Individual variability, end-point effects and possible biases in electrophysiological research

Neocortical dynamics and signals reflect spatiotemporal cortical synchronization as well as functional condition and modulation by subcortical projecting structures that determine the brain state. However, neurotransmitters interact and neuronal function and signals also reflect non-neuronal factors that should (but not necessarily are, or can) be taken into proper consideration in neuroscience. Examples in this regard are the CNS modulation by spontaneous or induced changes in metabolism (e.g. glucose, ammonia), blood flow, pO(2), pCO(2), hormones (thyroxine; sexual hormones, ATCH) and neurohormonal interaction, maturation, aging, temperature, light-dark regulation of serotonine, histamine and dopamine/melatonin/vitamin B12 secretion and interaction, etc. Hormones also regulate the (functional) maturation and differentiation between sexes both during development and in adult life. These factors interact with each other and individually or collectively can account for unexplained or underestimated individual variability; on occasions, may become critical variables in animal or human research. In specific instances, variability is the information of interest. However, physiological variability due to neuronal or non-neuronal factors can interact with the experimental setups or affect electrophysiological signals to extents that can result in biased findings in otherwise controlled studies. An "inverse problem" difficult to approach may ensue in both animal and human research; in several instances, the specificity of brain signals for the investigated function may require experimental validation.