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Step 5 – Evoked response display and mapping

Having computed the averaged evoked potentials (.p files), it is possible to display them using erpa, a highly user-friendly graphic program. Erpa results from 20-years interactions between electrophysiologists and software developers, and thus covers a comprehensive and thorough viewing of all aspects of the data: time curves, topography (potential and scalp current density mapping) and measurements.
Typing the command line erpa will open a screen window with a general menu bar presenting 6 buttons: ‘Files’, ‘Curves’, ‘Maps’, ‘Models’, ‘Transform’, ‘Quit’. Each button commands a pull-down menu allowing to interactively specify the display parameters. (‘Models’ is inoperative at the moment).

.p file loading.
The first step is to click the ‘Files’ button in the general menu bar, then the ‘Load’ button to select the .p files to display. It is possible to ‘load’ / ‘unload’ several .p files to process several responses on the same display (superposition of time curves, juxtaposition of topographic maps…). If two or more files do not have the same length (time samples), erpa can align them (‘Align’ button). It is also possible to specify the amplitude units (μV, fT…).

.p curves processing (general menu bar, ‘Curves’ button).
The time curves of the .p files loaded can be drawn over their whole latency range or within specified latency windows (click ‘Time window’). Clicking the ‘Draw curves’ button displays the .p curves in a new screen window, with a secondary menu bar at the top of the screen. This menu bar offers a number of options as:

  • select a subset of files to draw among the initially loaded files (‘Select Files’ button)
  • select display options (‘Display’ button) allowing among others to specify:
    • the time and amplitude scales (‘Scales’),
    • a personalized presentation format (‘Curves Layout’) that can be edited, saved, or loaded, allowing to have the same personalized curve presentation for a set of data,
    • the color and type of lines for the different .p files (‘Curves attributes’),
    • the file legends (by default, the legend is the .p file name),
    • positivity up or down
  • zooming on one electrode (‘Zoom’ button): this wil open a new window
  • spectrum analysis (‘Tools’ button, then ‘Spectrum’)
  • perform various measurements (‘Measures’) at one latency (amplitudes at each channel) or between two latencies (peak amplitudes and latencies, mean amplitudes, amplitude ranges, spatial or time RMS…). The latencies are selected by one or two cursors made active/inactive by clicking the ‘cursor’ button at the bottom left of the curve display. The measures can be saved in external files to be processed via standard statistic programs.

In addition, when one cursor is selected, it is possible to draw one spatial map per .p file at the cursor latency by clicking the ‘Map’ button. This opens a new screen window, at the bottom of which the ‘Display Options’ and ‘Map Options’ buttons allow to select the maps parameters (voltage or SCD maps, type of views (left, right, top, back…), amplitude scales (global, per file, value), … see more information below). Moving the latency cursor allows to dynamically follow the topographic changes of the different .p responses.
If two cursors are selected, it is possible to draw the spatiotemporal maps between the two latencies delimited by the cursors.

Topographic mapping (general menu bar, ‘Maps’ button).