Rapid Bidirectional Reorganization of Cortical Microcircuits

Mature neocortex adapts to altered sensory input by changing neural activity in cortical circuits. The underlying cellular mechanisms remain unclear. We used blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to show reorganization in somatosensory cortex elicited by altered whisker sensory input. We found that there was rapid expansion followed by retraction of whisker cortical maps. The cellular basis for the reorganization in primary somatosensory cortex was investigated with paired electrophysiological recordings in the periphery of the expanded whisker representation. During map expansion, the chance of finding a monosynaptic connection between pairs of pyramidal neurons increased 3-fold. Despite the rapid increase in local excitatory connectivity, the average strength and synaptic dynamics did not change, which suggests that new excitatory connections rapidly acquire the properties of established excitatory connections. During map retraction, entire excitatory connections between pyramidal neurons were lost. In contrast, connectivity between pyramidal neurons and fast spiking interneurons was unchanged. Hence, the changes in local excitatory connectivity did not occur in all circuits involving pyramidal neurons. Our data show that pyramidal neurons are recruited to and eliminated from local excitatory networks over days. These findings suggest that the local excitatory connectome is dynamic in mature neocortex

Sensory inputs from whisking movements modify cortical whisker maps visualized with functional magnetic resonance imaging

Rodents vary the frequency of whisking movements during ex-ploratory and discriminatory behaviors. The effect of whisking frequency on whisker cortical maps was investigated by simulating whisking at physiological frequencies and imaging the whisker representations with blood oxygen level--dependent (BOLD) func-tional magnetic resonance imaging. Repetitive deflection of many right-sided whiskers at 10 Hz evoked a positive BOLD response that extended across contralateral primary somatosensory cortex (SI) and secondary somatosensory cortex (SII). In contrast, synchro-nous deflection of 2 adjacent whiskers (right C1 and C2) at 10 Hz evoked separate positive BOLD responses in contralateral SI and SII that were predominantly located in upper cortical layers. The positive BOLD responses were separated and partially surrounded by a negative BOLD response that was mainly in lower cortical layers. Two-whisker representations varied with the frequency of simulated whisking. Positive BOLD responses were largest with 7-Hz deflection. Negative BOLD responses were robust at 10 Hz but were weaker or absent with 7-Hz or 3-Hz deflection. Our findings suggest that sensory inputs attributable to the frequency of whisking movements modify whisker cortical representations

Stock price synchronicity and liquidity

10.1016/j.finmar.2012.09.007Journal of Financial Markets163414-43