Pulsed infrared light alters neural activity in rat somatosensory cortex in vivo - PubMed (original) (raw)

Pulsed infrared light alters neural activity in rat somatosensory cortex in vivo

Jonathan M Cayce et al. Neuroimage. 2011.

Abstract

Pulsed infrared light has shown promise as an alternative to electrical stimulation in applications where contact free or high spatial precision stimulation is desired. Infrared neural stimulation (INS) is well characterized in the peripheral nervous system; however, to date, research has been limited in the central nervous system. In this study, pulsed infrared light (λ=1.875 μm, pulse width=250 μs, radiant exposure=0.01-0.55 J/cm(2), fiber size=400 μm, repetition rate=50-200 Hz) was used to stimulate the somatosensory cortex of anesthetized rats, and its efficacy was assessed using intrinsic optical imaging and electrophysiology techniques. INS was found to evoke an intrinsic response of similar magnitude to that evoked by tactile stimulation (0.3-0.4% change in intrinsic signal magnitude). A maximum deflection in the intrinsic signal was measured to range from 0.05% to 0.4% in response to INS, and the activated region of cortex measured approximately 2mm in diameter. The intrinsic signal magnitude increased with faster laser repetition rates and increasing radiant exposures. Single unit recordings indicated a statistically significant decrease in neuronal firing that was observed at the onset of INS stimulation (0.5s stimulus) and continued up to 1s after stimulation onset. The pattern of neuronal firing differed from that observed during tactile stimulation, potentially due to a different spatial integration field of the pulsed infrared light compared to tactile stimulation. The results demonstrate that INS can be used safely and effectively to manipulate neuronal firing.

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Figures

Fig. 1

Experimental setup for infrared neural stimulation and optical imaging. (A) Schematic diagram of the experimental setup. Experiment is controlled by Redshirt imaging software that acquires images and determines when stimuli are presented by signaling a separate computer running LabVIEW software that is responsible for stimulus presentation. The LabVIEW control computer triggers stimuli by sending TTL pulses to either the piezoelectric controller or the laser. The optical fiber used to deliver infrared light is positioned on cortex or just above cortex through a window created through agar and the piezoelectric bender is positioned on targeted forepaw digit or whiskers. Image shows an example of a fiber positioned on cortex through a port created in the agar. (B) Imaging protocol flow chart of one trial for a given condition.

Fig. 2

Typical intrinsic imaging response to vibrotactile stimulation of contralateral forepaw digits. (A) Blood vessel map. Black boxes are region of interests where time course data was calculated for D2, D4, and no stimulation conditions. (B & C) Activation maps in response to stimulation of D2 and D4 respectively. Darkening in image indicates activation. (D) D4 – D2 subtraction map, darkened area represents selective D4 and lightened area selective D2 activation. (E–G) Time courses of intrinsic signals taken from region of interests demarcated by black and white boxes in (A–D). Traces in green, blue, and red indicate responses to D2, D4, and no stimulation conditions. ROI 1 corresponds to a D2 region of cortex, ROI 2 corresponds to a D4 region of cortex and ROI 3 corresponds to a non-activated region of cortex. Black bar represents the timing of the stimulus. Stimulation parameters: 3 sec train, 8 Hz. Imaging Parameters: 10 fps, 21 trials. A = anterior, M = medial. Scale bar next to (D) indicates clipping range of % change in signal in respective images

Fig. 3

INS evoked intrinsic optical signals in somatosensory cortex. INS evokes intrinsic optical signals in somatosensory barrel field (A–D) and forepaw cortex (E–H) in separate experiments (632 nm). (A & E) Blood vessel maps indicating ROI locations (ROI 1&3 = site near INS stimulation, ROI 2&4 = site distant from INS stimulation) and fiber location (Fiber). (B & F) Activation maps obtained to laser stimulation in barrel field cortex (B) and forepaw cortex (F). (C & G) Time course of signals in region near laser stimulation (ROI 1 and ROI 3). (D & H) Time course of signals distant from laser stimulation site (ROI 2 and ROI 4). Blue line reflects signal evoked by laser stimulation. Red line shows control (no stim) time course. There is no appreciable optical response at this distance from laser stimulation. Laser parameters: λ = 1.875 μm, repetition rate = 100 Hz, pulse train duration = 500 ms, pulse width = 250 μs, radiant exposure = 0.55 J/cm2, spot size diameter = 400 μm. Imaging Parameters: 5 fps, ITI = 8 s, Trials = 40. A = anterior, M = medial. Black bar in (C–H) represents the timing of the stimulus. Scale bars next to (B & F) indicate clipping range of respective images.

