God Soul Mind Brain

moment? How about now?”), whether a scientist is asking you a question verbally (“Are you aware of the red dot briefly flashed on the computer screen?”) or whether you are performing in an experiment in which your responses are non-verbal (you press a button whenever you are consciously aware of a tone presented over a pair of headphones), all of these cases involve signal detection. They involve a yes-no decision. A great deal is now known about decision-making and how it is implemented in the brain, and some of that knowledge can be transferred directly to the topic of conscious awareness.

    A common method of studying decision-making comes from experiments originally on the monkey brain, performed by many researchers including New-some, Shadlen, Glimcher, and other colleagues. In the typical experiment, a monkey decides whether a shimmering cloud of dots on a computer screen is drifting more to the right or to the left. In some versions of the experiment the monkey presses a button on the right or the left to answer the question; in other versions he glances to the right or the left while his eye movement is tracked by a video system. In either case, if he gets the right answer, he is rewarded with a drop of apple juice. (Monkeys love apple juice.) The decision—right or left—appears to be made in the following manner. One region of the brain reconstructs the visual motion. That region of the brain generates signals that correspond to the proposition “it’s moving to the right” or “it’s moving to the left.” The signals, however, are weak and inconsistent, because the cloud of dots that the monkey is looking at is shimmering in a way that obscures the direction of motion. The signals are received by a different brain area called an integrator. The integrator sums up the signals over time. If a particular signal, indicating a particular direction, is reliable enough to push the integrator and cause it to cross a threshold, then the decision is made.

    The power of this formulation of decision-making is that it is easily generalized from the extremely specific case of a monkey making a visual decision to any type of decision-making. Something in the brain generates a signal; the signal is received by an integrator; and depending on the consistency of the signal over time, the integrator is pushed to threshold, triggering a response.

    Applying this model of decision-making to the domain of conscious awareness results in the following hypothesis: some region of the brain generates a signal that, in the context of the experiment, represents the piece of information: “I have conscious awareness of thing Y.” That signal is sent to an integrator. If the signal is reliable enough to cause the integrator to cross a threshold, then the person reports that, yes, conscious awareness of thing Y is present. Everything that is known about decision-making suggests that conscious awareness depends on a signal that is explicitly generated somewhere in the brain and that feeds into a decision-making integrator somewhere else in the brain.

    Here I would like to make a subtle but important distinction between detecting thing Y and detecting conscious awareness of thing Y. When a person reports being consciously aware of a visual stimulus, the task is one of signal detection. The signal to detect, however, is not the visual stimulus; it is the special signal of conscious awareness of the visual stimulus. Suppose you ask a person to press a button every time he detects a visual stimulus flashed on a screen. The person can perform that task while zoning out, or wandering in thought to the point that the button-press response has become automatic and the person lacks conscious awareness of the visual presentations. Very often a person becomes better at the task when conscious awareness has fled somewhere else in this fashion. In testing for conscious awareness of thing Y, the decision the person must make is not whether information about thing Y is present in the brain and actionable, but whether the special property of conscious awareness has been attached to thing Y.

    This formulation begs the question: what system in the brain generates the signal of conscious awareness and attaches it to object Y? Why would a brain explicitly compute such information? We already know that the brain generates the signal of awareness and assigns it to models of other people’s minds. The human brain comes already equipped with