The Brain Basis for the Continuity of Thought
If we could pause your mind at this instant and look carefully inside your brain, we would see that some brain cells are active and others are inactive. How long these neurons continue to fire after we unpause your mind is determined by how much input they are getting from other active neurons. If they are not being sent more than the requisite number of messages from their peers, they slow down or turn off. Some of the currently active neurons will remain active for only a few milliseconds, others for large fractions of a second and others for several seconds. None remain active indefinitely, but rather they each persist for different durations. The pattern of activity in the brain is constantly changing, but because some individual neurons persist during these changes, particular features of the overall pattern will be conserved over time. In other words, the distribution of active neurons in the brain transfigures gradually from one configuration to another, instead of continually changing all at once. I believe that the persistence of certain neurons allows the temporary maintenance of mental imagery which is a central hallmark of consciousness and working memory. I also believe that this persistence lends continuity to the train of thought.
Six years ago I was waiting at a bus stop wondering how my mind is different from that of other animals. I realized that my thoughts can extend further in the sense that I can carry a complex concept out to its logical conclusion. I can take more information with me through time before I lose it and forget what it was I was just thinking about. Psychologists agree that working memory, or the ability to preserve information and perform manipulations on it, is more highly developed in humans. Influenced by the various lengths of different pine needles on a Douglas fir at the bus stop, I concluded that human thoughts were somehow “longer.” But if thought has a length associated with it, then it must have a beginning and an end too. I wondered for a while if thoughts really do begin and end, and if so, on what time scales. I now believe that it is possible to answer these questions using the reasoning in the previous paragraph.
Thoughts have length in a sense, but thoughts do not have a clear beginning or an end. Thoughts are “longer” in humans because they are composed of elements (that correspond to individual neurons, or neural assemblies) that remain active for longer periods than they do in other animals. Our large prefrontal cortex and association areas keep some neurons online for several seconds at a time, whereas in our pets, for example, most neurons remain active only very briefly. So it is not that individual human thoughts are longer, it is that our thoughts are composed of elements that remain coactivated for longer. The neurons that persist stop and go at different intervals. It is not the case that all of the neurons that persist turn on and off simultaneously. In fact, the beginning of the activity of one neuron will actually overlap with the tails of others. The neurons act like racecars that join in and drop out of a race intermittently. Their behavior is staggered, insuring that we continually have a cascade of cognitive elements that persist through time. Thus there is no objective stopping or starting point of thought. Instead, thought itself is composed of the startings and stoppings of huge numbers of individual elements that, when combined, create a dynamic and continuous whole.
Sensory neurons in the back of the brain do not usually remain active for long. It is the anterior, association areas, especially the prefrontal cortex that contains neurons that stay online for seconds and even minutes at a time. These neurons, by remaining active, can mete out sustained signaling to other neurons, insisting that the representations that they code for are imposed upon the processing of other neurons that are firing during their span of activity. This is why the prefrontal cortex is associated with working memory, mental modeling, planning and goal setting. The longest, most enduring element or neuron would correspond to what the individual is most focused on, the underlying theme or element that stays the same as other contextual features fluctuate.
Thought changes incrementally during its course. We picture one scenario in our mind’s eye and this can often morph into a related, but distinctly different scenario. Our brain is constantly keeping some elements online whether they are representations of things that are concrete and tangible or abstract and conjunctive. I think that neural continuity as described here is an integral element of consciousness and may be a strong candidate for the “neural correlate of consciousness.” Philosophers and neuroscientists have identified many different elements of brain function (thalamocortical loops and reentrant cortical projections) and attempted to explain how these may lead to conscious experience. I think that the present concept of “continuity through differential temporal persistence of distributed neural activity” is instructive and I even feel that it is the core aspect of conscious experience, qualia and phenomenality.
Figure A shows two time points and the change in activation over time. Undoubtedly the longer the separation in time between time one and time two, the fewer reactivated elements. Figure B shows the time course for eight hypothetical neurons. Note how some remain activated for longer than others and that they overlap frequently.