Connections of Anterior Thalamic Visual Centers in the Leopard Frog, Rana pipiens.

The amphibian retina projects to two discrete regions of neuropil in the anterior thalamus: the neuropil of Bellonci and the corpus geniculatum. These retinorecipient areas are encompassed within a larger zone of surrounding neuropil we call the NCZ (for neuropil of Bellonci/corpus geniculatum zone). The NCZ is characterized electrophysiologically by a distinctive tonic oscillatory response to blue light; it appears to be a visual module involved in processing the stationary visual environment. Using horseradish peroxidase (HRP), we mapped the connections of the NCZ. Retrogradely labeled cell bodies are found in: (1) the contralateral anterior thalamus; (2) both retinas; and (3) the posterior medial dorsal thalamus (PMDT). Anterogradely labeled fibers are found in: (1) the contralateral anterior thalamus; (2) the ipsilateral PMDT; (3) the ipsilateral neuropil lateral to the posterior tuberculum in the ventrolateral posterior thalamus; and (4) the ipsilateral anterior medulla. There are no direct connections between the NCZ and the telencephalon, the tectum, or the suprachiasmatic nucleus. Applying HRP to the PMDT, we found that its inputs are limited to the contralateral and ipsilateral NCZ and the contralateral PMDT. Thus, PMDT appears to be a satellite of the NCZ. Blue light elicits tonic oscillatory electrical responses in the PMDT quite similar to the responses to blue light in the NCZ. We discuss how the leopard frog NCZ and the mammalian ventral lateral geniculate nucleus share anatomical and physiological properties.

Representation of the visual field in the anterior thalamus of the leopard frog, Rana pipiens.

We used physiological and anatomical methods to elucidate how the visual field is represented in the part of the dorsal anterior thalamus of the leopard frog that receives direct retinal projections. We recorded extracellularly while presenting visual stimuli, and characterized a physiologically defined region that encompasses the retinal projections as well as an extended zone beyond them. We probed the area systematically to determine if the zone is organized in a visuotopic map: we found that it is not. We found that units in this region respond only to stimuli in the contralateral half of the visual field, which is similar to what is seen in the dorsal lateral geniculate nucleus in mammals. When we backfilled retinal ganglion cells from application of HRP to the anterior thalamus, we found labeled cells only in those parts of the retina corresponding to the contralateral hemifield, confirming our physiological observations.

Light and shadow: visual recognition of the stationary environment by leopard frogs.

We determined how leopard frogs respond to non-moving aspects of the environment. We have discovered that these frogs are attracted to dark, stationary, opaque objects. This attraction depends on the relative reflectance of the object, i.e., the darker the block, the more attractive it is, and the attraction is found under both bright and dim ambient light levels. Larger blocks are more attractive than smaller blocks, but frogs are still attracted to blocks much smaller than themselves. Previous studies have shown that frogs are also attracted to sources of light. Using a choice experiment, we show that the probability a frog will choose a dark object versus a light source depends on the intensity of the light source relative to the intensity of the ambient light. The frog only moves toward a light source when it is at least 20 times brighter than the brightest object in the environment. These findings help to clarify the frog's "phototactic" nature.

Leopard frogs move their heads, but not their eyes: implications for perception of stationary objects.

Movement of an image on the retina is necessary for the persistence of vision in vertebrates. Leopard frogs (Rana pipiens) do not show any obvious independent eye movements that could sustain perception of stationary objects when the animal itself is stationary. However, video recordings of normal, awake leopard frogs made through a dissecting microscope reveal that the animal's whole head oscillates with an amplitude of 10-100 µm in step with the breathing cycle. The retinal image shifts produced by these breathing movements could ensure continuous perception of the frog's stationary environment.