In transcranial magnetic stimulation, strong magnetic fields generated by specific coils placed on top of the scalp induce currents in the underlying tissue that extend to cortical tissue. Whilst transcranial magnetic stimulation has been traditionally considered as a method that disrupts the functioning of the targeted cortical locations, thus allowing causal testing of the roles of cortical areas in perceptual and cognitive functions by measuring performance decrements in specific behavioral tasks, there are also reports of performance enhancements following transcranial magnetic stimulation. Such findings have been generally considered to be surprising, but given that observations of performance enhancement have kept accumulating, they cannot be written off as erroneous chance findings.
In their recent review article, Drs. Bruce Luber and Sarah Lisanby (2013) systematically go over the findings on performance enhancement observed in transcranial magnetic stimulation studies. They nicely divide the reported effects into three classes, 1) some non-specific effects caused possibly by increased arousal due to the loud clicks and muscle stimulation, 2) some specific performance enhancement effects caused by taking out of areas that would normally compete with and interfere processing in the areas that are important for the task at hand, and 3) “genuine” performance enhancement effects. The authors also nicely discuss critical factors that give rise to performance enhancement such as strength, and specific rates, of stimulation.
Overall, this review article provides a very nice introduction to these less-well known but highly interesting effects of transcranial magnetic stimulation. The observations that transcranial magnetic stimulation can enhance perception and cognition are indeed highly promising also from the perspective of clinical research and ultimately development of novel clinical treatment and rehabilitation methods. Given that transcranial magnetic stimulation is a relatively non-specific method that stimulates rather extensive cortical areas, advances in stimulation technology, especially in case of patients where non-invasive methods are not the only option, will undoubtedly provide fascinating additional possibilities in the future that results obtained with transcranial magnetic stimulation pave way for in a very important way.
Reference: Luber, Bruce, Lisanby, Sarah H., Enhancement of human cognitive performance using transcranial magnetic stimulation (TMS). NeuroImage (2013), e-publication ahead of print. http://dx.doi.org/10.1016/j.neuroimage.2013.06.007
Learning based on rewarding outcomes of one’s behaviors is one of the most fundamental, and at the same time one of the most evolutionarily oldest, types of learning. Previous research has shed light on the potential underlying neural mechanisms, with non-human primate studies showing how dopaminergic neurons in ventral tegmental area and substantia nigra pars compacta increase their firing during unexpected rewards and decrease their firing during unexpected reward omission, with little change in activity upon occurrence of rewards that are well anticipated. These findings have strongly suggested that dopaminergic activity serves the purpose of providing reward prediction error signals that serve the purpose of teaching associations between rewards and preceding behaviors/events. Direct causal evidence for this mechanism has, however, been lacking due to methodological limitations.
In their recent study, Elizabeth Steinberg et al. (2013) optogenetically activated rat ventral tegmental area dopaminergic neurons to causally test the hypothesis that these neurons provide the prediction-error signal that guides associative learning. Specifically, the authors activated ventral-tegmental area dopamine neurons during both blocking (i.e., when there already is a fully reward-predicting stimulus present in the environment another temporally coinciding stimulus is not learned as a reward predictor under ordinary circumstances) and extinction (i.e., absence of a reward following reward-cued stimulus results in gradual un-learning). In both conditions, optogenetic activation of dopamine neurons that produced an artificial reward prediction error signal modulated learning in a manner consistent with the hypothesis that ventral-tegmental area dopamine neurons underlie associative learning.
These highly important findings provide yet another demonstration of the power offered by causal neuroscience methods in yielding evidence that confirms theoretical assumptions based on previously observed correlations between brain responses and behavioral effects. Indeed, the hypothesis that ventral-tegmental area dopaminergic neurons drive learning of reward-cue associations is not a new one, yet the definitive causal test of this hypothesis has been wanting. Therefore, the findings of Steinberg and her colleagues constitute a very important step forward in our understanding of the neural mechanisms that underlie reinforcement learning.
Reference: Steinberg EE, Keiflin R, Boivin JR, Witten IB, Deisseroth K, Janak PH. A causal link between prediction errors, dopamine neurons and learning. Nature Neuroscience (2013) e-publication ahead of print. http://dx.doi.org/10.1038/nn.3413