Contralateral

"Contralateral" refers to the anatomical or functional relationship between structures or processes on opposite sides of the body or brain. The term originates from Latin roots: "contra-" meaning "against" or "opposite," and "-lateral" meaning "side."

A classic example of contralateral organisation is the primary motor and primary sensory cortices' representation of the body, where neurons in the right hemisphere primarily control or receive sensory input from the left side of the body, and vice versa^[1].

This contralateral arrangement also applies to the visual system. Each hemisphere of the brain processes visual information from the contralateral (opposite) visual field. For instance, information from the left visual field of both eyes is processed in the right hemisphere and vice versa ^[3].

The process of contralateral control often involves a feature of nervous system architecture called decussation, which is the crossing over of nerve tracts from one side of the body to the other ^[4]. A well-known example of decussation occurs in the medulla oblongata, where the majority of motor fibres from the motor cortex cross over to the other side before continuing down the spinal cord^[5]. This means that motor commands from the left hemisphere of the brain control the right side of the body, and vice versa ^[6].

Decussation also occurs in sensory pathways. For example, most sensory information (such as tactile and temperature information) crosses over to the opposite side at the level of the spinal cord before it ascends to the brain^[8]. This is why a stroke affecting one side of the brain can result in sensory loss or paralysis on the opposite side of the body^[9].

The reasons for contralateral control are still not fully understood, but there are a number of theories:

1. Evolutionary advantage: Some researchers believe this setup might have offered an evolutionary advantage. For instance, contralateral control might improve coordination or reaction times in response to threats or opportunities in the environment^[10].

2. Increased neural connectivity: Contralateral organisation increases the potential for connectivity between the two halves of the brain, which could potentially facilitate more complex cognitive processes^[11].

3. Symmetry breaking: During embryological development, asymmetry in certain biological processes leads to the contralateral setup^[12]. This is somewhat akin to our preference for using one hand (right or left) over the other^[13].

4. Enhanced spatial awareness and coordination: Contralateral control could enhance spatial awareness by making it easier to coordinate movements in three-dimensional space^[14]. For example, reaching across your body to grab an object with your opposite hand requires complex coordination that might be facilitated by contralateral brain organisation^[15].

Broca's and Wernicke's areas, two primary areas of the brain involved in language production and comprehension, are typically located in the left hemisphere for right-handed individuals, and most left-handed individuals as well^[16]. Damage to these areas on the left side of the brain can cause language impairments, but these would not typically result in contralateral symptoms because language processing is not typically a function that is divided between the two sides of the body in the same way that sensory perception and motor control are^[17]. However, right-sided neglect or inattention (neglect of attention to the right side of space) can occur with left hemisphere strokes, as spatial attention networks are typically right-lateralised^[18].