# Windowing Raw Sample Data

#### Austin Hurst

#### 2026-02-22

    # Import libraries required for the vignette
    library(eyelinker)
    library(dplyr)
    library(stringr)
    library(tidyr)
    library(ggplot2)

In most situations, the raw sample data from the eye tracker isn’t very useful on its own: generally, you’re interested in changes in eye position and/or pupil size within certain periods of time (e.g. change in pupil size following a stimulus onset).

To split the raw signals into segments based on task events we typically need to extract and parse the message data, which contains time-stamped messages sent by the experiment program during the experiment runtime.

This vignette provides a basic example of how to extract events from trial messages and align them with raw sample data.

    # Read in example data
    fpath <- system.file("extdata/mono250.asc.gz", package = "eyelinker")
    asc <- read_asc(fpath)

## Extracting Trial Events From Messages

Trial event messages are stored in the `msg` table of the imported data, which contains the text and timestamps of all messages sent to the tracker by the experiment program during each trial. Let’s take a look at the messages in our example data:

    asc$msg

    ## # A tibble: 29 × 3
    ##    block    time text
    ##    <dbl>   <dbl> <chr>
    ##  1     1 5886023 -6 Initial_display
    ##  2     1 5886023 -5 !V DRAW_LIST ../../runtime/dataviewer/js/graphics/VC_1.vcl
    ##  3     1 5886504 0 Display_initial_time_out
    ##  4     1 5886514 -14 Target_display
    ##  5     1 5886514 -14 !V IAREA FILE ../../runtime/dataviewer/js/aoi/IA_1.ias
    ##  6     1 5886816 0 Saccade_target
    ##  7     1 5886822 -6 End_trial_display
    ##  8     2 5888666 -12 Initial_display
    ##  9     2 5888666 -12 !V DRAW_LIST ../../runtime/dataviewer/js/graphics/VC_2.vcl
    ## 10     2 5889154 0 Display_initial_time_out
    ## # ℹ 19 more rows

As you can see, each trial contains a sequence of messages relating to events that happen during the trial (e.g. `Initial_display`). To get this into a more useful format, we can use pattern-matching functions from the `stringr` package to identify the onsets of trial phases based on the message text:

    trial_phases <- asc$msg %>%
      mutate(
        phase = case_when(
          str_detect(text, "Initial_display") ~ "onset",
          str_detect(text, "Target_display") ~ "target_on",
          str_detect(text, "End_trial") ~ "trial_end"
        )
      ) %>%
      subset(!is.na(phase)) %>%
      select(-c(text)) %>%
      mutate(
        phase = as.factor(phase)
      )

The `case_when` block here checks for partial matches in the message text corresponding to our events of interest and creates a new column with their phase names (defined on the right of the `~`). `case_when` will fill any messages that don’t match one of the defined phases as NA, so we just filter out NA phases to get the timestamps for the events we actually care about. Let’s look at the resulting data frame:

    trial_phases

    ## # A tibble: 12 × 3
    ##    block    time phase
    ##    <dbl>   <dbl> <fct>
    ##  1     1 5886023 onset
    ##  2     1 5886514 target_on
    ##  3     1 5886822 trial_end
    ##  4     2 5888666 onset
    ##  5     2 5889164 target_on
    ##  6     2 5889454 trial_end
    ##  7     3 5891689 onset
    ##  8     3 5892180 target_on
    ##  9     3 5892454 trial_end
    ## 10     4 5895296 onset
    ## 11     4 5895797 target_on
    ## 12     4 5896082 trial_end

Much easier to follow! Note that converting ‘phase’ to a factor isn’t strictly necessary, but since factor columns take up quite a bit less memory that string columns in R and raw sample data can be millions of rows long it’s generally a good idea.

## Aligning Samples With Phases

Now that we’ve got a table of trial events and their onsets, all we need to do is connect this info with the raw sample data.

For data recorded at 1000 Hz this is straightforward, but for 500 Hz and 250 Hz recordings we need to do some extra work to make sure the event timestamps line up properly with the sample timestamps. This is because messages are always recorded at a resolution of 1000 Hz regardless of the sampling frequency, meaning you can end up with messages that happen in between samples instead of corresponding directly to them (e.g. samples at times 10000, 10004, 10008, etc. and message at time 10002).

To fix this, let’s write a little utility function to sync message timestamps to their nearest samples and use it on our table of events:

    sync_timestamps <- function(df, raw) {

      # Get tracker sample rate and block onset timestamps from raw pupil data
      ms_per_sample <- median(lead(raw$time)[1:10] - raw$time[1:10])
      block_onsets <- raw %>%
        group_by(block) %>%
        summarize(onset = time[1])

      # Adjust df so that its timestamps map onto the nearest sample timestamp
      df <- df %>%
        left_join(block_onsets, by = "block") %>%
        group_by(block) %>%
        mutate(
          time = round((time - onset) / ms_per_sample) * ms_per_sample + onset
        ) %>%
        select(-c(onset))

      df
    }

    trial_phases <- sync_timestamps(trial_phases, asc$raw)

If you’re working with 1000 Hz recordings, you can skip the above step completely.

Now that the event timestamps are properly aligned, let’s connect the trial phases with the raw sample data!

    rawdat <- asc$raw %>%
      left_join(trial_phases, by = c("block", "time")) %>%
      group_by(block) %>%
      fill(phase, .direction = "down")

`left_join` here joins the the trial phases we defined above to their corresponding blocks and timestamps in the raw gaze data. Then, for each block we use `fill` to fill in the gaps between trial events with the correct phase label.

