Visual pathways in the brain, and what happens when they break

Each of our senses follows the same basic neural pathway to transmit information to the brain. First, energy is converted into a neural impulse. Then, those impulses are transmitted along receptor neurons that transduce the information for the brain. Finally, the receptors send the signal to the cerebral cortex which interprets the information. The visual pathway begins with light rays passing through the cornea and centering at the fovea and back of the retina. When the light rays focus at the back of the eye, they are processed by specialized cells known as rods and cones. These photoreceptors transduce the photons into action potentials to be passed along the optic nerve to eventually reach the thalamus and then the cerebral cortex.

Once these impulses reach the occipital lobe of the cerebral cortex, the brain must manage and sort the information. Each eye sees a different perspective of the image, and thus sends different information along its optic nerve. The cortex is responsible for creating a single, stable image from this data, and then processing it. Visual analysis occurs through two principal paths – the dorsal “where” pathway, and the ventral “what” pathway. The former pathway runs to the parietal lobe of the brain, while the latter leads to the temporal lobe.

When either of these pathways fails, there are consequences for the way we perceive our world. One such disorder is known as neglect syndrome. When asked to redraw a picture, patients copy only one side of the model, while ignoring the other part. On the other hand, patients with visual agnosia can draw the whole object, but they cannot recognize what they have created. Though they see the image, it does not translate into an idea in their minds.

Two patients with neural damage, Angela and Zelda, were asked to perform a series of tasks designed to test the visual pathways in the brain. In the first task (Figure 1), each was shown pairs of faces and asked to determine whether they were identical. In the second task (Figure 2), Angela and Zelda were asked to trace a series of pictures onto transparent paper.

Figure 2. Tracing test. Patients were asked to trace over pictures on transparent paper. First set of trials used pictures of known objects, second trial asked patients to trace abstract lines. Data shown as percentage traced in $3$‍ minutes.

Data adapted from:

Goodale, M. (1994). Separate neural pathways for the visual analysis of object shape in perception and prehension. Current Biology, 604-610

A patient was asked to copy a picture of a clock and produced the following image: Doctors immediately suspected a lesion in the right hemisphere. What does this imply about optic nerves organization?