Imaging fast neural activity in the brain during epilepsy with electrical impedance tomography

Electrical impedance tomography (EIT) is a medical imaging technique which reconstructs images of the internal conductivity of an object using boundary measurements obtained by applying current through pairs of non-penetrating surface electrodes. EIT is able to image impedance changes which arise during neural activity at a high spatiotemporal resolution through the rat cerebral cortex and therefore represents a novel method for understanding neuronal network dynamics in epilepsy. Additionally, it holds therapeutic potential for improving the presurgical localisation of epileptogenic foci in individuals with drug-resistant epilepsy. This thesis was aimed at developing EIT for imaging epileptiform activity in vivo and assessing its potential for clinical use. Chapter 1 is a review of existing functional neuroimaging modalities, the principles of EIT and previous studies that have used EIT for imaging epileptic events. In Chapter 2, the safety of continuous current application to the rat cortical surface at 10-100 μA and 1725 Hz, parameters that are representative of fast neural EIT protocols, was verified by histological evaluation. Chapter 3 details the development of two acute rat models of focal epilepsy, the cortical and hippocampal epileptic afterdischarges models, for assessing the feasibility of imaging epileptiform activity with fast neural EIT using epicortical electrode arrays. In Chapter 4, EIT was used to image the propagation of ictal spike-and-wave activity through the cerebral cortex at a resolution of 2 ms and ≤300 µm. In order to enable imaging of epileptiform discharges in deeper subcortical structures, the optimal carrier frequency for current application was determined in Chapter 5. Results demonstrated that the maximal signal-to-noise ratio of fast neural impedance changes during ictal discharges is obtained at 1355 Hz. Finally, in Chapter 6, epileptiform activity in the hippocampus was imaged, with a localisation accuracy of ≤400 µm, using epicortical impedance measurements obtained at this optimised carrier frequency

Relationship between synaptic dysfunction and degeneration in a rodent model of dementia

Synaptic degeneration is currently the best biomarker correlate of cognitive decline in dementia. In the years prior to dementia onset, many neurophysiological changes are occurring hypothesised to preserve cognitive function, including alterations in synaptic and neuronal function. This thesis aims to characterise the early synaptic and neurophysiological alterations occurring in a mouse model of tauopathy-driven neurodegeneration (rTg4510). This work was performed in the somatosensory cortex, a well characterised region of the brain in the mouse, which serves as a prototypical model of the neocortex. The work presented in Chapters two and three revealed alterations in synaptic glutamatergic receptor function (reduced NMDA:AMPA receptor ratio) and intrinsic neuronal properties in prodromal tauopathy in rTg4510 mice, using in vitro whole-cell patch clamp electrophysiology. Increased dendritic branching proximal to the soma was seen in these recorded neurons following post hoc imaging of their structure. In more advanced stages of tauopathy, reductions in putative AMPA receptor-mediated spontaneous synaptic activity was observed. Significant reductions in glutamatergic receptor expression and synaptic markers was detected in both prodromal and more advanced tauopathy, quantified from isolated synaptosomes. To characterise how glutamatergic receptor dysfunction manifested in vivo, recording paradigms were optimised for in vivo two-photon targeted whole-cell patch clamp electrophysiology, outlined in Chapter four. This technique was used to simultaneously record subthreshold synaptic properties, network activity, and evoked synaptic responses in the rTg4510 model in early neurodegeneration in Chapter five. Whilst spontaneous network activity was similar between genotypes, there was an observable increase in the fast peak response of evoked activity. This work suggests that synaptic dysfunction is a feature of both prodromal and advanced tauopathy, with different functional and biochemical correlates manifesting at different stages of disease progression. Further characterisation of these processes, and how this contributes to symptomatic decline, can provide a basis to develop novel therapeutic strategies to alleviate tau-mediated synaptic and neuronal dysfunction prior to widespread cell loss

Understanding loss of consciousness under general anaesthesia using multimodal neuroimaging

