Multi-frequency electrical impedance tomography and neuroimaging data in stroke patients.

Electrical Impedance Tomography (EIT) is a non-invasive imaging technique, which has the potential to expedite the differentiation of ischaemic or haemorrhagic stroke, decreasing the time to treatment. Whilst demonstrated in simulation, there are currently no suitable imaging or classification methods which can be successfully applied to human stroke data. Development of these complex methods is hindered by a lack of quality Multi-Frequency EIT (MFEIT) data. To address this, MFEIT data were collected from 23 stroke patients, and 10 healthy volunteers, as part of a clinical trial in collaboration with the Hyper Acute Stroke Unit (HASU) at University College London Hospital (UCLH). Data were collected at 17 frequencies between 5 Hz and 2 kHz, with 31 current injections, yielding 930 measurements at each frequency. This dataset is the most comprehensive of its kind and enables combined analysis of MFEIT, Electroencephalography (EEG) and Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) data in stroke patients, which can form the basis of future research into stroke classification.

A Versatile and Reproducible Multi-Frequency Electrical Impedance Tomography System.

A highly versatile Electrical Impedance Tomography (EIT) system, nicknamed the ScouseTom, has been developed. The system allows control over current amplitude, frequency, number of electrodes, injection protocol and data processing. Current is injected using a Keithley 6221 current source, and voltages are recorded with a 24-bit EEG system with minimum bandwidth of 3.2 kHz. Custom PCBs interface with a PC to control the measurement process, electrode addressing and triggering of external stimuli. The performance of the system was characterised using resistor phantoms to represent human scalp recordings, with an SNR of 77.5 dB, stable across a four hour recording and 20 Hz to 20 kHz. In studies of both haeomorrhage using scalp electrodes, and evoked activity using epicortical electrode mats in rats, it was possible to reconstruct images matching established literature at known areas of onset. Data collected using scalp electrode in humans matched known tissue impedance spectra and was stable over frequency. The experimental procedure is software controlled and is readily adaptable to new paradigms. Where possible, commercial or open-source components were used, to minimise the complexity in reproduction. The hardware designs and software for the system have been released under an open source licence, encouraging contributions and allowing for rapid replication.

Oppositional transcranial direct current stimulation (tDCS) of parietal substrates of attention during encoding modulates episodic memory.

Effective learning requires that attentional resources be focused on target information and withheld from irrelevant events in the learner's surroundings. This requires engagement of the brain substrates of selective attention and the concurrent disengagement of brain substrates of orienting toward changes in the environment. In the present study, we attempted to modulate activation of cortical substrates of attention during learning by physiological intervention, using transcranial direct current stimulation (tDCS). To effect adversarial modulation, we applied anodal stimulation directed toward left intraparietal sulcus/superior parietal cortex (IPS/SPL; a substrate of selective attention) and cathodal stimulation directed toward right inferior parietal cortex (IPL; a substrate of orienting). Such stimulation during study of verbal materials led to superior subsequent recognition memory relative to the opposite polarity of stimulation. To our knowledge, this is the first application of direct current stimulation to parietal regions implicated in different forms of attention in an oppositional manner in order to modulate learning in a verbal recognition memory task. Additionally, these results may have practical implications for the development of interventions to benefit persons with various types of attentional deficits.