Investigating Near-Threshold Perception During Ultrasonic Deep Brain Stimulation

Participants assigned to the fMRI arm will complete five visits, with a different brain region targeted at each visit. Each visit will include seven consecutive 10-minute scanning periods: one pre-LIFUP baseline fMRI scan, five fMRI scans conducted during LIFUP using five predefined combinations of duty cycle and pulse repetition frequency, and one post-LIFUP baseline fMRI scan. The five stimulation parameter combinations will be labeled Combination 1 through Combination 5. The order of the five parameter combinations will be randomized within each visit. The total planned scanning time for these seven periods is approximately 70 minutes, excluding structural imaging, participant positioning, transducer placement, safety checks, and other required study procedures.

LIFUP will be used to stimulate specific brain regions and assess their causal involvement in the control of conscious state and contents.

Whole-brain functional images will be acquired using a multiband gradient-echo echo-planar imaging sequence with the following parameters: 60 slices, repetition time (TR) = 800 ms, echo time (TE) = 30 ms, flip angle = 52°, multiband acceleration factor = 6, slice thickness = 2.4 mm, in-plane resolution = 2.4 × 2.4 mm, FOV = 216 × 216 mm, and image matrix = 90 × 90. MRI acquisition and associated study procedures will be completed within a scanning visit lasting no more than approximately 2 hours.

Participants assigned to the MRS arm will also complete five visits, with a different brain region targeted at each visit. Each visit will include a pre-LIFUP baseline MRS acquisition, an excitatory LIFUP condition, a post-excitatory non-LIFUP MRS acquisition, an inhibitory LIFUP condition, and a post-inhibitory non-LIFUP MRS acquisition. Each MRS acquisition period will last approximately 13 minutes and will include approximately 5 minutes of glutamate plus glutamine measurement and 8 minutes of gamma-aminobutyric acid (GABA) measurement. The order of the excitatory and inhibitory LIFUP conditions will be randomized across participants.

LIFUP will be used to stimulate specific brain regions and assess their causal involvement in the control of conscious state and contents.

Single-voxel proton magnetic resonance spectroscopy will be performed. A nominal 15 × 15 × 10 mm spectroscopy voxel will be positioned at the prespecified brain target using the participant's anatomical MRI. Glutamate plus glutamine (Glx) will be measured using a short-echo-time spectroscopy sequence with an acquisition time of approximately 5 minutes. GABA will be measured using an edited spectroscopy sequence with an acquisition time of approximately 8 minutes. An unsuppressed water-reference scan will also be acquired for signal correction and metabolite quantification. Voxel placement, shimming, and other sequence parameters may be adjusted slightly based on target anatomy, scanner requirements, and data quality while maintaining the planned procedures and approximate acquisition times.

Participants assigned to the glymphatic MRI arm will complete up to five visits, with a different prespecified brain region targeted at each visit. Tissue-to-fluid water exchange, velocity spectrum imaging, and cerebral perfusion using arterial spin labeling will be assessed before, during, and after LIFUP. The planned imaging periods total approximately 1.5 hours per visit, excluding participant positioning, structural imaging, transducer placement, safety checks, and other required study procedures.

LIFUP will be used to stimulate specific brain regions and assess their causal involvement in the control of conscious state and contents.

The study team will collect three separate measurements related to glymphatic function. (i) Multi-delay pseudo-continuous arterial spin labeling (PCASL) MRI with a 3D stack of spirals (SOS) gradient and spin echo (GRASE) read-out. Labeling durations from 500 to 2000 ms, TR = 6 sec., background suppression. FOV = 24 cm with 3x3x5 mm resolution. (ii) Tissue-to-fluid Water exchange rate using T2 selective saturation labeling pulses using the same resolution parameters. (iii) Velocity Spectrum Imaging using velocity selective preparation pulses followed by spherical spiral projection imaging (FOV = 24 cm, velocity bandwidth = [-1, 1] cm/s, isotropic spatial resolution = 3.5 mm).

Changes in task-evoked blood oxygen level-dependent responses to visual stimuli will be measured using functional MRI before, during, and after LIFUP across the five stimulation parameter conditions.

The perceptual response criterion, c, will be calculated from participants' recognition responses using Signal Detection Theory to quantify their tendency to report conscious recognition of a visual stimulus.

For each scanning session, the study team will calculate sensitivity (d') and criterion (c) using subjective reports of recognition as "yes" or "no".

Perceptual sensitivity, d', will be calculated from participants' visual discrimination responses using Signal Detection Theory to quantify their ability to distinguish intact object images from control stimuli.

For each scanning session, the study team will calculate sensitivity (d') and criterion (c) using subjective reports of recognition as "yes" or "no".