Classical I.P. Pavlov Theory and the Principle of Dominance
According to I.P. Pavlov, the formation of a conditioned reflex was primarily associated with the activity of the cerebral cortex. The process was viewed as the interaction between two excitation arcs: the conditioned reflex arc and the unconditioned reflex arc.
The foundation of the closure mechanism relies on dominant relationships in the cortex:
- Unconditioned reinforcement creates a strong focus of excitation in the cortex, which becomes dominant.
- The neutral (conditioned) stimulus creates a weaker focus of excitation.
- The dominant center, according to its properties, "attracts" the excitation caused by the conditioned signal.
- A functional pathway—the temporary connection—arises between the representations of the conditioned signal and the unconditioned reinforcement.
The connection is called "temporary" because it lacks rigid genetic determination and rapidly disintegrates (extinguishes) if the conditioned signal ceases to be accompanied by reinforcement.
Dynamics of Connection Formation (Using the Salivary/Feeding Reflex as an Example)
The process can be divided into three successive stages illustrating the interaction of cortical centers:
- Stage A (Action of the neutral stimulus): For example, a flash of light stimulates the retina. The signal travels via subcortical centers to the visual cortex, forming a focus of excitation there. There is as yet no connection with the feeding center, so only an orienting reflex ("What is it?") occurs.
- Stage B (Action of the unconditioned stimulus): Ingesting food stimulates taste receptors. The signal travels via the brainstem (medulla oblongata) to the salivary glands and also irradiates into the cortex, creating a powerful cortical focus of feeding excitation. The unconditioned reflex is executed.
- Stage C (Closure): With repeated pairing (light precedes food), two foci simultaneously exist in the cortex. The strong feeding focus "attracts" impulses from the visual focus. A functional bridge is formed. Now, excitation from the visual cortex directly passes along the established pathway into the feeding center, eliciting salivation in response to light.
Conditioned Reflex in Light of Modern Neurophysiology
Modern data have significantly expanded classical views, proving that the entire brain, rather than isolated cortical points, participates in the closure of a temporary connection.
A crucial addition was the discovery of the properties of the brainstem reticular formation. It exerts generalized ascending activating influences on the cerebral cortex. Cortic-subcortical interactions ensure the reverberation (circulation) of excitations. Furthermore, it has been experimentally proven that basic conditioned reflexes can be established even in decorticated animals (animals without a cerebral cortex). This ultimately refuted the theory regarding the exclusive role of the cortex in closing absolutely all temporary connections.
P.K. Anokhin's Convergence Theory
These modern concepts formed the basis of convergence theory, which explains the closure mechanism at the cellular level. The formation of the connection occurs when various excitations converge on the same neurons.
The process includes the following stages:
- Motivation (initial background): A focus of motivation (hunger) arises in subcortical structures (e.g., the hypothalamus). It exerts an ascending influence on the cortex, increasing the excitability of specific neuronal populations (creating a functional pre-tuning).
- Arrival of signals: Excitations from the conditioned signal (light) and unconditioned reinforcement (food) sequentially arrive at the cortex.
- Convergence: A meeting of three excitation streams occurs on the membranes of pre-tuned neurons: motivational, sensory (conditioned), and reinforcing.
Molecular Mechanisms of Memory Fixation
The convergence of all types of excitations on a cortical neuron is only the first step. For the temporary connection to become robust, cellular and molecular-level changes are required.
Stimulus convergence triggers a cascade of intracellular biochemical reactions involving secondary messengers. The signal is transmitted to the cell nucleus, targeting its genetic apparatus:
- Genome activation occurs (expression of immediate-early genes).
- Transcription (RNA synthesis on a DNA template) and translation processes are initiated.
- As a result, specific memory proteins are synthesized de novo.
These synthesized proteins integrate into synaptic membranes or alter the structure of the neuron. This exact process ensures long-term plasticity and the consolidation of the memory trace (engram), serving as the intimate structural mechanism for forming a temporary connection.