Primer Objetivo: Caracterizar la herramienta integrada y los agentes que intervienen en el proceso de autoevaluación institucional en el componente de gestión

Results indicate that the increase in test level, and thereby SL, of carriers by 15 dB, resulted in a significant increase in response amplitudes for individual EFR carriers as well as the overall number of detections at each test level. The increase in response amplitude leads to a higher probability of detection as the criteria for detection (F ratio) is based on the response amplitude relative to the noise amplitude. The level-dependent changes in the present study support previous findings that demonstrate a positive relationship between level and response amplitudes (Lins, Picton, Picton, Champagne, & Durieux-Smith, 1995; Picton et al., 2005, 2003; Vander Werff & Brown, 2005). The change in response amplitudes due to level can be explained by a combination of increased neural firing rates (Sachs & Abbas, 1974) and spread of excitation in the peripheral auditory system (Moore, 2003; Moore & Glasberg, 1987). Spread of excitation recruits more sensory cells, and thereby nerve fibers and synapses, resulting in an increase in input to the brainstem generators of EFRs (Picton et al., 2003; Purcell & Dajani, 2008).

The pattern of intensity-response amplitude slope of speech EFRs is similar to AM tones (Lins et al., 1995; Vander Werff & Brown, 2005); high frequency carriers tend to show a shallower slope than lower frequency carriers. However, the absolute magnitudes of the slopes are higher than that for AM tones. The estimated slopes

for low (500 Hz), mid (1 & 2 kHz) and high (4 kHz) frequency AM tones were ∼1.4, 1.3, and 0.84 respectively (Lins et al., 1995; Vander Werff & Brown, 2005). The steeper slope for this study’s broadband stimuli could be due to a larger excitation area on the basilar membrane, compared to AM tones with only three frequency components. For example, in the case of vowels, it may be that the higher stimulus level allows for better interaction between many harmonics causing a larger increase in response amplitude. The difference in slope across carrier frequencies could be explained, at least in part, on the basis of excitation patterns on the basilar membrane. With increase in level, excitation patterns show more basal spread of activity towards frequencies higher than the stimulus frequency (Moore, 2003; Moore & Glasberg, 1987). Since the basal spread is likely to be greater for low frequency stimuli, the growth is probably steeper for low frequency stimuli relative to high frequency stimuli (Lins et al., 1995).

Response detection time

Response detection time based on sweep length and carrier recording time indicate lower detection times for fricatives, on average (Figures 6-5A & 6-5B). Particularly, the fricative /S/ has the least carrier recording time (Figure 6-5B) likely due to higher response amplitudes relative to other carriers (see Figure 6-4). Average noise floors are variable across carriers too (see Figure 6-4), however, the differences in amplitude are larger than the differences in noise estimates. Variations in response amplitudes lead to differences in F ratios and therefore the time to reach the critical

F ratio. Also, detection times tend to be longer at test levels of 50 dB SPL relative to 65 dB SPL. This can also be explained in terms of differences in response

amplitudes while noise estimates are similar; shorter detection times at the higher test level are facilitated by higher amplitude responses at 65 dB SPL (see Figure 6-4). The carrier recording times for vowels appear to be longer than detection

times reported by Aiken and Picton (2006), possibly due to a floor effect in the present study as responses were not analyzed under 50 sweeps.

A direct comparison with other aided protocols such as CAEPs is difficult to make due to limited data available on test time. Test time computed based on the stop criterion on ‘accepted’ sweeps and inter-stimulus level may range from under 2 to 4 minutes per carrier (Carter, Dillon, Seymour, Seeto, & Van Dun, 2013; Van Dun, Carter, & Dillon, 2012). Average carrier recording time required for a significant detection (Figure 6-5B) are comparable to these estimates. However, the effective testing time necessary using the current stimulus would likely be shorter due to simultaneous presentation of vowel carriers. For the testing conditions in the present study, the average test time necessary for a significant detection using the stimulus /susaSi/ fell under 10 minutes for all carriers (see Figure 6-5A). It is

encouraging to note the clinically feasible test times of the EFR paradigm using the stimulus /susaSi/ to infer neural representation of low, mid and high frequency dominant carriers in the auditory system. Note that the detection times are representative only of responses that were significantly higher than the noise. Due to the use of a fixed number of sweeps, detection times do not represent responses that may have taken over 300 sweeps to be detected (e.g., responses that might have been detected at 320 sweeps are not represented).

In summary, results of Experiment I suggest that the EFR paradigm shows changes in amplitude and detection with improvement in SL. Detection times at

suprathreshold levels demonstrate clinically feasible test times. Alternate scoring rules such as combining carriers within a frequency band improves detection rates. Although combining carriers within a frequency band reduces carrier-specific information, it may prove to be an advantage clinically to demonstrate that detection is possible in each band in possibly shorter test times. The test could

continue for longer to obtain carrier-specific detections, if time permits.