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The Lateralizer

The Lateralizer is a tool for stimulus presentation and response recording in a divided visual field (DVF) paradigm, enabling students to investigate theories of asymmetries between the two cerebral hemispheres. Subjects are instructed to classify stimuli into two categories (A and B), and response time is recorded separately when the stimuli are presented on the left or right visual fields.

Application download in zip file with targets

About Asymmetry

It's been all around us for years; it is a cliche in general advice to managers, bankers, and artists; it's in cartoons. It is in advertisements. United Airlines offers reasons to fly boths sides of you coast to coast. The music for one side, and the good value for the other. The Saab automobile company offered their turbo-charged sedan as "a car for both sides of your brain," the rationale presumably being the cheap-to-run side and the exciting high-performance side. ... Many popular writers and educators have written that the right hemisphere is the key to expanding human thought, surviving trauma, healing autism, and more. It's going to save us. It's the seat of creativity, of the soul, and even great casserole ideas. The right side, no doubt.

Robert Orstein, The Right Mind

Despite a great deal of misinformation from folk-psychology, there are functional differences (asymmetries) between the right and left hemispheres, as observed from split-brain patients, individuals with unilateral brain damage, and neuroimaging studies. One way to easily explore these asymmetries (with normal subjects) is by using the divided visual field technique (DVF).

The visual system is "wired" so that information presented to the left visual field travels first to the right hemisphere, and information presented to the right visual field goes to the left hemisphere. If one cortical hemisphere is specialized to perform a particular function, then it might have an advantage when it receives information first. So by "lateralizing" visual stimuli, and comparing response times to these stimuli when they're presented to the different visual fields, we can explore theories of asymmetries between the cortical hemispheres.


The Lateralizer program is designed to run a categorization task on the left and right visual fields. Subjects are instructed to classify stimuli into two categories (A and B), and response time is recorded separately when the stimuli are presented on the left or the right.

The first step in designing an experiment using Lateralizer is to devise your categorization task. Think of a categorization task that would require cognitive processes that you think might be lateralized to one side or the other. Once you've decided on the task, you'll need to generate stimuli (or "exemplars") for the two different categories.

If you'll use images as your stimuli, you'll need to add the stimuli to the "targets" folder, which should be in the same directory as the Lateralizer.jar file, and then restart the Lateralizer application. If you'll use text as your stimuli, simply check "Use Text Instead:" and type your text. Click the "Do Experiment" tab to specify the number of trials, and click "Start Experiment" to begin.

For each trial, Lateralizer will present a centralized fixation point, then display your stimuli for 200ms. Lateralizer records the amount of time (since stimulus onset) that the subject takes to classify the target as either Category A or Category B.

Once the experiment is complete, Lateralizer will report the raw data obtained from each stimulus presentation.

Where To Go Next

If you find differences between the left and right visual fields, that's great, but try not to get too excited. While you've found evidence of a functional asymmetry between the two different hemispheres, you haven't proven that an asymmetry exists. As described by Ivry and Robertson (in their book The Two Sides of Perception, 1998):

These [divided visual field] methods have their limitations. A critical assumption has been that differences in performance with lateralized stimuli nearly always reflect functional differences between the two hemispheres. This is an extremely strong assumption. Researchers have tended to ignore or downplay the fact that asymmetries in brain function cannot be directly observed with these methods. It would require a leap of faith to assume that there is a straightforward mapping between lateralizing a stimulus and producing disproportionate activation throughout the contralateral hemisphere. Normal subjects have an intact corpus callosum, which provides for the rapid transfer of information from one hemisphere to the other.
In other words, the difference that you observed could be caused by something other than one hemisphere's advantage in your classification task. For instance, consider which hand your subjects were using to respond to the stimuli: If subjects always used their right hand to respond, you might expect that they'd be faster when the stimuli went to the left hemisphere because the left hemisphere "controls" the right hand. To avoid this confound, you should balance the response hand used across your subjects (or across trials).

Still, if you observe a reliable difference in classification time when stimuli are lateralized, your results can't be dismissed as meaningless. What might cause the difference between left and right stimulus presentations? Consider different general theories of the differences between the hemispheres, or theories of information processing between the two hemispheres.