A Novel Approach for Documenting Phosphenes Induced by Transcranial Magnetic Stimulation

This video will demonstrate a standard procedure for obtaining a phosphine threshold value using transcranial magnetic stimulation and a method for mapping and recording the size and location of perceived phosphines. TMS is a non-invasive neurostimulation technique that is used to alter neuronal activity through the application of a brief magnetic pulse. The application of TMS provides a safe and non-invasive means for disrupting neuronal activity with a high degree of spatial and temporal specificity.

When stimulating visually sensitive regions of cortex, a brief visual atopic sensation known as a phosphine is produced. The location of the initial point of stimulation is first identified. The center of the TMS coil is placed over the point of stimulation so that the handle is tangential to the surface of the scalp.

The stimulation intensity is adjusted until a threshold value is determined. To record the observed TMS induced phosphine, the participant is seated in front of the laser tracking and painting system or lt. A-P-A-T-M-S pulse delivered to the right occipital cortex produces a phosphine located in the contralateral visual field.

The participant is instructed to outline the perceived phosphine using a laser pointer. The path of the laser pointer is tracked using a standard webcam and recorded using the ltap software. The laser path may be projected onto the projection screen so that the participant is able to gauge his or her accuracy results of the ltap system.

Confirm that the location of perceived phosphines is dependent reflecting the small receptive field size and organization of the occipital cortex. Hi, my name is Seth Kin Frankston from the Laboratory of Cerebral Dynamics in the Department of Anatomy and Neurobiology at the Boston University School of Medicine. Hi, my name is Peter Free also from the Lab of Cerebral Dynamics.

Today we'll be showing your procedure for how to obtain a phosphine threshold value using TMS as well as a method for recording perceived phosphines. We use this procedure in our laboratory to study visual processing and cortical excitability and human participants. So let's get started.

To begin seat the participant comfortably in a dimly lit room. Place a swim cap on their head and adjust it for comfort and provide the participant with earplugs to locate and mark the initial point of stimulation. First, identify the inion and then move about two centimeters roly and about two centimeters laterally.

Using a standard figure eight TMS coil, place the center of the coil directly over the initial point of stimulation. The coil should be held tangential to the surface of the scalp oriented medial to lateral with the handle facing away from the midline. Apply a single pulse at 60%of intensity output.

The foot pedal is used to remotely trigger TMS pulse. Ask the participant if they perceived any visual sensation. Did you see anything?

No.If no phosphine is reported, reposition the coil and redeliver a pulse. If the participant still does not report a phosphine continue delivering pulses every seven to 10 seconds, increasing the intensity by 5%after every five attempts until the participant reports a phosphine or 100%intensity is reached. Did you perceive anything?

Yes.When the participant reports a phosphine, ask him or her to describe in detail what they perceived including the color, shape, and location. Can you describe what you saw? Yes, it's, it's, it's only in my left visual field and it's triangular, and one of the points is right in the center of my visual field.

Note that phosphines should appear in the visual field contralateral to the hemisphere that is being stimulated. Once a location is found where phosphines are reliably reported, mark it on the swim cap. Now that we've successfully induced phosphines, let's see how to obtain a threshold value to begin deliver a series of pulses at the same coil, location and orientation.

As in the previous section, ask the participant to confirm the presence of the induced phosphine Once confirmed, deliver pulses every seven to 10 seconds. After each pulse, instruct the participant to report whether a phosphine was perceived using the following criteria. Yes, if they experience an unambiguous phosphine, no in the absence of any visual perception, and maybe if they're not sure, a maybe response should not be counted as a positive response.

When determining the threshold value, continue to adjust the stimulation intensity until a phosphine threshold is determined. The threshold is defined as the lowest intensity of stimulation at which the participant reports unambiguous phosphines for at least 50%of delivered pulses at a given intensity value, typically using a series of 10 pulses. Now let's see how to determine the size and location of phosphines using the ltap system.

The laser tracking and painting system or ltap, is a system that records the location and size of perceived phosphines in real time. The ltap system provides a stable and customizable environment for quantification and analysis of phosphines. A computer running the ltap software records the laser using a webcam.

The location is recorded and the data is then projected back onto the screen via a rear facing projector. To start seat the participants so that the nasion or the point between the eyes is approximately 30 centimeters from the center of the projection screen to acclimate the subject to the ltap system, give them the laser pointer and instruct them to practice tracing figures that resemble perceived phosphines. When the participant is acquainted with the ltap system, deliver a series of TMS pulses to the same location as in the previous steps.

Have the participant trace each phosphine in. They report onto the projection screen. Using the laser pointer to further characterize phosphines deliver TMS pulses at 120%of the previously determined threshold.

While participants direct their gaze toward various fixation points, have participants trace reported phosphines so that they can be quantified and analyzed. Now we will show you some representative phosphines that were outlined and recorded. Using the laser tracking and painting system, the location and size of the perceived phosphines are shown when the participant was instructed to direct their gaze towards a central upward and rightward fixation point, a series of pulses were delivered sufficient to induce 10 phosphines in each condition.

As you can see, the location of the perceived phosphine is dependent upon the direction of gaze. This is consistent with the small receptive field size and organization of occipital cortex. We have just shown you how to obtain a phosphine threshold value and how to record the perceived phosphine.

When doing this procedure, it's important to remember to one, maintain a consistent coil position and two, that the safety and comfort of the participant is paramount. So that's it. Thanks for watching and good luck with your own experiments.