draw) is, but I just can’t picture it” (Riddoch and Humphrey, 1987). This finding also showed that when JB tried to use his knowledge of naming the pictures he instead used gestures. The type of knowledge he accessed is linked to the structural rather then the semantic properties.
Other studies and patients have been observed to show double dissociations in a variety of tasks. One in particular study has shown that these neurological patients can suffer from topographical memory loss and can get lost quite easily. A study carried out by Bisiach and Luzzatti (1978) showed that the patients could not report buildings to the left of their mental image even though they knew the location of the buildings and that they existed there.
Findings showed that the knowledge about the arrangements of the buildings was intact yet it interfered with the ability to express this within the task. These types of studies are in support of spatial knowledge and spatial thought within topographical memory; there are two separate types of this memory. In one form spatial knowledge is preserved; an example of this type of form is a studied patient of Whiteley and Warrington (1977) this patient JC tends to get lost easily as they cannot recognise familiar buildings as landmarks. Patients like these can describe the buildings they are looking at but when they look at them they seem unfamiliar, JC described his street as looking unfamiliar. Problems like these seem like a form of visual recognition ability yet in contrast with the findings of JC another patient had opposite problems. Patient MA found landmarks and buildings familiar in her surrounding but could not get to them without constantly getting lost, MA showed intact memory apart from remembering routes. In contrast to these patients De Renzi et al (1977) observed patient RA who had severe amnesia but had no difficulties in remembering routes and finding his way. These studies have supported that impairments of spatial knowledge fall into different types yet they have not been distinguished to which parts of the brain or why these double dissociations occur.
While it is clear that patients do acquire double dissociations and contrast each others impairments, the real issue is which part of the brain is affected and why these double dissociations occur. Neuropsychologists have found that there is evidence that different functional components of the visual system are organised into dissociable systems concerned primarily with the analysis of different types of visual pattern (objects, faces, words, etc) or with space perception. (Ellis & Young, pp 85). This indicates why components concerned with visual pattern are in the same area of the brain that the separate systems regarding objects and faces etc are also in different components and are separate. This shows that if one type of area or component associated with words is damaged they may still be able to read but not write; this ultimately depends on which parts have been affected.
Evidence for these types of impairment has been supported using various sets of case studies, with discussion to the different types of components for various objects. There are many different demands for recognition; for example when we wake in the morning and begin to make a cup of tea for ourselves we look for our mug and assign it to a category ‘my mug’. While looking for the tea we associate if the tea belongs to us or our flatmates. Distinguishing between these belongings and objects we are still able to associate them with the correct categories in our everyday life and make choices while the demands of face recognition are very different. These types of demands are different because there have different types of perceptual mechanisms needed to cope with recognitions tasks that demand between- categories or within category discriminations (Ellis and Young, pp 109). There is also evidence that shows dissociations between impairments and the discriminations involved in this contributes to be an important factor when realising brain structure.
Support for the neural structure of the brain regarding recognition of words can be supported in terms of dyslexia. There are many types of this including peripheral, neglect, attentional, alexia, surface and deep. Each of these disorders is a disorder of reading and they are consequences of brain injury and are acquired as dyslexia’s. This identification is a visual process and includes three components specific to reading and this is known as the phonological route. The components are 1) Visual Analysis System; this system identifies the component letters of the words and notes their posisitioning 2) Visual Input Lexicon; this is each word as activated by in the input lexicon and experienced readers have these as they become familiar with thousands of words. The lexicon receives the input from the letter recognisers in the Visual Analysis System and, in turn, activates stored representations of their meanings in the semantic system (Ellis and Young, pp, 193). 3) Grapheme – phoneme conversion, this route enables a reader to be able to pronounce unfamiliar words. Letters are firstly identified and are converted into sounds (grapheme to phoneme) this basically translates unfamiliar letter strings into phoneme strings.
Each type of dyslexia has occurred due to some form of damaged at various stages in the route. This is when the problems occur and recognition in reading tasks is affected. To be able to distinguish between the impairments and how to overcome them, an explanation of the routes is discussed in order to understand the ability to read. The route discussed is the phonological route and the second is the lexicon route. The lexicon route also begins with either the auditory analysis system or the visual analysis system depending on whether the word is heard or written. The next stage is input lexicon where again there are representations of the words that are familiar. The semantic system contains the meanings of the familiar words and the speech output lexicon contains the representations of spoken words, which are only activated when the word is going to be spoken. The last level is also the phoneme level which in regards is the same as the phonological loop; when these levels are achieved speech occurs. Problems may occur in different levels of the loop resulting in the types of dyslexia’s, a problem resulting from brain injury could be that the patient may recognise words but not know or remember what they mean. This type of impairment may occur in the semantic system of the lexicon route.
