Professional and Linguistic Assessments Board (PLAB) Practice Exam

A lesion in the thalamus primarily affects which type of sensation?

1. Mixed sensory and motor

2. Pure motor function

3. Pure sensory

4. Reflex actions

The correct answer is: Pure sensory

A lesion in the thalamus primarily affects pure sensory function because the thalamus serves as the main relay station for sensory information traveling to the cerebral cortex. It processes and transmits sensory inputs such as touch, pain, temperature, and proprioception from the body to the appropriate areas in the brain. Damage to the thalamus can lead to various sensory deficits depending upon which specific thalamic nuclei are affected. To clarify the context of the other options: mixed sensory and motor functions refer to areas of the brain where both types of signals are integrated, and a lesion in the thalamus does not primarily impact motor control directly, as motor pathways mostly travel through different areas, including the motor cortex and basal ganglia. Pure motor function is not significantly affected by thalamic lesions since motor pathways are largely independent of thalamic processes. Reflex actions, which are automatic responses, are mediated by spinal pathways and the brainstem rather than the thalamus, thus they are not directly impacted by lesions in this area. Understanding the thalamus's role in sensory pathways clarifies why pure sensory sensation is the primary area affected by lesions in this brain region.