Despite major advancements in consciousness science over the past few decades, how anaesthesia causes loss of consciousness remains incompletely understood and the translation from basic science to clinical practice has been limited. Recent concurrent electroencephalography (EEG) and functional magnetic resonance imaging (FMRI) evidence indicates that a state of complete perception loss is achieved under general anaesthesia when slow wave activity in the brain reaches saturation. Slow wave activity saturation (SWAS) is therefore a potentially clinically relevant end point for titration of anaesthesia. We have developed a prototype system for titrating anaesthesia to SWAS within an individual. The system features EEG and anaesthesia data input, modelling of slow wave power and detection of SWAS, and visualisation of the model output in a graphical user interface. The prototype system was applied in a patient pre-surgery study (Study 1) which focused on clinical translation of SWAS, and a healthy volunteer EEG-MRI study (Study 2) which focused on experimental validation of SWAS. We successfully applied the prototype SWAS system in twelve patients (Study 1) and twenty-three healthy volunteers (Study 2). No subjects in either Study 1 or Study 2 were behaviourally responsive when held at SWAS and none had recall of events from when they were held at SWAS. In Study 2, we also acquired measures of cerebral blood flow (CBF), and resting and task-related FMRI data during wakefulness and when held at SWAS. When held at the SWAS state, CBF was significantly elevated compared to wakefulness. Furthermore, we found that the brain was unresponsive to external stimulation when held at SWAS even when controlling for the observed CBF changes. Finally, we found that this was accompanied by disruption of functional connectivity in the thalamocortical system and in known resting state networks. Our findings provide further evidence that SWAS is a state of perception loss and a clinically relevant target for surgical anaesthesia

Characterising the frequency response of impedance changes during evoked physiological activity in the rat brain

OBJECTIVE: Electrical impedance tomography (EIT) can image impedance changes associated with evoked physiological activity in the cerebral cortex using an array of epicortical electrodes. An impedance change is observed as the externally applied current, normally confined to the extracellular space is admitted into the conducting intracellular space during neuronal depolarisation. The response is largest at DC and decreases at higher frequencies due to capacitative transfer of current across the membrane. Biophysical modelling has shown that this effect becomes significant above 100 Hz. Recordings at DC, however, are contaminated by physiological endogenous evoked potentials. By moving to 1.7 kHz, images of somatosensory evoked responses have been produced down to 2 mm with a resolution of 2 ms and 200 μm. Hardware limitations have so far restricted impedance measurements to frequencies  2 kHz using improved hardware. APPROACH: Impedance changes were recorded during forepaw somatosensory stimulation in both cerebral cortex and the VPL nucleus of the thalamus in anaesthetised rats using applied currents of 1 kHz to 10 kHz. MAIN RESULTS: In the cortex, impedance changed by -0.04 ± 0.02 % at 1 kHz, reached a peak of -0.13 ± 0.05 % at 1475 Hz and decreased to -0.05 ± 0.02 % at 10 kHz. At these frequencies, changes in the thalamus were -0.26 ± 0.1%, -0.4 ± 0.15 % and -0.08 ± 0.03 % respectively. The signal-to-noise ratio was also highest at 1475 Hz with values of -29.5 ± 8 and -31.6 ±10 recorded from the cortex and thalamus respectively. Signficance: This indicates that the optimal frequency for imaging cortical and thalamic evoked activity using fast neural EIT is 1475 Hz

Thalamocortical Oscillations in Sleep and Anaesthesia

The last 20 years have seen a substantial advancement in the understanding of the molecular targets of general anaesthetics however the neural mechanisms involved in causing loss of consciousness remain poorly understood. Thalamocortical oscillations are present in natural sleep and are induced by many general anaesthetics suggesting that modulation of this reciprocal system may be involved in the regulation of consciousness. Dynamic changes of thalamocortical oscillations in natural sleep and anaesthesia were investigated in rats chronically implanted with skull screw and depth electrodes in the cortex and thalamus. The hypothesis that discrete areas within the thalamus are responsible for regulation of arousal was tested. The anaesthetics propofol and dexmedetomidine but not midazolam produced switches in delta frequency at loss of righting reflex (LORR). This switch in frequency mirrored that seen within non-rapid eye movement sleep (NREM), whereas the onset of NREM was characterized by a switch from theta to delta in the EEG. Depth recordings during NREM indicated that the switch into a NREM state occurred in the central medial thalamus (CMT) significantly before the cingulate, barrel cortex and ventrobasal nucleus (VB), and that the CMT switch corresponded to the switch seen in the global EEG. Dexmedetomidine hypnosis showed a delta frequency shift that occurred simultaneously within the thalamus and cortex, and furthermore that the thalamus exhibited phase advancement over the cortex at the point of LORR. In conclusion, globalised changes within the thalamocortical system occur for propofol and dexmedetomidine LORR in the rat. This change represents a transition within drug free NREM and may implicate a common pathway responsible for a decrease in arousal. Furthermore, the phase advancement of the intralaminar thalamus over the cortex at LORR suggests a crucial role for this part of the thalamocortical system for regulating consciousness

