Variability has always been an issue for experimental psychology, but it is only recently that it has been appreciated as information that is interesting in its own right. In cognitive and psychophysical assessments variability is not just sampling error, it is something produced by a nervous system that is organized and which embodies many different forms of memory. When viewed from a dynamical perspective, the residuals from typical cognitive methodologies are often found to be correlated as a particular type of fractal, 1/f noise. 1/f noises have been observed in a number of seemingly unrelated contexts (quasar light, river and tide height, traffic flow, to name a few) and its appearance is believed to be meaningful, especially as a signature of dynamical complexity. Notably, people who have attention-deficit hyperactivity disorder (ADHD) produce noises in speeded response tasks that are distinctly not of the 1/f variety . The work proposed here is designed to ascertain why this is so, and to use the disorder as a wedge to gain experimental leverage into an understanding of 1/f noise in normal human behavior. The two specific theories evaluated are 1) the possibility that disruptions of the ATP cycle in neuronal energy regulation create attentional dysfunction, and 2) that dopamine dysfunction and its attendant disruption of the reward system in learning affect the timescales over which temporal associations are constructed. The pairing of behavioral and cognitive assessments will allow a better understanding of the causal mechanisms of noise production in ADHD task performance. The significance of this research is twofold. First, 1/f noise combines aspects of order and disorder in a way that its presence is thought to signify aspects of dynamic complexity. Discovering what produces it in human cognition would be tantamount to describing what kind of thing thought is at the systems level. This is one of the unique places in psychology where meaningful contact can be made with fields as diverse as statistical physics and physiology. Second, identification of the cognitive deficits in ADHD has lagged behind advances in establishing its neurobiology. Exploiting the normal 1/f nature of behavior in or to identify core cognitive deficits in ADHD will clarify the nature of the disorder and may suggest directions for treatment.
This project concerned the temporal dynamics of cognition and particularly the integrative processes that compromise what is colloquially known as "the present moment". The present moment was defined operationally in our research as the maximum period of time that can be spanned before temporally distributed experiences fragment. An example is the feeling of rhythm. This feeling decays if individual hand, foot, or body movements are separated by more than about 1.5 or 2 seconds. This is well known in so far as metronomes do not typically allow largo tempi slower than 40 beats per minute. The first goal of our research was to assess a range of cognitive operations that are time sensitive and determine if the 1.5 to 2 second timescale was invariant over this range. Initial evidence is that priming, cueing, and apparent motion perception all are constrained by this common timescale. The development of these methods allowed us to test a hypothesis about what ADHD is. The dogma on ADHD is that it is an attention deficit disorder, hence its name. However, it is well known to those who read the literature critically that this point of view has not been experimentally justified. Recent evidence that dopamine pathways have abnormal PET and fMRI signatures in ADHD led us to conjecture that the processes of mental integration would have less time to operate in people with this syndrome. In other words we hypothesized that people with ADHD have a shorter present moment. In so far as vigilance requires sustained attention, attention does have a time based component and disruptions in the processes that permit mental continuity will look like attention disorders. However we hypothesize that not just vigilance is affected in ADHD, but all the behavioral and perceptual outcomes that are time sensitive. So for example we propose that people with ADHD would not need a metronome that goes down to 40 bpm - their integrative processes being already compromised at delays of 1.5 seconds. So ADHD is conceived here to be as much a rhythm disorder as it is an attention disorder. Using the same methods that we benchmarked in normal controls we did indeed discover that in those paradigms where we could reliable measure a present moment people with ADHD do have shorter time horizons. This work has the promise to give new direction to this field and clarify diagnosis which has historically been a source of controversy.
