It's long been known that putting an image of the environment onto the retina of the eye is just a small piece of the visual perception system. A striking example of this can be seen in the phenomenon of prism adaptation. Prism adaptation experiments have been conducted since at least the 1800s. These experiments involve changing the usual correspondence between the light hitting the eye and the environment, by placing prisms (among other optical devices tested) in front of the eyes.
In the simplest case, this can shift the direction of light rays by a constant angle, so that, at first objects in the world seem to be moved by that amount. But over a relatively short period of time, one can become used to the perturbation, so that one can move around with nearly as much ease as before (behavioral adaptation), and the world may even begin to "look right" (experiential adaptation). Because it's easier to measure the behavioral changes, most experiments have concentrated on how motor tasks, most commonly reaching, are affected by prism adaptation.
In addition to altering the apparent position of objects, prisms have side effects, such as changing straight edges to curves (primarily affecting vertical lines, given the prisms in this experiment) which can cause the world to appear "warped" as one moves through it. Another alteration is "chromatic aberration", the classic "prism effect" where white light separates into spectral colors, especially around light sources. It seems to be possible to adapt to these effects, too, although that won't be the focus of this module.
The following laboratory exercise is designed to assess how the usual hand-eye coordination can be changed, in a short time, by altering the visual input with prism goggles. It is adapted from Cohen (1973), to use modern technology (such as touchscreens), and be runnable in one day.
Before beginning the laboratory exercise, make sure that the touchscreen is configured properly.
First check the settings in "TBMorph" (there should be an icon for this on the desktop -- it is one of the two that look like a fingerprint, with a big red arrow in the middle). Double-clicking this should bring up a window labeled "Pointer Device Properties *** Beta Build ***".
All values should be set to defaults, except for three. The default settings are, in the "Settings" screen:
And in the "Advanced" screen:
The three settings you may have to change are:
In the "Settings" screen:
If you change any of these, remember to click the "Apply" button, and then click "OK".
If these are set differently, individual touches could be interpreted as drags of the cursor, which can cause experiments using the touchscreen to flash a series of stimuli on the screen without waiting for new touches. If you see that, it's probably because of incorrect settings.
After verifying that the settings in "TBMorph" are correct, double-click on the "Calibrate icon" on the desktop (it is the other icon that looks like a fingerprint, this one with a cross-hair with a red dot in the middle). This will bring up a calibration screen with a cross in the upper left, then the lower left, then the upper right, then the lower right. You should touch these crosses as soon as they come up.
If the cross doesn't disappear soon after you touch it, your touch is not being detected. This could occur for a couple of reasons:If you still have problems, contact the Lab Coordinator.
After calibrating, the software will take a couple of seconds to set itself up, then the touchscreen should respond to your touches by moving the "arrow" of the mouse cursor to the touch location.
You'll need to run the experiment at one of the touchscreens in a darkened room (so that you can't see your hand during certain parts of the experiment). The experiment, in the basic form described here, should take less than ten minutes to complete for each participant.
You'll need a pair of the prism goggles (base right, 30 prism-diopter), for part of the experiment. See Fig. 1 for the relation between prism diopters and deviation angle for a prism: Prism diopters = 100*tan(deviation angle).
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| Figure 1. Relationship between Prism Diopters and Deviation Angle |
Open the folder entitled "Prism Adaptation Lab". Within that folder, you'll see the SuperLab script "prism.script" and a folder entitled "Stimuli" which contains all the stimulus files.
To run in the experiment, sit about 55 cm from the screen (use a meter stick to check), and rest the hand you are using to touch the screen on the edge of the table, when not touching the screen. Make sure that the face of the touchscreen is 20 cm from the table edge (so that all participants are reaching the same distance).
To begin the laboratory exercise, first make sure to quit out of other applications. Make sure the touchscreen monitor is set to 640x480 resolution (otherwise the stimuli will either not fit on the screen, or will be displayed in a frame in the middle of the screen).
Open the "Prism Adaptation" folder on the desktop, and then double-click on the "prism.script" file inside that folder. This should start SuperLab running on the prism experiment. (If the computer is low on memory, you may find that the "Experiment Editor" display is empty at this point. You would then have to go to the File menu in SuperLab, and "Open" the prism.script from there.)
DO NOT MAKE ANY CHANGES within the SuperLab Experiment Editor display and DO NOT press on the buttons at the bottom of the window.
When the SuperLab Experiment Editor window opens, go to the Experiment pull-down menu, and select Run. A different window will now open. This window allows you to enter the participant's name. Type in a unique name or designation for the participant and press the Run button in the window.
Another window will appear. This window allows you to specify where the results of the laboratory exercise will be saved, and what will be the name of the file in which the results will be saved. You can select the hard disk (on the Desktop, for instance, in a folder labeled with the experimenter's name) for now, but you should remember to copy the results file to a backup location (writable CD, for instance) as soon as possible. Data files and other personal files on the computer's hard drive are liable to be erased.
Second, type in a unique name for the data file that will be created, replacing the default file name that automatically appears. Each data file must have a different, unique name. If two files have identical names, the more recent file will replace and destroy the older file.
After the location for saving the data file and the data file's name have been specified, the experiment automatically begins. Instructions will appear on the screen. Starting at this time, and continuing until all the trials of this laboratory exercise have occurred, the computer keyboard will become inoperative. Only input from touching the screen or from the mouse will have any effect.
