Have We Learned Anything New About The Functions Of The Frontal Lobe In The Last Five Years Discuss The frontal lobe is thought to be the latest area of the brain to develop and is largest in humans. It is therefore suggested that the area plays a key role in differentiating humans from other hominids (Crespo-Facorro et al 1999; Fuster, 1997).
For well over a century research has investigated the functioning of the frontal region of the human brain (Della Sala et al, 1998).
In 1964 Tuber wrote of the riddle of frontal lobe function in man, today it is believed that this riddle is still yet to be solved (Darling et al, in press).
Confusions arise as in the first instance, as definitions of the frontal lobe are not universally clear. In primates the prefrontal cortex is used for the frontal lobe whilst implicitly excluding the motor cortex and premotor cortex (Fuster, 1997).
The boundaries of the frontal lobe are therefore traced in various ways, depending on the methods and criterion for definition. Fuster defined the prefrontal cortex as being the rostral part of the brain, the part of the cortex that receives fibres from the medio dorsal nucleus of the thalamus. Parcellation of the cerebral cortex into functionally distinctive areas is by no means unanimous (Crespo-Facorro, 1999).
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However some broad general areas have been discovered. These have been divided into the motor, premotor and prefrontal areas. The premotor area may also include the supplementary motor area on the lateral and medial surfaces of the cortex.
The third are is the prefrontal cortex has many subdivisions within itself. These are classified as the dorsolateral prefrontal cortex, the inferior prefrontal cortex (including the orbital frontal cortex) and the medial frontal cortex. A recent MRI based parcellation method (Crespo-Facorro, 1999) used topographical features of the frontal cortex to produce a map that subdivides the area into 11 subregions. These are shown in Figure 1. The areas include: supplementary motor area (SMA), rostral anterior cingulate gyrus (racing), caudal anterior cingulate gyrus (c-A ciG), superior cingulate gyrus (SC iG), medial frontal cortex (MFC), straight gyrus (SG), orbitofrontal cortex (OFC), pre central gyrus (PCG), superior frontal gyrus (SFG), inferior frontal gyrus (IF), and Middle frontal gyrus (MFG).
The frontal lobe is a large and highly differentiated region of the brain that is reciprocally connected to other cortical and subcortical brain areas.
The prefrontal cortex is the only neo cortical region that directly projects to the hypothalamus (Fuster, 1997).
Different sub areas have different connections. For example the orbital prefrontal cortex is connected to the medial thalamus, hypothalamus, ventromedial caudate and amygdala. The dorsolateral prefrontal cortex is connected to the lateral thalamus, dorsal caudate nucleus, hippocampus and neo cortex. The question of the functions of the frontal lobe are best summarise by Fuster (1997, pp. 4).
The precise nature of apparently multiple functions of the prefrontal cortex is still unclear and inevitably the reviewer of the subject is obliged to compile and attempt to relate large numbers of diverse and seemingly unrelated facts However despite this wall of problems some general themes have emerged concerning the frontal lobe. In the last 20 years or so, evidence has converged upon the view that frontal regions of the brain rather than themselves implementing specific operations such as memorizing, learning or reasoning, are concerned with the deployment and co-ordination of such functions. According to Fuster, the frontal lobe itself does nothing but coordinate with other cortices. It is only with regard to the commonality of cognitive functions at the service of assorted actions that the prefrontal cortex may be considered functionally whole The frontal lobes have come to be viewed as having an executive function. Many frontal lobe tests such as the Wisconsin Card Sorting Test and verbal fluency test have supported the idea of an executive. However the idea of there being a single executive has recently been questioned, for example Burgess (1997) has argued for the fractionation of the executive into multiple components, suggesting that there may be distinct executives for verbal and spatial materials (Della Sala, 2000).
The Research paper on The Relationship Between Dysfunction of the Prefrontal Cortex and Antisocial Behaviour
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McCarthy & Warrington, 1990) found that lesioned patients can be impaired on one executive test but not on others. For the purposes of this essay I have chosen two distinct research articles. One (Duncan and Owen, 2000) attempts to tackle the general role of the frontal lobes in cognition. The second (Stuss, 2001) examines how a specific area of the frontal lobe has been implicated in the ability to infer mental states in others. Both these articles examine primarily the prefrontal cortex. I have chosen the study by Duncan and Owen (2000) because it offers broad insight into what has been learnt over the last five years.
