Basal optic complex in the frog (Rana pipiens): a physiological and HRP study

The basal optic projection in the frog Rana pipiens has been investigated by single-unit extracellular recording and horseradish peroxidase (HRP) histochemistry. We approached the projection from the ventral side of the brain and recorded single units in the basal optic projection proper as well as in the adjacent dorsomedial region (jointly called the basal optic complex). We found a) units responsive to stimuli moving in a vertical direction, b) an approximately equal number of units responsive to stimuli moving in a horizontal direction, and c) a smaller number of units responsive to changes in ambient light and moving stimuli without direction selectivity. Directional units display significant maintained activity and usually decrease their firing rate in response to stimulus motion in a direction opposite to that which elicits the maximal increase in firing rate. Receptive-field sizes for directional units ranged from 10 to 60 degrees. All units displayed vigorous excitatory response to a wide variety of moving stimuli within the velocity range of 0.2-10 degree/s. HRP histochemistry shows that in addition to the retina, the basal optic complex is connected to three principal areas: the ipsilateral tegmental griseum centrale, the ipsilateral dorsal ventrolateral nucleus of the anterior thalamus, and the ipsilateral posterior thalamic nucleus. In addition, a pathway was observed consisting of two groups of cells that send axons to the ipsilateral rostroventral medulla. This pathway originates a) in cells whose somata lie within the dorsomedial aspect of the basal optic complex (BOC); and b) in cells whose somata lie immediately outside the BOC in the adjacent gray, with apical dendrites extending into the BOC. Some of these fibers continue to the level of the spinal cord. Injection of HRP into the rostroventral medulla led to retrograde labeling of cells of the BOC.