Examining Local Network Processing using Multi-contact Laminar Electrode Recording

Summary

A fundamental issue in our understanding of cortical circuitry is how networks in different cortical layers encode sensory information. Here we describe electrophysiological techniques utilizing multi-contact laminar electrodes to record single-units and local field potentials and present analyses to identify cortical layers.

Abstract

Cortical layers are ubiquitous structures throughout neocortex^1-4 that consist of highly recurrent local networks. In recent years, significant progress has been made in our understanding of differences in response properties of neurons in different cortical layers^5-8, yet there is still a great deal left to learn about whether and how neuronal populations encode information in a laminar-specific manner.

Existing multi-electrode array techniques, although informative for measuring responses across many millimeters of cortical space along the cortical surface, are unsuitable to approach the issue of laminar cortical circuits. Here, we present our method for setting up and recording individual neurons and local field potentials (LFPs) across cortical layers of primary visual cortex (V1) utilizing multi-contact laminar electrodes (Figure 1; Plextrode U-Probe, Plexon Inc).

The methods included are recording device construction, identification of cortical layers, and identification of receptive fields of individual neurons. To identify cortical layers, we measure the evoked response potentials (ERPs) of the LFP time-series using full-field flashed stimuli. We then perform current-source density (CSD) analysis to identify the polarity inversion accompanied by the sink-source configuration at the base of layer 4 (the sink is inside layer 4, subsequently referred to as granular layer^9-12). Current-source density is useful because it provides an index of the location, direction, and density of transmembrane current flow, allowing us to accurately position electrodes to record from all layers in a single penetration^{6, 11, 12}.

Protocol

1. NAN microdrive construction

We use the U-Probe in combination with the NAN electrode drive system. Building this system requires 2-3 hours but once constructed it is very simple to modify. We begin by assembling the NAN tower, which includes a 4-channel base (Figure 2a), the NAN chamber (Figure 2b), the grid with 1 mm spacing (Figure 2c), 1-4 screw microdrives (Figure 2d), 1-4 guide tubes (Figure 2e, 500 μm diameter and cut to about 5-7 cm), and 1-4 microdrive towers (Figure 2f). For simplicity, we will describe the procedure for building the NAN system with one tower and one U-Probe. After some training, this procedure typically takes 2-3....

Discussion

Multi-unit recordings have become standard for analyzing how neural networks in the cortex encode stimulus information. Given the recent advancements in electrode technology, the implementation of laminar electrodes enables an unprecedented characterization of local cortical circuits. Although multi-electrode recordings offer useful information about neural population dynamics, multiple laminar electrodes enable greater resolution and more information about the specific location of neurons. Since the cortex is organize.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Nan microdrive system	NAN Instruments	NAN-S4	Figure 2. Custom clamps are needed to use the U-Probe. Everything mentioned with exception of the U-Probe is provided by NAN instruments.
Screw microdrives	MIT Machine shop		Anything that is able to secure a guide tube to the NAN grid should be appropriate.
Stainless Steel Guide Tubes	Small Parts, Inc.	B00137QHNS (1) or B00137QHO2 (5)	These are 60 in long and cut to size in the laboratory using a Dremel hand drill
Plexon U-Probe	Plexon	PLX-UP-16-25ED-100-SE-360-25T-500	See U-Probe specifications available at www.plexon.com Also see Figure 1.
Table 1. Hardware.
NAN software	NAN Instruments		Computer interface requires an additional serial port to accommodate the Plexon system and the NAN hardware
Offline Sorter, FPAlign, PlexUtil, MATLAB programs	Plexon		Under ’Installation Packages’
Neur–xplorer	NeuroExplorer		Under ’Resources’
CSDplotter Version 0.1.1	Klas H. Petterson
Table 2. Software.