THE ROLE OF TORUS LONGITUDINALIS IN EQUILIBRIUM ORIENTATION MEASURED WITH THE DORSAL LIGHT REFLEX

The torus longitudinalis (TL) is a paired elongated structure reciproc ally connected with the optic tectum in actinopterygian fishes. Electr ophysiological recordings have revealed a sustained neural discharge i n TL inversely related to luminance [Northmore, 1984]. This photometri c response may play a role in the comparison of light levels between t he two eyes, which must occur for the dorsal light reflex (DLR) to ori ent 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 an d constant light sources on opposite sides of a test tank. The DLR was measured with respect to angle (degree) and duration (seconds) of bod y tilt about the longitudinal axis. After baseline readings were obtai ned, the intertectal commissure was cut and TL was removed bilaterally . Surgical control fish had only the intertectal commissure cut. Follo wing surgery, TL ablatees showed a significantly reduced DLR compared to controls. The results are consistent with the suggestion by Northmo re [1984] that TL may be involved in the processing of luminance infor mation.