40 Hz ASSR or N1-P2 for Threshold Estimation?

Both of these auditory evoked potentials responses arise from generators in the thalamo-cortical region of the central auditory system and so you would associate them with objectively assessing thresholds in adults e.g. medico-legal or non-organic hearing loss, or in those with learning difficulties or other challenges that preclude behavioural threshold finding techniques.

Either approach would be perfectly feasible and acceptable. At least two peer-reviewed studies have compared the techniques for time, accuracy and other clinical considerations (Van Maanen and Stappells 2005; Tomlin et al 2006). Although of similar study design, they make opposing recommendations. Van Maanen and Stappells (2005) suggest that 40 Hz ASSR would be the test of choice whereas Tomlin et al (2006) suggest that the N1-P2 response would be the test of choice. Although of similar design, some elements of the study designs may be factors leading to these conclusions. For example, Van Maanen and Stappells provide clear objective “stopping” criteria for the ASSR but not the N1-P2 responses (“by eye”).

What follows is a short list of some of the positive and negative points of the 40 Hz ASSR when set against the N1-P2 response as a way to objectively estimate hearing threshold in adults:

• ASSR is a fully objective response detection method
  • Eliminates tester bias
  • Reduces workload
• ASSR allows multiple simultaneous stimuli: thus theoretically faster than N1-P2 (x2-3)
• High arousal (patient awake) is desirable for both ASSR and N1-P2 measures, but ‘adaptation’ of response is not relevant in ASSR.
• Stimulus interleaving/switching is not necessary in ASSR, but may be useful in N1-P2.
• CE-Chirp stimuli used in ASSR produces more synchronously active neurons, which should produce a more robust physiological response but one would expect lower frequency specificity than tonebursts used in N1-P2 measures. When differentiating sharply sloping or “notched” audiograms (common through noise exposure, for example) the frequency specificity of long-duration (e.g. 80ms) tone bursts may be desirable.
• ASSR has (arguably) a lower legal precedent than N1-P2 in some regions, which may be relevant in medico-legal scenarios.
• At high stimulus levels, multiple simultaneous stimuli in ASSR may lead to unwanted cochlear interactions. In N1-P2 testing multiple stimuli are not presented simultaneously so this is not a factor.
• Although ASSR is fully objective, the tester cannot utilise waveform morphology in decision making. 

References and caveats
Van Maanen, A and Stappells, DR. (2005) Comparison of multiple auditory steady-state responses (80 versus 40 Hz) and slow cortical potentials for threshold estimation in hearing impaired adults. IJA 44, 613-624.

Tomlin D et al. (2006) A comparison of 40 Hz auditory steady-state response (ASSR) and cortical auditory evoked potential (CAEP) thresholds in awake adult subjects. IJA 45, 580-588.