Differential regional expression of brain-derived neurotrophic factor following olfactory fear learning

We examined brain-derived neurotrophic factor (BDNF) mRNA expression across the olfactory system following fear conditioning. Mice received 10 pairings of odor with footshock or equivalent unpaired odors and shocks. We found increased BDNF mRNA in animals receiving paired footshocks in the multiple regions examined including the posterior piriform cortex (PPC) and basolateral amygdala (BLA). This was in contrast to the unpaired and odor-alone treatments, where BDNF mRNA was increased in the olfactory bulb (OB) and anterior piriform cortex (APC) only, but not the higher olfactory areas. We propose that odor exposure increases expression of BDNF in the OB and APC while the PPC and BLA increase BDNF mRNA only when associative learning occurs.     

Learning categories at different hierarchical levels: a comparison of category learning models

Three formal models of category learning, the rational model (Anderson, 1990), the configural-cue model (Gluck & Bower, 1988a), and ALCOVE (Kruschke, 1992), were evaluated on their ability to account for differential learning of hierarchically structured categories. An experiment using a theoretically challenging category structure developed by Lassaline, Wisniewski, and Medin (1992) is reported. Subjects learned one of two different category structures. For one structure, diagnostic information was present along a single dimension (1-D). For the other structure, diagnostic information was distributed across four dimensions (4-D). Subjects learned these categories at a general or at a specific level of abstraction. For the 1-D structure, specific-level categories were learned more rapidly than general-level categories. For the 4-D structure, the opposite result was observed. These results proved highly diagnostic for evaluating the models—although ALCOVE provided a good account of the observed results, the rational model and the configural-cue model did not.     

Olfactory fear conditioning induces field potential potentiation in rat olfactory cortex and amygdala

The widely used Pavlovian fear-conditioning paradigms used for studying the neurobiology of learning and memory have mainly used auditory cues as conditioned stimuli (CS). The present work assessed the neural network involved in olfactory fear conditioning, using olfactory bulb stimulation-induced field potential signal (EFP) as a marker of plasticity in the olfactory pathway. Training consisted of a single training session including six pairings of an odor CS with a mild foot-shock unconditioned stimulus (US). Twenty-four hours later, the animals were tested for retention of the CS as assessed by the amount of freezing exhibited in the presence of the learned odor. Behavioral data showed that trained animals exhibited a significantly higher level of freezing in response to the CS than control animals. In the same animals, EFPs were recorded in parallel in the anterior piriform cortex (aPC), posterior piriform cortex (pPC), cortical nucleus of the amygdala (CoA), and basolateral nucleus of the amygdala (BLA) following electrical stimulation of the olfactory bulb. Specifically, EFPs recorded before (baseline) and after (during the retention test) training revealed that trained animals exhibited a lasting increase (present before and during presentation of the CS) in EFP amplitude in CoA, which is the first amygdaloid target of olfactory information. In addition, a transient increase was observed in pPC and BLA during presentation of the CS. These data indicate that the olfactory and auditory fear-conditioning neural networks have both similarities and differences, and suggest that the fear-related behaviors in each sensory system may have at least some distinct characteristics.     

Distinct neural mechanisms mediate olfactory memory formation at different timescales

Habituation is one of the oldest forms of learning, broadly expressed across sensory systems and taxa. Here, we demonstrate that olfactory habituation induced at different timescales (comprising different odor exposure and intertrial interval durations) is mediated by different neural mechanisms. First, the persistence of habituation memory is greater when mice are habituated on longer timescales. Second, the specificity of the memory (degree of cross-habituation to similar stimuli) also depends on induction timescale. Third, we demonstrate a pharmacological double dissociation between the glutamatergic mechanisms underlying short- and long-timescale odor habituation. LY341495, a class II/III metabotropic glutamate receptor antagonist, blocked habituation only when the induction timescale was short. Conversely, MK-801, an N-methyl-D-aspartate (NMDA) receptor antagonist, prevented habituation only when the timescale was long. Finally, whereas short-timescale odor habituation is mediated within the anterior piriform cortex, infusion of MK-801 into the olfactory bulbs prevented odor habituation only at longer timescales. Thus, we demonstrate two neural mechanisms underlying simple olfactory learning, distinguished by their persistence and specificity, mediated by different olfactory structures and pharmacological effectors, and differentially utilized based solely on the timescale of odor presentation.     

