Visual stimulus.

A small (4 × 6-in.) LCD screen was placed at a roughly 45° angle with respect to the mouse’s anteroposterior axis, in the mouse’s right visual field. Roughly 7.6 min of a visual stimulus were played, consisting of sinusoidal drifting gratings (generated with Psychtoolbox) and/or a natural scene of reeds blowing in the wind (from the Chicago Motion Database, courtesy of Stephanie Palmer’s laboratory, University of Chicago) to elicit neural activity. The drifting grating presentation consisted of 10 s of gray screen, followed by the presentation of a sinusoidal drifting grating of a particular orientation (12 different orientations separated by 30° each, 1 cycle per second, 1/720 cycles per pixel) for 2 s, followed by 1s of gray screen, with each orientation (and subsequent gray screen) repeated 12 times in a pseudorandom fashion. A photodiode was placed in the lower left of the LCD screen and was digitized along with the patch signal, for synchronization. A typical recording session consisted of one or more 7.6-min presentations in succession. For neurons in which the drifting gratings and natural scene were presented in succession, the recording that exhibited the greatest mean extracellular spike amplitude (as explained above) was analyzed, with the exception of one recording that had the greatest spike amplitude but appeared to exhibit drift in its extracellular signal (that is, the extracellular spike amplitude decreased over time throughout the recording, although this was not quantified in detail). The absolute value of the difference of the mean spike amplitude between multiple recordings of the same neuron was, on average, 7.4% (4.1%) (n = 7 neurons from 5 mice).