Retinotopic mapping of adult human visual cortex with high-density diffuse optical tomography

Abstract

Functional neuroimaging is a vital element of neuroscience and cognitive research and, increasingly, is an important clinical tool. Diffuse optical imaging is an emerging, noninvasive technique with unique portability and hemodynamic contrast capabilities for mapping brain function in young subjects and subjects in enriched or clinical environments. We have developed a high-performance, high-density diffuse optical tomography (DOT) system that overcomes previous limitations and enables superior image quality. We show herein the utility of the DOT system by presenting functional hemodynamic maps of the adult human visual cortex. The functional brain images have a high contrast-to-noise ratio, allowing visualization of individual activations and highly repeatable mapping within and across subjects. With the improved spatial resolution and localization, we were able to image functional responses of 1.7 cm in extent and shifts of <1 cm. Cortical maps of angle and eccentricity in the visual field are consistent with retinotopic studies using functional MRI and positron-emission tomography. These results demonstrate that high-density DOT is a practical and powerful tool for mapping function in the human cortex.

Fig. 1.

High-denisty DOT system. (a) Schematic of the high-density imaging grid with 24 sources (red) and 28 detectors (blue). Measurement pairs are represented by green lines. We used first-, second-, third-, and fourth-nearest-neighbor pairs at source–detector separations of 13, 30, 40, and 48 mm, respectively. (b) Schematic showing the placement of the imaging grid over the visual cortex. (c) Detected light level vs. source–detector separation on a human subject averaged over 1 sec (≈10 image frames). All first-, second-, third-, and fourth-nearest-neighbor pairs were detected simultaneously and were well above the noise floor (dotted line).

Fig. 2.

Detection of visual cortex activations. (a) Schematic showing placement of activation images on a human subject. (Inset) The visual stimulus presented is a reversing, radial grid (10-Hz reversal) that is black and white on a 50% gray background and that extends over a polar angle of 70° and a radial angle of 0.5–1.7°. (b) An axial image slice with a cortical activation (subject 4). In this paper, images are displayed as two-dimensional coronal projections (as in c) of a cortical shell covering a depth 10 ± 2 mm below the scalp surface (the region between dotted lines with arrows showing direction of view). (c) Time course showing the temporal response of ΔHbO₂. The stimulus occurred during t = 0–10 sec. (The color scale applies to a–c.) (d) The high-density grid allows imaging of functional activations from individual stimuli, shown here for nine of the same stimulus (ΔHbO₂, red; ΔHb_R, blue; ΔHb_T, green) (subject 5).

Fig. 3.

Eccentricity mapping in the visual field was evaluated within and across five subjects. (a) Schematics of the visual stimulus. (Right) The full screen subtended a radial angle ± 12°. The peripheral lower-right stimulus (B) extended over a polar angle of 70° and a radial angle of 2.5–10.5°. (Left) An expanded view of the central view area showing the central lower-right stimulus (A) that extends over a polar angle of 70° and a radial angle of 0.5–1.7°. The central lower-left stimulus (C) mirrors stimulus A. (b–f) Contours at 75% maximum for peak (ΔHbO₂) activations A (red), B (green), and C (blue) for three sessions for each of five subjects show the repeatability of the measurements and highlight their spatial resolution. Each of the five subjects was imaged in three distinct sessions, with nine of each stimulus (A–B–C) presented in each session. The activations for each session and stimulus type were block-averaged. Contours were obtained from projection images (as in Fig. 2c) averaged over t = 7–15 sec.

Fig. 4.

Data above half-maximum value overlays of contrast-weighted average activations (n = 5 subjects) for all three stimuli (A, red; B, green; C, blue; with yellow being the overlap of the red and green channels) and contrasts (ΔHbO₂, ΔHb_R, and ΔHb_T). The central lower-right (A) and central lower-left (C) stimuli produce activations that are centered at (−21 mm, 0 mm) and (19 mm, 2 mm), respectively, relative to the center of the image. This separation is well matched with fMRI studies. The peripheral lower-right (B) stimulus results in an activation shifted medially and superiorly (center at −16 mm, 7 mm), consistent with angular and eccentricity maps of the human visual cortex from PET and fMRI studies.

Fig. 5.

Polar angle mapping with DOT. (a) Functional responses to an angularly swept grid (subject 5). The grid (60° polar angle, 0.5–10° radial angle, reversed at 10 Hz) was rotated in steps of 10° (polar angle) each second. The successive stimulus and activation images have a phase shift of 90° from the previous image. Color maps were normalized within each frame to plus or minus maximum contrast (0.34, 0.60, 0.45, and 0.32 μM, top to bottom respectively). (b) Half-maximum contours of the four activations show the left–right symmetry of the mapping for the angular sweep.