| Discarding Noise and Artefacts |
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Typically, the first step in sorting spike clusters involves discarding bad clusters, i.e. clusters that contain either multi-unit signals of low amplitude (noise) or electrical signals produced by the experimental setup (artefacts).
The Overview Dipslay is ideal for this. Start by selecting the first cluster in the Cluster Panel. By combining information displayed in the three views, you can easily determine if this cluster contains well-isolated action potentials, noise, or artefacts:
action potentials emitted by a well-isolated neuron have a characteristic V-shaped waveform and their autocorrelogram has a stereotypical shape with a clear refractory period (in the previous Figure, the autocorrelogram of cluster 10 is typical of a hippocampal pyramidal cell, while that of cluster 17 is typical of a hippocampal interneuron);
noise has a low amplitude and although its auto-correlogram looks somewhat like that of a well-isolated neuron, it lacks a refractory period;
artefacts usually have a high amplitude and their waveforms and auto-correlograms look very different from those of neuronal signals.
If the cluster contains artefacts, delete it by selecting -> or by clicking on the red trash icon
. This will move all the spikes to cluster 0 (which by convention contains all artefacts). If the cluster contains noise, delete it by selecting -> or by clicking on the grey trash icon
. This will move all the spikes to cluster 1 (which by convention contains the noise). Otherwise, move on to the next cluster.
Once the bad clusters have been removed, you may wish to renumber the remaining clusters in sequence: to do this, select ->.
All these operations can be undone: simply select -> to revert to the previous state. Conversely, after cancelling an operation selecting -> will apply the changes again. To set the number of operations that can be undone, see the General Settings.
One very convenient way to speed up these operations is to use the keyboard shortcuts: press Shift-DEL to delete artefacts, DEL to delete noise, or the Right Arrow to move on to the next cluster (deleting a cluster also automatically moves on to the next cluster).
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