Combined transcranial magnetic stimulation and electroencephalography (TMS-EEG) enables noninvasive neurophysiological investigation of the human cortex. A TMS paradigm of short-latency afferent inhibition (SAI) is characterized by attenuation of the motor-evoked potential (MEP) and modulation of N100 of the TMS-evoked potential (TEP) when TMS is delivered to motor cortex (M1) following median nerve stimulation. SAI is a marker of cholinergic activity in the motor cortex; however, the SAI has not been tested from the prefrontal cortex. We aimed to explore the effect of SAI in dorsolateral prefrontal cortex (DLPFC). SAI was examined in 12 healthy subjects with median nerve stimulation and TMS delivered to M1 and DLPFC at interstimulus intervals (ISIs) relative to the individual N20 latency. SAI in M1 was tested at the optimal ISI of N20 + 2 ms. SAI in DLPFC was investigated at a range of ISI from N20 + 2 to N20 + 20 ms to explore its temporal profile. For SAI in M1, the attenuation of MEP amplitude was correlated with an increase of TEP N100 from the left central area. A similar spatiotemporal neural signature of SAI in DLPFC was observed with a marked increase of N100 amplitude. SAI in DLPFC was maximal at ISI N20 + 4 ms at the left frontal area. These findings establish the neural signature of SAI in DLPFC. Future studies could explore whether DLPFC-SAI is neurophysiological marker of cholinergic dysfunction in cognitive disorders.
Neuroplasticity and long-term potentiation (LTP) in the dorsolateral prefrontal cortex (DLPFC) are considered important mechanisms in learning and memory, and their disruption may be related to the pathophysiology of several neuropsychiatric disorders. Paired associative stimulation (PAS) is a brain stimulation paradigm that produces enhanced activity in the human motor cortex that may be related to LTP. In a group of 15 healthy participants, we report on the potentiation of cortical-evoked activity in the human DLPFC using the combination of PAS and electroencephalography. In contrast, a PAS control condition did not result in potentiation in another group of nine healthy participants. We also demonstrate that PAS-induced potentiation of cortical-evoked activity is characterized by anatomical specificity that is largely confined to the site of stimulation. Finally, we show that PAS results in potentiation of θ- and γ-activity and θ-phase–γ-amplitude coupling. These neurophysiological indices may be related to working memory, an important function of the DLPFC. To our knowledge, this is the first report of potentiation of cortical-evoked activity in the DLPFC. As this potentiation may be related to LTP, our findings provide a model through which neuroplasticity in health and disease states in the frontal cortex can be studied.
Morphological features of the dendritic arborization can affect neuronal responses and thus the input-output function of a particular neuron. In this study, morphological data of eight fully reconstructed thalamocortical (TC) neurons from the ventroposterolateral (VPL) nucleus of adult cats have been analyzed. We examined several geometrical and topological parameters, which have been previously shown to have a high impact on the neuron firing pattern and propagation of signals in the dendritic tree. In addition to well-known morphological parameters such as number of dendritic trees (8.3 ± 1.5) and number of branching points (80–120), we investigated the distribution of dendritic membrane area, branching points, geometrical ratio, asymmetry index, and mean path length for all subtrees of the TC neurons. We demonstrate that due to extensive branching in proximal and middle dendritic sections, the maximum value of the dendritic area distribution is reached at 120–160 μm from the soma. Our analysis reveals that TC neurons are highly branched cells and their dendritic branching pattern does not follow Rall's 3/2 power rule; average values at proximal vs. distal dendritic sections were different. We also found that the dendritic branching pattern of each subtree of the cell had a wide range in symmetry index, whereas the mean path length did not show a large variation through the dendritic arborizations.