If ABR using a click stimulus produces a response between 2-4 kHz, then why should filtering low frequencies in infants affect the ABR response?
Stimulation of the cochlea with clicks activates hair cells from the base to the apex. However, the ABR is generated when auditory neurons innervating inner hair cells in the basal region begin firing. Actually, it's possible to record a normal ABR even in patients with very poor hearing in the 2000 to 4000 Hz region but normal hearing for higher frequencies. We rarely see such patients in the clinic.
The frequency specificity of the stimulus and cochlear activity is not directly correlated with the spectrum of the ABR. In other words, the energy in the ABR (from about 100 Hz up to at least 1000 Hz) is related to neural firing in the auditory nerve and auditory regions of the brainstem, not the region of the cochlea that is stimulated. The ABR spectrum is essentially the same whether the stimulus is a 8000 Hz tone burst or a 500 Hz tone burst, or a click.
The same concept also applies to behavioral hearing. The response to a pure tone at a specific frequency is tonotopic from the cochlea to the cortex. Certain hair cells or neurons respond depending on the frequency. However, the nature of the response (e.g., the firing rate, etc) is the same for all frequencies.
For an in-depth discussion of this topic, please view Dr. Hall’s recorded course on AudiologyOnline, Application of ABR in Objective Assessment of Infant Hearing