Infrared neural stimulation of primary visual cortex in non-human primates

Abstract

Infrared neural stimulation (INS) is an alternative neurostimulation modality that uses pulsed infrared light to evoke spatially precise neural activity that does not require direct contact with neural tissue. With these advantages INS has the potential to increase our understanding of specific neural pathways and impact current diagnostic and therapeutic clinical applications. In order to develop this technique, we investigate the feasibility of INS (λ=1.875μm, fiber diameter=100-400μm) to activate and modulate neural activity in primary visual cortex (V1) of Macaque monkeys. Infrared neural stimulation was found to evoke localized neural responses as evidenced by both electrophysiology and intrinsic signal optical imaging (OIS). Single unit recordings acquired during INS indicated statistically significant increases in neuron firing rates that demonstrate INS evoked excitatory neural activity. Consistent with this, INS stimulation led to focal intensity-dependent reflectance changes recorded with OIS. We also asked whether INS is capable of stimulating functionally specific domains in visual cortex and of modulating visually evoked activity in visual cortex. We found that application of INS via 100μm or 200μm fiber optics produced enhancement of visually evoked OIS response confined to the eye column where INS was applied and relative suppression of the other eye column. Stimulating the cortex with a 400μm fiber, exceeding the ocular dominance width, led to relative suppression, consistent with involvement of inhibitory surrounds. This study is the first to demonstrate that INS can be used to either enhance or diminish visual cortical response and that this can be done in a functional domain specific manner. INS thus holds great potential for use as a safe, non-contact, focally specific brain stimulation technology in primate brains.

© 2013.

Figures

Fig. 1

Methods for infrared neural stimulation and optical imaging. (A) Schematic diagram of the experimental setup. Experiment is controlled by VDAQ/NT imaging software that acquires images and determines when stimuli are presented through signaling separate computers running ViSaGe software (visual stimulus presentation) and LabVIEW software (INS presentation). ViSaGe software generates the monochromatic drifting square-wave gratings that are presented to the animal through electromagnetic shutters placed over the eyes. The LabVIEW control computer triggers laser stimuli by sending TTL pulses to the laser at the desired repetition rate. Intensity of INS was set manually by adjusting the diode current and pulse width of the laser. The optical fiber used to deliver infrared light is positioned on the artificial dura covering the cortex in the desired stimulation location. Image shows an example of a fiber positioned on the cortex through the imaging system. (B) Imaging protocol flow chart of one trial for a given condition.

Fig. 2

Increasing radiant exposure of INS increases intrinsic reflectance signal magnitude. (A.) Blood vessel map. Region of interest demarcated by white box. (B–D) Activation maps in response to INS demonstrate increases in increased intrinsic optical signal for (B) 0.5 J/cm2, (C) 0.78 J/cm2, and (D) 1.3 J/cm2. (E) Time course of intrinsic signal in response to different radiant exposures. Black bar: INS stimulus on. Error bars: standard error of the mean. (F) Peak magnitude of intrinsic reflectance signal change. Statistical significance: p-Values obtained from paired two-tailed t-test. INS stimulation parameters: λ = 1.875 μm, pulse width = 250 μs, pulse train length = 500 ms, fiber size = 100 μm. Imaging parameters: 5 fps, ITI = 8 s, number of trials = 40 (1.3 J/cm2), 22 (0.78 J/cm2), and 20 (0.50 J/cm2). Scale bar in A: 1 mm. A = anterior, L = lateral. Data from Monkey 1 left hemisphere.

Fig. 3

Excitatory single unit response evoked by INS of the primary visual cortex. (A) Single unit response near INS site located in left eye ocular dominance column. Electrode depth at 340 μm. (B) Single unit response in right eye ocular dominance column to INS located in left eye ocular dominance column near INS site. Electrode depth at 650 μm. Gray bars: INS stimulation on. INS stimulation parameters: λ = 1.875 μm, pulse width = 250 μs, radiant exposure = 0.57 J/cm2, fiber size = 200 μm, repetition rate = 200 Hz, stimulation duration = 500 ms. Data from Monkey 2.