## Putting It Together

Now that we’ve identified the event phases for each sample in the raw data, let’s try using it to help visualize the changes in pupil size across trials.

First, we’ll drop all samples without a corresponding phase (all samples prior to the trial onset message in this case) and get the time relative to trial onset for each trial:

    rawdat <- rawdat %>%
      subset(!is.na(phase)) %>%
      group_by(block) %>%
      mutate(
        trialtime = time - time[1]
      )

Before we plot, let’s see what our data looks like now:

    rawdat

    ## # A tibble: 818 × 8
    ## # Groups:   block [4]
    ##    block    time    xp    yp    ps cr.info phase trialtime
    ##    <dbl>   <dbl> <dbl> <dbl> <dbl> <chr>   <fct>     <dbl>
    ##  1     1 5886021  511.  385.  1026 ...     onset         0
    ##  2     1 5886025  510.  384.  1024 ...     onset         4
    ##  3     1 5886029  511.  384.  1027 ...     onset         8
    ##  4     1 5886033  511.  384.  1022 ...     onset        12
    ##  5     1 5886037  511   384.  1020 ...     onset        16
    ##  6     1 5886041  512.  385.  1019 ...     onset        20
    ##  7     1 5886045  512.  386.  1013 ...     onset        24
    ##  8     1 5886049  511.  386.  1017 ...     onset        28
    ##  9     1 5886053  511   386.  1017 ...     onset        32
    ## 10     1 5886057  511.  386.  1016 ...     onset        36
    ## # ℹ 808 more rows

Now we can take the data and plot it out, color-coding by phase:

    # Plot pupil size across the first 4 trials, color-coding by phase
    ggplot(subset(rawdat, block < 5), aes(x = trialtime, y = ps, color = phase)) +
      geom_line() +
      facet_wrap(~ block) +
      labs(color = "Phase") +
      ylab("Pupil Size") +
      xlab("Trial Time (ms)")

## Extracting Windows Around Events

The above approach is useful for visualizing eye data and for subsetting data based on event onsets, but what if you want to get a specific window of time around an event?

Let’s say you wanted to grab the region of data starting 100 ms before and ending 200 ms after the target onset for each trial. First, we’ll define some helper functions for extracting epochs from event data and testing whether a given timestamp is within them:

    get_epochs <- function(msg, start, end = NULL, pad = c(0, 0)) {
      end <- ifelse(is.null(end), start, end)
      epochs <- msg %>%
        group_by(block) %>%
        # Select start/end event messages for each trial
        filter(grepl(start, text) | grepl(end, text)) %>%
        # If start != end and trial has no end event, discard trial
        filter(n() >= ifelse(start == end, 1, 2)) %>%
        # Get start and end timestamps for each trial, adding padding
        summarize(
          start = time[grepl(start, text)] - pad[1],
          end = time[grepl(end, text)] + pad[2],
        )
      epochs
    }

    within_epoch <- function(time, epochs) {
      time %within% cbind(epochs$start, epochs$end)
    }

### Defining Epochs

Our `get_epochs` function takes a data frame of messages (`msg`) and returns a data frame of epochs (i.e. windows of time) based on strings matching start and (optionally) end events in those messages. It also lets us define a padding window (in milliseconds) to add to either end of the epoch:

    # Get the window of data between the trial start and end events for each trial
    trial_windows <- get_epochs(asc$msg, start = "Initial", end = "End")

    # Get the window of data 100 ms pre-target to 200 ms post-target for each trial
    target_windows <- get_epochs(asc$msg, start = "Target", pad = c(100, 200))

As illustrated above, `start` and `end` identifiers do not need to match the full text of their corresponding event messages, just enough of the text to be uniquely identifying.

### Extracting Epochs

Now that we’ve defined some epochs, we can use our `within_epoch` helper function to subset the sample data based on these windows of time:

    raw_trial <- asc$raw %>%
      filter(within_epoch(time, trial_windows))

    raw_target <- asc$raw %>%
      filter(within_epoch(time, target_windows))

Let’s check that our epochs are working correctly by verifying the length of the target windows we extracted for each trial:

    raw_target %>%
      group_by(block) %>%
      summarize(duration = max(time) - min(time))

    ## # A tibble: 4 × 2
    ##   block duration
    ##   <dbl>    <int>
    ## 1     1      296
    ## 2     2      296
    ## 3     3      296
    ## 4     4      300

Note that due to asynchrony between message timestamps and sample timestamps at sample rates under 1000 Hz (as explained earlier), the window intervals may be off by a sample (4 ms in this case) unless additional corrections are made.

### Epochs With Other Event Types

Using the same helper functions, we can also subset eye events recorded by the tracker such as blinks, fixations, and saccades based on when they occur in the trial. For example, let’s check the data for any saccades that started between target onset and trial end:

    post_target <- get_epochs(asc$msg, start = "Target", end = "End")

    asc$sacc %>%
      filter(within_epoch(stime, post_target))

    ## # A tibble: 4 × 11
    ##   block   stime   etime   dur   sxp   syp   exp   eyp  ampl    pv eye
    ##   <dbl>   <dbl>   <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <fct>
    ## 1     1 5886725 5886773    52  509.  384   241.  376.  7.57   401 L
    ## 2     2 5889357 5889405    52  514.  384.  243   366.  7.68   439 L
    ## 3     3 5892369 5892405    40  515.  385.  796   377   7.92   371 L
    ## 4     4 5895997 5896033    40  510.  373.  800.  380   8.16   391 L