The greater plasticity of cerebral hemispheres, attributed to a large number of cortical neurons, results in milder deficits compared to brainstem or spinal cord lesions of the same size. Patients may be unaware of deficits, especially in complex behaviors like aphasia or apraxia. Right hemisphere lesions often lead to patients being less aware of their deficits.

 

Neurological deficits can be inconsistent due to reduced attention span influenced by factors like time of day, lack of stimuli, or a noisy environment. Aphasia, a multimodal deficit, may present with apparently normal language. Multimodal deficits, such as alexia, should be prioritized for localization.

​

Cortical plasticity, mediated by extensive multisynaptic arrays, is susceptible to metabolic disturbances, affecting neurotransmitter processing. For instance, pneumonia may impair comprehension in mild sensory aphasia.

​

Characteristic features of cortical lesions, like hypoxic laminar necrosis, include seizures and multimodal motor-sensory deficits. Subcortical lesions may cause aphasic syndromes but are usually less pronounced. White matter lesions manifest as weakness, spasticity, visual deficits, "pure" motor syndromes, and urinary incontinence.

​

The output from the brain is ultimately motor, and observations of motor performance are crucial for assessing lesions. Motor performance relies on alertness, cortical attention, perception of sensory stimuli, and the ability to carry out motor sequences.

 

Attention:

• Attention requires a certain level of alertness.

• Inferior parietal lobe and premotor areas plan attentional strategies.

• Cingulate gyrus injects relevance into the attention network.

• Thalamus, especially the pulvinar, and reticular nucleus play key roles in attention.

• Right hemisphere provides attentional mechanism for both sides of the body.

Sensory Inattention:

• Unimodal visual inattention, extinction to double simultaneous stimulation, and spatial neglect may occur.

• Right inferior parietal lobule lesions are apt to cause spatial neglect.

• Lesions disrupting cortico-striatal-nigral-collicular pathways may cause hemispatial neglect.

Global Inattention:

• Inability to concentrate on a task, motor and verbal "impresistance."

• Right hemispheric lesions are more likely to cause motor imperistance.

• Distractibility and perseveration (motor or verbal) may occur.

• Akinetic mutism may result from lesions in the mesial aspect of both hemispheres.

Emotional Disturbances:

• Hypothalamus mediates primitive emotional responses.

• Emotional expression depends on arousal, vegetative functions, memory retrieval, affective response to stimuli, and intact motor systems.

• Lesions in various brain regions lead to specific emotional disturbances.

• Right hemisphere is often associated with positive emotions, left hemisphere with negative emotions.

Memory Disturbances:

• Amnestic syndrome characterized by impaired registration of new information (anterograde amnesia) and variable retrograde amnesia.

• Medial temporal lobes, thalamic nuclei, and connections are critical for memory.

• Korsakoff psychosis may confabulate with mesial thalamic damage.

• Unilateral strokes may impair storage of language-related or nonverbal information.

​

In summary, the plasticity of cerebral hemispheres allows for varied presentations of deficits, and attention, emotional responses, and memory can be selectively affected by lesions in different brain regions.