E44 Active exploration of the environment

Topic

Active exploration involves deliberately selecting sensory actions to maximize the information gathered from the environment. Unlike passive perception, where the organism simply receives available signals, here it actively controls the information it receives. It may move its head to smell or hear better, perform saccadic eye movements, change its position, emit signals that interact with the environment—such as through echolocation or electrolocation—or move about to view the surroundings from different perspectives.

The choice of these actions is driven by information gain. An action is considered more informative the greater the reduction in uncertainty it yields regarding the state of the environment. This gain can be understood as the difference between the uncertainty present before acting and the uncertainty expected to remain after observing the action's outcome. An organism exploring optimally, therefore, selects the sensory action that promises the greatest reduction in uncertainty. This active search constitutes a form of epistemic curiosity—or information seeking—observable in both mammals and insects when they encounter novel environments.

However, gathering information competes with another objective: leveraging existing knowledge to obtain an immediate reward. This tension gives rise to the exploration-exploitation dilemma. Exploitation means choosing known options that already yield good results; exploration involves dedicating resources to uncertain options that might prove superior. One mechanism for balancing these strategies adds an "exploration bonus"—linked to the degree of uncertainty surrounding an option—to the expected reward of each action. Less-tested options receive a larger bonus, which diminishes as the action is repeated and more information is acquired. Thus, choosing an alternative progressively reduces uncertainty regarding it and makes other, less-explored options relatively more attractive, thereby closing the loop between action, learning, and further exploration.

Active sniffing in rodents demonstrates how this abstract principle can manifest as a physiological mechanism. Active inhalation significantly increases the speed at which an odor reaches the receptors compared to passive breathing. The concentration of the odorant at the receptors progressively approaches the ambient concentration, doing so more rapidly as the inhaled airflow increases. As the two concentrations converge, the rate of increase diminishes, resulting in saturation-like behavior. Furthermore, rodents synchronize sniffing with their breathing—at a rate of approximately 4 to 12 cycles per second—thereby coupling the acquisition of olfactory information with the periodic intake of fresh air into the nasal cavity.

Active electrolocation in weakly electric fish offers a completely different solution to the same problem. The fish generates an electric field around its body and detects disturbances that arise when an object with a conductivity different from that of the surrounding water enters the field. The magnitude of the disturbance depends on the difference in conductivity, the object's volume, and its distance from the fish, decreasing sharply as that distance increases.

The spatial distribution of these disturbances across the electric receptors on the skin enables the fish to gather information regarding the object's size, shape, distance, and electrical properties. Gymnotiform fish do not remain passive in the face of this signal; instead, they perform repeated exploratory movements and turns around the object. These movements deliberately alter the received electrical pattern, allowing for the acquisition of additional information and thereby maximizing the utility of electrolocation.

Overall, active exploration transforms the organism from a passive receiver into an agent that decides what information to extract from the environment. Information gain provides the criterion for selecting actions; epistemic curiosity drives the reduction of uncertainty; and the exploration-exploitation dilemma regulates the effort devoted to investigating new alternatives versus utilizing known options.

Rodent sniffing and fish electrolocation represent distinct physiological solutions to the same general principle: acting upon the environment—or upon one's own sensory organs—to control the information received. In this way, active exploration allows for the reduction of uncertainty in a targeted and efficient manner, continuously balancing the search for new information with the utilization of knowledge the organism already possesses.

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