The importance of this paper is that from it describes the problems and questions that researchers have come across in attempting to clearly define the functions of the frontal lobe. Duncan & Owen (2000) believe that there must be some regional specialisation in a brain structure as large and complex as the prefrontal cortex. They point out that unfortunately there is only modest evidence for this. The problem is that any small region of the frontal cortex is connected not only to immediate surrounding regions, but also networks of small, structured patches of cortex that are widely spread through the frontal lobe (Puca k et al 1996).
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They theorise that this connectivity may suggest functional modules.
These modules rather than consisting of specific regions may consist of widely distributed parts. They point out that such specialisation may have not been shown in the past due to the use of course level resolutions used in such studies not being able to pick up such a distribution of modules. They use this theory to show how dividing prefrontal functions into components has been hindered in past research. They point out that this over generality in current conceptions of functions such as executive control, and working memory lead to few strong testable predictions.
It is shown that recent functional imaging techniques have indicated a regional differentiation in the prefrontal cortex. However this regional specificity in cognitive functions appears to take the form of co-recruitment of the same areas rather than task dependent regional differentiation. In this way the areas of the mid-dorsolateral (areas within and surrounding the middle and posterior parts of the inferior frontal sulcus), mid-ventrolateral (areas dorsal and anterior to the Sylvian fissure) and dorsal anterior cingulate areas can be seen to form a networked module of prefrontal regions recruited to solve diverse cognitive problems. After an initial study finding such results, this authors recruited studies that focused on the purest possible manipulations of tightly defined demands (pp. 477).
Using a strict inclusion criterion, five types of studies were included for analysis.
These included response conflict. Frontal executive functions is said to have a role in suppression of inappropriate responses. Included in this was the aforementioned str oob test. Secondly task novelty was investigated. It is suggested that the frontal executive functions are especially important in early intentional learning rather than later automatic skills. The studies involved the comparison of initial learning of unfamiliar tasks with the later well practiced performance.
Third and fourth were working memory tasks. As working memory is a major theme in current accounts of frontal lobe funciton (pp. 477).
These study types were divided between looking at the working memory in terms of number of elements and in terms of delay before recall. The final type of study included was perceptual difficulty. Perceptual demand has not been conventionally associated with executive or working memory and were included as a comparison to more standard frontal tasks.
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These included studies of stimulus degradation and of viewpoint convention. The results show tight definitions of the activation regions. Principally on the medial surface, activations were almost entirely restricted to the dorsal part of the anterior cingulate. Other prominate clusters appeared in the mid-dorsolateral region in both hemispheres and the mid-ventrolateral regions especially in the right hemisphere. Importantly there was a similarity of activation for different demands. According to Duncan and Owen All five demands are associated with a similar pattern of activations in the dorsal anterior cingulate and in both mid-dorsolateral and mid-ventrolateral regions (pp.
479) whatever the functions of these regions, they seem to be recruited by modest increases in demands as diverse as response selection, working memory maintenance and stimulus recognition This data is supported by the fact that each individual experiment showed the full pattern of joint activity I the given regions. However the authors do not rule out the possibility of finer specialisations within the network. The authors state that this may be the case if finer examination was used, for example single neurone analysis. They also theorise that specialisation may be of degree, the implication being that broadly distributed frontal neurones have some relevance to any given activity, but from one activity to another these neurones may have somewhat different peaks. Some finer specialisation cued recall verses free recall. Have we learned anything new from such as study indeed the data confirms that the understanding of Prefrontal functions is a difficult problem (pp 481) Duncan & Owen claim it is very hard to be precise about the function of a region when that region is important in such a diversity of behaviour (pp.
Known before how the frontal lobe was functionally interrelated. This is one example of how little has been learnt in the last five years Shallice (2001) believes that despite findings that a large number of different subprocesses are frontally localised, this has not lead to much closure on the nature of the individual processes involved. The problems as has already been mentioned is the subprocesses involved may be too abstract to map onto perceptual output. Also tasks which activate prefrontal regions may involve a number of subprocesses therefore making it hard to observe completions of stages in normal performance. One area of recent work is that involving theory of mind.