Opioids depress cortical centers responsible for the volitional control of respiration

Respiratory depression limits provision of safe opioid analgesia and is the main cause of death in drug addicts. Although opioids are known to inhibit brainstem respiratory activity, their effects on cortical areas that mediate respiration are less well understood. Here, functional magnetic resonance imaging was used to examine how brainstem and cortical activity related to a short breath hold is modulated by the opioid remifentanil. We hypothesized that remifentanil would differentially depress brain areas that mediate sensory-affective components of respiration over those that mediate volitional motor control. Quantitative measures of cerebral blood flow were used to control for hypercapnia-induced changes in blood oxygen level-dependent (BOLD) signal. Awareness of respiration, reflected by an urge-to-breathe score, was profoundly reduced with remifentanil. Urge to breathe was associated with activity in the bilateral insula, frontal operculum, and secondary somatosensory cortex. Localized remifentanil-induced decreases in breath hold-related activity were observed in the left anterior insula and operculum. We also observed remifentanil-induced decreases in the BOLD response to breath holding in the left dorsolateral prefrontal cortex, anterior cingulate, the cerebellum, and periaqueductal gray, brain areas that mediate task performance. Activity in areas mediating motor control (putamen, motor cortex) and sensory-motor integration (supramarginal gyrus) were unaffected by remifentanil. Breath hold-related activity was observed in the medulla. These findings highlight the importance of higher cortical centers in providing contextual awareness of respiration that leads to appropriate modulation of respiratory control. Opioids have profound effects on the cortical centers that control breathing, which potentiates their actions in the brainstem

Single whisker representations in the circuitry of the cerebellar cortex

The cerebellum plays a crucial role in sensorimotor processing, yet little is known about its contribution towards sensory signal processing. The whisking behaviour in rodents is a model behaviour for sensorimotor function and the Crus I and II lob ules of the cerebellum have been shown to represent perioral stimulation, linearly encoded whisker setpoint and be important in the generation of whisker movement. To target our investigation of the sensory representation, I have used a very nar rowly defined stimulus, the deflection of a single whisker, to investigate the re sponse in cerebellar cortex neurons using whole-cell and cell-attached patch-clamp. In the first step of cerebellar cortex processing, I found the convergence in individual granule cell of mossy fibre inputs at latencies indicating a direct pathway through the trigeminal nuclei and a cortico-pontine path for the single whisker signal. Mo lecular layer interneurons were found to be highly precisely and rapidly excited by the early direct path input from granule cells. Lateral inhibition was also displayed by a molecular layer interneuron with an inhibitory response. The sole output of the cerebellar cortex, the Purkinje cells, exhibited simple spike responses often combining excitatory and inhibitory phases in the majority of rec orded cells despite the narrowness of the stimulus and the wide recording location. A complex spike response was measured in half of the Purkinje cells with a simple 7 response and never in Purkinje cell without simple spike responses. This separation of the Purkinje cell population into neurons receiving both mossy fibre and climbing fibre input on the single whisker deflection and those that only receive mossy fibre input suggest different mechanisms (e.g. plasticity) and functions. Together, these findings quantify the sensory input to the cerebellar cortex follow ing a single whisker deflection and the downstream processing of this signal.Open Acces

Insects have the capacity for subjective experience

To what degree are non-human animals conscious? We propose that the most meaningful way to approach this question is from the perspective of functional neurobiology. Here we focus on subjective experience, which is a basic awareness of the world without further reflection on that awareness. This is considered the most basic form of consciousness. Tellingly, this capacity is supported by the integrated midbrain and basal ganglia structures, which are among the oldest and most highly conserved brain systems in vertebrates. A reasonable inference is that the capacity for subjective experience is both widespread and evolutionarily old within the vertebrate lineage. We argue that the insect brain supports functions analogous to those of the vertebrate midbrain and hence that insects may also have a capacity for subjective experience. We discuss the features of neural systems which can and cannot be expected to support this capacity as well as the relationship between our arguments based on neurobiological mechanism and our approach to the “hard problem” of conscious experience