2.5. Evoked population spikes
Population spike responses from the CA3 and CA1 (the recording electrodes placed in the CA3 and CA1 pyramidal layers) were obtained via electrical stimulation using a bipolar tungsten electrode (Resistance in aCSF: ~0.1 MΩ) placed at the apical dendrites of the proximal CA1 stimulating the Schaffer collaterals. Such stimulation configuration leads to an antidromically evoked population spike in the CA3 pyramidal cell layer and a secondary orthodromic population spike via recurrent axon collaterals between CA3 pyramidal cells and between CA3 pyramidal cells and interneurons (Behrens et al. , 2005; Fanoet al. , 2012; Çalışkan et al. , 2015). The same stimulation leads to an orthodromic population spike in the CA1 pyramidal layer. An input-output (IO) curve was obtained using five intensities ranging from 10 µA to 50 µA. The population spike area was calculated by integrating the area above the of the population spike curve (mV.ms). Normalized collateral associative network activation in the CA3 was measured via dividing the antidromic population spike area to the orthodromic population spike area. Data analysis was performed off-line using Spike2 software (Version 8, Cambridge Electronic Design, Cambridge, UK).