Assessing Pupil-linked Changes in Locus Coeruleus-mediated Arousal Elicited by Trigeminal Stimulation

The locus coeruleus is a deep brain structure that regulates behavioral arousal, attention, and cognitive performance. Its activity can be recorded indirectly by measuring the pupil size. This approach allows the verification of where the stimuli that boost cognitive performance, such as the gemina stimulation, modulate LC activity as the diameter changes in the pupil size.

Experimental evidence suggests that neurodegenerative diseases are associated with locus coeruleus dysfunction. The present approach will pave the way for the modulation of the geminal input to the locus coeruleus for therapeutic purposes. Demonstrating the procedure with Tommaso Banfi will be Vincenzo De Cicco, a professional physician and odontologist.

To perform the experiment, a commercially available piece of chewing gum, referred to as a soft pellet, and a silicon rubber pellet, referred to as the hard pellet, are needed. A tangram puzzle must also be prepared for the haptic task. For protocol one, provide a paper displaying three 10 by 10 numerical matrices to the subject, and ask the subject to sequentially scan the matrix lines while using a pencil to tick as many target numbers indicated above each matrix within 15 seconds to measure the subject's baseline cognitive performance.

Then manually evaluate the performance index, scanning rate, and error rate. To measure the baseline pupil size, seat the subject at an optimal working distance of 56 millimeters from a corneal topographer pupillographer, and acquire single, separate camera shots of the left and right pupils under a constant illumination of 40 lux. To evaluate the pupil size during a tangram haptic task, place one of the pieces from the puzzle into the subject's hand, and ask the subject to return the piece to the tangram while recording the subject's pupil size.

To measure the pupil size during the haptic task, acquire the photos while the subject performs the second of two task repetitions at the beginning of the puzzle surface exploration. Evaluating the left and right pupil size by direct acquisition of the values displayed by the software in millimeters. Then subtract the pupil size at rest from the pupil size during the haptic task to calculate the task-related mydriasis and obtain the average value for both the right and left pupils.

Next, ask the subject to chew a self-administered soft pellet, letting the subject spontaneously choose both the rate of chewing and the preferred mouth-chewing side. After one minute, provide the subject with a new soft pellet, and ask the subject to switch the chewing side for one more minute of chewing. Immediately after, end the chewing exercise, evaluate the subject's performance in the matrices test as demonstrated while measuring the pupil size both at rest and during the haptic task.

30 minutes following the end of the chewing exercise, evaluate the subject performance and pupil size, both at rest and during the haptic task. For protocol two, equip the subject with the wearable pupillometer eye tracker endowed with a 3D-printed glass frame structure, and adjust the position of the two infrared cameras mounted on the frame so that the subject's eyes are in focus with the field of view of the cameras. Using a calibrated logarithmic light sensor mounted on the wearable pupillometer frame to continuously and simultaneously record the environmental illumination level, acquire images of the subject's pupils at rest at a 120 Hertz sampling rate within the wearable pupillometer software for 20 seconds.

To obtain the baseline cognitive performance data, record the size of the pupils while the subject performs the Spinnler-Tognoni test. Manually elevate the left and right pupil sizes at rest and during the Spinnler-Tognoni test by averaging the acquired values for each pupil. Then calculate the task-related mydriasis as demonstrated to obtain the average values for both the left and right pupils.

Next, ask the subject to chew a self-administered soft pellet, letting the subject spontaneously choose both the rate of chewing and the mouth-chewing side. After one minute, provide the subject with a new soft pellet, and ask the subject to switch the chewing side for an additional minute of chewing. Immediately at the end of the chewing exercise, evaluate the pupil size at rest in both the performance and the pupil size during the matrices test.

30 minutes following the end of the chewing exercise, evaluate both the pupil size at rest and both the performance pupil size during the matrices test. In this representative example, the performance index was increased soon after having chewed either a hard or a soft pellet. However, after 30 minutes, the increased performance persisted only for the hard pellet.

Two control conditions, such as a lack of activity and the handgrip exercise, had a negative effect on performance, with a tendency to recover after 30 minutes. Qualitatively similar changes were observed for the task-related mydriasis in the same subject displayed in the previous plot. In the continuous acquisition mode of the instrument, final individual samples are representative of the final average value since the pupil reaches a stable size within five seconds from when the light is off.

In single subjects, a strong correlation was observed between the performance and the task-related mydriasis after chewing hard and soft pellets. A correlation is also evident when the corresponding changes are normalized for the baseline values. Even stronger evidence of LC involvement in the stimulating effects of chewing on cognitive performance can be obtained by correlating the chewing-induced changes in the performance index with the change in mydriasis observed during the execution of the same matrices test.

Using the procedure, it is very important to carefully control environmental lighting as the external light can be a major determinant to the pupil's size, confounding the results. This procedure might find several applications, for example, we are currently studying whether the correction of an occlusal assymetry may boost performance by affecting locus coeruleus activity.