Examining Differences in Working Memory of Normal Adults and Adults with ADHD using the fMRI - Research Paper Example

Examining Differences in Working Memory of Normal Adults and Adults with ADHD using the fMRI Introduction Most studies that try to understand the differences between individuals with and without ADHD are focused on children. This does help us in the design and carrying out of intervention programs that could help provide a more normal existence for these children. But while the vast majority of children with ADHD are identified at a relatively young age at present; this was not always the case. The procedures and training for detecting conditions like ADHD were not as well developed a couple of decades ago; and a number of children with ADHD did not receive adequate treatment and care. These children have now become part of the country’s adult population; and are still coping with the ramifications of suffering from ADHD. Understanding how the brains of these individuals respond to stimuli as compared to normal adults will help us evaluate the need for intervention programs and in the design of these said programs. This study shall attempt to provide some information in this direction. The main aim of the study will be to examine differences in the activation levels in the pre-frontal cortex of adults with ADHD and control adults from the normal population. The study shall also attempt to explain the reasons for any differences found; and attempt to suggest further work that would help in providing adults with ADHD with the help they may need. Review of Literature Working Memory Working memory is a term coined in an attempt by Miller, Galanter and Pribram (1960) to compare the human mind to the working of a computer. Theorists have drawn a number of similarities between The STM store described by Atkinson-Shriffrin (1968) and the working memory. The distinctive characteristic of this memory system is that it is used in active processing, and not relatively passive storage. In the early 20th century, the work of Hitzig and Ferrier indicated that the prefrontal and frontal cortex were responsible for the cognitive tasks; a concept verified later by Jacobsen in 1935. The Baddeley and Hitch (1974) model describes the working memory as a multi-component unit. According to them, the working memory has a central executive function that supervises and structures information, chooses stimuli for processing and oversees the working of two slave systems - The phonological loop and The visuo-spatial sketchpad. In 2000, Baddeley added a further component that provides an explanation for the capacity to integrate information from the two slave systems before it is processed by the central executive. He called this the episodic buffer. Though limited, the working memory does process somewhat more information than Miller’s (1956) seven unit estimate. It has been verified that the process of using ‘chunks’ does increase the amount that may be stored in working memory; and Baddeley (2001) has proposed that there are about four chunks that are processed by the central executive at one time. According to Cowan (1995), the working memory is essentially an extension or part of the long term memory store. It has two components or embedded levels. The first holds activated material from long term memory while the second which is the “focus of attention” is a limited capacity level that can hold up to four such activated elements of information at once. Oberauer (2006) has added to this model by suggesting a third component that holds only one piece of information and is embedded in the second level. Ericsson and Kintsch (1995) have proposed a third understanding of the working memory. According to them, a limited view of the working memory cannot explain carrying out complex tasks like reading and writing long texts. They postulate that we hold information in short term memory and then keep it active in long term memory by linking it through retrieval structures. They called this the Long Term Working Memory. Gobet (2000) has categorized these functions into generic, domain related and episodic retrieval structures. Tests of working memory Daneman and Carpenter (1980) designed that first span detection task to measure working memory. Other tasks that have been used to test the working memory are change detection tasks (Luck & Vogel, 1997; etc) and N-back tasks (Kirchner, 1958). The working memory is a representation of the efficiency of the executive functions; and is also seen as a function of the human frontal brain (Kane & Engle, 2002). Others like Halford, though, have argued that the capacity of the working memory is best represented by its ability to form and understand relationships between elements of information. The working memory, like any mental capacity, responds to training. Klingberg and colleagues (2002; 2005) have shown that working memory capacity can be increased in children with ADHD. Through their work; they have established short term effects of the training; but they found no long term effects. They did establish that training led to an increase in prefrontal and parietal activity. Klingberg (2009) claims that the enhancement of memory is due to excess neural activation; and an individual’s ability’s can be affected by training. While there is evidence that the optimal working memory performance does seem to be linked to the neural ability and activation (Berry et al, 2009); a number of these studies have been disputed on the basis of methodology used. Age has been found to be an important factor in the extent to which working memory functions efficiently; with a gradual increase occurring over childhood; and a decline happening towards old age (Salthouse, 1994). This change is more quantitative than qualitative; and the capacity of the working memory has been found to be an important indicator of ability in the developmental years. Adults with ADHD Nearly 60% of children with ADHD exhibit a significant amount of symptoms even as adults (Valdizan, 2009). These are seen more significantly in individuals who have not received treatment as children. ADHD in adults leads to disruptions in work and personal life; and can put individuals in relatively dangerous situations even during routine tasks like driving and household work. The frustrations experienced by these individuals can drive them towards the use of alcohol or other intoxicants as a coping mechanism, and also are more likely to report symptoms of anxiety or depression (Swensonl et al, 2004). Adults with ADHD may be identified by using the Adult ADHD Self-Report Scale (ASRS) Symptom Checklist  developed by Kessler in 2005 for WHO. fMRI Functional magnetic resonance imaging or fMRI is a technique that measures the change in blood flow to the brain as associated with neural activity. The popularity of the fMRI is mainly due to its wide availability and low invasiveness. The absence of any kind of radiation reduces concern about complications; and makes it a valuable instrument for research. The fMRI works on the principle that blood flow to the brain is associated with changes in activity. This occurs since the brain does not have reserves of glucose or oxygen; and when activation increases; the neurons require an increased amount of both. Thus, it becomes necessary for blood flow to increase to areas that require a greater amount of glucose and oxygen – the hemodynamic response. This results in increased local ratio of oxyhemoglobin to deoxyhemoglobin; providing a contrast for monitoring Blood-oxygen-level dependence (BOLD). The changes in hemoglobin from the oxygenated to deoxygenated states allow us to trace where the oxygenated blood is highest. The direction of the changes and the intensity of the changes can tell us about the levels of activation and the