Phase-dependent synaptic changes in the hippocampal CA1 field underlying extinction processes in freely moving rats

Recent studies focus on the functional significance of a novel form of synaptic plasticity, low-frequency stimulation (LFS)-induced synaptic potentiation in the hippocampal CA1 area. In the present study, we elucidated dynamic changes in synaptic function in the CA1 field during extinction processes associated with context-dependent fear memory in freely moving rats, with a focus on LFS-induced synaptic plasticity. Synaptic transmission in the CA1 field was transiently depressed during each extinction trial, but synaptic efficacy was gradually enhanced by repeated extinction trials, accompanied by decreases in freezing. On the day following the extinction training, synaptic transmission did not show further changes during extinction retrieval, suggesting that the hippocampal synaptic transmission that underlies extinction processes changes in a phase-dependent manner. The synaptic potentiation produced by extinction training was mimicked by synaptic changes induced by LFS (0.5 Hz) in the group that previously received footshock conditioning. Furthermore, the expression of freezing during re-exposure to footshock box was significantly reduced in the LFS application group in a manner similar to the extinction group. These results suggest that LFS-induced synaptic plasticity may be associated with the extinction processes that underlie context-dependent fear memory. This hypothesis was supported by the fact that synaptic potentiation induced by extinction training did not occur in a juvenile stress model that exhibited extinction deficits. Given the similarity between these electrophysiological and behavioral data, LFS-induced synaptic plasticity may be related to extinction learning, with some aspects of neuronal oscillations, during the acquisition and/or consolidation of extinction memory.     

Transposed-letter priming effect in Hebrew in the same-different task

We investigated the interaction between morphological structure and transposed-letter priming using the same-different task with Hebrew, a Semitic language in which morphology has been shown to play a key role in visual word recognition. In contrast to the results observed with lexical decision (e.g., Velan & Frost, 2009, 2011), a transposed-letter priming effect was observed irrespective of the morphological structure of the words. We take these results to suggest that morphological decomposition occurs only in the service of lexical access. We discuss further a unique feature of written Arabic, another Semitic language, to explain the apparent conflict between our findings and those reported by Perea, Abu Mallouh, García-Orza, and Carreiras (2010).     

Does the familiarity bias hypothesis explain why there is no masked priming for “NO” decisions?

Traditional "activation" views of masked priming explain the identity priming effect in terms of facilitation due to ‘pre-activation’ of stored representations. Norris and Kinoshita's (2008) Bayesian Reader theory of masked priming instead explains priming in terms of the evidence that the prime contributes towards the decision required to the target. In support of the Bayesian Reader account, Norris and Kinoshita showed that the absence of priming for nonwords in the lexical decision task and for targets requiring a Different decision in the same–different match task can be explained based on a single principle. Against this, Bowers (2010) argued that the absence of priming should be explained instead by a combination of sublexical priming and "familiarity bias". As evidence, Bowers cited Bodner and Masson’s (1997) finding that nonword priming did emerge with targets presented in visually unfamiliar cAsE-AlTeRnAtEd format. We present evidence that this finding was due to the use of an ambiguous letter in case-alternated format; when using unambiguous letters, we consistently failed to find priming of case-alternated nonwords. We suggest that the Bayesian Reader, rather than the familiarity bias hypothesis, explains the absence of priming.     

Effects of semantic context and event-related potentials: N400 correlates with inhibition effect

Two experiments investigated the modulation of event-related brain potentials (ERPs) by semantic context. A prime-target pair was visually presented in each trial of a lexical decision task. For word targets, three types of relatedness conditions were employed: (1) Related word condition (e.g., school-teacher); (2) Neutral word condition (e.g., [symbol: see text] - number); (3) Unrelated word c((e.g., hospital-potato). In Experiment 1, the reaction time for unrelated targets was longer than that for neutral targets (inhibition effect) which was longer than that for related targets (facilitation effect). The N400 amplitude in the unrelated targets was larger compared to those in the related and neutral targets, which did not differ. In Experiment 2, where only the facilitation effect was obtained, the N400 amplitude did not differ among conditions.     

How lexical decision is affected by recent experience: Symmetric versus asymmetric frequency-blocking effects

In a lexical decision task (LDT) in which list composition is manipulated, a typical finding to date has been a slowdown for easy items (e.g., high-frequency words) but little speedup for hard items (e.g., low-frequency words) when they are mixed together. This asymmetric frequency-blocking effect contrasts with the symmetric pattern (both a speedup for hard items and a slowdown for easy items when they are mixed together) observed with the naming task. In the present study, we investigated the mechanism responsible for the asymmetric blocking effect in the LDT within a model of blocking effect proposed by Mozer, Kinoshita, and Davis (2003), termed the adaptation-to-the-statistics-of-the-environment (ASE) model. Experiments 1A and 1B showed that when the same high- and low-frequency words were used, consistent with the existing literature, an asymmetric blocking effect was found in the LDT and a symmetric blocking effect was found in the naming task. Within the ASE model, a symmetric versus asymmetric blocking effect can be explained in terms of different asymptotic rates in subjective estimates of error probability. Experiments 2 and 3 tested and confirmed a prediction of the model based on this assumption that a speedup of hard items would be observed in an LDT with hard items whose subjective error probability asymptotes near zero (low-frequency words with high familiarity ratings that subjects could be certain were words). Implications of the model for task differences in reaction times are discussed.     

Involvement of IP3 receptors in LTP and LTD induction in guinea pig hippocampal CA1 neurons

The role of inositol 1, 4, 5-trisphosphate receptors (IP3Rs) in long-term potentiation (LTP) and long-term depression (LTD) was studied in CA1 neurons in guinea pig hippocampal slices. In standard solution, short tetanic stimulation consisting of 15 pulses at 100 Hz induced LTP, while three short trains of low-frequency stimulation (LFS; 200 pulses at 1 Hz) at 18-min intervals or one long train of LFS (1000 pulses at 1 Hz) induced stable LTD in both the slope of the field EPSP (S-EPSP) and the amplitude of the population spike (A-PS). Bath application of 2-aminoethoxydiphenyl borate (2-APB), an IP3R antagonist, or of alpha-methyl-4-carboxyphenylglycine (MCPG), a wide-spectrum metabotropic glutamate receptor antagonist, during weak tetanic stimulation significantly increased the magnitude of the LTP in both the S-EPSP and A-PS. Three short trains of LFS or one long train of LFS delivered in the presence of 2-APB or MCPG did not induce LTD, but elicited LTP. Based on these results, we conclude that, in hippocampal CA1 neurons, IP3Rs play an important role in synaptic plasticity by attenuating LTP and facilitating LTD.