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).