Vision in Toads - Feature Detectors and The Visual System

Feature Detectors and The Visual System

To understand the neural mechanisms underlying the toad’s behavioral responses, Ewert performed a series of recording and stimulation experiments. First and foremost, the results allowed him to understand the way the visual system is constructed and connected to the central nervous system. Secondly, he discovered areas of the brain that were responsible for differential analysis of stimuli.

First, the retina is connected to the optic tectum by at least three types of ganglion cells, each with an excitatory receptive field and a surrounding inhibitory receptive field, but they differ in the diameter of their central excitatory receptive fields. Diameters in Class II (R2) ganglion cells are approximately four degrees visual angle. Those in Class III (R3) cells are about eight degrees and Class IV (R4) ganglion cells range from twelve to fifteen degrees. As stimuli move across the toad’s visual field, information is sent to the optic tectum in the toad’s midbrain. The optic tectum exists as an ordered localization system, in the form of a topographical map. Each point on the map corresponds to a particular region of the toad’s retina and thus its entire visual field. Likewise, when a spot on the tectum was electrically stimulated, the toad would turn toward a corresponding part of its visual field, providing further evidence of the direct spatial connections.

Among Ewert’s many experimental goals was the identification of feature detectors, neurons that respond selectively to specific features of a sensory stimulus. Results showed that there were no “worm-detectors” or “enemy-detectors” at the level of the retina. Instead, he found that the optic tectum and the thalamic-pretectal region (in the diencephalon) play significant roles in the analysis and interpretation of visual stimuli (summarized in Ewert 1974, 2004; Ewert and Schwippert 2006).

Electrical stimulation experiments demonstrated that the tectum initiates orienting and snapping behaviors. It contains many different visually sensitive neurons, among these Type I and Type II neurons (later named T5.1 and T5.2, respectively). Type I neurons are activated when an object traversing the toad’s visual field is extended in the direction of movement; Type II neurons, too, but they will fire less when the object is extended in a direction that is perpendicular to the direction of movement. Those T5.2 neurons display prey-selective properties; see prey feature detectors. The discharge patterns of these neurons – recorded in freely moving toads – “predict” prey-catching reactions, e.g. the tongue flip of snapping. Their axons project down to the bulbar/spinal motor systems, such as the hypoglossal nucleus which harbors the motor neurons of the tongue muscles. In combination with additional projection neurons, prey-selective cells contribute to the ability of the tectum to initiate orienting behavior and snapping, respectively.

The thalamic-pretectal region initiates avoidance behavior in the toad. More specifically, electrical triggering the thalamic-pretectal region initiates a variety of protective movements such as eyelid closing, ducking and turning away (Ewert 1974, 2004). Various types of neurons in this region are responsible for the avoidance behaviors and they are all sensitive to different types of stimuli. One type of neurons (TH3) is activated by large threatening objects, especially those ones that are extended perpendicularly to the direction of motion. Another type (TH6) is activated by a looming object moving toward the toad. Still other types (TH10) respond to large stationary obstacles, and there are also neurons responding to stimulation of the balance sensors in the toad’s ear. Stimulation of (a combination of) such types of neurons would cause the toad to display different kinds of protective behaviors.

Lesioning experiments led to the discovery of pathways extending between the tectum and the thalamic-pretectal region. When the tectum was removed, orienting behavior disappeared. When the thalamic-pretectal region was removed, avoidance behavior was entirely absent while orienting behavior was enhanced even to predator stimuli. Furthermore, prey-selective properties were impaired both in prey-selective neurons and in prey-catching behavior (Zupanc 2004). Finally, when one half of the thalamic-pretectal region was removed, the disinhibition applied to the entire visual field of the opposite eye. These and other experiments suggest that pathways, involving axons of type TH3 cells, extend from the pretectal thalamus to the tectum, suitable to modulate tectal responses to visual stimuli and to determine prey-selective properties due to inhibitory influences.