The Behavior of Animals. Группа авторов

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2 stimulus perception

      H. BURGHAGEN AND J.-P. EWERT

      INTRODUCTION

      Driving a car during rush-hour, we are exposed to a flood of information that bombards our sensory systems through various channels: visual, auditory, vibratory, somatosensory, etc. If the central nervous system (CNS) were to respond to all this information simultaneously, chaos would develop. Thus, on the one hand, the CNS must be ready to collect information from different sensory channels and to process these in parallel and concurrently; on the other hand, it must be selective: perceiving the right thing in the right place at the right time—say, a traffic sign—and responding to it appropriately, for example, by stepping on the brakes. This involves localization, identification, and decision-making. In general, all animals employ their sensory instruments for the translation of perception into action in order to select a specific goal-oriented skill.

In this chapter, we start with a survey of sensory modalities and show that sense organs and corresponding neural networks (sensory maps) provide animals with their own sensory worlds. In the light of current investigations in different animal species, including humans, we select examples showing that Niko Tinbergen’s ideas and concepts have paved the way for ethological and neuroethological studies over the last six decades. We go on to describe quantitative relationships between stimulus and behavioral response, including discussion of the concepts of sign-stimulus, innate releasing mechanism (IRM), heterogeneous summation, supernormal releaser, and the influences of attention and motivation. Our intention is to show that various classical ethological concepts can be redefined, filled with physiological content, and thus integrated into our current knowledge.

      Moving from the behavioral to the neurophysiological level of analysis, we explore stimulus perception and the behavior that ensues, from which some general principles across species emerge. In the CNS, there are stimulus-response mediating pathways and neural loops that modulate, modify, or even specify that mediation. Using neuronal correlates of releasing mechanisms as well as neural network modeling, which operate as sensori-motor interfaces, we discuss sensory structures involved in feature detection including olfaction in insects, configurational visual object perception in toads and monkeys, and visual perception in primates.

      Stimulus Reception

      Sensory information to be processed comes from outside an organism’s CNS and must get first in contact with the nervous system via its receptors. This information concerns basic sensory modalities:

      Particular perceptual capabilities include electroreception (response to electrical energy) and magnetoreception (response to energy of a magnetic field). Nociception, the reception of pain, involves specific cell physiological responses to severe tissue damage caused by thermal, kinetic, and/or chemical energy. The form of energy to which the receptor cell responds determines the sensory modality. Within a sensory modality (e.g., vision), different stimulus qualities (color) and stimulus quantities (brightness) can be distinguished.

A stimulus, sensed by a receptor cell, is transduced by intracellular chemical processes (see Chapter 5). These lead to a change in the (receptor) membrane potential, which—depending on cell type—can generate nerve impulses. To respond to weak stimuli, receptors may have their own amplifying system. In rod photoreceptor cells of the vertebrate retina, for example, one photon absorbed by one molecule of rhodopsin gives rise to a signaling cascade of intracellular biochemical events that activate 6×10⁶ molecules of cyclic guanosine monophosphate, cGMP. This intracellular messenger influences the ion channels of the cell membrane leading to the receptor potential. Scent receptors in mammals have similar properties.

      Receptor cells provide organisms with information on their own sensory worlds

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