A matrix of seven independently moveable electrodes (Thomas Recording, Marburg, Germany) was used to record simultaneously from multiple cortical cells and sites in V1. The seven electrodes were arranged in a straight line with each electrode separated from its neighbor by ~300 μm. Each electrode consisted of a platinum/tungsten core (25 μm in diameter and 1 μm at the tip) covered with an outer quartz-glass shank (80 μm in diameter), and had an impedance value of 1–4 MΩ. The multi-electrode matrix was precisely positioned before recordings so the tip of the matrix was ~3 mm above the cortical surface. In order to keep all seven electrodes at roughly similar cortical depth, we independently moved each electrode from the surface of V1 until we got from each electrode a detectable high frequency response that was driven by the visual stimulus. The high frequency response (hash) represents multi-unit spike activity, and usually finding the first hash provided a very good estimation of the upper part of layer 2/3, and helped us to align all seven electrodes at a similar cortical depth (Fig. 5A for example). Once all electrodes were placed in the superficial layer of V1, we built a chamber with bone wax to surround the multielectrode matrix and filled the chamber with a large amount of agar to seal the craniotomy entirely. We found this procedure largely enhanced the stability of recordings throughout the experiment. Then we conducted our experiments advancing the electrode matrix at ~100-μm intervals in the cortex. This setup allowed simultaneous recordings of multiple neurons within the same cortical layer, all at nearby visual eccentricities. Electrical signals from the seven electrodes were amplified, digitized, and filtered (0.3–10 kHz) with a preamplifier (Tucker-Davis Technologies, FL, model number: RA16SD) configured for multi-channel recording. The Tucker-Davis system was interfaced to a computer (Dell, TX) running a multi-channel version of the OPEQ program (designed by Dr. J.A. Henrie) to acquire both spike and local field potential data. Visual stimuli were generated also with the custom OPEQ program running in a Linux computer (Dell, TX) with a graphics card with Open GL optimization. Data collection was synchronized with the screen refresh to a precision of <0.01 ms. Stimuli were displayed on a 20-in monitor (IIyama HM 204 DTA flat Color Graphic Display, Cheltenham, UK; pixels: 1024 × 768; frame rate: 100 Hz; mean luminance: 59.1 cd/m2) with a screen viewing distance of ~114 cm. The basic attributes of each cell were estimated using small drifting sinusoidal gratings surrounded by a gray background (both the gratings and the gray background had a mean luminance of 59.1 cd/m2).