## Materials and Methods
### Stimuli
#### 2.3.2 Stimulus Recording  
All words were pronounced by a male, native-English speaker (from the United States of America, age = 28 years old). The original speech recordings were stored in \"wav\" format with a sampling rate of 44.1 kHz, 16-bit quantization on a mono channel. Each word was recorded three times, from which one was chosen as a stimulus to be used in the experiment. This selection was based on having limited fluctuation in the speech signal amplitude (within \(-\) 3 or + 3 dB overall) and the phonemes were checked by use of the Cambridge Dictionary online [36]. Furthermore, we added an empty duration of 10 ms before each word and 5 ms after the end of each word. In order to generate mosaic speech, we were not able to proceed from any existing mosaicization program for visual images [37]. Since, for sound, an uncertainty principle works between time and frequency, the inverse of time, it is essentially, not just technically, impossible to cut both the horizontal and the vertical axis of the sound spectrogram into pieces very accurately. We therefore carefully constructed an algorithm by which a temporal resolution of 20 ms was secured, and frequency resolution of the narrowest critical band was secured as well, considering the purpose of the present study (see [PERSON] et al., 2018 [14]). The recorded words were thus transformed into mosaic speech stimuli with an in-house made program written in the \"J\" programming language. Using this computer program, we separated first the original speech signal into 20 band-pass filters, mimicking the auditory periphery, which is considered to work as if made of non-overlapping but closely packed frequency bands called _critical bands_, covering a frequency range of 50-7000 Hz [38] (Figure 1a,b). All waveforms in these band-pass filters were cut into temporal segments of the _original mosaic block duration_ (OMBD). An example of 40-ms segments is shown in Figure 1c. As mentioned earlier, in our previous study [14], it was found that the intelligibility of Japanese mosaic speech was near-perfect (>95%) if the mosaic block duration was 20 or 40 ms. Unpublished data from our preliminary experiment [34] indicated that the intelligibility of mosaicized English speech was on a high plateau of about 70% when the mosaic block duration was 20 or 40 ms. Therefore, we selected OMBDs of 20 and 40 ms for the present study. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Group** & **Word** & **Group** & **Word** & **Group** & **Word** & **Group** & **Word** \\ \hline \multirow{4}{*}{A} & bush & /bql/lw/r  
As the next step in generating mosaic speech, the total amount of sound energy in each temporal segment in each frequency band was calculated by squaring and adding up instantaneous amplitudes. A white noise of the same duration as that of the speech signal was then generated. It went through  
Figure 1: Examples of the mosaic speech stimuli used in this study. (**a**) The waveform and (**b**) the spectrogram of the original speech for the word “mouse”, pronounced by a male native-English speaker. (**c**) An example of mosaic speech with an original mosaic block duration (OMBD) of 40 ms. Each individual block consisted of one mosaic block duration on the horizontal axis and one frequency band on the vertical axis. (**d**) An example of compressed mosaic speech with an OMBD of 40 ms compressed into a mosaic block duration (MBD) of 20 ms. (**e**) An example of stretched mosaic speech consisting of an OMBD of 40 ms stretched into an MBD of 80 ms.  
the same band-pass filters, and the waveforms in these band-pass filters were cut into temporal segments in the same way. For each temporal segment, cosine-shaped rise and fall times of 4 ms were used. The total amount of sound energy in each temporal segment in each frequency band was calculated in the same way. There was a time-frequency correspondence between the speech signal and the white noise, because they had the same duration and the same frequency range.