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Post-Stimulus Time Histogram (PSTH)
As with the amplitude Histogram, the Post-Stimulus (or Peri-Stimulus) Time Histogram (PSTH) is not really a waveform average. Instead, it shows the temporal distribution of threshold events. The X axis shows the same time span as a conventional waveform display. At each sample time point, the Y-height shows the percent of frames that contained a threshold event at that time.
Threshold events are those that have the specified slope polarity when they cross the specified threshold, as set by the controls beneath the PSTH selector button.
With a repetitive waveform and a stable Trigger setting, the default positive slope and zero threshold level will result in a threshold event at each positive zero crossing. If the wave repeats exactly on each frame, the PSTH will show a 100% spike at each of these times. If there is a little jitter in the timing, the peaks may be broader and of lower amplitude (since 100% of the frames don't have an event that lands on the same time point.) With a lot of jitter or with no trigger sync, the peaks may appear at random positions and at very low levels.
The PSTH will allow you to quickly pick out timing problems. If the input is a pulse train, this will easily show skipped or extra pulses.
Note that the threshold detector needs a "running start" to determine the slope. So if the threshold is set to zero and the waveform phase is such that each frame starts exactly at zero, the detector will not recognize the first sample as a threshold event. If you want to count it, you should set the threshold level a little above zero, say 2% or so. Or you can set the Trigger Delay to show a few samples prior to the actual trigger event. Or, if the Daqarta Generator is creating the stimulus, you can set the main tone Phase to start a little before the zero crossing, say -10 degrees or so, and use Gen Sync triggering.
One typical use for a PSTH is in electrophysiological studies. Neurons typically have some baseline "spontaneous" firing rate; with no stimulus applied, the PSTH would show a low-level random pattern, because there would be no correlation between the neural discharge spikes and any stimulus signal.
Now if a stimulus is applied, such as a tone burst that repeats once per frame, the neuron will be much more likely to discharge at the start of the stimulus. It won't necessarily fire at the start of each and every stimulus, since it may have fired spontaneously just before that, and not yet recovered. But there should be a big peak in the PSTH at the stimulus time and shortly thereafter.
During the time the stimulus remains on, the neuron may show an elevated firing rate. The PSTH would show a lower-level region after the onset peak, decaying after the stimulus offset.
For some acoustic neurons, the discharge spikes from an ongoing tone are more likely to occur at a certain phase of the tone. The particular phase won't be known in advance, since there will be acoustical and neural delays between the stimulus generator and the neural spike mechanism. But this phase-locking behavior will show up as a series of peaks or clumps in the ongoing portion of the PSTH.
When you set a negative Trigger Delay, you can see events that led up to the trigger event. This is a Peri-Stimulus Time Histogram, and can be used for "Reverse Correlation" studies. Typically, the trigger is not really a stimulus, but is itself a response whose cause you want to determine.
For example, you might want to know what causes the firing of a particlar target neuron, so you trigger on its spikes. Then you probe a candidate neuron on the other input channel, and look at the PSTH. If spikes from the candidate are clustered about some particular time prior to the target firing (giving a peak in the PSTH), that probably indicates the candidate is at least influencing the target. The amount of the delay can be used to estimate the presence and number of intermediate connections. The size of the PSTH peak can give information about the strength of the contribution. On the other hand, if the candidate spikes are scattered at random times (low, broad PSTH), you can conclude that there is no connection.
The same concept can be used for mechanical or electrical testing, such as to identify the cause of a spurious event. (In fact, you can use it for software debugging if you can arrange for the spurious event and the candidate to generate external electrical pulses that can serve as triggers.) The PSTH doesn't care how infrequent the spurious events may be... it can run for days, weeks, or months if needed. As with the neural example, a PSTH peak indicates a clear correlation, and the delay between the peak and the spurious event may give clues about other steps in the chain.
WavgMode=PSTH or WavgMode=3 sets PSTH mode. See Macro Notes under Waveform Averager Mode for the complete list of mode numbers and names.
PSTHslope=Pos or PSTHslope=0 sets the slope to positive, while PSTHslope=Neg or PSTHslope=1 sets negative. PSTHslope=x toggles between the two.
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