Neuronal oscillations and their inter-regional synchronization is believed to be one of the mechanisms that enables interaction between various brain regions. There are other types of connectivity, such as structural connectivity which refers to direct anatomical links between brain regions or effective connectivity which gives information about the causality or directed influence (region A caused the activation of region B).Īnother important approach to estimating functional connectivity is to look at spatial relationships in the EEG. Such functional integration or connectivity is typically defined as the statistical inter-dependencies between the functional properties (for example electromagnetic activation or BOLD response) of these brain areas. This comes down to devising ways to identify which regions are interacting or independent at any period of time, and essentially to construct maps of functional connectivity. working independently or separate from each other) is considered to be fundamentally important for understanding how the brain drives cognitive function and behavior. working together) versus functionally segregated (i.e. How electrical activity flows in the brain to create groups of cells that are functionally integrated (i.e. What is Coherence and what are the practical considerations, limitations and interpretations of this measure when applied to EEG? The most important advantage of coherent radar systems is that even very small phase shifts of the echo signal are detectableĪnd the Doppler effect may used to reduce the influence of fixed clutter.Coherence is a commonly used measure in EEG analysis. Therefore, the radar must measure the phase changes from pulse to pulse in order to detect a Doppler frequency. This is far too little to be measured directly as an oscillation. These are only very few (often only one!) oscillation periods of the Doppler frequency to be measured. On the target, only a few hits can be achieved by surveillance radars. Advantagesĭoppler frequencies are frequencies in the lower audio range. Which is why this procedure is also-called “coherent on receive”. However, the next transmitted pulse terminates this coherence, Is forced by the phase of the current transmit pulse to continue oscillating with this phase shift. This phase can be conserved for reference purposes by a controlled damped oscillation over an entire reception period. coherence! Pseudo-Coherent RadarĮven non-coherent radar sets are able to determine a phase position of the echo signal.Įven if the transmitter starts with a random phase, Low-power master generators with subsequent high-power amplification produce a constant phase relationship between the transmission pulses, i.e. The modulation of the power output stage of the transmitter does not affect the phase relation of the transmitted pulse. Radar sets in which the phase relationship is so stable are called fully coherent. The single transmission pulses then consist of partial sections of this continuous oscillation.
#Mean phase coherence generator#
The condition is that this master generator (also-called coherence oscillator) delivers a phase-stable continuous oscillation. In this system, the transmitter consists of a high power amplifier and is fed by a highly stable continuous oscillation of the Self-oscillating transmitters have a random phase from pulse to pulse and are therefore not coherent! Coherent Radar ProcessingĪnother transmitter system is the PAT ( Power- Amplifier- Transmitter). This phase shift, which occurs during the start of the oscillation, is a purely random process. This transmitter begins to oscillate with a different phase shift with each transmission pulse. If such a transmitter is switched on and off by the almost right-angled high-voltage modulation pulse, Non-coherent Radar ProcessingĪ given transmitter system is the self-oscillating POT ( Power Oscillator Transmitter). Whether radar is coherent or not is determined by the type of transmitter.ĭifferent systems can work in the radar as transmitters, which are either coherent, partially coherent or incoherent. In case of incoherence, these phase shifts are statistically distributed. Oscillations and electromagnetic waves are described as coherent if their phase relationships are constant. Coherence describes the phase relationships between the transmitted and the received pulses.
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