Study Uncovers Mechanism Behind Visual Impairment in Traumatic Brain Injury

A healthy neuron.
A healthy neuron. Credit: NIH

Traumatic brain injury can lead to long-term visual impairment, which researchers have found is caused by a dramatic drop in the number of neurons in the visual cortex. Their findings were published in Communications Biology.

Traumatic brain injury (TBI) is associated with mechanical brain damage and a wide range of neuronal abnormalities.  Injuries to the posterior occipital cortex are common in humans, and can result in visual impairment. Up to 75% of current or former soldiers live with permanent visual dysfunction or cortical blindness. 

The human brain possesses surprising neuroplasticity, which allows other areas of the brain to take over the functions of a damaged area.

Such neuroplasticity is also characteristic of the sensory areas of the visual cortex, which is final component of the visual pathway, responsible for receiving and processing visual impressions. The primary visual cortex (V1) is reached by the nerve fibres of the optic radiation, which carry nerve impulses from the retinas of both eyes.

Until now, scientists knew little about the effects of TBI on long-term visual circuit function. Using mice, a team of researchers examined how neurons respond to visual stimuli two weeks and three months after mild injury to the primary visual cortex (V1). V1 neurons normally show sensitivity to different features of a visual stimulus, such as colour or direction of movement. The preprocessed data is transmitted to subsequent areas of the visual cortex. This study showed that although the primary visual cortex remained largely intact after the brain injury, there was a 35% reduction in the number of neurons. This loss largely affected inhibitory neurons rather than excitatory neurons, which inhibit or stimulate action in the target cells, respectively.

After TBI, fewer than half of the isolated neurons were sensitive to visual stimuli (32% at two weeks after injury; 49% at three months after the event), compared with 90% of V1 cells in the control group. Up to a threefold decrease in neuronal activity was seen after the brain injury, and the cells themselves had worse spatial orientation. The overall results mean that even minor, superficial brain injuries cause long-term impairment in the way visual stimuli are perceived, persisting several months after the event.

Such a deeper understanding of the functional impairments in damaged visual cortex could provide a basis for developing circuit-level therapies for visual cortex damage.

Source: Institute of Physical Chemistry of the Polish Academy of Sciences