Case studies supporting the structures of the human brain have been studied and observed using patients with neglect dyslexia. Patients are observed and studied by Kinsbourne and Warrington (1962). These patients tend to neglect the left half of space. The types of visual errors these patients made during reading consisted of getting the endings of the words correct but the beginnings incorrect such as ‘level’ as ‘novel’. Patients who suffered from visual field defects where half of the space was left blind were known to have ‘left homonymous hemianopia’. Patient VB often read the right side of the sentence, ignoring the left and made 8% of errors when reading words.
Findings showed that the errors were influenced by visual factors and errors included the deletion of initial letters. Shallice (1981a), Ellis, Flude and Young (1987a) argued that the impairments should be located in the visual analysis system. Two functions usually identify the letter and then position it, yet in this case VB would replace the initial letter rather than delete it and this would change the representation of the overall word in the speech output lexicon. Ellis et al (1987a) suggested that errors found in neglect dyslexia arise when a more general visual neglect happens to compromise the reading process. In other words, there is nothing about neglect dyslexia which is specific to reading and can happen during any other every day activities.
A supporting assumption in terms of brain injury and brain structures is explained by a number of assumptions of surgical procedures in epilepsy operations. An approach by Coltheart (1980b: 1983) reported that word recognition is mediated by a visual input lexicon in the deep dyslexic patients right hemisphere, while speech output still occurs in the left hemisphere. Coltheart (1980b: 1983) has supported his assumptions by observing and comparing deep dyslexic readings. Case studies which he used consisted of 1) capacities of right hemisphere ‘spilt- brain’ patients whose two hemispheres had been surgically spilt to relieve epilepsy 2) The language skills of patients who had had their left hemisphere completely surgically removed and 3) the capacities for the normal right hemisphere.
Support can be gained for these assumptions; a finding concerning deep dyslexic patients has found that they cannot read non – words and neither can the right hemisphere for the spilt- brain patients (Zaidel and Peters, 1981) or normal subjects (Young, Ellis, & Bion, 1984) On the other hand the right hemisphere can contribute to the recognition concrete words (Ellis et al, 1974). While there have been positive comparisons on the other hand there have also been criticisms raised by Marshall and Patterson (1983) and Patterson and Besner (1984). Two patients studied by Patterson and Besner (1984) showed that reading performance by two deep dyslexics were superior to that of the right hemispheres of any spilt brain patients. However, findings have shown that only 5 out of the studied 44 patients have genuine right hemisphere language, this may have been due to early left hemisphere language (Gazzaniga, 1983). Finally there has been no evidence to show that any differences have been found between the two visual half fields of normal people in their capacity.
In conclusion to these findings it may be possible to have a second system which may be suppressed in intact normal individuals and hence only becomes evident after brain injury (Landis, Regard, Graves, & Goodglass, 1983).
In conclusion to this discussion there have been many supporting findings to suggest different areas of the human brain have different functions and levels when it comes to working. Several case studies of brain injured patients have given an insight to the many disorders and impairment that can occur due to damage. As findings have shown when carrying out daily tasks, a person needs to progress through several routes to achieve their destined goal, for example to read, a person may need to travel down the phonological loop, but if a link appears to have broken or been damaged there may be problems and this is were impairments arise. The case studies have demonstrated through many tasks about how the neural structure can be affected and how this changes behaviour. More support has been gained about double dissociations and the types of cognitive processes that work in order to conduct these activities.
On the other hand neuropsychologists have wondered if the impairments seen in brain injured people are just exaggerations of tendencies to error seen in normal people. This statement supports the assumption of subtractivity which it is assumed that entirely new cognitive processes do not arise following brain injury (Ellis and Young, pp, 139). This criticism suggests a lot more research and consideration needs to be approached when answering this question; there is little supporting evidence surrounding this statement, yet some has been found in order to support it.
In comparison to these findings optic aphasia challenges these theories that postulate a common set of semantic representations for known objects that can be accessed from any sensory modality (Ellis and Young, pp 54). ‘The patients knows what seen and felt objects are’, this patient can only name the ones that have been handled. In order to understand these theories we need to understand that the sections of the semantic system which are linked together to carry out daily tasks can disconnect from each other. This possibility of separation of visual and verbal semantic processes is suggested by Schwartz, Marin, and Saffran (1979) who studied a patient with a progressive dementing disease and found that WLP could mime the use of objects despite a severe impairment of semantic memory. WLP could not identify these objects verbally and this supports that while there is evidence of separate areas of neural structure they can also be separated.