Fig. 4

Infrared neural stimulation generates focal responses in primary visual cortex. (A.) Blood vessel map. Dashed blue line: V1/V2 border. Solid blue lines: Ocular dominance borders based on B. White dots: Regions of interest from which time courses in F–I were taken. Red outline: fiber optic tip targeted in the right eye column. (B) Ocular dominance map (dark pixels: left eye, light pixels: right eye). (C–E) Activation maps in response to (C) 100 Hz pulse train, 500 ms, (D) 200 Hz pulse train, 500 ms, and (E) 200 Hz pulse train, 1000 ms. Overlay of ocular dominance borders in E shows specificity of activation of the right eye column (dark pixels) and adjacent left eye column (light pixels). (F–I) Time course of intrinsic signals taken from regions of interest 1–4 in A. Traces in blue, red, and green: time course of response to INS in D, E, and F, respectively. (F) ROI1: response at fiber tip location in the right eye column reveals a strong intensity dependent negative reflectance change. (G) ROI2: response to location in adjacent left eye column reveals an opposite (positive) reflectance change. (H) ROI3: smaller response at location further away from fiber tip. (I) ROI4: null response at control location distant from fiber tip. INS parameters: λ = 1.875 μm, pulse width = 250 μs, radiant exposure = 0.6 J/cm2, fiber size = 100 μm. Imaging parameters: 5 fps, ITI = 8 s, trials = 40. A = anterior, L = lateral. Stimulation begins at 0.4 s following imaging onset and ends at 0.9 s (500 ms train) or 1.4 s (1 s train). Scale bar in B: 500 μm, applies to A–E. Data from Monkey 1 left hemisphere.

Fig. 5

Intrinsic optical response to visual stimulation potentiated by INS. (A) Blood vessel map. Left eye OD columns demarcated by red outline and right eye OD columns demarcated by blue outline. Location of INS demarcated by yellow circle. Scale bar = 1 mm. Middle panel: ocular dominance column activation maps generated by visual stimulation of both eyes (vision, Le–Re). Right panel: ocular dominance columns generated by INS during visual stimulation of both eyes (Combined, LeC–ReC). (B & C) Hypothesized potentiation (B) and attenuation (C) of intrinsic signal from INS targeted in the left eye column. (D & E) Time course of intrinsic signal for visual and combined stimulation for (D) left and (E) right eye OD columns demarcated in activation maps in A. (F & G) Averaged reflectance change corresponding to 4–6 s of time courses in D & E indicates that INS potentiates visually evoked intrinsic signals for left eye OD column but attenuates signal in right eye OD columns. p value comparing visual to combined stimulation obtained from paired two-tailed t-test. Error bars represent standard error of the mean. INS parameters: λ = 1.875 μm, radiant exposure: 1.3 J/cm2, repetition rate = 200 Hz, pulse width = 250 μs, pulse train length = 500 ms, fiber size = 100 μm. Visual stimulation parameters: Prestim gray = 1 s, stimulation duration = 3.5 s, spatial frequency = 1 Hz, temporal frequency 3 Hz, square wave grating. Imaging parameters: 5 fps, ITI = 8 s, trials = 40. Subtraction maps in B and C correspond to frames 20 to 30 (4 to 6 s). Scale bar in A: 1 mm. (A = anterior, L = lateral). Data from Monkey 1 left hemisphere.

Fig. 6

Modulation of visual signal by INS dependent on fiber size. (A) Schematic displaying diameter difference of 100, 200, and 400 μm fibers. (B & C) Hypothesized intrinsic responses in right and left OD columns in V1 for INS alone in a right eye OD column and in combination with visual stimulation for each optical fiber diameter tested. (D–F) Blood vessel maps displaying placement of 100 (D), 200 (E), and 400 (F) μm fibers in relation to ocular dominance columns (white outlines: right eye; green outlines: left eye). Green dot: location of fiber tip. Error of fiber placement 2, repetition rate = 200 Hz, pulse width = 250 μs, and pulse train length of 500 ms. Black scale bar = 500 μm. (G–I) Activation maps generated by INS alone with 100, 200, and 400 μm diameter fibers. Intrinsic signal magnitude increases with increased fiber diameter. (J–L) Ocular dominance maps generated by visual stimulation combined with INS. (M & N) Intrinsic signal time course for INS alone for an ROI positioned in a right (ROI1 (M)) and left ROI2 (N) OD column for each fiber diameter. (O & P) Intrinsic signal time course evoked by combined stimulation during left eye stimulation (Le + laser-blank). Time course magnitude is enhanced in the right OD column with the 200 μm diameter optical fiber. INS parameters: λ = 1.875 μm, radiant exposure = 0.64 J/cm2, repetition rate = 200 Hz, pulse width = 75–300 μs, pulse train length = 500 ms. Trials = 38 for 100 and 200 μm fiber, trials = 25 for 400 μm fiber. Subtraction maps in (J–L) correspond to frames 15 to 25 (3 to 5 s). (A = anterior, L = lateral). Data from Monkey 1 right hemisphere.