Laboratory for Visual Neuroethology
The laboratory of Bill Saidel (MIT, 78) is focused on the study of neuronal structures involved in visual sensory reception in fish and correlative behaviors. I have focused on this particular fish because it lives at the boundary of the air-water interface and reflects the evolution of simultaneous biophysical constraints for vision in water and air simultaneously.
Pantodon is a lovely little fish that lives just below the water’s surface in Nigeria. As a consequence of its optical ecology, its eye views into air and water simultaneously. We are currently studying two specific behaviors, feeding and the shadow reflex, and relating them to its brain organization and the visual electrophysiology of cells at different loci of its visual pathway. In particular, we have found a diencephalic nucleus (nucleus RostroLateralis) that is concerned only with vision in air. This nucleus receives direct visual input from the ipsilateral ventral retina and indirect visual input from cells of the optic tectum (bilaterally) that are postsynaptic to the ventral retina. This nucleus is found in less than a dozen of the >20,000 species of fish. Interestingly, it is found in Anableps anableps, another and unrelated species that simultaneously sees in air and water.
The eye of Pantodon is unusual because its visual field is closely reflected in eye and retinal structure. The fundus of the eye is bisected by a modified falciform process that extends from the nasal pole almost to the temporal pole. Blood vessels emerge perpendicular to the falciform process to run on the surface of the retina. A small region of continuity extends from ventrotemporal retina to dorsotemporal retina at the temporal pole of the ora serrata. The embryonic fissure extends from the nasal pole to the optic disc. I am currently studying the non-symmetrical postembryonic addition of cells in the retina as a probe to understand the morphological pressures that might account for the extension of the falciform process into the temporal hemiretina. The ratio of temporal / nasal >>3.
The visual field is tripartite. The ventral hemiretina views into the air. The water surface area through which all aerial rays pass is known as Snell’s window. The dorsal retina views into the water column and the region between (which has an unusual retinal structure, see below) views the aquatic environment as seen in a reflection from the aquatic side of the air-water interface. The retina reflects this tripartite visual field in the following manner:
The falciform process bisects the retina in the nasal hemiretina (left) and sits on the surface of the temporal retina. The black tissue is due to highly melanized chorioid epithelial cells that penetrate the retina via the optic nerve at the disk. Cones are wider and larger in the aquatic viewing retina (right,top) than the aerial viewing retina (right, bottom).
Along with Dr. Hong Y. Yan of the T.H. Morgan School of Biology, University of Kentucky, we have been studying the feeding behavior of this fish using videotape techniques. We found that the fish feeds monocularly and it only feeds on targets at the water surface. [In the numerous years of observing this fish, I have seen only 1 example of feeding within the water column.] It only captures a target from within 1 cm of an eye. It is fairly clear that the sensory information involved with feeding is a combination of vision and lateral line with visual input under bright illumination acting in an inhibitory manner. Either vision alone or lateral line alone are sufficient for feeding purposes, but the combination in low illumination provides a more favorable sensory environment. Bright light seems to inhibit the feeding process.
Although the fish will jump at targets in the air, it does not jump at the targets. It jumps at the image of the target in Snell’s window. The figure at the left just below demonstrates that the fish does not jump at the hanging cricket in air. Rather, as you can see in the image below, the fish jumps at the image of the cricket in Snell’s window.
Each frame is a composite of a top view (above) and a side view (below) taken simultaneously (with the aid of an overhead mirror). The small arrows in frame 4 identify the hanging cricket. The large arrows in figure 4 identify the vector of movement. As can be seen in frame 5, the fish (albeit blurred), is jumping at a target adjacent to its eye (ie., within Snell’s window).
Since feeding only occurs from stimulation of the ventral retina, we searched for a neurological correlate or neural element that can be found in the visual pathway concerned with the superior visual field. In the optic tectum, we found a distinctive neuron (below) whose dendrites appear to integrate visual and lateral line input. Since these cells are predominately (>85%) found in the tectual representation of the aerial visual field, we feel that these cells signal a potential feeding target. Interestingly and as a corollary, we also studied the distribution of these cells in a fish (goldfish) that feeds throughout its visual field and sure enough, the cells are found throughout the tectum. We are currently searching through the tectum of other fish with uniquely peculiar feeding habits for the tectal distribution of these cells.