Cognitive Studies/Psychology/Visual Studies 201:

Prism Adaptation (One-Day Experiment)

Laboratory Module

by Douglas R. Elrod, Cornell University


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.

Background Significance

Prism adaptation is a much-researched instance of dynamic perceptual/motor reorganization. Uncovering how it works will doubtless help us to understand how (and whether) humans deal with the challenge of new interfaces. Humans are rather flexible to various forms of rearrangement. Adaptation can occur even without active movement (but that often facilitates it). Feedback about errors can also facilitate adaptation. And tactile information can also increase prism adaptation.

Reading Assignment

Laboratory Exercise

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.

Setting up the Touchscreen

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:

And in the "Events" 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:
  1. The wrong display is being interpreted to be the touchscreen. In this case the calibration screen comes up on the non-touchscreen monitor. In general, we will want the "startup screen" (the one with the menu bar at the top) to be assigned to the touchscreen, because some of the software we will be using assumes that. If that's not true, it can be changed in the "Display" System preference (ask the Lab Coordinator to change this, if this is locked).
  2. The USB cable from the touchscreen is not plugged in. Locate this cable and be sure it's plugged into the open USB jack on the back of the keyboard (the jack on the other side of the keyboard has the mouse plugged into it).

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).
Prism diopter definition
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.

Pre-exposure trials:
These are performed while the participant is NOT wearing prism goggles. (If "control" goggles, without prisms, are available, they should be worn.)

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.

Prism-exposure trials:
The participant wears the (30 diopter, base right) prism goggles for this stage. These trials are the same as the pre-exposure trials, except that, instead of the final high-pitched tone, the screen turns white, allowing the participant to see where his or her finger landed. These trials are separated by inter-trial intervals of 2.5 seconds. Participants always use their dominant hand to touch the screen in this stage.

There are 60 prism-exposure trials.

Post-exposure trials:
These are just like the pre-exposure trials. They are performed while the participant is NOT wearing prism goggles. (If "control" goggles, without prisms, are available, they should be worn.) As in the pre-exposure stage, participants alternate which hand touches the screen, starting with their dominant hand.

There are 20 post-exposure trials.

At the end of the experiment, a screen appears saying "Touch the screen to finish." At this time, touching the screen anywhere will produce a return to the normal SuperLab environment, and the keyboard will be useable.

Possible Extension:

Adaptation has been investigated in many different contexts. Held (1965), for instance, describes a yoked active-passive kitten experiment. This might be difficult to do with prism-goggled humans, though. One might attempt the "passive transport" of a prism-goggled participant, however, as a replacement for the "Prism-Exposure" trials. These participants could be pushed around in chairs while wearing the prism goggles, and compared with other prism-goggle wearers who are allowed to walk around freely.

Data Analysis

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: Thought Questions: