Alexia without agraphia, also known as pure alexia, is a neurological disorder presenting a striking paradox. Individuals with this condition lose their ability to read, yet their capacity for writing remains intact. This creates a baffling situation where a person can fluently write a sentence and then be entirely unable to read what they have just written. The disorder is a classic example of a “disconnection syndrome,” where distinct brain regions are unable to communicate with each other.
The Core Neuroanatomical Lesions
Alexia without agraphia is linked to damage in two precise locations within the brain. The first injury occurs in the left occipital lobe, impacting the primary visual cortex. This area processes visual information from the right side of one’s field of view. Damage here creates a blind spot in the right visual field, a condition known as right homonymous hemianopia.
The second site of damage is the splenium, the most posterior section of the corpus callosum. The corpus callosum is a massive bundle of nerve fibers that acts as a communication bridge between the brain’s left and right cerebral hemispheres. The splenium is tasked with transferring visual information between the two hemispheres.
These two lesion sites are frequently caused by a single vascular event. A stroke within the territory of the left posterior cerebral artery (PCA) is the most common cause because the PCA supplies blood to both the left occipital lobe and the splenium. A blockage in this artery can produce the combination of brain damage required to induce the syndrome.
The Disconnection Mechanism of Reading
The inability to read in pure alexia stems from these lesions isolating the brain’s language centers from visual information about words. For most right-handed individuals, the hubs for language processing, including reading, are in the left cerebral hemisphere. A region in the left hemisphere known as the angular gyrus is involved in recognizing written words by linking their visual form to their meaning.
Damage to the left visual cortex creates a direct problem. Written text in the right visual field cannot be processed by the left hemisphere because its visual receiving area is destroyed. Patients with this deficit cannot see words on the right side of a page or screen.
An issue also arises with information from the left visual field. This visual data is received and processed by the intact visual cortex in the right hemisphere. Under normal circumstances, this information would be transferred across the splenium of the corpus callosum to the language centers in the left hemisphere.
However, because the splenium is also damaged, this transfer is blocked. The processed visual information from the right hemisphere becomes trapped, unable to reach the left hemisphere’s angular gyrus. The result is that the language centers in the left hemisphere are effectively blind to all written material. The angular gyrus, though undamaged, is isolated from the necessary visual data, leading to the inability to read.
Preservation of Writing and Associated Deficits
The preservation of writing makes this condition distinctive. Writing is orchestrated by different neural pathways that do not depend on the damaged visual structures. Language formulation for writing begins in the left hemisphere’s language centers and is then sent to the motor cortex, which controls the hand movements for writing. These pathways remain completely intact.
This explains why a patient can write a message but be unable to read it moments later. The breakdown occurs when the patient attempts to use their visual system to process the text, as the disconnection prevents the information from reaching the language centers for comprehension.
The brain damage often leads to other associated neurological deficits. The most common is right homonymous hemianopia, the loss of the right visual field, which is a direct consequence of the left visual cortex lesion. Another common issue is color anomia, the inability to name colors.
This is not a problem of color perception, as patients can match colors correctly. It is another disconnection phenomenon, where the perception of color cannot be transmitted to the language centers to retrieve and speak the color’s name.
Modern Diagnostic and Atypical Localization
The diagnosis is confirmed using neuroimaging techniques. Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) scans are effective at visualizing the structural damage. These tools allow neurologists to identify the characteristic lesions in the left occipital lobe and the splenium, providing anatomical evidence for the diagnosis.
While the dual-lesion model is a common explanation, research has revealed variations in the location of the damage. Studies show a single, strategically placed lesion can sometimes cause the syndrome without directly damaging the splenium. This occurs when the lesion is in the left occipitotemporal cortex, affecting a region known as the Visual Word Form Area (VWFA).
The VWFA is a specialized area for the rapid recognition of whole words. A lesion confined to the VWFA or the white matter tracts connecting it to language areas can disconnect visual processing from language comprehension. This demonstrates that the primary factor is the interruption of the pathway between visual perception and linguistic interpretation, which can occur at slightly different points.