FIG. 2. The opponent responses of middle temporal area (MT) and V1 neurons as a function of Δ*x.* Each panel plots the mean opponent response as a function of Δ*x*, averaged across neurons with a given range of preferred speeds. *Left* and *right*: responses when the stimulus moved at 16 and 32°/s. Data for V1 and MT are shown in black and gray. Legends give the number of neurons with preferred speeds in the given range and their mean preferred speed. Before averaging, responses were normalized, so that each neuron's opponent response was one for a Δ*t* of 4 ms. Error bars show SEs across neurons and are typically smaller than the symbols. Fits are sigmoidal, and the vertical lines show Δ*x*_{max}, defined as the value of Δ*x* for which the *y* value of the fits is 0.5. For a stimulus speed of 16°/s, the values of the Δ*x*_{max} for MT and V1 were 0.44 and 0.52° (2–8°/s), 0.45 and 0.60° (8–12°/s), and 0.65 and 0.61° (12–25°/s). For a stimulus speed of 32°/s, the values of Δ*x*_{max} were 0.43 and 0.59° (3–11°/s) and 0.76 and 0.82° (11–30°/s).