Note: If for any reason you need to terminate the experiment prematurely, hold down the command-key (Apple key) and click the mouse.
Reminder: It is important to turn off all unnecessary applications (including a screen saver if one is present) before running an experiment.
What follows is a description of the SuperLab script, "Prism.script".
First of all, the participant is asked whether he/she is primarily left-handed or right-handed. There are three stages to the experiment itself. The first is pre-exposure.
Each pre-exposure trial consists of a small white cross briefly (200 ms) appearing at a random location on the screen, followed by a field of white crosses (to reduce retinal afterimages) displayed for 400 ms, and then a black screen. When the black screen appears, a 500 Hz tone plays, which is the signal for the participant to touch the screen at the location where he or she saw the first white cross. In this stage, participants alternate which hand touches the screen, starting with their dominant hand.
When the participant touches the screen (or after 4 seconds, whichever is first), a higher-pitched tone (600 Hz) is heard, and after an inter-trial interval of 3 seconds, the next trial begins.
There are 20 pre-exposure trials.
There are 60 prism-exposure trials.
There are 20 post-exposure trials.
There are two kinds of dependent variables that are measured during the experiment. Location of the screen-touch, x: horizontal, and y: vertical, and the time it took to make the touch (from when the tone played to when the screen was touched).
There were several types of independent variables that were manipulated during the experiment. One is the location (x,y) of the target crosses. The other, of primary interest, is what stage of the experiment the participant is in (pre-exposure, prism-exposure, or post-exposure). Also of interest is which hand the participant used to touch the screen (dominant hand, or non-dominant hand?). Note that when the experiment begins, the participant is asked to indicate which hand is his/her dominant one.
One also shouldn't forget the trial order. Early trials in a stage may not have adapted as much as later trials.
Regarding the target locations, that may well affect the time to touch the target, because the higher targets are farther away from the hand's starting position. This relationship is explored further in the Touchscreen Manual Module.
The experiment as it is described here is a within-subject design. You may choose to introduce a between-subject variable by splitting your subjects into two groups, for instance, and having one group adapt to the prism goggles in one way, and the other group adapt in a different way.
By comparing the error (x-touch - x-actual) between the three conditions, you can see how (and whether) the participant adapts to wearing the prism goggles, and if an aftereffect (which causes errors in the reverse direction of the prism shift) occurs after the participant takes off the goggles.
Given that we were using prisms rated at 30 prism diopters, you can determine the angle that objects are shifted, relative to normal vision (see Fig. 1). You can thereby predict where a subject would be expected to touch the screen, while wearing the prism goggles, before prism adaptation occurs. How does this compare to the initial touch-locations in the prism-adaptation stage?
Also, by comparing the performance of each hand during the post-exposure trials, you can see if the adaptation transfers to the (non-dominant) hand that wasn't used during the prism-adaptation stage.
When looking at the pattern of results, one should keep in mind that speed of performance is often inversely related to accuracy. This is the Speed-Accuracy tradeoff. Is there evidence of this in your data?
Other possible analyses:After you have completed the Laboratory Exercise (including any alterations to the prism-adaptation stage), you will be ready to write a final report in journal article format. (Select a journal that represents an interest of yours. Follow its 'Instructions to Authors' and its general format in preparing your report.) General guidelines can be found here.
Each participants' data will be stored in a file that can be opened using Microsoft Excel. It is also possible to read these files as text using other programs such as SimpleText or TeachText.
The participant code is at the top of the file, followed by the script name ("Prism.script"), and the date and time when the experiment was run. Then there's a blank line, and the data columns follow, headed by the column-names which are listed below.
Each trial of the spreadsheet takes up one row of the spreadsheet, with successive trials going from top to bottom.
The columns in the datafile, from left to right, are as follows:
The row near the top with trial name "Left-right ques" is where the participant indicated their handedness. This information is contained in the x-coordinate of the touch-location. Code x-values below 320 (the left half of the screen) as indicating "primarily left-handed" and values 320 or above (the right half of the screen) as indicating "primarily right-handed".
Given that, during the pre- and post-exposure stages, each participant alternated using their hands to touch, starting with his or her dominant hand, you should add a column indicating which hand was used on each trial. Perhaps call it "Hand", with values "left" or "right".
Note: It's best to edit a copy of the data file, rather than the original data file itself, given the risk of deleting data!
At the time this module was written (2004), glass wedge prisms of the appropriate strength (around 30-diopters) proved difficult to obtain. Therefore, prism goggles were constructed using a thin Fresnel-prism film (3M Press-On Optics), oriented to produce a base-right prism effect (objects appear to the left of where they would appear if the film weren't there). Because many different diopter-ratings are available, it was possible to purchase precise 30-diopter films. It should be noted that the current state of this technology seems to be necessarily more "blurry" than glass prisms; so participants may not be able to do activities requiring high acuity, such as reading, while wearing the prism goggles.
The film normally self-adheres, using warm water, to glasses (spectacles). To make prism goggles, it was applied to the clear eyepieces of flip-up welding goggles (which have tinted eyepiece-covers that can be flipped up out of the way). These were chosen because of their opaque frames, so that non-refracted areas don't conflict with the view through the film, unlike most "safety goggles" whose sides would have to be covered over.