Conscious unity, split perception

We take it for granted that our entire brain only produces one conscious agent, despite the fact that the brain actually consists of many different, more or less independent modules. But how is this possible?

The classic answer to this riddle is cortical connectivity. Separate brain regions only give rise to one conscious agent because the different parts continuously exchange massive amounts of information. This serves as the cornerstone of two current leading theories on consciousness: the Global Workspace Theory, and the Information Integration Theory. Both theories assert that once cortical connectivity between brain parts breaks down, the brain no longer gives rise to only one conscious agent. On the contrary, the theories assert that in such a scenario, each part would create an independent conscious agent.

This is an interesting philosophical thought experiment, but obviously it is hard to conduct experiments in real life where cortical connectivity is severely disrupted just to test our ideas about consciousness. However, starting in the 1940s, some patients suffering from intractable epileptic seizures have been treated by surgically severing the corpus callosum. As a result, communication between the left and the right cerebral hemisphere ground to a halt (and seizures had less chance to propagate through the brain). These split-brain patients provide the ideal testing ground for the role of cortical connectivity in consciousness. If cortical connectivity is the key, then conscious unity should break down in split-brain patients, resulting in two independent conscious agents (one per cerebral hemisphere). Indeed, numerous reports have confirmed this. Although the patients feel normal, and behave normally in social settings, controlled lab tests reveal the following: when an image is presented to the left visual field, the patient indicates verbally, and with his right hand, that he saw nothing. Yet, with his left hand he indicates that he did see the object! Conversely, if a stimulus appears in his right visual field, he will indicate awareness of this stimulus when he responds verbally or with his right, yet with his left hand he will report that he saw nothing. This exactly fits the notion that in a split-brain patient the two separated hemispheres each become an independent conscious agent. The left hemisphere perceives the right visual field, controls language and the right side of the body. The right hemisphere experiences the left visual field and controls the left hand. This, and other discoveries on split-brain patients, earned Roger Sperry the Nobel Prize in Medicine in 1981.

Thus, case closed. The two major theories of consciousness are correct: Without cortical connectivity, different brain parts can no longer create one conscious agent. Instead each part creates its own independent conscious agent.

There is only one slight problem: there are split-brain patients who do not show this pattern of results. Pinto and colleagues re-investigated the fundamental question of conscious unity in split-brain patients. They noticed that even in the existing literature on split-brain patients, the results are much more complicated than the clear-cut picture presented in reviews and textbooks. Moreover, no research had quantified exactly how much the location of stimulus presentation (and thus the hemisphere with which the stimulus was perceived) influenced the ability of the split brain patient to respond with their left hand, right hand, or verbally. The existing literature only stated, based on qualitative observations that split-brain patients indicated awareness with the left hand only to the left visual field, and with the right hand/verbally to the right visual field. Based on these observations, it is impossible to determine whether this effect was absolute, indicating the presence of two independent conscious agents, or a large but not absolute effect, which would indicate that separating cortical connections does not result in completely independent conscious agents.

Pinto and colleagues investigated this question in two split-brain patients. To their surprise, the canonical claims were not replicated. In both patients, across many tasks, there was no response type to visual field interaction. Both patients indicated awareness of stimuli throughout the entire visual field (left and right) irrespective of response type. So even if a patient responded with the left hand, he had no problem indicating the presence of a stimulus in the right visual field, and vice versa. Yet, the patients did differ from healthy controls, in the sense that they could not compare stimuli across visual fields (although they could make this comparison within a visual field).

This finding turns the previously neat picture upside down. Not only do split-brain patients behave normally, and introspectively feel normal, careful tests actually show that conscious unity is preserved: even though there is no cortical connection between the two hemispheres, there seems to be one conscious agent, that is in control of the entire body and who can respond to stimuli everywhere in the visual field. Thus, the lesson learned from split-brain patients is opposite to the previous conclusions: it suggests that cortical connectivity is not needed for conscious unity.

Instead, the authors suggest that the best way to think of split-brain patients is in terms of conscious unity, but split perception. One agent perceives the entire visual field and controls the entire body, but the streams from the left and right visual field remain unintegrated (and hence the patient cannot compare stimuli across the visual fields). This does mean that consciousness remains a mystery. If conscious unity does not depend on cortical connectivity, what then are its essential requirements? Whatever the explanation, it seems clear that these new split-brain findings pose a serious challenge for the currently leading theories on consciousness.

Featured image credit: “Right brain” by Allan Ajifo. CC BY-2.0 via Wikimedia Commons.