The Split Brain

1. Patient and control recruitment

1. There are a variety of patients with disconnection syndromes, including complete and partial surgical callosotomies and congenital conditions, such as agenesis of the corpus callosum (ACC), in which the corpus callosum does not fully develop. There are multiple tracts that connect the two hemispheres; the largest is the corpus callosum, but some fibers cross at the anterior commissure, hippocampal commissure, and posterior commissure.
   Note that these different varieties of disconnection may lead to different behavioral outcomes in this testing.
2. For the purposes of this experiment, preselect the patient through the use of neuroimaging to confirm the absence of connecting fibers.
   1. Standard MRI and diffusion imaging, which can be used to image white matter tracts, are particularly useful. Knowing which connecting fibers are present in the patient helps with the interpretation of the results. In this demonstration, a patient with a complete callosotomy has been selected.
3. Make sure that the patient has been fully informed of the research procedures and has signed all the appropriate consent forms.
4. Recruit 20 participants of the same age and gender as the patient, matched for intelligence, using scores on the Wechsler Adult Intelligence Scale (WAIS).

2. Data collection

1. In order to present visual stimuli to the left or right hemisphere alone, the stimuli must be properly presented to one visual field. Note that this is not equivalent to presenting the stimuli to one eye. Each eye projects to both hemispheres of the brain; for example, the part of the left eye that sees the left visual field is processed by the right hemisphere, but the part of the left eye that processes the right visual field is seen by the left hemisphere. Therefore, to present an image to the left hemisphere, present it entirely within the right visual field, which is to the right of where the patient is looking.
   1. To achieve this lateralization, use a chinrest to maintain the eyes approximately 22 in. from the computer screen. Place the patient's chin comfortably within the chinrest, facing the screen.
   2. Have a small cross remain in the center of the screen to provide a location for the patient to fixate their eyes.
   3. Instruct the patient to maintain their fixation on this cross throughout the experiment, even as images appear to the left or right side of it.
   4. Explain to the patient that when an image appears, they should say the name of the object out loud.
2. Present images of well-known objects briefly on the left or right side of the screen to project them to the right or left hemispheres of the brain, respectively. 50 images are presented in random order from a set of objects that include easily recognizable drawings, such as an apple, a ball, a broom, and a chicken.
   1. Present the images for less than 150 ms to ensure proper lateralization. This is enough time to see the stimulus, but fast enough so the patient isn't able to move their eyes to see the stimulus in central vision.
   2. Ask the patient to name the objects presented on the screen out loud, and record their responses. This is a test of verbal linguistic capability and should reveal the differences in speaking ability between the hemispheres.
   3. If the patient is unable to name any of the objects, ask the patient to draw the object, without looking at the paper, with the hand ipsilateral to (on the same side as) the stimulus. This serves as a non-linguistic measure of knowledge of the stimulus.
      1. The hand ipsilateral to the stimulus is controlled by the hemisphere that saw the stimulus. For example, when the stimulus is presented in the left visual field, it is processed by the right hemisphere. The right hemisphere is largely responsible for the control of the left hand.
      2. Make sure the patient does not look at their hand while it is drawing to maintain the isolation of the stimulus to one hemisphere.
      3. When the patient finishes drawing an object, ask them to look at the object and say what it is out loud. This confirms that the patient knows the name of the object when it is presented in central vision, even if they are unable to name it when it is presented to a single hemisphere.
3. Repeat the procedure for each control participant.

3. Data Analysis

1. To analyze the patient's performance, compare the data from the left and right visual half-field with each other. To do this, tabulate the number of correct and incorrect responses in each visual field, and test the likelihood of obtaining a difference as large as the one observed using a chi-square test of independence.
2. Compare the data from the patient with the data from the age, gender, and intelligence-matched control population to determine deficits in patient behavior. To do this, compile the average score for each person's left visual field and right visual field separately, and compare the distributions using a repeated-measures analysis of variance test (ANOVA).