Greg Detre
Wednesday, 24 May, 2000
Rolls � B&B III
Neuroimaging
branching study in Nature
Kolb & Whishaw � ch 10
Passingham
Fuster
prefrontal lobotomies, pioneered by Moniz (Moniz, 1936; Fulton, 1951) � argued that anxiety, irrational fears and emotional hyperexcitabilty in humans might be treated by damage to the frontal lobes
widespread use of this procedure � although irrational anxiety or emotional outbursts were sometimes controlled � but intellectual deficits and other side effects were often apparent (Rylander, 1948; Valenstein, 1974)
still had pain, but it no longer bothered them (Freeman & Watts, 1950; Melzack & Wall, 1996)
Using imaging technology, scientists from the National Institute of Neurological Disorders and Stroke (NINDS) found that a specific type of multitasking behavior, called branching, can be mapped to a certain region of the brain that is especially well developed in humans compared to other primates. The study will appear in the May 13, 1999, issue of the journal Nature.1
"The results of this study suggest that the anterior prefrontal cortex, the area of the brain that is most developed in humans, mediates the ability to depart temporarily from a main task in order to explore alternative tasks before returning to the main task at the departed point," says Jordan Grafman, Ph.D., Chief of the Cognitive Neuroscience Section at the NINDS and a co-author of the study.
The investigators used functional magnetic resonance imaging (fMRI), which measures changes in blood flow to the brain, to view the brains of volunteers while they performed branching tasks. The region of the brain that is involved in multitasking is called the fronto-polar prefrontal cortex (FPPC).
Tasks performed by the volunteers involved exercises to test working memory, attentional focus, and a combination of the two. All of the subjects, who were healthy, normal volunteers, participated in all of the task groups. The task groups consisted of a control task, a delayed-response task, a dual-task, and a branching conditions task. Dual-task involves changing focus between alternative goals successively. The investigators predicted that subject performance on the individual delayed-response task and dual-task conditions would not activate the FPPC. They did predict that the branching task which involves problem solving and planning would stimulate activity in the FPPC. According to the fMRI data, their predictions were correct. The FPPC was activated only during those tasks that involved an interaction between working memory and attentional focus decisions.
The FPPC is the region of the brain that controls complex problem solving and is especially well developed in humans as compared to other primates. The study showed that the FPPC selectively mediates the human ability to multi-task.
Functional magnetic resonance imaging (fMRI) was used to examine the role of the prefrontal cortex (PFC) in both long-term memory (LTM) encoding and working memory (WM) tasks involving a variety of material types (words, faces, and pictures). Encoding was studied in a task requiring intentional memorization of items for a later recognition test. WM was studied in the two-back condition of the n-back task. Bilateral PFC in the inferior frontal gyrus (IFG) was found to be jointly activated in both encoding and WM. This region also showed material-specific lateralization in both tasks, with the left hemisphere more active for words and the right hemisphere more active for faces. PFC regions were also found that were selective to either encoding or WM. Right dorsolateral PFC was selectively activated during WM, but showed no material-specificity, while left anterior PFC was selectively activated during encoding of faces and pictures. Activity in medial temporal lobe was also observed, with the left hemisphere engaged by both memory tasks, and the right hemisphere showing significant activity only during encoding. The finding of PFC regions jointly activated during both encoding and working memory tasks suggests that these regions may subserve cognitive processes important for both short-term active maintenance and long-term memorization. Conversely, the finding of selective activation in specific PFC regions and medial temporal lobe suggests that these brain areas may be functionally specialized. Moreover, the results indicate that a complete characterization of the cognitive functions performed by PFC and other brain areas will be best served by integrating findings across multiple memory domains.
Previous research has identified the prefrontal cortex (PFC) as a brain region that is critical for cognitive control. Currently, theorists remain divided about whether to view the PFC as primarily a coordinative, mnemonic, or inhibitory structure. A theory is presented that attempts to resolve some of the apparent conflicts between the predominant views on PFC control functions. In this theory, PFC is proposed to actively maintain representations of context information. These maintained representations provide a mechanism of control by serving as a top-down bias on the local competitive interactions that occur during processing. As such, it is suggested that PFC performs both mnemonic and inhibitory functions in the service of control, and that each is preferentially observable under different task situations. A series of behavioral, computational, and neuroimaging studies are presented that demonstrates how this theory can account for a wide range of data associated with performance of a simple cognitive control paradigm.
Primates are unique among mammals in possessing a region of
dorsolateral prefrontal cortex with a well-developed internal granular layer.
This region is commonly associated with higher cognitive functions. Despite the
histological distinctiveness of primate dorsolateral prefrontal cortex, the
work of Rose, Woolsey, and Akert produced a broad consensus among neuroscientists
that homologues of primate granular frontal cortex exist in nonprimates, and
can be recognized by their dense innervation from the mediodorsal thalamic
nucleus (MD). Additional characteristics have come to be identified with
dorsolateral prefrontal cortex, including rich dopaminergic innervation and
involvement in spatial delayed-reaction tasks. However, recent studies reveal
that these characteristics are not distinctive of the dorsolateral prefrontal
region in primates: MD and dopaminergic projections are widespread in the
frontal lobe, and medial and orbital frontal areas may play a role in delay
tasks. A re-evaluation of rat frontal cortex suggests that the medial frontal
cortex, usually considered to homologous to the dorsolateral prefrontal cortex
of primates, actually consists of cortex homologous to primate premotor and
anterior cingulate cortex. The lateral MD-projection cortex of rats resembles
portions of primate orbital cortex. If prefrontal cortex is construed broadly
enough to include orbital and cingulate cortex, rats can be said to have
prefrontal cortex. However, they evidently lack homologues of the dorsolateral
prefrontal areas of primates. This assessment suggests that rats probably do
not provide useful models of human dorsolateral frontal lobe function and
dysfunction, although they might prove valuable for understanding other regions
of frontal cortex.
what�s the difference between prefrontal and frontal?
where is the orbitofrontal � what�s it normally called?
is a lobe solid or cortical (i.e. surface)?
= surface
the visual regions of TE