Effects of visual deprivation on the development of auditory sensitivity during formation of the freezing reaction in pied flycatcher nestlings.

Recording of evoked potentials from the higher center of the auditory system - field L of the nidopallidum - was used to study the formation of auditory sensitivity in normally developing and visually deprived pied flycatcher nestlings aged 6-9 days. Restriction of visual afferentation was found to produce significant reductions in the absolute threshold of auditory evoked potentials in the frequency range of the species-specific food call (1-3 kHz) during the period at which vision acquires a role in providing sensory support for feeding behavior in control nestlings (six days). In the frequency range of the species-specific alarm call (4-5 kHz), the thresholds of auditory evoked potentials were significantly lower than those in controls during the period at which vision acquires a role in providing sensory support for defensive behavior (8-9 days). Taking account of previous data showing decreases in the efficiency of acoustic signals in evoking freezing reactions in visually deprived nestlings, it is suggested that defensive behavior develops not simply as a response to the acoustic alarm call or tones imitating it, but as a system whose complete formation and functioning require integration of a series of factors, including visual afferentation.