The role of torus longitudinalis in equilibrium orientation measured with the dorsal light reflex.

Abstract

The torus longitudinalis (TL) is a paired elongated structure reciprocally connected with the optic tectum in actinopterygian fishes. Electrophysiological recordings have revealed a sustained neural discharge in TL inversely related to luminance [Northmore, 1984]. This photometric response may play a role in the comparison of light levels between the two eyes, which must occur for the dorsal light reflex (DLR) to orient the body in the environment. Based on the luminance response of TL, it was hypothesized that TL ablation would result in a reduced DLR. Individual goldfish (Carassius auratus) were observed with variable and constant light sources on opposite sides of a test tank. The DLR was measured with respect to angle (degree) and duration (seconds) of body tilt about the longitudinal axis. After baseline readings were obtained, the intertectal commissure was cut and TL was removed bilaterally. Surgical control fish had only the intertectal commissure cut. Following surgery, TL ablatees showed a significantly reduced DLR compared to controls. The results are consistent with the suggestion by Northmore [1984] that TL may be involved in the processing of luminance information.