F Draicchio

Gravity load related asymmetries in the sagittal vestibulo-collic reflex

EMG recordings of the neck muscles (biventer cervicis, complexus, splenius, longus capitis) of decerebrate cats were obtained during pitch and roll stimulations (sinusoidal stimulation: 30 degrees p-p amplitude, 0.2 Hz frequency). Most of the EMG responses to pitch showed activation peaks leading the position stimuli by 56 degrees and inhibition peaks leading by 11 degrees. Conversely, in response to roll the activation peak led by 16 degrees and the inhibition peak by 10 degrees. The activation peaks of the pitch responses were, thus, more asymmetric and more leading than those of the roll responses. Consequently, the harmonic distortion coefficient was significantly higher in pitch than in roll. Moreover, when the vertical semicircular canals were activated in absence of otolithic modulation, the pitch and roll responses maintained the same difference in timing observed in the presence of otolithic coactivation. It appeared that the simultaneous stimulation of both anterior semicircular canals (pitch) induces a greater lead than that of combined anterior and posterior canals (roll). Thus the timing of neck muscle responses to vestibular stimulation depends on the pair of activated vertical semicircular canals.

Otolithic control of anticompensatory fast phases (AFPs) in cats

Anticompensatory Fast Phases (AFPs) to vestibular step stimulation in the vertical and horizontal planes and spontaneous saccades were recorded in cats in the upright and 90 degrees side-down positions. With increasing stimulus velocity, the AFPs of the Horizontal Vestibulo-Ocular Reflex (HVOR) increased in their cumulative amplitude as the mean AFP amplitude increased. The AFPs of the vertical vestibulo-ocular reflex (VVOR) were smaller than those of HVOR and showed only a slight increase in their cumulative amplitude with increasing stimulus velocity due to a frequency increase. AFP amplitude and frequency showed features similar to the HVOR when the VVOR was evoked in 90 degrees side-down position. The trend of AFP trajectory was to be aligned with the horizon regardless both the stimulus plane and the animal position. Also the spontaneous saccade trajectory tended to lie close to the horizon in the upright and in 90 degrees side-down position as well. The distribution area of the AFP final points was elliptic with the major axis oriented in the horizontal plane. Its projection onto the retina corresponded to the high receptor density area. The ellipse orientation did not change with respect to the space up to 30 degrees of head inclination. Beyond this limit the major axis of the ellipse tended to be progressively less aligned with the horizon. The same trend was observed for spontaneous saccades.