Auditory evoked potentials of the plainfin midshipman fish (Porichthys notatus): implications for directional hearing.

The plainfin midshipman (Porichthys notatus) is an acoustically communicative teleost fish. Here, we evaluated auditory evoked potentials (AEPs) in reproductive female midshipman exposed to tones at or near dominant frequencies of the male midshipman advertisement call. An initial series of experiments characterized AEPs at behaviorally relevant suprathreshold sound levels (130-140 dB SPL re. 1 µPa). AEPs decreased in magnitude with increasing stimulus frequency and featured a stereotyped component at twice the stimulus frequency. Recording electrode position was varied systematically and found to affect AEP magnitude and phase characteristics. Later experiments employed stimuli of a single frequency to evaluate contributions of the saccule to the AEP, with particular attention to the effects of sound source azimuth on AEP amplitude. Unilateral excision of saccular otoliths (sagittae) decreased AEP amplitude; unexpectedly, decreases differed for right versus left otolith excision. A final set of experiments manipulated the sound pressure-responsive swim bladder. Swim bladder excision further reduced the magnitude of AEP responses, effectively eliminating responses at the standard test intensity (130 dB SPL) in some animals. Higher-intensity stimulation yielded response minima at forward azimuths ipsilateral to the excised sagitta, but average cross-azimuth modulation generally remained slight. Collectively, the data underscore that electrode position is an essential variable to control in fish AEP studies and suggest that in female midshipman: (1) the saccule contributes to the AEP, but its directionality as indexed by the AEP is limited, (2) a left-right auditory asymmetry may exist and (3) the swim bladder provides gain in auditory sensitivity that may be important for advertisement call detection and phonotaxis.