Examining punishment at different explanatory levels

Experimental studies on punishment have sometimes been over-interpreted not only for the reasons Guala lists, but also because of a frequent conflation of proximate and ultimate explanatory levels that Guala's review perpetuates. Moreover, for future analyses we may need a clearer classification of different kinds of punishment.     

Attribute coding at different levels of processing

Schoolchildren, aged 16 years showed poorer incidental learning for nouns if the orienting task was one of finding rhymes for the items compared to assigning each word to a semantic category. On a recognition task including homophone and synonym foils, rhyming subjects made significantly more phonemic errors, whereas categorizing subjects produced a significantly greater number of semantic confusions. Subjects instructed to construct images for each of the nouns showed a performance pattern similar to the categorizing group. The results are interpreted in terms of Herriot's (1974) view that different levels of processing are associated with the coding of different clusters of stimulus attributes.     

Enhanced proliferation of progenitor cells following long-term potentiation induction in the rat dentate gyrus

The dentate gyrus (DG) is among the few areas in the mammalian brain where production of new neurons continues in the adulthood. Although its functional significance is not completely understood, several lines of evidence suggest the role of DG neurogenesis in learning and memory. Considering that long-term potentiation (LTP) is a prime candidate for the process underlying hippocampal learning and memory, these results raise the possibility that LTP and neurogenesis are closely related. Here, we investigated whether or not LTP induction in the afferent pathway triggers enhanced proliferation of progenitor cells in the DG. LTP was induced by tetanic stimulation in perforant path-DG synapses in one hemisphere, and the number of newly generated progenitor (BrdU-labeled) cells in the DG was quantified. Compared with the control hemisphere (stimulated with low-frequency pulses), the LTP-induced hemisphere contained a significantly higher number of newly generated progenitor cells in the dorsal as well as ventral DG. When CPP, an NMDA receptor antagonist, was administered, tetanic stimulation neither induced LTP nor enhanced progenitor cell proliferation, indicating that NMDA receptor activation, rather than tetanic stimulation per se, is responsible for enhanced progenitor proliferation in the control animal. Our results show that tetanic stimulation of perforant path sufficient to induce LTP increases progenitor proliferation in adult DG in an NMDA receptor-dependent manner.     

Processing letters in words at different levels

Subjects indicated whether two letters, two words, or a letter and the first letter of a word were the same. Letter targets were matched more quickly than word targets when the stimuli were presented simultaneously. When the target and comparison stimuli were separated by a 3-sec interval, word targets were matched more quickly than a letter and a letter in a word. It was also shown that the physical similarity of the targets and comparison stimuli had a greater effect in the simultaneous matching conditions. These findings are consistent with a model of word processing in which letters are individually compared prior to word identification at a physical level of processing. At a higher level of processing, words may be encoded as a unit, and the identification of the letters within the word may require a decoding of the word unit.     

Stimulus-naming effects at different stages of motor paired-associate learning

In a combined verbal and motor paired-associate task, nonsense names and button-pushing responses were learned for three random shapes, with verbal training beginning after 3, 12, or 24 motor learning trials. Earlier studies indicated that concurrent verbal and motor learning produce a strong tendency for the naming and button-pushing responses to be learned in “consistent” pairs. According to the theoretical analysis presented, the verbal-motor consistency is explained by the development of name-button associations. It was predicted that these associations affect both motor errors and correct responses, that their effects are greater for correct responses, and that attenuation of the effects due to delay in verbal training is gr, eater for motor errors. The results are in good agreement with the predictions, and they suggest that name-button associations develop rapidly even under delayed verbal training.     

Representation at different levels in a conceptual hierarchy

The present study examines the influence of hierarchical level on category representation. Three computational models of representation – an exemplar model, a prototype model and an ideal representation model – were evaluated in their ability to account for the typicality gradient of categories at two hierarchical levels in the conceptual domain of clothes. The domain contains 20 subordinate categories (e.g., trousers, stockings and underwear) and an encompassing superordinate category (CLOTHES). The models were evaluated both in terms of their ability to fit the empirical data and their generalizability through marginal likelihood. The hierarchical level was found to clearly influence the type of representation: For concepts at the subordinate level, exemplar representations were supported. At the superordinate level, however, an ideal representation was overwhelmingly preferred over exemplar and prototype representations. This finding contributes to the increasingly dominant view that the human conceptual apparatus adopts both exemplar representations and more abstract representations, contradicting unitary approaches to categorization.     

