From evoked potentials to cortical currents: Resolving V1 and V2 components using retinotopy constrained source estimation without fMRI.

Despite evoked potentials' (EP) ubiquity in research and clinical medicine, insights are limited to gross brain dynamics as it remains challenging to map surface potentials to their sources in specific cortical regions. Multiple sources cancellation due to cortical folding and cross-talk obscures close sources, e.g. between visual areas V1 and V2. Recently retinotopic functional magnetic resonance imaging (fMRI) responses were used to constrain source locations to assist separating close sources and to determine cortical current generators. However, an fMRI is largely infeasible for routine EP investigation. We developed a novel method that replaces the fMRI derived retinotopic layout (RL) by an approach where the retinotopy and current estimates are generated from EEG or MEG signals and a standard clinical T1-weighted anatomical MRI. Using the EEG-RL, sources were localized to within 2 mm of the fMRI-RL constrained localized sources. The EEG-RL also produced V1 and V2 current waveforms that closely matched the fMRI-RL's (n = 2) r(1,198) = 0.99, P < 0.0001. Applying the method to subjects without fMRI (n = 4) demonstrates it generates waveforms that agree closely with the literature. Our advance allows investigators with their current EEG or MEG systems to create a library of brain models tuned to individual subjects' cortical folding in retinotopic maps, and should be applicable to auditory and somatosensory maps. The novel method developed expands EP's ability to study specific brain areas, revitalizing this well-worn technique. Hum Brain Mapp 37:1696-1709, 2016. © 2016 Wiley Periodicals, Inc.

Multifocal pupillographic perimetry with white and colored stimuli.

PURPOSE: We investigated issues that could impair the capacity of multifocal pupilliographic perimetry to detect visual field damage. Differential blue light absorbance causes between-subject variance so we compared stimuli with differing blue content. We also quantified declining response gain at higher stimulus intensities (saturation), which can reduce sensitivity to changes in the visual field. METHODS: Independent stimuli were delivered to 44 regions of both eyes whereas pupil responses were recorded under infrared illumination. Luminance-response functions were measured at 88 locations for white, yellow, and red stimuli at luminances ranging from 36 to 288 cd/m2. Response saturation was quantified by fitting power functions: Response =α Luminance, z<1 indicating declining response gain. Experiments were conducted on 2 groups containing 16 and 18 different normal subjects. The second experiment was designed to confirm the results of the first and to include red stimuli. RESULTS: Response saturation occurred in all visual field regions: the mean exponents ranged from 0.57 ± 0.01 to 0.74 ± 0.02 (mean ± SE), that is up to 30 SE away from an exponent of 1 (no saturation). The stimulus-response functions appeared to be determined by luminance rather than color. Signal to noise ratios and regional visual field sensitivities were similar for all stimulus colors. CONCLUSIONS: Response saturation was a feature of all visual field locations. Stimuli with reduced blue light content produced the same signal to noise ratios as white stimuli. Given that these stimuli would not be affected by variable lens brunescence, they might be preferable for perimetry.

Multifocal pupillographic visual field testing in glaucoma.

PURPOSE: This preliminary study investigated a means of concurrently assessing the visual field defects of both eyes by recording pupillary responses to multifocal stimuli. METHODS: Twenty normal subjects and 26 primary open angle glaucoma patients, age and sex matched, were examined by slit-lamp, Humphrey Field Analyser II achromatic 24-2 perimetry and fundus photography. The patients had moderate to severe fields in at least one eye. Two stereoscopically arranged displays presented an array of 24 stimulus regions per eye extending from fixation to 30 degrees eccentricity. Pupil responses were recorded by video cameras under infrared illumination. Four stimulus conditions were tested: each stimulus region containing either a single or a 2 x 2 array of patches, presented either steadily for 133 ms or flickered at 15 Hz for 266 ms. Mean presentation rate was 1/s/region. The 4-min duration stimuli were presented in 8 segments of 30 s. Segments did not need to be repeated unless more than 15% of a segment record was lost as a result of blinks or fixation losses. RESULTS: The 48 stimuli produced 96 direct and consensual responses per subject. The single patch, non-flickered stimulus condition produced the best diagnostic performance, an area under the curve of 84%. The contraction amplitudes for that stimulus gave a median z-score of 3.2. CONCLUSIONS: The method produced diagnostic accuracy approaching that of automated perimetry, but unlike perimetry provides standard errors for every point in each field as well as information on response delay and efferent defects. Only one pupil needs to function to measure both visual fields.