Why some visual stimuli don’t reach consciousness

Why are some images consciously perceived while others are not? A team of international researchers has discovered how and when a visual stimulus reaches consciousness. This discovery, published on 22 march 2018 in Science, is of major importance for the development of a prosthesis for the blind.

Much of what we look at, we never actually see. Why this happens is a major challenge for neuroscience. The research group, led by Pieter Roelfsema (Netherlands Institute for Neuroscience, KNAW), investigated the processing of a visual stimulus that is hard to see in the visual and frontal cortex of monkeys. The stimulus was a small, increasingly dim light spot, which flickered on a screen for 50 milliseconds. The stimuli were initially registered in the visual cortex and then sent on to the frontal cortex, where they had to elicit a minimum level of activity before they could reach consciousness. Stimuli not perceived were insufficiently transferred from visual to frontal cortex. As a result, the minimum level of activity in the frontal cortex was not achieved, and the stimuli failed to reach consciousness. The same effects occurred when neurons in the visual cortex were artificially activated with weak electrical currents. The artificially induced visual percepts could only reach consciousness when the activity was efficiently propagated to the higher cortical areas.

The fate of a weak stimulus

The researchers were able to predict the fate of a weak stimulus on the basis of the brain state, even before it appeared. When the brain was in the right state, the information transfer was better, which increased the chances of conscious perception. “This new information about how and when a stimulus reaches the consciousness brings us one step closer to the development of a visual cortical prosthesis based on electrical stimulation of the cortex”, says Pieter Roelfsema.