Asymmetry of charge relaxation times in quantum dots: The influence of degeneracy

Using time-resolved transconductance spectroscopy, we study the tunneling dynamics between a two-dimensional electron gas (2DEG) and self-assembled quantum dots (QDs), embedded in a field-effect transistor structure. We find that the tunneling of electrons from the 2DEG into the QDs is governed by a different time constant than the reverse process, i.e., tunneling from the QDs to the 2DEG. This asymmetry is a clear signature of Coulomb interaction and makes it possible to determine the degeneracy of the quantum dot orbitals even when the individual states cannot be resolved energetically because of inhomogeneous broadening. Our experimental data can be qualitatively explained within a master-equation approach

Voluntary Modulation of Hemodynamic Responses in Swallowing Related Motor Areas: A Near-Infrared Spectroscopy-Based Neurofeedback Study.

In the present study, we show for the first time that motor imagery of swallowing, which is defined as the mental imagination of a specific motor act without overt movements by muscular activity, can be successfully used as mental strategy in a neurofeedback training paradigm. Furthermore, we demonstrate its effects on cortical correlates of swallowing function. Therefore, N = 20 healthy young adults were trained to voluntarily increase their hemodynamic response in swallowing related brain areas as assessed with near-infrared spectroscopy (NIRS). During seven training sessions, participants received either feedback of concentration changes in oxygenated hemoglobin (oxy-Hb group, N = 10) or deoxygenated hemoglobin (deoxy-Hb group, N = 10) over the inferior frontal gyrus (IFG) during motor imagery of swallowing. Before and after the training, we assessed cortical activation patterns during motor execution and imagery of swallowing. The deoxy-Hb group was able to voluntarily increase deoxy-Hb over the IFG during imagery of swallowing. Furthermore, swallowing related cortical activation patterns were more pronounced during motor execution and imagery after the training compared to the pre-test, indicating cortical reorganization due to neurofeedback training. The oxy-Hb group could neither control oxy-Hb during neurofeedback training nor showed any cortical changes. Hence, successful modulation of deoxy-Hb over swallowing related brain areas led to cortical reorganization and might be useful for future treatments of swallowing dysfunction

Feedback screen.

<p>On the background of the feedback screen, either green or gray stripes moved from the right to the left side of the screen with a constant speed. During the resting period, the white dot in the middle of the screen overlapped a gray stripe of the moving background and was vertically fixed in the center of the screen (first, second and fifth example screens in the figure). During the feedback trials, the white dot overlapped a green stripe of the moving background and participants tried to move the white dot up by motor imagery of swallowing (third and fourth example screens in the figure). When they were successful, the number of reward points increased (green reward points indicated number of reward points obtained in the actual feedback trial, gray reward points indicated sum of all obtained reward points of all previous feedback trials).</p

Topographical maps of deoxy-Hb.

<p>Grand average topographical maps of oxy- and deoxy-Hb during motor execution (ME) and motor imagery (MI) for the deoxy-Hb group, presented separately for the task interval (second 5–15 after task onset) and the pause interval (second 15–25 after task onset) and for the pre- and post-test. In the upper left map, the 48 NIRS channel locations are additionally marked.</p

NIRS time course during MI and ME of swallowing.

<p>Mean activation changes in oxy- and deoxy-Hb in response to motor execution (ME, black lines) and motor imagery (MI, gray lines) over the inferior frontal gyrus, presented separately for the pre- (full lines) and post-measurement (dotted lines) and the oxy-Hb group (left panel) and deoxy-Hb group (right panel).</p

NIRS NF training performance.

<p>A) NF training performance (relative concentration changes in oxy- and deoxy-Hb over the bilateral inferior frontal gyrus during the NF training task, average second 5 to 20 after task onset) over the seven NF training sessions and the results of the regression analysis, presented separately for the oxy-Hb group (upper panel) and deoxy-Hb group (lower panel). B) NIRS time course during the first, fourth and last NF training session averaged for NIRS channels over the bilateral inferior frontal gyrus, presented separately for both groups. Note that the NF training task started at second 0 and ended after 17–23 seconds.</p

Position of the optode probe set on the head.

<p>(A) Channel configuration of the optode probe set (2x4x4). The gray numbers in the white rectangles represent the NIRS channel number and the corresponding Brodmann areas are shown in black. The NIRS probe set of 48 channels was positioned over right and left motor cortex. Red and blue circles illustrate positions of NIRS sensors and detectors, respectively. According to the international 10–20 placement system we used Cz, T7 and T8 as marker positions for ensuring replicable placements of the optodes. (B) Projections of the 48 NIRS channel positions (white points) on the cortical surface over the right hemisphere. NIRS positions are overlaid on a MNI-152 compatible canonical brain that is optimized for NIRS analysis according to a procedure of Singh et al. (2005) [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0143314#pone.0143314.ref063" target="_blank">63</a>].</p