Competitive interactions in somatosensory cortex for concurrent vibrotactile stimulation between and within hands.

We investigated competitive neural interactions for processing resources during sustained spatial attention in somatosensation. Participants received concurrent vibrotactile stimulation at different frequencies to either non-adjacent left hand fingers (within-hand) or fingers on two hands (between-hands) for 4500 ms to elicit steady-state somatosensory evoked potentials (SSSEPs). They attended to either one or both stimulated locations, respectively, and responded to rare events embedded in the ongoing stream. Behavioral and electrophysiological results revealed competitive interaction of simultaneous stimulation with overall smaller SSSEP amplitudes and poorer behavioral performance in the within-hand condition. Spatial proximity of stimulation did not affect the magnitude of attentional gain, it resulted in the occurrence of an intermodulation frequency indicating integrative stimulus processing in the within-hand condition. Obviously, while this integrative stimulus processing resulted in or was a result of greater intra-hemispheric competitive interactions, attention acted additively as a signal gain, which was independent of spatial competition.

Test-retest reliability of concurrently recorded steady-state and somatosensory evoked potentials in somatosensory sustained spatial attention.

We investigated the test-retest reliability of sustained spatial attention modulation of steady-state somatosensory evoked potentials (SSSEPs) and the N140 component of the somatosensory evoked potentials (SEPs). Participants attended to one or both hands to perform a target detection task while concurrent mechanical vibrations were presented for 4500ms to both hands in two recording sessions. Results revealed that the amplitude and the attentional modulation of SSSEPs had high test-retest reliability, while the test-retest reliability for the N140 component was low. SSSEPs for stimuli with focused and divided attention had about the same amplitude. For the N140 component only the stimuli with focused attention were significantly enhanced. We found greater habituation effects for the N140 compared to SSSEP amplitudes but attentional modulation was unaffected in both signals. Given the great test-retest reliability of SSSEP amplitude modulation with attention, SSSEPs serve as an excellent tool for studying sustained spatial attention in somatosensation.

Competitive interactions of attentional resources in early visual cortex during sustained visuospatial attention within or between visual hemifields: evidence for the different-hemifield advantage.

Performing a task across the left and right visual hemifields results in better performance than in a within-hemifield version of the task, termed the different-hemifield advantage. Although recent studies used transient stimuli that were presented with long ISIs, here we used a continuous objective electrophysiological (EEG) measure of competitive interactions for attentional processing resources in early visual cortex, the steady-state visual evoked potential (SSVEP). We frequency-tagged locations in each visual quadrant and at central fixation by flickering light-emitting diodes (LEDs) at different frequencies to elicit distinguishable SSVEPs. Stimuli were presented for several seconds, and participants were cued to attend to two LEDs either in one (Within) or distributed across left and right visual hemifields (Across). In addition, we introduced two reference measures: one for suppressive interactions between the peripheral LEDs by using a task at fixation where attention was withdrawn from the periphery and another estimating the upper bound of SSVEP amplitude by cueing participants to attend to only one of the peripheral LEDs. We found significantly greater SSVEP amplitude modulations in Across compared with Within hemifield conditions. No differences were found between SSVEP amplitudes elicited by the peripheral LEDs when participants attended to the centrally located LEDs compared with when peripheral LEDs had to be ignored in Across and Within trials. Attending to only one LED elicited the same SSVEP amplitude as Across conditions. Although behavioral data displayed a more complex pattern, SSVEP amplitudes were well in line with the predictions of the different-hemifield advantage account during sustained visuospatial attention.

Effects of overt and covert attention on the steady-state visual evoked potential.

Flickering stimuli evoke an oscillatory brain response with the same frequency as the driving stimulus, the so-called steady-state visual evoked potential (SSVEP). SSVEPs are robust brain signals whose amplitudes are enhanced with attention and thus play a major role in the development and use of non-invasive Brain-Computer Interfaces (BCIs). We compared the modulation of SSVEP amplitudes when subjects directly gazed at a flickering array of static dots (overt attention) to when they covertly shifted attention to the dots keeping their eyes at central fixation. A discrimination task was performed at the attended location to ensure that subjects shifted attention as instructed. Horizontal eye movements (allowed in overt attention but to be avoided in covert attention) were monitored by the horizontal electrooculogram. Subjects' behavioural performance was significantly reduced in covert attention compared to overt attention. Correspondingly, attentional modulation of SSVEP amplitudes by overt attention was larger in magnitude than for covert attention. Overt attention also changed the topographical distribution of SSVEP amplitudes on the scalp. Stimuli elicited the largest amplitudes at central occipital electrodes when they were overtly attended and at contralateral parieto-occipital sites when they were covertly attended. Accordingly, source analysis revealed clear centrally located sources in early visual areas in overt attention, regardless of the attended visual hemifield. Taken together these results affirm that overt and covert attention have qualitatively and quantitatively different effects on SSVEP responses as well as on task performance. Moreover, our results suggest that navigating SSVEP-BCIs with overt attention is more reliable and highlight some of the challenges in developing BCIs for patients who have lost the ability to move their eyes.

Regulation of cytosolic pH and lactic acid release in mesangial cells overexpressing GLUT1.

BACKGROUND: Anaerobic glycolysis leads to the formation of lactate and H+ and thus imposes a significant challenge on cytosolic acid/base regulation. Cytosolic acidification, on the other hand, is known to inhibit flux through glycolysis and lactate formation. To explore the interplay of cytosolic pH and glycolysis, rat mesangial cells transfected with the glucose transporter GLUT1 (GLUT1 cells) were compared with those transfected with beta-galactosidase (LacZ cells). METHODS: In the presence of extracellular glucose, the glycolytic rate was one order of magnitude higher in GLUT1 cells than in LacZ cells. Cytosolic pH (pHi) was significantly higher in GLUT1 than LacZ cells, an effect abolished in the presence of Na+/H+ exchange inhibitor ethylisopropylamiloride (1 micromol/L). RESULTS: Addition of 40 mmol/L lactate led to marked cytosolic acidification, which was in both cell types blunted by O-methyl-glucose (20 mmol/L) and completely abolished by 100 micromol/L phloretin and 1 mmol/L p-chloromercuribenzene-sulphonic acid (p-CMBS) and in LacZ cells only by glucose (20 mmol/L). The functional characterization points to the involvement of a lactic acid transporter from the monocarboxylate transporter (MCT) family, particularly MCT1. Reverse transcription-polymerase chain reaction (RT-PCR) indeed disclosed the expression of MCT1 and MCT2 in both GLUT1 and LacZ cells. CONCLUSION: Overexpression of GLUT1 leads to cytosolic alkalinization of mesangial cells depending on functional Na+/H+ exchanger but not on Na+ independent H+ transport.