The overall goal of this divided visual field event related potential methodology is to observe the unique contribution of each hemisphere of the brain to the processing of emotional stimuli as measured by the late positive potential, or LPP. This method can help to answer key questions in the field of lateralized processing of emotion such as, What are the unique contributions of each hemisphere to the motivated attention network for processing unpleasant versus pleasant stimuli? The main advantage of this technique is that it allows you to assess the processing differences between the two hemispheres of the brain with a fine temporal resolution.
Demonstrating the procedure will be Nick Lunny, a grad student in my lab. Zach Gemelli, an undergraduate research assistant in my lab will be playing the role of the participant. Begin by escorting the participant into the testing room and having them read and complete an informed consent.
Then have the participant complete a demographic survey to provide sex, age, handedness, native language, vision and neurological history in order to assess inclusion for the study. Next, apply electroencephalography, or EEG electrodes, onto the participant's scalp according to the international 10-20 system. Then, seat the participant in a dark, electrically shielded sound attenuated room.
Use a chin rest to stabilize the head and minimize movement. Then, instruct the participant to passively view the image stimuli without shifting their eyes away from the center of the screen. Instruct the participant that a recognition quiz will follow the stimuli, so it is important that they pay attention.
Finally, display a fixation cross at the center of the screen to help the participant fixate. For the central visual field, CVF paradigm, have the participant passively view the image stimuli in four experimental blocks of 45 trials each at the center of the screen. Note each block has an equal number of stimuli from the valence arousal conditions.
On the other hand, if the participant is randomized to the divided visual field, DVF paradigm, randomly present each image, once in the left visual field and once in the right visual field. Pair each image stimulus with the simultaneous presentation of a solid brown rectangle identical in stimulus dimensions on the opposite side of the fixation in order to reduce reflexive saccades to the stimulus. Next, present each stimulus and its paired brown rectangle with their inside edge three degrees of visual angle from the fixation.
Finally, backward mask both the stimulus and the brown rectangle using the same criteria and procedure as was done for the CVF paradigm. The late positive potential, or LPP, in the early epic was significantly larger to unpleasant and pleasant images, which did not differ from each other, compared to neutral images. And high arousing images evoked larger LPP responses than medium arousing images.
In the late epic, the LPP was significantly larger to high arousing unpleasant images compared to all other images. Further, the LPP was found to be larger in the early epic compared to the late epic for all image presentations except for high arousing unpleasant images presented to the left hemisphere. The left hemisphere engages high arousing unpleasant images for longer, possibly to create an action plan in response to the images.
For the divided visual field procedure, it is important that the participant's eyes stay fixated at the center of the screen to optimize lateralized stimulus processing. For successful divided visual field presentations, carefully control stimulus size, presentation duration and location on the screen. Balance the presentation of your stimulus with the filler stimulus and use backward masking to optimize lateralized stimulus processing.
The divided visual field procedure can be used with vent related potentials to investigate lateralized processing of emotion as well as investigate the unique and interactive effects of valence and arousal on the motivated attention network. Divided visual field procedures, in combination with event related potential techniques, can also be used to answer additional questions about a variety of theories of lateralized processing and visual spacial processing systems in the brain.
