Electrophysiological recordings and behavioural procedures

We recorded neurons in three male macaque monkeys (Macaca mulatta, 8–12 years of age). After initial training, monkeys were implanted with a head holder and recording chambers above V1 under general anaesthesia and sterile conditions (for details of surgical procedures, see41). Single-cell discharges were recorded extracellularly using a tungsten-in-glass electrode flanked by two pipettes41,42. Drugs were applied iontophoretically through these pipettes using the NeuroPhore BH-2 System (Digitimer). Stimulus presentation and behavioral control were managed by Remote Cortex 5.95 (Laboratory of Neuropsychology, National Institute for Mental Health, Bethesda, MD). Neuronal data was collected by Cheetah data acquisition (Neuralynx) interlinked with Remote Cortex. The waveforms of all spikes that exceeded a threshold set by the experimenter were sampled at 32 kHz. Offline sorting of these spike samples was carried out based on waveform features (Neuralynx spike sorting software, Version 2.51). Average waveforms of recorded cells are shown in Fig. 1B. While we aimed to isolate single cells, on occasions these may be somewhat contaminated by spikes from additional cells, as is generally the case with extracellular recordings. Recordings were performed once the first isolated spikes were encountered after crossing the dura. This was usually the case within 100–300 μm after the first background hash activity occurred. Once a recording was finished and the animal continued to work we advanced the electrode pipette by at least 150 μm, until we encountered the next well isolated spike, and performed the next recording. In rare cases we performed a third recording on a recording day. Thus, almost all out recordings were performed within the first 1000 μm of crossing the dura, and therefore most likely within the supragranular layers (Fig. 1C).

(A) The fixation task. Monkeys fixated a central point (red) and passively viewed a sequence of 4 stimuli displays. Each display consisted of a central target Gabor presented in the RF (dashed circle) of the neuron under study, either in isolation or together with two collinear flankers. Each display lasted 700 ms, with a 300 ms blank interval. Note that the circle outlining the RF is for illustrative purposes and was not part of the stimulus display. (B) Normalized mean waveforms of the cells recorded. (C) Median, quartile, and range of recording sites in V1, reconstructed according to depth records in relation to first spiking activity.

Monkeys were trained to keep fixation (eye window 1.2° in diameter) while a small oriented Gabor was presented in the periphery of their visual field, with or without two collinear flankers (Fig. 1). The fixation point (FP, 0.1° diameter) was presented centrally against a grey background (21 cd/m2) on a 20″ analogue cathode ray tube monitor (100 Hz, 1,600 × 1,200 pixels, 57 cm from the animal). Eye position was recorded with an infrared based camera system (Thomas Recording GmBH) and sampled at a rate of 250 Hz.

A trial started as soon as the monkey’s eye position was within a fixation window centred on the fixation point. After 500 ms four Gabor stimuli, matching the preferred orientation and spatial frequency of the neuron, were presented in succession. Stimulus presentation time was 700 ms followed by 300 ms gaps between presentations (details about the stimuli are given below). At the end of the four presentations, the fixation point disappeared and monkeys were rewarded if their eye position had been within the fixation window for the trial duration. If the monkey broke fixation before the FP disappeared the condition was repeated later in the block.