Fig. 4

Intrinsic signals produced by different rates of INS. (A) Blood vessel map. Location of fiberoptic is indicated by arrow. Orange pixels indicate significant pixels in t-test between 100 Hz stimulation and blank condition. (B–E) Activation maps of laser repetition rates: 50 Hz (B), 100 Hz (C), 150 Hz (D), 200 Hz (E). (F) Time course of response resulting from laser stimulation conditions 50 Hz (red), 100 Hz (blue), 150 Hz (yellow), and 200 Hz (aqua blue) and blank conditions. (G) Laser repetition rate versus the peak amplitude of the intrinsic signal. Relationship fit with an exponential equation. Laser parameters: λ = 1.875 μm, repetition rates = 50, 100, 150, 200 Hz, pulse train duration = 500 ms, pulse width = 250 μs, radiant exposure = 0.55 J/cm2, spot size = 400 μm. Imaging parameters: 40 Trials, 5 f/s. A = anterior, M = medial. Black bar in (F) represents the timing of the stimulus. Scale bar next to (E) indicates clipping range of images (B–E).

Fig. 5

Increased INS radiant exposure leads to an increase in intrinsic signal magnitude. (A) Blood vessel map showing location of ROI (red box) and fiber location (tip barely in FOV). (B & C) Activation maps from stimulation with 0.14 J/cm2 and 0.48 J/cm2. (D) Time course of signal for different radiant exposures. (E) Radiant exposure versus peak amplitude of the intrinsic signal. Relationship fit with a linear equation. Laser parameters: λ = 1.875 μm, repetition rate = 200 Hz, pulse train duration = 500 ms, pulse width = 250 μs, spot size = 400 μm, radiant 0.14 (blue), 0.21 (green), 0.32 (red), 0.37 (purple), 0.48 J/cm2 (orange). A = anterior, M = medial. Black bar in (D) represents the timing of the stimulus. Scale bar next to (C) indicates clipping range of images.

Fig. 6

Spatial distribution of intrinsic signal in response to INS. (A) Blood vessel map with sampled ROIs overlaid. Color of box in map corresponds to color of time course trace displayed in (B). (B) Intrinsic signal time courses at different distances from the INS stimulation location. Dashed black line corresponds to the no stimulation condition collected from the red-boxed ROI. (C) Peak amplitude of the intrinsic signal as a function of distance from the fiber. Relationship fit with an exponential equation. (D) Time mosaic of optical images to illustrate the spatiotemporal aspects of the INS induced intrinsic signal. Images were temporally binned by two decreasing the effective frames per second to 2.5 Hz. Laser parameters: λ = 1.875 μm, repetition rate: 200 Hz, pulse train duration = 500 ms, pulse width = 250 μs, radiant exposure = 0.55 J/cm2, spot size = 400 μm. Imaging parameters: 40 Trials, 5 f/s. Black bar in (B) represents the timing of the stimulus. A= anterior, M = medial.

Fig. 7

INS induces an inhibitory neural response and does not alter neuronal response to tactile stimulation. (A) Image of somatosensory cortex corresponding to barrel field showing electrode and fiber placement. Fiber stimulation site to electrode distance was approximately 1 mm. Electrode tip was placed 50 μm into cortex. (B) PSTH showing modulation of neural response to INS (30 trials). The laser-induced inhibition of neural activity had a duration of approximately 1.5 s and was followed by a rebound. (C) PSTH of vibrotactile stimulation generated by a piezoelectric bender deflecting contralateral whiskers once at each arrow. Laser and whisker stimulation were interleaved. (D) PSTH of vibrotactile stimulation after INS. (C & D) Demonstrate INS did not cause a loss of cortex functionally responding to sensory stimulation. Laser parameters: λ = 1.875 μm, repetition rate = 100 Hz, pulse train duration = 500 ms, pulse width = 250 μs, radiant exposure = 0.019 J/cm2, spot size = 1200 μm. Hatched bar in (B) represents the timing of the stimulus. A= anterior, M = medial.

Fig. 8

Inhibitory effect of INS on neural activity is consistent over many trials. (A–J) PSTH mosaic of ten segments (40 trials per segment). The laser induced inhibition is strongest in the first 120 trials, then weakens but is evident through trials 361–400. (K) PSTH summation of all segments (A – J). Laser parameters: λ=1.875μm, repetition rate = 200 Hz, pulse train duration = 500 ms, pulse width = 250 μs, radiant exposure = 0.0549 J/cm2, spot size = 850 μm. Hatch bars represent the timing of the stimulus.

Fig. 9

Repeatability of the neural INS inhibitory effect. (A) PSTH of single unit in response to INS. (B) A second PSTH from the same unit taken approximately 30 minutes later. Laser parameters: λ = 1.875 μm, repetition rate= 200 Hz, pulse train duration = 500 ms, pulse width = 250 μs, radiant exposure = 0.078 J/cm2, spot size= 850 μm. (C,D) PSTHs from other experiments showing inhibition from different cells. Laser parameters: λ = 1.875 μm, repetition rate =100 Hz, pulse train duration = 500 ms, pulse width = 250 μs, radiant exposures: 0.043 J/cm2 and 0.12 J/cm2, spot sizes= 880 μm and 840 μm. Inhibition was observed regardless of baseline activity of a single unit. Hatch bars in represent the timing of the stimulus.

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Pulsed infrared light alters neural activity in rat somatosensory cortex in vivo - PubMed (original) (raw)