Schreuder et al. Listen, you are writing! PROCEDURE Subjects sat in a reclining chair, facing a screen with a ﬁxation cross at ∼1 m. They were surrounded by six speakers at ear height, evenly distributed in a circle with 60˚ distance between them (see Figure 1A) which is well over the resolution of spatial hearing in the azimuth plane (Grantham et al., 2003). Circle radius was ∼65 cm and speakers were calibrated to a common stimulus inten- sity of ∼58 dB. The room was neither electromagnetically shielded, nor were extensive sound attenuating precautions taken. Before the experiments, subjects were asked to minimize eye movements and muscle contractions during recording periods. The experimental protocol is visualized in Figure 1B. A run, or sentence, consisted of several trials. Each trial, or selection, consisted of several iterations. During one iteration all speaker locations were stimulated exactly once in a pseudo-random order, before proceeding to the next iteration. This pseudo-random sequence was generated such that (1) between two stimuli from one speaker, there were at least two other stimuli and, (2) two successive stimuli never came from neighboring speakers. A single stimulus is hereafter referred to as a subtrial. The stimulus onset asynchrony (SOA) was set to 175 ms. The amount of iterations/trial inﬂuences the efﬁciency of the system. A high number is likely to increase the accuracy, but at the same time it increases the time needed for a selection. Vice versa, a low number will allow for quicker selections, but they are likely to contain more errors. Therefore, two settings were tested: (1) a ﬁxed number of 15 iterations and, (2) a method which allows for determining the number of iterations on runtime. During a trial, the subject’s task was to focus attention to one (target) of the six directions and mentally count the number of appearances. During the calibration phase the target was given to the subject explicitly. During the writing phase the target direction was not given explicitly, but had to be inferred from the spelling interface. All 21 subjects performed a ﬁrst session (Session 1), and those that reached a performance level that allowed them to write the given sentence were invited for a second session (Session 2). SPELLING INTERFACE For online writing, an adapted version of the hex-o-spell speller (Blankertz et al., 2006) was created in which a character can be selected in two steps. First a group of characters is selected (group “F–J” in Figure 1B) by focusing on the corresponding direction. In the second step the characters of this group are divided over ﬁve of the directions and an individual letter can be selected. Choosing the sixth direction returns the user back to the ﬁrst selection step, thus preventing a wrong letter selection. Which (group of) let- ter(s) corresponds to a direction – and which direction had been selected – could be read out to the user using speech synthesis (Schröder and Trouvain, 2003). The copy text and progress were presented visually. SESSION 1 First, subjects were familiarized with the sounds, after which about 30 min of calibration data were recorded. Subjects performed 48 trials, 8 for each direction. At the start of a trial the target direction was indicated both visually (one of six on-screen dots highlighted) and auditory (by playing the direction speciﬁc cue from the target location). After that, stimulation was purely auditory and sub- jects were asked to count the number of target appearances in the trial and report them. Each calibration trial consisted of 15 itera- tions. However, in order to have a varying number of targets and thus have a more challenging task, a prequel consisting of varying FIGURE 1 | Experimental protocol. (A) Subjects were surrounded by six speakers at ear height. Speakers were equally spaced with 60˚ angle between neighbors, with a circle diameter of 65 cm. (B) Visualization of the experimental protocol in the online phase. In Session 1, the number of iterations j was set to 15. For Session 2, j was determined by the dynamic stopping method. No labels were read aloud (RL) in Session 2. www.frontiersin.org October 2011 | Volume 5 | Article 112 | 3