Web-based perception experiments
Experiments can be hosted on the Department of Linguistics server, and then distributed to listeners by email link, or via Amazon Mechanical Turk.
An example is here: example experiment
A Javascript library audexp.js makes it realatively easy to implement the following four typical kinds of experiments:
- Identification (id) - a single audio file is played, and a two-alternative forced choice (2AFC) is given.
- Discrimination (ax) - two audio files are played, and a 2AFC is given.
- Rating (r) - a single audio file is played, and a rating number (from 1 to 7) is given.
- Contrast Rating (cr) - two audio files are played, and a rating number is given.
Your html code must have four features:
1) after loading audexp.js, load a javascript file that defines:
- an array (or two arrays) of filenames that will be presented.
- a variable called 'block'
- here's an example of such a file
- this .js file was created with a small perl script from a .csv spreadsheet.
- the html header contains the following two lines to get the audexp.js library and the experiment-specific list of sound files.
<script src="js/audexp.js"></script> <script src="js/blocktest_list.js"></script>
In the experiment linked above, the link is to a php script which dynamically constructs the html code that is delivered the user's web browser. The word 'test' is passed to the php script in the URL: http://linguistics.berkeley.edu/~kjohnson/rating_exp.php?list=test
The list variable is read from the php $_GET variable:
<?php
$block = $GET["list"]; ?>
In the php script, then the javascript lines above are generated with a
<script src="js/audexp.js"></script> <script src="js/block<?php echo $block; ?>_list.js"></script>
2) a call to load the experiment when the page is loaded
- this call specifies the type of experiment (in the script below this is 'id')
- whether to randomize the order of presentation of the list of sound files
- the interstimulus interval for 'ax' and 'cr' types
- and the intertrial interval for all experiments.
<body onload="load('id',false,500,2000);">
3) Three or four span elements that will be used to give feedback to listeners.
- Show warnings to the listener
<span id="wr"></span>
- indicate that an audio file is playing
<span id="f1">sound 1</span>
- indicate that a second audio file is playing (for ax, and cr type experiments)
<span id="f2">sound 2</span>
- give feedback to the lister, showing which key they pressed.
<span id="key">#</span>
4) A <form ...> element named "dataform"
- the order of the input items determines the column order in the output file.
<form method="POST" id="dataform" action="process.php?p=id&n=1">
<input type="hidden" name="subject" value=<?php echo $subj; ?> />
<input type="hidden" name="trial" />
<input type="hidden" name="list" />
<input type="hidden" name="file1" />
<input type="hidden" name="filedur" />
<input type="hidden" name="mystatus" />
<input type="hidden" name="loadtime" />
<input type="hidden" name="response" />
<input type="hidden" name="rt" />
</form>
Saving data to a server file
Finally, in (4) above you may have noticed there was reference to a file: process.php. Here is that file.
<?php
$prefix = $_GET["p"];
$n = $_GET["n"];
$datafile = $prefix . "_data" . $n . ".csv";
if ($_SERVER["REQUEST_METHOD"] == "POST") {
$data = "empty";
foreach($_POST as $key=>$value) { // we don't have to know the keys
if ($data == "empty") { $data = $value; }
else {$data = $data . "," . $value;}
}
$data = $data . "\n";
$ret = file_put_contents($datafile,$data,FILE_APPEND| LOCK_EX);
if ($ret) {
echo "saved data";
} else {
echo "there was a problem writing the data";
}
} else {
http_response_code(403);
echo "invalid submission";
}
?>
- This script constructs a filename from the parameters that you pass to it, and then stores the data from a single key press response as a line in the data file.
- for example, "process.php?p=id&n=1" translates into the filename "id_data1.csv"
- The <input..> elements of "dataform" (# 4 above) are now saved as columns in the data file.
- The data file must already exist on the server
- This script adds lines, but does not create a new file
- The data file must be writable, for example: chmod a+w id_data1.csv
- You can read the data file directly into R from your server address:
data <- read.csv("http://linguistics.berkeley.edu/~kjohnson/talkers/id_data.csv",header=TRUE);
