Omniplex acquisition system

For each recording session, one quartz-glass insulated platinum/tungsten microelectrode (Thomas RECORDING GmbH, Giessen, Germany) was advanced into the posterior wall of the superior temporal sulcus, targeting area MT using the TREC Mini Matrix also used for iron deposition experiments. Signals from the microelectrodes were pre-amplified by a factor of 19 and wide-band filtered from 0,034 Hz to 50 kHz by an analog pre-amplifier built into the Mini Matrix. Then the signals were amplified and recorded with a sampling rate of 40 kHz and 16-bit precision using an Omniplex acquisition system (Plexon, Dallas, TX, USA).

Electrophysiological recordings were carried out on lightly anesthetized animals (0.5–1.5% isoflurane) immediately following surgical implantation and once per week for 13 weeks afterward. Spontaneous wideband recordings (0.1–7000 Hz) were collected using 15-channel Michigan style SMP arrays (IC-5-16E, Qualia, Inc.) and an Omniplex acquisition system (Plexon, Inc., Dallas, TX, USA) from all 15 recording sites simultaneously at 40,000 Hz for 10 min. Wideband data were processed using a four-pole Butterworth high pass filter with a cutoff frequency of 250 Hz. Individual waveforms (spikes) were identified by filtered continuous data crossing a threshold of −4σ, based on the root mean square (RMS) of the filtered continuous signal. Single units were manually identified from collections of spikes using 2D principal component space, but were excluded from further analysis if they did not contain at least 100 individual spikes, or if >3% of spikes violated a 1.5-ms minimum refractory period. The signal to noise ratio (SNR) was calculated by dividing the mean peak-to-peak voltage of each unit (Vpp) by the RMS noise of its associated channel. The RMS noise was calculated as the RMS of the filtered continuous signal after removing all samples exceeding the 4σ threshold.

All recordings were performed in rats breathing 1% isoflurane in oxygen. Signals were amplified and digitized on an RHD2132 headstage (Intan) and streamed to a PC using an Omniplex acquisition system (Plexon) at a rate of 40,000 samples per second per channel. All recordings were performed using a ground skull screw as reference. LFPs were extracted from raw signals online using the bandpass filter with a passband of 0.1-300 Hz. Offline, LFPs were decimated to 1 kHz and filtered using a custom acausal FIR 0.1-200 Hz bandpass filter. Noisy channels were removed by visual inspection of the signals. Before subsequent analyses, LFPs were rereferenced to the mean computed over all clean channels on the laminar probe. An example 5 min segment of rereferenced LFP is shown in Figure 3A. All data analysis was completed using custom-built MATLAB (The MathWorks) code unless otherwise stated. In total, 29.88 h of recordings were used to generate all data in this manuscript. Recording durations from single rats included in analyses ranged from 4 to 7.25 h, with a median of 5.5 h per animal.