Auditory threshold in ND and ED rats was defined as the stimulus intensity in dB that evoked peak–to–peak waves amplitudes greater than two standard deviations (SD) from background activity (Cediel et al., 2006 (link); Garcia-Pino et al., 2009 (link); Alvarado et al., 2012 (link), 2014 (link), 2016 (link); Fuentes-Santamaría et al., 2012 (link), 2013 (link), 2014 (link), 2017 (link); Melgar-Rojas et al., 2015a (link)). Evoked responses were recorded from 80 dB sound pressure level (SPL) in 5 dB descending steps, while background activity was defined as the basal activity recorded prior stimulus onset. During recordings, the maximum level of intensity was set at 80 dB to minimize any possible additional noise overstimulation (Gourévitch et al., 2009 (link); Alvarado et al., 2012 (link), 2014 (link), 2016 (link); Fuentes-Santamaría et al., 2012 (link), 2013 (link), 2014 (link), 2017 (link); Melgar-Rojas et al., 2015a (link)). For statistical purposes, in any frequency where no auditory evoked responses were obtained at 80 dB, the auditory threshold was set at that intensity level (Subramaniam et al., 1992 (link); Trowe et al., 2008 (link); Alvarado et al., 2012 (link), 2014 (link), 2016 (link); Fuentes-Santamaría et al., 2012 (link), 2013 (link), 2014 (link), 2017 (link); Melgar-Rojas et al., 2015a (link)).
The threshold shift in both ND and ED animals was determined as the differences between the time points: 6–8M and 12–14M, minus the auditory thresholds at 3M (Alvarado et al., 2012 (link), 2014 (link), 2016 (link); Fuentes-Santamaría et al., 2012 (link), 2013 (link), 2014 (link); Melgar-Rojas et al., 2015a (link)).
The percentage of variation of the threshold shift was calculated using the following formula (Meredith and Stein, 1983 (link); Alvarado et al., 2007a (link),b (link), 2009 (link), 2016 (link); Fuentes-Santamaría et al., 2017 (link)):
Where ATTP is the auditory threshold in the time-points 6–8M and 12–14M, and ATCC is the auditory threshold at 3M (control condition).
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