The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

The phenomenon of blindsight is a particularly clear indicator that there may be a grain of truth in Freud’s theory of the unconscious. Recall from Chapter 2 that someone with blindsight has damage to the V1 area in the visual cortex, and as a result cannot see anything. She is blind. She experiences none of the qualia associated with vision. If you project a spot of light on the wall in front of her, she will tell you categorically that she does not see anything. Yet if asked to reach out to touch the spot, she can do so with uncanny accuracy even though to her it feels like a wild guess. She is able to do this, as we saw earlier, because the old pathway between her retina and her parietal lobe is intact. So even though she can’t see the spot, she can still reach out and touch it. Indeed, a blindsight patient can often even guess the color and orientation of a line (vertical or horizontal) using this pathway even though she cannot perceive it consciously.

This is astonishing. It implies that only the information streaming through your visual cortex is associated with consciousness and linked to your sense of self. The other parallel pathway can go about its business performing the complex computations required for hand guidance (or even correctly guessing color) without consciousness ever coming into the picture. Why? These two paths for visual information are made up of identical-looking neurons, after all, and they seem to be performing equally complex computations, yet only the new pathway casts the light of consciousness on visual information. What’s so special about these circuits that they “require” or “generate” consciousness? In other words, why aren’t all aspects of vision and vision-guided behavior similar to blindsight, chugging along with competence and accuracy but without conscious awareness and qualia? Might the answer to this question give clues to solving the riddle of consciousness?

The example of blindsight is suggestive not only because it supports the idea of the unconscious mind (or several unconscious minds). It also demonstrates how neuroscience can marshal evidence about the innermost workings of the brain in order to make its way through the cold-case file, so to speak, addressing some of the unanswered questions about the self that have plagued philosophers and scientists for millennia. By studying patients who have disturbances in self-representation and observing how specific brain areas malfunction, we can better understand how a sense of self arises in the normal human brain. Each disorder becomes a window on a specific aspect of the self.

First, let’s define these aspects of the self, or at the very least, our intuitions about them.

1. Unity: Despite the teeming diversity of sensory experiences that you are deluged with moment to moment, you feel like one person. Moreover, all of your various (and sometimes contradictory) goals, memories, emotions, actions, beliefs, and present awareness seem to cohere to form a single individual.

2. Continuity: Despite the enormous number of distinct events punctuating your life, you feel a sense of continuity of identity through time—moment to moment, decade to decade. And as Endel Tulving has noted, you can engage in mental “time travel,” starting from early childhood and projecting yourself into the future, sliding to and fro effortlessly. This Proustian virtuosity is unique to humans.

3. Embodiment: You feel anchored and at home in your body. It never occurs to you that the hand you just used to pick up your car keys might not belong to you. Nor would you think you’re in any danger of believing the arm of a waiter or a cashier is in fact your own arm. However, scratch the surface and it turns out your sense of embodiment is surprisingly fallible and flexible. Believe it or not, you can be optically tricked into temporarily leaving your body and experiencing yourself in another location. (This happens to some extent when you view a live, real-time video of yourself or stand in a carnival hall of mirrors.) By wearing heavy makeup to disguise yourself and looking at your own video image (which doesn’t have to do a left-right reversal like a mirror), you can get an inkling of an out-of-body experience, especially if you move various body parts and change your expression. Furthermore, as we saw in Chapter 1, your body image is highly malleable; it can be altered in position and size using mirrors. And as we will see later in this chapter, it can be profoundly disturbed in disease.