Neural Correlates of Consciousness
Further insights into the biological basis for consciousness come from studies of the so- called neural correlates of consciousness—specific states of the brain that correspond to the exact content of someone’s conscious experience. In one study, for example, the researchers exploited a phenomenon known as binocular rivalry (Tong, Nakayama, Vaughan, and Kanwisher, 1998). In the study, one picture is placed in front of one of aperson’s eyes and another, entirely different picture is placed in front of her other eye. Inthis setup, the visual system is unable to handle both stimuli at once,or to fuse the stimuli into a single complex perception. Instead, thevisual system seems to flip-flop between the stimuli so that, for awhile, the person is aware of only one picture, then for a while awareof only the other, and so on. Notice therefore that this is a setting in which the physical situation doesn’t change—the two pictures arealways present. What is changing is the person’s consciousexperience—that is, which picture she’s aware of. This allows us to askwhat changes take place in the brain when the experience changes.
In this study, the researchers placed a picture of a face in front oneof the participant’s eyes and a picture of a house in front of the other eye. We know from many other studies that when people are looking at faces, neuroimaging reveals high levels of activity in a brain region called the fusiform face area (FFA). We also know that when people are looking at houses, there’s a lot of activity in a brain region known as the parahippocampal place area (PPA).
But what exactly does this brain activity—in the FFA or PPA—indicate? If these brain areas respond simply to the available stimuli, then the pattern of activity should be constant in the Tong et al. procedure. The stimuli, after all, were present all the time. But if these brain areas reflect the participants’ conscious perception, then activity should fluctuate—with a change in brain activity each time the binocular rivalry produces a new perception.
In the Tong et al. study, participants pressed buttons to indicate at each moment which picture they were aware of seeing—the house or the face. At the same time, the researchers used fMRI to keep track of the activity levels in the FFA (again, normally responsive to faces) and the PPA (normally responsive to places).
The results are summarized in Figure 6.9. Immediately before the moments in which the participant reported a conscious switch from seeing the face to seeing the house, activity in the FFA went down and activity in the PPA went up. At moments in which the participant reported the reverse switch, the activity levels in these two brain areas showed the opposite pattern.
Apparently, then, activity levels in the FFA or PPA change whenever the participant’s conscious experience changes. Put differently, activity in the FFA doesn’t indicate “a face is in view.” Instead, activity here seems to indicate “the participant is aware of seeing a face.” Likewise for the PPA; activity here seems to indicate “the participant is aware of seeing a house.” In this fashion, it does seem that that we can use brain scans to identify some of the biological correlates of specific conscious states. (For related results, including some with other species, see Kim & Blake, 2005; Haynes, 2009; Koch, 2008; Logothetis, 1998; Rees & Frith, 2007.)
A different example concerns—remarkably—the conscious sensation of “free will.” In a classic study, participants watched a dot moving in a circular pattern on a computer screen, and they were asked to move their hands occasionally (Libet, 1983; also see Haggard & Eimer, 1999; Wegner, 2002). It was up to the participants to decide when they would move their hands; but they were asked to note the dot’s position at the exact moment when they
chose to make this movement, and later they were asked to report this position. This response tells us in essence when the conscious decision to move actu-ally took place, and we can compare that moment to when the movement itself occurred.
Not surprisingly, there was a brief gap—about 200 millisec-onds—between the moment of decision and the actual move- ment. It took a fraction of a second, it seems, to translate the decision into an action (Figure 6.10). The real surprise was that recordings of brain activity showed a marked change—a so-called readiness potential—almost a half-second before participantsreported any awareness of a decision to move. In other words, the participants’ brains had launched the action well before
the participants themselves felt they had initiated the action. This result seems to imply that the feeling of “I will move my hand now” is not the cause of brain activity, as common sense might suggest. Instead, it’s the result of brain activity— in particular, brain activity in the pre-motor and anterior cin-gulate cortices (Lau et al., 2004). In other words, the “decision to move” and the initiation of action happen out-side of awareness, and the person (consciously) learns only a moment later what they’ve just decided. (For some complica-tions and possible challenges to this result, see Banks & Isham, 2009; Desmurget et al., 2009; Haggard, 2009; Obhi, Planetta, & Scantlebury, 2009.)