Limits of phase and amplitude sensitivity in the torus semicircularis of Eigenmannia

Eigenmannia can detect modulations in the time disparity of signals received by different regions of the body surface as small as several hundred nanoseconds. This study presents recordings of single units in the torus semicircularis that are sensitive to time disparities (differential-phase) between a sinusoidal signal received by the head region and a similar signal received by the body surface caudal to the fish's pectoral fins. The sensitivity of units to differential phase, measured by the change in spike rate per unit change in time disparity, was greatest when small phase modulations, rather than stationary phase differences, were presented. Thresholds of differential-phase coders ranged from 6.5 microseconds to several hundred microseconds, with approximately 20% of the units having thresholds in the 5-10 microseconds range. For most cells, sensitivity to small modulations of differential-phase was relatively unaffected by time disparity 'offsets' within a range of several hundred microseconds. A threshold of 5-10 microseconds is still an order of magnitude higher than that measured in the Jamming Avoidance Response (JAR). Neurons that were sensitive to amplitude modulations (AMs) had thresholds as low as 0.05%. This value is comparable to that observed at the behavioral level.