activational effects on the auditory system. Similar effects may be present
in experimental animal systems, but our behavioral techniques are not yet
sensitive enough to detect them.
5.2. Electrophysiological Evidence for
Hormonal Control of Acoustic Perception
Auditory pathways in the CNS are highly conserved across species.Acoustic
information, conveyed via the eighth nerve, enters the medulla and is sent
to the superior olive, lateral lemniscus, and midbrain (inferior colliculus in
mammals and torus semicircularis in birds and frogs; see Gentner and
Margoliash, Chapter 7). From the midbrain, there are projections to the
thalamus and then to the forebrain. Electrophysiological recordings indicate that activity of the eighth nerve in response to sound can be sexually
distinct and modulated seasonally in some species of frogs. Narins and
Capranica (1976) used a combination of playback and electrophysiological
methods to demonstrate sex differences in auditory perception of the
Puerto Rican tree frog Eleutherodactylus coqui. In this frog, males and
females respond to different notes of the two-note male calls (Narins and
Capranica 1976). Males respond selectively to the monotonous, single-frequency “Co” note, whereas females preferentially approach the upsweep
“Qui” note (Narins and Capranica 1976). This behavioral difference is
accompanied by a sex difference in the tuning of primary auditory neurons
at threshold. Males have a larger number of single units that respond to
“Co,” whereas females have a larger number of single units that respond to
the “Qui” frequency (as judged by single-unit auditory tuning curves that
yield “best frequencies”; Narins and Capranica 1976). Thus, males may
respond to “Co” and females to “Qui” when they hear a distant calling male
because of this differential acoustic sensitivity.
Seasonal fluctuations in electrophysiological properties of auditory
neurons have also been described. In the fire-bellied toad (Rana temporaria), the grass frog (Bombina bombinba), and the gray tree frog (Hyla
chrysoscleis), single-unit activity in the torus semicircularis (TS) is
depressed during the winter in comparison with the summer, suggesting that
seasonal variation in circulating levels of gonadal steroids may modulate
activity of auditory neurons (Walkowiak 1980; Hillery 1984). Corresponding to this finding, injection of estradiol into the third ventricle of the female
leopard frog produces an increase in the amplitude of auditory evoked
potentials in the TS (Yovanof and Feng 1983). Although E 2 does not
enhance behavioral receptiveness in females of this species (Diakow 1978),
it is possible that E 2 enhances perceptual responsiveness without influencing behavioral responsiveness.Taken together, these studies suggest that the
physiological functions of auditory neurons are modified by gonadal
steroids and that these changes may underlie the modification of auditory
perception in the context of acoustic communication.
6. Hormonal Control of Communication
309
in experimental animal systems, but our behavioral techniques are not yet
sensitive enough to detect them.
5.2. Electrophysiological Evidence for
Hormonal Control of Acoustic Perception
Auditory pathways in the CNS are highly conserved across species.Acoustic
information, conveyed via the eighth nerve, enters the medulla and is sent
to the superior olive, lateral lemniscus, and midbrain (inferior colliculus in
mammals and torus semicircularis in birds and frogs; see Gentner and
Margoliash, Chapter 7). From the midbrain, there are projections to the
thalamus and then to the forebrain. Electrophysiological recordings indicate that activity of the eighth nerve in response to sound can be sexually
distinct and modulated seasonally in some species of frogs. Narins and
Capranica (1976) used a combination of playback and electrophysiological
methods to demonstrate sex differences in auditory perception of the
Puerto Rican tree frog Eleutherodactylus coqui. In this frog, males and
females respond to different notes of the two-note male calls (Narins and
Capranica 1976). Males respond selectively to the monotonous, single-frequency “Co” note, whereas females preferentially approach the upsweep
“Qui” note (Narins and Capranica 1976). This behavioral difference is
accompanied by a sex difference in the tuning of primary auditory neurons
at threshold. Males have a larger number of single units that respond to
“Co,” whereas females have a larger number of single units that respond to
the “Qui” frequency (as judged by single-unit auditory tuning curves that
yield “best frequencies”; Narins and Capranica 1976). Thus, males may
respond to “Co” and females to “Qui” when they hear a distant calling male
because of this differential acoustic sensitivity.
Seasonal fluctuations in electrophysiological properties of auditory
neurons have also been described. In the fire-bellied toad (Rana temporaria), the grass frog (Bombina bombinba), and the gray tree frog (Hyla
chrysoscleis), single-unit activity in the torus semicircularis (TS) is
depressed during the winter in comparison with the summer, suggesting that
seasonal variation in circulating levels of gonadal steroids may modulate
activity of auditory neurons (Walkowiak 1980; Hillery 1984). Corresponding to this finding, injection of estradiol into the third ventricle of the female
leopard frog produces an increase in the amplitude of auditory evoked
potentials in the TS (Yovanof and Feng 1983). Although E 2 does not
enhance behavioral receptiveness in females of this species (Diakow 1978),
it is possible that E 2 enhances perceptual responsiveness without influencing behavioral responsiveness.Taken together, these studies suggest that the
physiological functions of auditory neurons are modified by gonadal
steroids and that these changes may underlie the modification of auditory
perception in the context of acoustic communication.
6. Hormonal Control of Communication
309