Westheimer (1967) reported a sensitization result for detection of a small spot of light. In the task, the subject must judge whether or not he detected a brief, small test spot when it was superimposed on a larger luminous disk. Westheimer was interested in measuring the test spot luminance whereby the subject could just see the test spot. This threshold was measured for a variety of disk diameters. The interesting finding is that test spot luminance threshold first increases with disk diameter (indicating detection requires a higher test spot luminance) and then decreases with disk diameter (indicating detection can be done with a lower test spot luminance).

This finding is important because it has been described as psychophysical evidence of the neural coding of visual information. Cells early in the visual pathways from the retina to cortex have an on-center, off-surround receptive field. This means that a small spot of light centered on the cell's receptive field is excitatory, while a similar spot of light in the surround is inhibitory.

More significantly, a very small disk of light centered on the receptive field could excite only part of the center region, and thereby give an weak excitatory response. As the disk grew larger the excitatory response would grow stronger, as more of the center region is filled with light. As the disk grew larger still, the response would grow weaker, as the disk covers larger regions of the inhibitory surround. The general shape of the neural response looks much like the shape of test spot luminance threshold found by Westheimer.

Several researchers have suggested that the results found by Westheimer (commonly called the Westheimer effect) are directly the result of the on-center, off-surround receptive field structure. In particular, detecting an additional test spot on a disk requires noting some change in the cell's response to the disk alone. Presumably, the stronger the cell's response to the disk, the more difficult it is to detect a change.

This lab allows you to participate in a version of Westheimer's experiment.


Click on the "Experiment" button to start to the experiment. A window will appear, which will present all stimuli. A trial is started by pressing the space bar. Centered on the screen will be a small gray disk. After a variable delay (0.5 to 1.5 seconds), a very small test spot will appear for approximately 50 milliseconds at the center of the disk. Half a second after the test spot disappears text asks you to enter your response. Your job is to change the luminance of the test spot so that you can just barely detect it.

You increase the luminance of the test spot for the next trial by pressing "i". You decrease the luminance of the test spot for the next trial by pressing "k". Each set of trials begins with the test spot having the same luminance as the disk, and is therefore undetectable. You must increase the test spot luminance until you can just detect it. After you find you can see the test spot you should probably decrease its luminance to see if you can still detect it. Keep making adjustments to find the test spot luminance where you can detect the spot, and where any lower luminance renders it undetectable.

Once you find this threshold luminance value, press the space bar. The disk diameter will change and you repeat the above process to find a new test spot luminance threshold. This process will be repeated 10 times for different disk diameters. After completion of all disks, a graph will appear that plots the RGB threshold values as a function of disk diameter in pixels. You should find that the curve rises and then falls.

Sit back a bit from the monitor. Resist the urge to move closer to the monitor for the smaller disks, doing so will screw up your data. Take breaks whenever you need to, this is a demanding experiment, particularly for the smallest disks. Try to get rid of any glare on the monitor.

You can download source code for this experiment at

Copyright Purdue University 1998