abftools

abftools is a package for reading and analysing abf data in R. The tasks are splitted into three major parts: data acquisition, data processing and plotting.

Overview

Currently, a few R packages provide loading of abf files, notably abf2 and readABF.

• abf2 by Matthew Caldwell provides functions to load gap-free ABF2 files and basic plotting functions.

• readABF by Stanislav Syekirin provides a better support of loading both ABF and ABF2 files.

What makes abftools different to aforementioned packages are:

• abftools not only provides functions to load ABF2 files, but also a set of analysis and plotting functions that help streamline data processing of ABF2 data.

• abftools is designed to be data-first and performance-focused.

  • data-first: An abf object is designed to be a 3d array with dimensions of abf[time, episode, channel], regardless of op mode be it episodic, gapfree or event driven variable length. This guarantees consistent subsetting across all abf objects and compatibility of R base functions and 3rd party functions. Compared to nested list with meta available immediately to userspace, getter and setter functions are provided to access attributes that are still relevant.

  • performance-focused: abf objects are usually large, and dealing with large objects are slow. Thus calculation critical functions are implemented with performance in mind and profiled/optimised to save even microseconds of CPU time.

Task: Data Acquisition

Abf files can be loaded by calling abf2_load() and abf2_loadlist() (to load multiple files). To facilitate easier batch loading by abf2_loadlist(), two simple helper functions SelectSample() and ExcludeSample() are provided to select desired files from an index file which contain a filename column and multiple conditional columns. By uing syntax like SelectSample(df, cond1 = value1, cond1 = value2, ...), one can achieve simply data index management without loading extra packages.

abftools currently does not support writing/saving an abf objects to abf files. We do not believe writing abf2 file structure our top priority since it's proprietary and not even documented. Please resort to other serialisation methods at the moment.

Base data structure

abftools is built around the data class abf. An abf object is essentially a 3d array with some optional attributes attached to it.

Dimensions definition of abf objects.

An abf object has dimensions of c(time, episode, channel) since the logic
of accessing data by channel, and processing data by episodes (sweeps) along time and such dim arrangement can help improve memory performance. Notice that the dimensions are different to abf files stored on disk (which is c(channel, time, episode)), so if data is loaded by other means it may not be compatible with abftools before permutation.

When loading non-episodic abf or single channel files, the 3d structures are still preserved i.e. indices are not dropped, so function calls are consistent for any op mode.

In addition to using dim(abf), you can also call nPts(), nEpi() and nChan() to acquire the dimensions of an abf object and improve code readability.

Subsetting abf objects.

You can subset an abf object by abf[time, episode, channel], just like any array. Please notice that subsetting an abf object consequently removes all attributes assigned to it, making the returned values not an abf object any more.

An alternative way of subsetting is using abf[[channel]] to extract channel data of interest. The differences between abf[,, channel] and abf[[channel]] are:

• You can subset multiple channels using abf[,, channel], however only one using abf[[channel]]. A warning is printed and only first channel will be extracted if multiple channels are supplied to [[.

• abf[,, channel] returns all episodic data while abf[[channel]] excludes those are marked "removed" using RemoveEpisode().

• abf[,, channel] returns "raw" slices of the subset data (some indices may be dropped), while abf[[channel]] does some extra steps to maintain returned shape, and attach proper colnames to the returned matrix by calling DefaultEpiLabel() for a clearer presentation.

Attributes of abf objects.

In most cases, you don't need to access attributes of an abf objects since they are managed by specific functions automatically. Here is a list of attributes currently assigned to an abf objects:

• class: should always be "abf".

• title: title of the abf object, defaults to the file name when loaded. You can access title by GetTitle() and SetTitle().

• mode: op mode of the abf object, defined by voltage clamp protocol. Accesed by GetMode() and you should not change this attr.

• ChannelName, ChannelUnit and ChannelDesc: channel names, units and descriptions. These attributes are parsed from Strings section of an abf file. Accessed by GetChannelName(), GetChannelUnit() and GetChannelDesc(), and can be set by SetChannelName(), SetChannelUnit() and SetChannelDesc()

• SamplingInterval: the sampling frequency defined by voltage clamp protocol. Accessed by GetSamplingIntv(abf).

• EpiAvail: a vector maintained to record whether an episode is marked as "removed", accessed/set by GetAvailEpisodes(), RemoveEpisode(), RestoreEpisodes().

• meta: a list of raw (not parsed) properties of the abf files. Most of the meta data is not maintained, however, meta$SynchArray is evaluated and maintained by some functions, espeically those related to var-len mode.

Task: Data Processing

As 3d data structures, when processing/analysing abf data the final goals are mostly "flatten" the object to some 2d forms, be it plotting or presenting statistics that can be printed on paper. Data processing in abftools follows the logic of map-reduce and is facilitated by functional programming, most notably the wrapper function wrap(). The following example demonstrates wrapping commonly used mean() to calculate averages of abf objects.

f <- wrap(mean)

#f() now calculates mean I-V of an abf object
f(abf, intv = c(7000, 7200))

	I (nA)	V (mV)
epi1	25918.6251	47.636025
epi2	19352.5715	29.722206
epi3	13000.3191	12.142201
epi4	6907.2745	-4.560712
epi5	863.5107	-20.718752
epi6	-5051.8364	-36.100685
epi7	-11433.0079	-52.206389
epi8	-18052.0749	-68.457909
epi9	-24555.7584	-84.004401
epi10	-31109.3744	-99.271597
epi11	-12021.4602	-53.548504

In this case, wrap(mean) maps function mean() to time domain of an abf object, calculating average values of desired time interval for very corresponding episodes and channels. Then reduces those values by as.data.frame().

Built-in statistical functions

Here is a breif overview of some most used statistical functions provided in abftools. For a full function list, please refer to package help.

• The frequently used mean(), sd_abf() and sem_abf() are provided which can be used directly on an abf object.

• IVSummary() calculates mean and sem of mean voltage/current channels of a list of abf objects and can be handy for repeat/replicate experiments.

• CmpWaveform() and FindSamplingInterval() find a representative time interval of an abf object. CmpWaveform() returns a list of intervals which the current/voltage
  record matches the waveform settings. FindSamplingInterval() has a more specific
  usage: it finds an time interval that the voltage channel matches waveform while
  the current channel is most stable, which can be useful for TEVC analysis. Combining fast plotting methods PeekChannel(), QuickPlot() for quick inspection, one can avoid time consuming manual cursor settings, especially for multiple files.

• SampleAbf() reduces data points of an abf object, a sampling function can also be used to provide more flexible sampling.

Low-level functions

In most cases, data processing of an abf object can be generalised as:

• Procedures that maintains time-episode-channel dimensions. This is implemented by samplend().

• Procedures that "collapse" a specific dimension and results in a 2d object. This is implemented by mapnd().

• Procedures that coerce a 3d structure to 2d form. This is implemented by MeltAbf().

Examples:

I-V plots of TEVC data:

#Step 1. Load data.
alist <- abf2_loadlist(filelist)
#Step 2. Find representative time intervals.
ilist <- FindAllSamplingIntervals(alist)
#Step 3. Calculate IV summaries.
ivs <- IVSummray(alist, ilist)
#Step 4. Plot I-V chart
QuickPlot(ivs, smooth = TRUE)

Custom plotting using ggplot:

channel_labels <- c("Current", "Voltage")
f <- wrap(max, epi_id_func = DefaultEpiLabel, chan_id_func = channel_labels,
          #add an abf id column to identify each abf object
          abf_id_func = GetTitle)
data <- lapply(abf_list, f)
ggplot(data.table::rbindlist(data), aes(Voltage, Current)) + geom_line(aes(colour = id))

Plotting

Plotting of an abf object can be easily achieved with the ggplot2 package. Results from wrapped functions (to plot channel vs channel) and MeltAbf() (to plot channel vs time) are compatible with ggplot2.

Predefined PlotChannel() and PlotAllChannel() plot channel time series of an abf object. MultiPlotChannel() arranges multiple channel time series of a list of abf objects into a single plot. QuickPlot() provides a unified interface to plot various types depending on supplied objects. Please refer to package help for a full list of plotting functions.