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This is defined as an awareness of the likely content of other peoples minds (Wellman and Wooley, 1990).
In the past the right hemisphere damage has been associated with actions that require inference or attribution (McDonald, 1993).
The frontal lobes have been shown to have a role in cognitive functions as well as social behaviour, personality, memories and self-awareness. Stuss et al (2001) mention one previous study that directly implicate the frontal lobes in the theory of mind. This study by Stone et al (1998) used lesioned patients. Past research has shown how damage to the left or right orbitofrontal/ vento medial areas consistently caused personality changes.
The authors also mention functional imaging data that has found the left medial frontal lobe to be active in theory of mind tasks. (Fletcher et al. , 1995).
They point out that while imaging data shows what areas are involved, they do not show which areas are necessary for the theory of mind. The paper used two main tasks. Both required patients to make inferences about the location of an object they couldnt see based on the direction the experimenter was pointing to.
The First involved verbal perspective taking. In this experiment patients had two people pointing to the location of a hidden object, only one of which could actually see the location of this object. Therefore they had to infer position from this person. It was found that frontal patients produced a much higher error rate on this task. It was also suggested that the right frontal lobe was the most critical region. The second task involved deception.
For this the hidden object had two possible positions. This time, the one experimenter always pointed to the wrong position. This time there was a striking right medial prefrontal difference between impaired and unimpaired patients. Bi frontal lesions involving medial regions impaired performance on the deception tas. The cognitive processes of the frontal regions are likely to play a network role in metarepesentation. The impairment in perspective taking did not appear to be a direct consequence of such cognitive deficits.
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The authors point out that cognitive features such as working memory and attention were controlled for. They also point out that the ventral medial frontal regions may be so important because connections with the amygdala and other limbic structures give them a key role in the neural network of behavioural modulation based on emotions and drives (Panda and Yeterian, 1996).
Further evidence for the importance of the frontal lobes in theory of mind is from functional imaging studies. Frith and Frith (1999) conducted a meta-analysis of such studies.
An updated version of this is shown in figure 1. Here the medial prefrontal cortex in particular the paracingulate sulcus has been shown to be involved in reports of mental states. Indeed in one of these studies, Gallagher et al (2000) the paracingulate cortex was the only region activated in both story and cartoon comprehension theory of mind tasks. This implies that the ability to attribute mental state is independent of modality with the medial frontal cortex. Within the last five years we have discovered that the frontal lobe is involved to some extent in wide reaching parts of behaviour. Ten to fifteen years ago there was little knowledge of the functions of the functions of the subregions of the human prefrontal cortex (Shallice, 2001; Fuster 1989).
With this time many activities have been found to be frontally localised. Fuster (1997) wrote that the prefrontal cortex was a doer as the posterior cortex is a sensor. In the last five years more and more areas of activation have been discovered. For example episodic memory, humour, aggression, TOM, (Henson et al, 1999; Stuss, xxxx, Hawkins & Trobst, 2000; Stuss et al 2001) have now been associated with the frontal lobe in the last five years. Undoubtedly in the next five years, more aspects of emotion and behaviour will be associated to some extent with the frontal lobes.
For example it is entirely plausible that love may be associated with the frontal cortex For example following their topographical MRI parcellation of the frontal lobes, Crespo-Facorro et al (2000) have implicated regional frontal abnormalities in schizophrenia. These abnormalities refer to cortical surface size abnormalities in the right straight gyrus and left orbitofrontal cortex. Have we learned anything new from such as study indeed the data confirms that the understanding of Prefrontal functions is a difficult problem and it is very hard to be precise about the function of a region when that region is important in such a diversity of behaviour (Duncan and Owen, 2000 pp 481) Yes we have leant new things but of functioning terms of cognition and emotion, it is clear that there are functionally separate subregions. Defining these subregions has been linked with function. However such is the Riddle of the frontal lobe that advancements within the last five years have simply added more to the riddle and we are only slightly nearer the answer of how the frontal lobe functions. We now know that certain emotional and various cognitive features are frontally clustered.
It is possible in cognition that certain subregions function as a network with other frontal regions Hawkins, K & Trobst, K (2000).
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