| dc.description.abstracteng | The detection and correct interpretation of motion in visual scenes is important in everyday tasks, e.g., for avoiding cars when crossing the street or for assessing the optic flow, induced by self-motion, when navigating through a room. The processing of visual motion starts in the retina where specialized neural circuits integrate the incoming signals and extract relevant features. Retinal ganglion cells, the output neurons of the retina, send the processed information to downstream brain areas.
Here, the retinal coding of motion signals was studied in the salamander, a widely used model system for analyzing retinal function. Signals from up to 400 ganglion cells were recorded simultaneously from the in-vitro retina with multi-electrode arrays, allowing the classification of cell types and thorough population analyses.
In the salamander retina, object-motion sensitive (OMS) ganglion cells have been identified which respond to the differential motion of an object on a moving background but are suppressed by global image motion. These cells might be relevant for detecting moving objects even during self-motion. Furthermore, many vertebrates possess direction-selective (DS) ganglion cells which preferably respond to a certain direction of drifting motion. They are thought to provide important information about the optic flow to higher brain areas. Yet, direction-selective ganglion cells have been absent in previous characterizations of the salamander retina.
Here, direction-selective ganglion cells could be identified in the retina of the axolotl salamander (Ambystoma mexicanum). Further, two distinct types of direction-selective ganglion cells could be discriminated. One might play a role in processing global image motion (standard DS cells), while the other is especially sensitive to object motion and may assist in detecting a moving object's direction (OMS-DS cells). Standard DS and OMS-DS cells differed in many fundamental properties, as their area of spatial integration and systems of preferred directions, and responded to different features of a composite motion stimulus. This suggests that the direction of global image shifts and of locally moving objects is processed in parallel via different pathways, reflected by the functional outputs of standard DS and OMS-DS cells, respectively.
The encoding of global motion direction by standard DS cells was additionally probed with more complex motion stimuli than traditional drifting gratings. This revealed that in complex visual scenes, standard DS cells simultaneously encode motion direction and strong local contrast changes caused by large translational movements independent of their direction. Populations of standard DS cells with different directional preferences could then partially compensate for the coding ambiguities of the individual cell, leading to a better readout of the motion trajectory than would be expected from single-cell responses. This synergy in the population readout illustrates that downstream brain areas could exploit combined inputs from standard DS cells with different preferred directions to decode global image motion more effectively. | de |