Greg Detre
Wednesday, 24 May, 2000
Rolls � B&B III
Prefrontal cortex��������������������������������� 13, 15, 124-72
areas���������������������������������������������������������������������� 124
in frontal lobe organisation�������������������� 207-12
possible functions�������������������������������� 208-9
the motor cortex is specialised for the control of the movements of limb and face (�manipulative movements�).
it is not essential for the control of non-learned movements, such as reaching and walking
although motor cortex is active during the normal performance of such movements, subcortical mechanisms have some control over them even in the absence of motor cortex
motor cortex provides a mechanism for the execution of the fine behavioural variants which are selected in voluntary action
the ability to perform discrete movements depends on the direct connections from motor cortex to the motor neurons in the spinal cord
the premotor areas can influence movement via their projections to the motor cortex and subcortical projections through the pyramidal tract
in truth, the premotor areas are influenced by projections from the parietal areas 5 and 7b and by subcortical projections from the thalamus
there are several indications that the lateral premotor cortex plays a role in the selection of manipulative movements.
1. PET scans show that there is greater activation in this area when subjects must select between movements rather than simply repeat the same movement
2. LPC lesions in monkeys and patients impair the learning and relearning of conditional tasks on which the subject must learn to select between movements on the basis of external cues. conditional motor tasks of this sort are all �recall� tasks in the sense that the subject must retrieve the movement that is appropriate to the context
3. finally, there are many cells in the lateral premotor area that increase their activity when a monkey is preparing to move, and this increase is selective depending on the movement to be made. many of the cells also show a selective increase in their activity during the time in which monkeys learn visual conditional motor tasks
Like the lateral premotor cortex, the medial premotor cortex plays a role in the selection of movement;
but the two areas differ in their specialisation. The lateral premotor cortex makes the greater contribution when the subject uses external cues to direct the movements, and the medial premotor cortex when no such cues are available. However, the specialisation is a matter of degree � it is by no means total.
The evidence comes from the analysis of the effects of lesions, from PET scanning and from unit recording. Considering first tasks with no external cues, monkeys with MPC but not LPC lesions are severely impaired at performing self-paced arm movements, and at relearning motor sequences. This contrasts with the pattern of results for tasks on which performance is directed by external cues. Monkeys� with LPC but not MPC lesions are impaired at relearning a visual conditional motor task, and monkeys with MPC lesions are only slightly impaired at making arm movements cued by an external signal.
In PET scanning experiments, the medial premotor cortex is activated when subjects perform a well-rehearsed sequence from memory, but there is more activation in the lateral premotor cortex when subjects must rely on external feedback to learn new sequences.
Recordings from single cells in monkeys show the same pattern. More cells are active in the medial premotor cortex when repetitive movements are self-paced, and in the lateral premotor cortex when these movements are externally triggered. Similarly, more cells are active in the medial premotor cortex when the animals perform motor sequences from memory and in the lateral premotor cortex when they learn new sequences as directed by visual cues.
Motor cortex governs movements of the limbs and face ,and the frontal eye fields the movements of the eyes. Just as the selection of limb and face movements depends on the premotor mechanisms of area 6, so the selection of eye movments depends on the premotor mechanims of the rest of area 8. Both monkeys and patients with lesions in lateral area 8 are poor at directing their eye on the basis of a learned context.
The selection of eye movements is to be described in terms of the objects of the search, the things or locations that we look for. Whereas area 6 selects our limb movements, area 8 selects things in the outside world. It is for this reason that area 6 receives proprioceptive information, and area 8 information from the external senses.
Area 8 forms part of the lateral premotor cortex and the dorsomedial eye field part of the medial (supplementary) motor cortex. It is proposed that area 8 is specialised for the selection of eye movements made when targets have been presented and the dorsomedial eye field for the selection of eye movements that are not determined by visual targets.
Areas 9 and 46 receive their main input from the parietal lobe which processes information about the animal itself, and about the space in which it moves and manipulates things.
Monkeys with lesions in area 46 fail to learn delayed response tasks. These are conditional tasks on which the animal must choose between locations on the basis of information in working memory. These impairments can be demonstrated on an oculomotor version of the DR task on which monkeys must direct their eye movements on the basis of locations in which they recently saw a spot of light. During the delay on this task, many cells in area 46 change their activity selectively according to the location of the target.
