Stimulus and optode placement effects on functional near-infrared spectroscopy of visual cortex

Abstract

Functional near-infrared spectroscopy has yet to be implemented as a stand-alone technique within an ophthalmology clinical setting, despite its promising advantages. The present study aims to further investigate reliability of visual cortical signals. This was achieved by: (1) assessing the effects of optode placements using the 10-20 International System of Electrode Placement consisting of 28 channels, (2) determining effects of stimulus size on response, and (3) evaluating response variability as a result of cap placement across three sessions. Ten participants with mean age [Formula: see text] years (five male) and varying types of hair color and thickness were recruited. Visual stimuli of black-and-white checkerboards, reversing at a frequency of 7.5 Hz were presented. Visual angles of individual checker squares included 1 deg, 2 deg, 5 deg, 9 deg, and 18 deg. The number of channels that showed response was analyzed for each participant, stimulus size, and session. 1-deg stimulus showed the greatest activation. One of three data collection sessions for each participant gave different results ([Formula: see text]). Hair color and thickness each had an effect upon the overall HbO ([Formula: see text]), while only color had a significant effect for HbD ([Formula: see text]). A reliable level of robustness and consistency is still required for clinical implementation and assessment of visual dysfunction.

Keywords: functional near-infrared spectroscopy; near infrared spectroscopy; visual cortex; visual stimulation.

Fig. 1

(a) Coronal illustration of optode layout of five sources (gray) and 12 detectors (white) based on the 10-20 system. (b) Illustration of functional block paradigm with a total of eight cycles for each trial. (c) A total of five trials (visual angles) were displayed, (i) 1 deg, (ii) 2 deg, (iii) 5 deg, (iv) 9 deg, and (v) 18 deg each day. Note: not to scale.

Fig. 2

Hemodynamic responses for three trials using the 1 deg checker size—one trial from each data collection session—for subject 2. All eight stimulus events from each trial are overlaid on a single graph for a representative channel. HbO curves range from red (first event) to yellow (eighth event) and HbD curves range from blue to green. The shaded region indicates the duration of the stimulus and the third session was found to produce significantly different ($p<0.05$) results than the other two for both HbO and HbD.

Fig. 3

Summary of qualitative statistics. A total of 120 stimulation events were analyzed for each of 10 participants. Each event possessed a total of 28 possible data channels to give a total of 3360 channels for each participant that were analyzed for the presence of an observable hemodynamic response. The number of (a) events and (b) channels in which a response was observed are illustrated. Note that the highest number of each was obtained for participants 2, 4, 7, and 10.

Fig. 4

Summary of quantitative results obtained across checker sizes—given in degrees—for the most consistent channel in each of the 10 participants. Associated error bars for the HbO (positive, red) and HbD (negative, blue) responses are given by vertical lines. Differences in response magnitudes only reached significance ($p<0.05$) for the second participant.

Fig. 5

Summary of quantitative results obtained across data collection sessions for the most consistent channel in each of the 10 participants. Associated error bars for the HbO (positive, red) and HbD (negative, blue) responses are given by vertical lines. Differences in response magnitudes for both HbO and HbD only reached significance ($p<0.05$) for two of the 10 participants.