Spontaneous and driven responses of semicircular canal primary afferents in the unanesthetized pigeon

Spontaneous activity from 120 vestibular semicircular canal (SC) primary afferents and driven responses from 20 SC primary afferents to 2.3-decade bandwidth (0.029-6.152 Hz) sum of sinusoidal angular rotations were characterized in 10 unanesthetized pigeons. The results were compared with those previously gathered from barbiturate anesthetized pigeons. The average spontaneous mean firing rate (MFR) of SC primary afferents in unanesthetized pigeons was found to be 168 impulses per second (I/s), n = 120, and is ~80% higher than that for SC primary afferents in anesthetized pigeons [93 I/s, n = 149 and 92 I/s, n = 124 The spontaneous discharge of SC primary afferents from unanesthetized pigeons was classified according to the coefficient of variation (CV) of 512-1,024 interspike intervals (ISIs) into regular (CV < 0.1), intermediate (0.1 ≤ CV < 0.4), and irregular (0.4 ≤ CV) categories. The percentages of SC primary afferents falling within each CV category were similar for unanesthetized and anesthetized pigeons. The relation between the spontaneous mean ISI and its standard deviation (SD) for SC primary afferents in the unanesthetized pigeon is best described by a power function model, which provides a significantly better fit than does a linear model. The parameters of this power function model are similar for SC primary afferents in unanesthetized and anesthetized pigeons. The spontaneous ISIs of individual SC primary afferents in the unanesthetized pigeon were found to be distributed either normally, log-normally, or according to the fist passage time of the Wiener-Levy (WL) process. The gain of anterior SC primary afferents at 0.25 Hz is similar for anesthetized (2.93 I·s^-1 ·deg^-1 ·s^-1, n = 14) (11) for unanesthetized (3.01 I·s^-1 ·deg^-1 ·s^-1, n = 14) pigeons. Bode plots constructed from the responses to sum of sinusoidal angular accelerations reveal that SC primary afferents in unanesthetized pigeons display a phase lead and gain enhancement at frequencies between 0.6 and 6 Hz. Also, the time constant (τ(L)) of a first-order transfer function relating SC primary afferent firing rate to head velocity is similar for anesthetized (mean = 10.24 s, range 4,45 s to 18,61 s) and for unanesthetized (mean = 7.66 s, range 4.76 s to 14.18 s) pigeons. However, the degree of adaptation, expressed here as a fractional exponent of s, appears to be greater in anesthetized (mean = 0.24, range 0.02 to 0.58) than in unanesthetized (mean = 0.09, range 0.00 to 0.16) pigeons.