How does the brain process vision?

The process of vision in the brain begins when a photon of light enters the eye and is detected by the retina, a sheet of neural tissue at the rear of the eye. The retina captures the light, converts it into electrical signals, and processes those signals to send visual information to the brain. There, the brain assembles the patterns of activity into a conscious visual experience. Interestingly, even without input from the periphery, the brain can create visual experiences, such as those during dreaming, indicating that seeing is primarily a brain phenomenon 1.

In the retina, photoreceptor cells transform light energy into electrical neural signals. These photoreceptors, together with the inner neural layers of the retina, extract varied features from the visual world through different processing operations, which include dozens of distinct cell types. Among these cell types, retinal ganglion cells have a crucial role, as they are responsible for transferring signals from the retina to the brain so that the process of vision can begin. These retinal ganglion cells represent the full visual scene but filter for different features such as spatial detail, motion, and color information. The retina provides approximately 20 different representations of the visual world, much like different photoshopped images or movies, each highlighting different aspects of the scene. The brain then integrates these diverse cues into a cohesive visual perception 2 3 4.

Moreover, even though information relayed from the retina to the brain is a 2D flat image, the brain can sense depth by considering factors like relative size and motion speed. For instance, nearer objects appear larger and move faster compared to distant objects. The brain also computes the slight differences in location where the image lands on each of our two eyes, using geometry and trigonometry to gauge the depth of objects. Astoundingly, vision utilizes about 40 to 50% of the brain's total capacity, highlighting its significance among the senses 5 6.