areas that have been activated. BOLD contrast is higher when oxygenated blood flows into an area; and low when the oxygen levels in the blood run low. The subtle changes that are captured by the fMRI can be tabulated and then statistically analyzed for trends in the data. These studies are still treated with caution though; since the numbers generated are very small; and the interpretations can often be led by the opinions of the researchers. Aim and Hypothesis of the Study Given this understanding; it becomes important to verify the particular areas of the brain that are affected in adults with ADHD. This information will give us an indication of the intervention strategies that can be applied to enhance the life experiences of these individuals. This study attempts to identify the differences in the activation of the pre-frontal cortex in respect to adults with ADHD and normal adults. Understanding the particulars of the differences – if any – in intensity and location of activation may provide valuable information towards understanding the needs of this population. Given that males and females exhibit different behavioral symptoms of Adult ADHD; it was believed that these populations should be tested differently. We base the following hypothesis on the fact that similar results have been seen with children. Thus, we may hypothesize: Male adults with ADHD will show significantly less activation in the pre-frontal cortex as compared to male adults from the normal population. Methods Participants Participants for this experiment in the adults with ADHD group would be selected on the basis of certain criteria: Participants shall be male; and in the age group of 25 to 35 years. All participants should be known to have average intelligence. All participants should be screened and found to have ADHD on the basis of the Adult ADHD Self-Report Scale (ASRS) Symptom Checklist (Kessler, 2005). Participants are eligible for fMRI study. Participants from the normal population should fulfil all the same criteria; except that they should be screened out as not having symptoms of ADHD on the Adult ADHD Self-Report Scale (ASRS) Symptom Checklist. Ideally; there should be a minimum of 15 participants in each group. This number provides a stable number to run the analyses for; and does leave some space for the possibility of participant drop-out. If possible; more participants shall be recruited to each group. Design and Procedure The study will have a between subjects design that compares adults with and without ADHD in the use of working memory. The differences in the neural activation for the two groups shall be examined; with emphasis on the differences in activation in the pre-frontal cortex. The testing session for each participant should take approximately ten minutes. The testing will involve asking the participant to respond to a visual change detection task while the MRI is being conducted. Complete and detailed instructions shall be given; and the participant shall be monitored at all times. It is necessary to use the least invasive techniques in order to protect the participants. The contrast agent shall be selected on the basis that it provides distinct images without posing any threat to the participants. If possible; invasive methods shall be completely avoided. BOLD imaging will be used in order to evaluate the activation of the brain centres as the participant completes the task. The study uses an event related stimulus presentation strategy. Each participant will be required to signal whether they observe a difference in two consecutively presented stimuli. They will be provided with a different key to press to indicate difference and no difference. The participant will be shown the initial picture in each trial; and after a lapse of 5 seconds, they will be shown the target stimulus. Each visual will be presented to the participant for 3 seconds. The participant will be warned before the beginning of each trail. This is particularly to ensure that the ADHD participant is prepared to respond to the stimulus. Statistical analysis of the fMRI results shall be conducted using the appropriate software. Thus; the statistical analysis for the fMRI results may be conducted using a t-test for between participant measures, or an independent t-test. This analysis shall be conducted using the appropriate software that conducts statistical analysis on data collected via fMRI. Stimuli used The experiment will use visual stimuli consisting of 20 trials for each participant. Of these, 10 trials will have a change from initial to target stimulus; while 10 trials will not. Trials will be presented in a randomised manner with care taken to ensure that no more than three consecutive trials have the same right response to avoid response set. Before the test trials start, the participant will be presented with four practice trials of which two will have ‘change’ as the right response and two will have ‘no change’. This experiment uses event related stimulus presentation; which allows for the detection of temporal changes in HRF (hemodynamic response function). The techniques also allows us the evaluate individual trails for distinctive response patterns as they correlate to brain functioning. This helps in the evaluation of different kind of responses saperately. Thus, it becomes possible to evaluate neural functioning for the mistakes differently from the correct responses, if required. The stimuli in each trial will consist of a series of geometric figures in different colours. The participant has to respond only to changes in shape and ignore any change in colour or location of the figures. This is an example of ‘No change’ This is an example of ‘change’. Pre-processing The images for magnitude and phase captured for each trail shall be stored as 4D NIfTI file; which can then be processed using an appropriate software package. Magnitude data will be analysed and compensated for movement in images. The images will then be spatially normalised and the data collected from the movement compensation and normalisation shall be applied to the data of phase images. Spatial smoothing strategies will be applied as required. Discussion The data collected from the Change Detection task will be analyzed to identify which participants showed higher levels of pre-frontal activation while performing the trails. The subtraction method was used in order to estimate the differences between images obtained for the two groups. This method assumes that the data for two groups can be added or subtracted from each other to identify the areas of neural activity that overlap and those that are unique to each group. If required, the data shall be analyzed for differences in the trails which were answered correctly and those that were not for both groups. It is expected that the evaluation of the images captured will indicate a distinct difference between the two groups. The group of normal male adults should show significantly more activity in the Medial frontal regions and in teh prefrontal regions. It is also possible that we may see a significant amount of activity in the parietal regions for the group of individuals with ADHD. A change detection task typically consists of two stimuli that are presented sequentially with a short gap in between.  Luck & Vogel, (1997) have described the process of conducting the change detection task. According to them, the two visuals differ on a single significant factor in a difference condition; and do not differ on that factor in the no difference condition. The aspect that needs to be focused on in this experiment will be the shape of the figures; and this will need to be made clear to the participants before hand in order to reduce confusion and mistakes rising from misinformation. 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