Fig. 1.

Neurophysiology methods. A: free-viewing visual search task. The top shows typical stimuli, and the bottom shows schematic trial structure. A textured noise pattern cued the onset of each search trial. Animals responded to the start cue by grabbing a touch bar. The search target (T) appeared at the center of the screen and could be inspected for 3–5 s. The search target was extinguished for a delay period (2–4 s), and an array of possible matches was presented (2–5 s). If any array contained the search target, the bar had to be released within 500 ms of array offset. If only distracters (X) were present in the array, no bar release was permitted. The delay-array cycle could repeat up to 7 times on each trial. During each experiment, the search target and distracters were circular image patches of equal size, selected at random from a single photograph. B: fixation task. A series of natural image patches were presented (over time, t) each centered on the spatial receptive field and covering 3× (in visual cortex areas V1 and V2) or 1.5× (in V4) the diameter of the receptive field. The stimuli changed at a constant rate (60 Hz in V1 and V2, 30 Hz in V4). Each trial ended if the animal failed to maintain fixation within a predefined window (typically 0.5°).