## 3 Results
### Findings  
#### 3.2.1 Epileptic Network Propagation  
In epilepsy, there is a disbalance between excitation and inhibition. Neurons become pathologically hyperexcitable and are therefore capable of repetitive generation of action potentials. When a significant number of neurons fire excessively in a hyper-synchronized manner, epileptic seizures manifest. The studies within this group evaluated functional changes that resulted from epileptiform activity. 2 PI allowed for two-photon calcium imaging (calcium concentrations [Ca\({}^{2+}\)]) and visualization of neurons within the local seizure onset zone in and ex vivo. In the majority of these studies, the fluorescent indicator GCaMP5G was used. This imaging allowed for mapping of patterns of neuronal recruitment and seizure propagation, sometimes in combination with electrographic recordings to verify 2 PI findings [35; 36; 37].  
[PERSON] et al. [32] used 2 PI visualization of neurons and interneurons [Ca\({}^{2+}\)] to find that, during focal seizure propagation, neuronal populations are recruited in a saltatory fashion. However, despite consistent recruitment pathways, propagating seizures showed temporal variability dependent on local interneuron GABAergic inhibition. This finding was supported by [PERSON] et al. [35], who used 2 PI [Ca\({}^{2+}\)] signal images to record inhibitory effects in pyramidal neurons and found that they correlated with inhibitory interneuron [Ca\({}^{2+}\)] during ictal discharges. They also described firing impairments of inhibitory interneurons preceding ictal discharges in neighboring pyramidal neurons. Similarly, [PERSON] et al. [31] and [PERSON] et al. [41] also reported that focal seizures were associated with heterogeneous inhibitory [Ca\({}^{2+}\)] interneuron activity outside the seizure focus. Furthermore, [PERSON] et al. [40] combined two-photon calcium imaging and glutamate uncaging with electrophysiological measurements in neocortical slices of epileptic patients. The dynamics of neuronal [Ca\({}^{2+}\)] activity in human tissue follow similar patterns to thosein experimental animal models, and combining electrophysiological recording techniques with 2 PI can be used to probe the functional and structural properties of human cortex in pathological conditions.  
[PERSON] et al. [39] cross-correlated [Ca2+] events to develop a spatial recruitment map of neuronal population during seizures in zebrafish. The cerebellum was found to be the main source of epileptiform transients. In addition, [PERSON] et al. [36] and [PERSON] et al. [37] investigated the effects of inducing interictal activity in experimental rodent models. [PERSON] et al. [36] demonstrated that interictal spiking resulted in widespread [Ca2+] alterations. These alterations in [Ca2+], in combination with electrophysiological registered field potential disruptions, were even found in the contralateral hemisphere as well.  
By using 2 PI visualization of GCaMP5G, [PERSON] et al. [37] showed that GABAergic neurons are preferentially recruited (as opposed to glutamatergic) during interictal activity in the cornu ammonis 1 (CA1) region of the epileptic mouse hippocampus. Finally, [PERSON] et al. [38] demonstrated the applicability of a K+-sensitive fluoroionophore FI3 to efficiently stain neurons and intra- and extracellular K+, allowing for 2 PI visualization of K+ dynamics during seizure activity.