Forgetting and learning potentiation: dual consequences of between-session delays in cognitive skill learning

In the cognitive skill literature, between-session delays have been treated either as having a negligible effect on performance or as causing forgetting. In contrast, in the procedural skill literature, overnight between-session delays can result in performance gains. In 5 multi-session data sets, the author demonstrates that neither of these 2 models holds for the case of cognitive skill learning. Instead, the delay between sessions appeared to yield both forgetting and enhanced potential for new learning. Two candidate classes of explanation are considered, and implications for the empirical law of learning are discussed.     

Hierarchical attention in discriminating objects at different levels of specificity

An implicit assumption of studies in the attentional literature has been that global and local levels of attention are involved in object recognition. To investigate this assumption, a divided attention task was used in which hierarchical figures were presented to prime the subsequent discrimination of target objects at different levels of category identity (basic and subordinate). Target objects were identified among distractor objects that varied in their degree of visual similarity to the targets. Hierarchical figures were also presented at different sizes and as individual global and local elements in order to investigate whether attention-priming effects on object discrimination were due to grouping/parsing operations or spatial extent. The results showed that local processing primed subordinate object discriminations when the objects were visually similar. Global processing primed basic object discriminations, regardless of the similarity of the distractors, and subordinate object discriminations when the objects were visually dissimilar. It was proposed that global and local processing aids the selection of perceptual attributes of objects that are diagnostic for recognition and that selection is based on two mechanisms: spatial extent and grouping/parsing operations.     

Spatial learning in the holeboard impairs an early phase of long-term potentiation in the rat hippocampal CA1-region

Long-term potentiation (LTP) and depression (LTD) are considered as cellular models for learning and memory. We studied the impact of holeboard training on LTP in the rat CA1 hippocampal region. In 7-week-old Wistar rats a recording electrode was chronically implanted into the hippocampal pyramidal cell layer of the CA1 of the right hemisphere and a stimulation electrode into the contralateral CA3 region.Two groups of animals received a spatial holeboard training of 10 or 15 trials over 2 days on a fixed pattern of baited holes. The last trial was performed 15 min after a primed burst stimulation of the contralateral CA3, which resulted in LTP in the ipsilateral CA1. A pseudo-trained group that received a 10 trial training with changing patterns of baited holes after each trial and a group that remained in the recording chambers during the experiments served as controls. Experimental rats significantly improved their spatial performance with increasing numbers of trials, indicated by decreasing times to pick up all food pellets and by decreasing numbers of reference memory errors. A learning-related impairment of CA1-LTP measured in both the population-spike amplitude as well as the fEPSP could be noted. These results show that specific (pattern-training), but not unspecific (pseudo-training) spatial information processing prior to electrical stimulation can severely affect LTP in hippocampal area CA1.     

Visual statistical learning at basic and subordinate category levels in real-world images

Visual statistical learning (VSL) has been proposed as a powerful mechanism underlying the striking ability of human observers to handle complex visual environments. Previous studies have shown that VSL can occur when statistical information is embedded at multiple levels of abstraction, such as at semantically different category levels. In the present study, we further examined whether statistical regularities at a basic category level (e.g., a regular sequence of a bird, then a car, and then a dog) could influence the ability to extract statistical regularities at the subordinate level (e.g., a regular sequence of a parrot, then a sports car, and then an Eskimo dog). In the familiarization phase, participants were exposed to a stream of real-world images whose semantic categories had temporal regularities. Importantly, the temporal regularities existed at both the basic and subordinate levels, or the regularities existed at only the subordinate level, depending on the experimental condition. After completing the familiarization, participants performed a surprise two-alternative forced choice (2AFC) task for a familiarity judgment between two triplets in which the temporal regularities were either preserved or not preserved. Our results showed that the existence of statistical regularities at the basic level did not influence VSL at the subordinate level. The subsequent experiments showed these results consistently even when the basic-level categories had to be explicitly recognized and when the stimuli were not easily categorized at their subordinate level. Our results suggest that VSL is constrained to learn a particular level of patterns when patterns are presented across multiple levels.