Abstract
Current source density (CSD) analysis assesses spatiotemporal synaptic activations at somatic and/or dendritic levels in the form of depolarizing current sinks. Whereas many studies have focused on the short (<50 ms) latency sinks, associated with thalamocortical projections, sinks with longer latencies have received less attention. Here, we analyzed laminar CSD patterns for the first 600 ms after stimulus onset in the primary auditory cortex of Mongolian gerbils. By applying an algorithm for contour calculation, three distinct mid and four late evoked sinks were identified in layers I, III, Va, VIa, and VIb. Our results further showed that the patterns of intracortical information-flow remained qualitatively similar for low and for high sound pressure level stimuli at the characteristic frequency (CF) as well as for stimuli ± 1 octave from CF. There were, however, differences associated with the strength, vertical extent, onset latency, and duration of the sinks for the four stimulation paradigms used. Stimuli one octave above the most sensitive frequency evoked a new, and quite reliable, sink in layer Va whereas low level stimulation led to the disappearance of the layer VIb sink. These data indicate the presence of input sources specifically activated in response to level and/or frequency parameters. Furthermore, spectral integration above vs. below the CF of neurons is asymmetric as illustrated by CSD profiles. These results are important because synaptic feedback associated with mid and late sinks—beginning at 50 ms post stimulus latency—is likely crucial for response modulation resulting from higher order processes like memory, learning or cognitive control.
Highlights
Population activity of neurons in the mammalian cortex and their interaction with subcortical as well as with other cortical regions can be evaluated using local field potentials (LFP)
LFPs were measured along 51 penetrations in the left primary auditory cortex (AI) of eight adult Mongolian gerbils, while
Our results provide a baseline for studies where precise quantitative physiological data are needed for modeling neuronal circuits involving different subcortical, intracortical, and corticocortical areas
Summary
Population activity of neurons in the mammalian cortex and their interaction with subcortical as well as with other cortical regions can be evaluated using local field potentials (LFP). These summed potentials are thought to reflect synaptic activity and the sequence of characteristic LFP waves is considered to indicate distinct stages in the cortical processing of information (Barth and Di, 1991; Medvedev and Kanwal, 2004). The current source density (CSD) analysis, which is based on the second spatial derivative of the field potentials along the radial depth, localizes synaptic inputs This method provides precise spatial and temporal information about the functional weights of synaptic activity (sinks) and points to the mechanism of their generation. It allows to trace the neuronal information flow (Nicholson and Freeman, 1975; Mitzdorf, 1985)
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