Supplementary Figure 1: Headphones and acoustic shadow of the head.

(a) Headphones used for stimulus delivery, and base (bottom). The base is chronically implanted to the skull of the animals and contains a strong miniature magnet which both holds the speakers in place during task performance and also allows easy attachment/detachment. The actual positioning of the speakers was adjusted under anesthesia to match the position of the pinnae for each rat individually relative to the base. The red material in the picture is a moldable glue (Sugru) used for strengthening the headphone structure. (b) Measurements of the acoustic shadow generated by the head. Broadband noise stimuli were played at 65, 70 and 75 dB SPL and sound level was measured placing the microphone by the ear canal of the ‘far’ ear. (c) The head plus near field positioning of the speakers causes an attenuation of 22 dB. (d) Because the head attenuation is not infinite, the sound at each ear contains a contribution from the intensity of both speakers. Thus, the intended (using the sound intensity only from the near speaker) and actual experienced intensity at each ear are not identical. We calculated the difference between the actual and experienced intensity assuming additivity of the squared pressure RMS from each speaker, and we used this to compute the difference between actual and intended ILD and ABL, which are shown in panel (d) as a function of intended ILD. Since these differences are always less than 0.1 dB (which is less than an order of magnitude smaller than the just noticeable difference (JND) for ILD of our animals) we have, for simplicity, ignored the difference between actual and intended levels throughout this study.