Monkeys with lesions in area 9 and 46 are impaired at selecting between objects on the basis of their past responses, and also at generating a series of actions. In PET scanning expeirments with human subjects, the dorsal prefrontal cortex is activated when the subjects generate a series of actions at will. In patients there is also a relation between psychomotor retardation and a decrease in regional cerebral blood flow the dorsal prefrontal cortex. This suggests a role for the dorsal prefrontal cortex in generating actions.
The ventral prefrontal cortex receives a multimodal input from the temporal lobe. Monkeys are impaired at learning what response to make, irrespective of the modality of the cue. There is also evidence suggesting that it may not be essential that there is a delay between the presentation of the cue and the opportunity to respond; however, this evidence is not conclusive. It is argued that the ventral prefrontal cortex selects the goal � e.g. an object � given the current context.
When monkeys learn visual concurrent discriminations, they can solve the problems by learning only about the associations between the stimuli and reward. Monkeys with ventral prefrontal lesions can learn such problems at a normal rate. Furthermore, when human subjects make perceptual judgements, there is no activation in the prefrontal cortex.
The ventral prefrontal cortex is heavily interconnected with the amygdala. Monkeys will learn to deliver rewarding stimulation to the orbital cortex or to deliver rewarding drugs. It is argued that the connections between the ventral prefrontal cortex and the amygdala are involved in the process by which responses are selected on the basis of their success.
The prefrontal cortex can influence the premotor areas by cortico-cortical connections and projections through the basal ganglia. Monkeys are severely impaired at relearning a visual conditional motor task if lesions are placed in the ventral thalamus so as to disrupt the influence of the basal ganglia on frontal cortex.
There are cells in the basal ganglia that fire well before movements when monkeys are repeating a movement from memory or deciding what movement to make.
The premotor areas also interact with the cerebellum via the ventral thalamus. Patients with cerebellar pathology are slow to learn conditional tasks. There is also activation of the lateral cerebellar cortex when subjects habitually produce the same words in response to cue words.
There are parallel cortical motor systems for movements of the limbs and eyes. There are also two premotor systems, both with a map of the whole body. These play a role in the selection of movements, and differ in their reliance on information about the external context.
The prefrontal cortex as a whole selects actions when the subject must make a new decision as to what to do. It is not yet clear whether prefrontal cortex is only engaged when there is no external cue at the time of the response.
If new decisions are required when a task is learned, prefrontal cortex is activated. But if the task can later be run off automatically, prefrontal cortex need no longer be engaged.
The exact role played by the basal ganglia is not clear. In animals without neocortex, such as amphibians, the basal ganglia form the telencephalic mechanism for determining responses. In mammals, the frontal cortical and basal ganglia are closely interrelated via a system of loops. There are suggestions that the ventral striatum may play a role in the process by which the probability of a response is altered as a result of positive or negative outcomes.
As in monkeys, the prefrontal cortex is also crtiically involved in the process by which the human brian generates and selects actions. Human beings can select between ideas, and the prefrontal cortex has been elaborated to allow the selection of mental responses.
Human beings can also plan future actions and select between them by mental trial and error. The consequences of this development are far-reaching, because it means that human beings can set themselves goals other than simply promoting their genes in the next generation. Human beings are capable of �voluntary� action in the most restrictive sense.
The frontal lobe selects between verbal responses. There are two premotor areas. Broca�s area is part of the lateral premotor cortex and the dorsomedial speech area part of the medial premotor cortex.
It is suggested that Broca�s area is critically involved when a word must be retrieved on the basis of an object, word, letter or meaning, and that the dorsomedial speech area is critically involved when words must be retrieved without external cues.
Strokes involving Broca�s area and the ventral prefrontal cortex impair the ability to use function words and morphological markers. It is claimed that the specialised mechanisms of the left hemisphere may have developed from an area in our primate ancestors that had related functions.
Several processes could be involved in the evolution of specialised mechanisms of the left frontal cortex of the human brain. The inputs and outputs could be altered, areas could be expanded, or areas could be differentiated into more specialised sub-areas.