Photic Stimulation

Imaging experiments were performed on a 7 T magnet (Magnex Scientific, Oxford, UK) equipped with a Siemens (Siemens, Erlangen, Germany) TIM console, Siemens Avanto body gradient hardware (rise time 200 μs and maximal gradient strength 40 mT/m), and an 8-kW RF amplifier (CPC, Hauppauge, New York). Imaging was performed on healthy volunteers who had signed a written consent form approved by the Institutional Review Board of the University of Minnesota. A 16-channel transmit/receive head coil (16 (link)) was used, with the RF power split evenly between the channels (Werlatone, Brewster, NY). To enable the use of visual stimulation hardware (Avotec Goggles), two transmission line elements centered in front of the eyes were shortened to 8 cm in length. The remaining 14 elements were each 16 cm in length. The elements were circumferentially distributed. All resonance elements were built using a 12mm-thick Teflon substrate between the coil conductor and the RF ground plane. The coil geometry allowed for maximal accelerations along the x and y direction, i.e., the directions perpendicular to the static magnetic field. The coil is similar to the “fixed-size elliptical coil” reported in Adriany et al. (17 (link)), where its performance was compared with a geometrically adjustable coil.
Simultaneous excitation of slices was performed on coronal planes using multiband excitation pulses, combining standard five-lobe sinc pulses with frequency offsets (phase ramps) applied to each constituent pulse to realize a spacing of 44mm between bands. This spacing was selected based on acceptable g-factor performance of unaliasing four simultaneously excited slices with the current RF coil employed. With a 2mm-thick slice and no interslice gaps, 22 executions of the four-band excitation were needed to cover the 44mm distance between the simultaneously excited slices, leading to a total of 22 × 4 unaliased slices to cover 176mm in the anterior-posterior direction in ~1 sec. For improved slice selection with the multiband RF pulses, a duration of 5.120 ms was used (the Siemens default duration for a traditional single-band RF-pulse is 2.560 ms). A 4-fold acceleration due to multiband excitation refers to a true 4-fold reduction in the acquired data but not necessarily the overall scan reduction, which can be less due to the lengthening of the RF pulses. Depending on the RF duration, the actual speed up factor due to multiband RF excitation for the four band studies was between 3.88 and 3.94.

Mice were anesthetized with isoflurane and held in place with a head post cemented to the skull. All incisions were infiltrated with lido-caine. A small craniotomy was made over barrel cortex approximately 200 μm anterior to the virus injection site. Extracellular single-unit and LFP recordings were made with tetrodes or stereotrodes. Intracellular recordings were conducted by whole-cell in vivo recording in current clamp mode. Stimulus control and data acquisition was performed using software custom-written in LabView (National Instruments) and Matlab (The Mathworks). Further electrophysiology methods and a description of the reversal potential calculation are given in Supplementary Methods.
Light stimulation was generated by a 473 nm laser (Shanghai Dream Lasers) controlled by a Grass stimulator (Grass Technologies) or computer. Light pulses were given via a 200-μm diameter, unjacketed optical fibre (Ocean Optics) positioned at the cortical surface 75–200 μm from the recording electrodes. For experiments using the broad range of light-stimulation frequencies (8, 16, 24, 32, 40, 48, 80, 100 and 200 Hz), we stimulated in bouts of 3 s of 1-ms pulses at 46 mW mm⁻² at each frequency in a random order. In a subset of these experiments, we stimulated at 31, 46 and 68 mW mm⁻².
Vibrissae were stimulated by computer-controlled movements of piezoelectric wafers (Piezo Systems). Vibrissa stimulations were single high-velocity deflections in the dorsal and then in the ventral direction (~6 ms duration). In most cases, adjacent vibrissae that yielded indistinguishable amplitude responses during hand mapping were deflected simultaneously. Vibrissa stimulations evoked layer 4 RS spike responses with an onset latency of 9.1 ± 0.08 ms. For RS cell response suppression experiments, light pulses were given on randomly interleaved trials. For gamma-phase experiments, we gave a series of trials each consisting of a 1-s series of 1-ms light pulses at 40 Hz, with a single whisker deflection after the thirtieth light pulse. The precise timing of the whisker deflection relative to the light pulses was varied across five phase points. Each of the five phase points was included in a random order across a minimum of 250 total trials.
Unit and LFP analysis used software custom-written in Igor Pro (Wavemetrics). For each stimulation frequency, we measured the relative power in an 8-Hz band centred on that frequency. For each recording site, we measured power from 5–10 LFP traces under each condition. Example power spectra are averages of the power spectra from 5–10 traces of unfiltered LFPs from individual experiments. Relative power was calculated by measuring the ratio of power within the band of interest to total power in the power spectrum of the unfiltered LFP. We also measured the power ratio: P_light/P_baseline, where P_light is the relative power in a frequency band in the presence of light stimulation and P_baseline is the power in that band in the absence of light stimulation. All numbers are given as mean ± s.e.m., except where otherwise noted.