Auditory imagery modulates frequency-specific areas in the human auditory cortex.

Neural responses in early sensory areas are influenced by top-down processing. In the visual system, early visual areas have been shown to actively participate in top-down processing based on their topographical properties. Although it has been suggested that the auditory cortex is involved in top-down control, functional evidence of topographic modulation is still lacking. Here, we show that mental auditory imagery for familiar melodies induces significant activation in the frequency-responsive areas of the primary auditory cortex (PAC). This activation is related to the characteristics of the imagery: when subjects were asked to imagine high-frequency melodies, we observed increased activation in the high- versus low-frequency response area; when the subjects were asked to imagine low-frequency melodies, the opposite was observed. Furthermore, we found that A1 is more closely related to the observed frequency-related modulation than R in tonotopic subfields of the PAC. Our findings suggest that top-down processing in the auditory cortex relies on a mechanism similar to that used in the perception of external auditory stimuli, which is comparable to early visual systems.

Transcranial focused ultrasound to the thalamus is associated with reduced extracellular GABA levels in rats.

OBJECTIVE: Transcranial focused ultrasound (FUS), with its ability to non-invasively modulate the excitability of region-specific brain areas, is gaining attention as a potential neurotherapeutic modality. The aim of this study was to examine whether or not FUS administered to the brain could alter the extracellular levels of glutamate and γ-aminobutyric acid (GABA), which are representative excitatory and inhibitory amino acid neurotransmitters, respectively. METHODS: FUS, delivered in the form of a train of pulses, was applied to the thalamus of Sprague-Dawley rats transcranially. Glutamate and GABA were directly sampled from the frontal lobe of the rat brain via a direct microdialysis technique before, during, and after the sonication. The dialysate concentrations were determined by high-performance liquid chromatography. RESULTS: The individual levels of the neurotransmitters sampled were normalized to the baseline level for each rat. In terms of the changes in extracellular glutamate levels, there was no difference between the FUS-treated group and the unsonicated control group. However, extracellular GABA levels started to decrease upon sonication and remained reduced (approximately 20% below baseline; repeated-measures ANOVA, p < 0.05, adjusted for multiple comparisons) compared to the control group. CONCLUSION: The ability to modulate region-specific brain activity, along with the present evidence of the ability to modulate neurotransmission, demonstrates the potential utility of FUS as a completely new non-invasive therapeutic modality.