15.2. The Left Frontal Lobe
Executive function, planning and proper behavioral choices given specific options are major functions of the left frontal lobe. Broca's aphasia and transcortical motor conduction aphasia occur with BA 44 and superior Broca's lesions. Immediately anterior to BA 44 is the frontal operculum and the area triangularis. Lesions in this area cause a particular lack of facility with writing (out of proportion to hand weakness) and cortical negative variant grammatical mistakes.
Various apraxias occur with frontal lobe lesions. An apraxia is the inability to perform an individual or sequential task with normal motor, sensory and coordinative abilities. A specific motor function is first conceived and is denoted as an engram. The electrophysiological correlate is the cortical negative. Variant these Bereitschaft (readiness) potentials are recorded over the prefrontal and supplementary motor cortices. This is then translated into the required movement. Disorganization of simple or sequential engrams is the core of apraxia. Various apraxias are seen with left frontal lobe lesions. These include ideomotor apraxia (engram encoded in area BA 6) in which there is inability to perform a single command such as saluting or using a comb. A patient with a colossal apraxia (right handed, left brain dominant) is unable to perform a simple task with the left hand. The command is decoded in Wernicke's area and projects to the left premotor area, crosses the corpus callosum (anteriorly) to synapse in the right prefrontal area and then projects to the right precentral gyrus (which controls the left hand). If the patient does not perform as well, motorwise as he should for the consequences of his deficit (a lack of facility of the movement) this is a limb-kinetic apraxia.
Patients with BA 6 premotor lesions may suffer the anterior alien hand syndrome. This syndrome, also known as "inter manual conflict" is evident when one hand undoes the work of the other. A patient may put his glasses on with the right hand only to have them removed with the left. Patients may suffer acute mutism and severe orobuccolingual apraxia with bilateral frontal opercular lesions. These are usually caused by seriatim prefrontal branch strokes of the middle cerebral artery.
Forced thinking, fatigue during the completion of a task, visual neglect to objects in the right hemi-field and directional hypokinesis may all occur with right premotor lesions. Constructional apraxia and perseveration, as well as failure of attentional mechanisms are also characteristic of left frontal deficits.
Left Frontal Lobe
Major Behavioral Deficits
- Poor planning
- Poor behavioral choices
- Poor executive function
- Broca's aphasia
- Acute mutism
- Bilateral frontal opercular lesions – aphemia
- Alien hand (inter manual conflict)
- Callosal apraxia
- Ideomotor apraxia
- Limb kinetic apraxia – paracentral lobule
- Writing apraxia (Exner's area)
- Eyelid opening apraxia
- Constructional apraxia
- Right directional apraxia (hypokinesis)
- Visual neglect of right hemispace
- Imitation-utilization behavior (environmental-dependency)
- Undue fatigue during the completion of a task
- Paratonia (inability to change position once the extremity is placed)
- Gait apraxia
Minor Behavioral Deficits
- Executing sequences
- Inability to follow consecutive hand movements
- Inability to copy facial movements
- Difficulty following multiple commands
- Inability to pair blink responses with voluntary saccades (the loop involved is from the posterior parietal cortex to the second the frontal convolution)
- Echopraxia
- Sitting apraxia
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