Representational gain in cortical area underlies increase of memory strength

Abstract

Neuronal plasticity that develops in the cortex during learning is assumed to represent memory content, but the functions of such plasticity are actually unknown. The shift in spectral tuning in primary auditory cortex (A1) to the frequency of a tone signal is a compelling candidate for a substrate of memory because it has all of the cardinal attributes of associative memory: associativity, specificity, rapid induction, consolidation, and long-term retention. Tuning shifts increase the representational area of the signal in A1, as an increasing function of performance level, suggesting that area encodes the magnitude of acquired stimulus significance. The present study addresses the question of the specific function of learning-induced associative representational plasticity. We tested the hypothesis that specific increases in A1 representational area for an auditory signal serve the mnemonic function of enhancing memory strength for that signal. Rats were trained to bar-press for reward contingent on the presence of a signal tone (5.0 kHz), and assessed for memory strength during extinction. The amount of representational area gain for the signal frequency band was significantly positively correlated with resistance to extinction to the signal frequency in two studies that spanned the range of task difficulty. These findings indicate that specific gain in cortical representational area underlies the strength of the behaviorally-relevant contents of memory. Thus, mnemonic functions of cortical plasticity are determinable.

Amygdala Strengthening of Cortical Memory Representations

“…Fifth, extinction of memory is a function of the amount of decrease in representational area [118]. In toto, these findings support the hypothesis that the brain has a memory code for the acquired behavioral significance of experiences, resulting in greater strength of memory: (a) the more important the stimulus, the greater the number of neurons that become tuned it and (b) the greater the number of tuned neurons, the stronger is the memory [87,95].…”

“…Specific shifts of frequency receptive fields accomplish such increased emphasis by increasing the number of cortical neurons that respond preferentially to signal tones. Indeed, increased memory strength is encoded by an increase in the area of tonal representation in the tonotopic map of A1 [87]. Therefore, we asked whether activation of the BLA following tone presentation could specifically modulate the representation of that stimulus in the auditory cortex.…”

“…Indeed, area expansions were found and there was a significant relationship to behavioral importance: the greater the level of importance, the greater the area of its representation [86]. Furthermore, the area of representation has been linked to the strength of memory: the greater the area of representational gain during learning, the greater the resistance to extinction, i.e., the stronger the memory [87].…”

“…Third, the number of neurons that become best tuned to a paired frequency increases directly as a function of the degree of the tone's behavioral meaning or importance [86]. Fourth, the strength of behaviorally validated memory (i.e., not merely cortical plasticity) is a function of the amount of increase of representation area for the signal stimulus in the primary auditory cortex [87]. Fifth, extinction of memory is a function of the amount of decrease in representational area [118].…”

Extinction reverses olfactory fear-conditioned increases in neuron number and glomerular size

“…Dynamic alterations in structural plasticity and neuroanatomical representation accompanying learning have been observed in other sensory systems; for example, studies in the primary auditory system have demonstrated that local tuning shifts produce a specific increase in the area of frequency representation within the tonotopic organization of primary auditory cortex (A1) (18,19). A learning-dependent shift in primary representational area may be a common feature across primary sensory systems.…”