Реферат: Have We Learned Anything New About The

Have We Learned Anything New About The Functions Of The Frontal Lobe In The Last Five Years? Discuss Essay, Research Paper

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 Teuber 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 mediodorsal

nucleus of the thalamus. Parcellation of the cerebral cortex into functionally

distinctive areas is by no means unanimous (Crespo-Facorro, 1999). 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 (rACiG), caudal anterior

cingulate gyrus (c-AciG), superior cingulate gyrus (SC iG), medial frontal

cortex (MFC), straight gyrus (SG), orbitofrontal cortex (OFC), precentral gyrus

(PCG), superior frontal gyrus (SFG), inferior frontal gyrus (IFG), 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 neucleus, hippocampus and neocortex.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 memorising, 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). 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 (Pucak et al 1996). 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 initialy 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 ?stroob? 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. 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 appered 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

specialistions 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? (pp481)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. 481). Known before how the frontal lobe was functionally

interrelated.This is one example of how little has been learnt in the

last five yearsShallice (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?..

This is defined as an awareness of the likely content of other people?s 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/ ventro

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 couldn?t 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. Bifrontal lesions involving medial regions impaired

performance on the decetion 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. 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 (Pandya 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 studes. 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 pp481)Yes we have leant new things but of function??In 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). Frontal lobe dysfunction

and aggression: Conceptual issues and research findings. 5, 2, 147-157Hensen in shalliceWellman, HM.,? and

Wooley HD.? (1990) From simple desires

to ordinary beliefs: the early development of everyday psychology. Cognition, 35, 245-275Crespo-Facorro,

B., Kim, J., Andreasen, N., O?Leary, D., Wiser, A., Bailey, J., Harris, G.,

Magnotta, V. (1999) Human Frontal Cortex: An MRI-Based Parcellation Method. NeuroImage

10, 5,.Della Sala, S., Gray, C., Spinnler, H., Trivelli, C.

(1998). Frontal Lobe Functioning in Man: The Riddle Revisited Archives

of Clinical Neuropsychology, Vol. 13, No. 8, pp. 663?682,Duncan, J & Owen, A. (2000). Common regions of the human frontal lobe

recruited by diverse cognitive demands. Trends

In Neuroscience. 23, 475-483.McCarthy, R. A., &

Warrington, E.K. (1990). Cognitive neuropsychology: A clinical introduction. London: Academic Press.Stuss, D., Gallup, G.,

Alexander, M. (2001). The frontal lobes are necessary for ?theory of mind?.. Brain, 124, 279-286Shallice, T. (2001).

Editorial: ?Theory of mind and the prefrontal cortex?.. Brain, 124, 279-286.Gallagher, H., Happe,

F., Brunswick, N., Fletcher, P., Frith, U., Frith, C. (2000). Neuroosychologica, 38, 11-21Fuster, J. M. (1997).

The Prefrontal Cortex: Anatomy, Physiology and Neuropsychology of the Frontal

Lobe (Third Edition). Lippincott-Raven. PhiladelphiaStone, VE, Baron-Cohen

S, Knight RT. Frontal lobe contributions to the theory of mind. In Stuss, D.,

Gallup, G., Alexander, M. (2001). The frontal lobes are necessary for ?theory

of mind?.. Brain, 124, 279-286.

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