The T5 Base Modulator Hypothesis: A Dynamic Model of T5 Neuron Function in Toads

Abstract

Prey-catching behavior in toads exhibits flexibility. Toads can be conditioned to regard objects which normally elicit escape behavior as prey. The range of acceptable sizes for prey varies with motivation. In spring, prey recognition is “exchanged” for female recognition introducing mating behavior. To account for this behavioral flexibility within a single dynamic model, T5 neuron function is modeled as a T5 base modulator consisting of two components. The first, called the T5 base, consists of a T5 unit and all of the influences upon it except for thalamic pretectal (TP) inhibition which constitutes the second component. The T5 base modulator is a TP modulated tectal structure in which the T5 base provides a basis for T5 neuron function while TP inhibition modulates that basis to achieve the desired characteristics (subclasses T5.1, T5.2, or T5.3) for an individual T5 neuron at a given time. A T5 neuron thus has the potential, e.g., to be either a prey or mate feature analyzer. In the absence of TP modulation, the T5 base is optimized for recognizing objects that are among the least prey-like objects that T5 neurons analyze. Three mechanisms for achieving this are explored. The T5 base modulator hypothesis does not argue against the division of T5 neurons into subclasses, but instead suggests that these subclasses can be regarded as (more or less) stable states of a modulated system capable of adaptability and changeability.