One-Trial Odor-Reward Association: A Form of Event Memory Not Dependent on Hippocampal Function

Hippocampal damage causes retrograde amnesia for objects' visual, but not odour, properties in male rats

Damage to the hippocampus produces profound retrograde amnesia, but odour and object discrimination memories can be spared in the retrograde direction. Prior lesion studies testing retrograde amnesia for object/odour discriminations are problematic due to sparing of large parts of the hippocampus, which may support memory recall, and/or the presence of uncontrolled, distinctive odours that may support object discrimination. To address these issues, we used a simple object discrimination test to assess memory in male rats. Two visually distinct objects, paired with distinct odour cues, were presented. One object was associated with a reward. Following training, neurotoxic hippocampal lesions were made using N-methyl-D-aspartate (NMDA). The rats were then tested on the preoperatively learned object discrimination problem, with and without the availability of odour or visual cues during testing. The rats were also postoperatively trained on a new object discrimination problem. Lesion sizes ranged from 67% to 97% of the hippocampus (average of 87%). On the preoperatively learned discrimination problem, the rats with hippocampal lesions showed preserved object discrimination memory when tested in the dark (i.e., without visual cues) but not when the explicit odour cues were removed from the objects. Hippocampal lesions increased the number of trials required to reach criterion but did not prevent rats from solving the postoperatively learned discrimination problem. Our results support the idea that long-term memories for odours, unlike recall of visual properties of objects, do not depend on the hippocampus in rats, consistent with previous observations that hippocampal damage does not cause retrograde amnesia for odour memories.

The chicken and egg problem of grid cells and place cells

Place cells and grid cells are major building blocks of the hippocampal cognitive map. The prominent forward model postulates that grid-cell modules are generated by a continuous attractor network; that a velocity signal evoked during locomotion moves entorhinal activity bumps; and that place-cell activity constitutes summation of entorhinal grid-cell modules. Experimental data support the first postulate, but not the latter two. Several families of solutions that depart from these postulates have been put forward. We suggest a modified model (spatial modulation continuous attractor network; SCAN), whereby place cells are generated from spatially selective nongrid cells. Locomotion causes these cells to move the hippocampal activity bump, leading to movement of the entorhinal manifolds. Such inversion accords with the shift of hippocampal thought from navigation to more abstract functions.

One-trial learning in larval Drosophila

Animals of many species are capable of “small data” learning, that is, of learning without repetition. Here we introduce larval Drosophila melanogaster as a relatively simple study case for such one-trial learning. Using odor-food associative conditioning, we first show that a sugar that is both sweet and nutritious (fructose) and sugars that are only sweet (arabinose) or only nutritious (sorbitol) all support appetitive one-trial learning. The same is the case for the optogenetic activation of a subset of dopaminergic neurons innervating the mushroom body, the memory center of the insects. In contrast, no one-trial learning is observed for an amino acid reward (aspartic acid). As regards the aversive domain, one-trial learning is demonstrated for high-concentration sodium chloride, but is not observed for a bitter tastant (quinine). Second, we provide follow-up, parametric analyses of odor-fructose learning. Specifically, we ascertain its dependency on the number and duration of training trials, the requirements for the behavioral expression of one-trial odor-fructose memory, its temporal stability, and the feasibility of one-trial differential conditioning. Our results set the stage for a neurogenetic analysis of one-trial learning and define the requirements for modeling mnemonic processes in the larva.

Ventral, but not dorsal, hippocampus inactivation impairs reward memory expression and retrieval in contexts defined by proximal cues

The hippocampus (HPC) has been widely implicated in the contextual control of appetitive and aversive conditioning. However, whole hippocampal lesions do not invariably impair all forms of contextual processing, as in the case of complex biconditional context discrimination, leading to contention over the exact nature of the contribution of the HPC in contextual processing. Moreover, the increasingly well-established functional dissociation between the dorsal (dHPC) and ventral (vHPC) subregions of the HPC has been largely overlooked in the existing literature on hippocampal-based contextual memory processing in appetitively motivated tasks. Thus, the present study sought to investigate the individual roles of the dHPC and the vHPC in contextual biconditional discrimination (CBD) performance and memory retrieval. To this end, we examined the effects of transient post-acquisition pharmacological inactivation (using a combination of GABA_A and GABA_B receptor agonists muscimol and baclofen) of functionally distinct subregions of the HPC (CA1/CA3 subfields of the dHPC and vHPC) on CBD memory retrieval. Additional behavioral assays including novelty preference, light-dark box and locomotor activity test were also performed to confirm that the respective sites of inactivation were functionally silent. We observed robust deficits in CBD performance and memory retrieval following inactivation of the vHPC, but not the dHPC. Our data provides novel insight into the differential roles of the ventral and dorsal HPC in reward contextual processing, under conditions in which the context is defined by proximal cues.

Role of the hippocampus in goal-oriented tasks requiring retrieval of spatial versus non-spatial information

The role of the hippocampus in non-spatial memory has been issue of some controversy. To investigate the nature of dorsal hippocampus engagement in spatial and non-spatial memory we performed discrete excitotoxic lesions of this region before mice (C57/BL6) were trained in one of two tasks that required the animals to retrieve a hidden food reward. In the visuospatial task animals had to remember a particular spatial location, independent of odor cues. In contrast, in a non-spatial olfactory task animals had to remember a particular odor, independent of spatial location. The mice were trained in one of these tasks over a period of three days. We found that lesions restricted to the dorsal hippocampus affected performance only in the spatial task. In contrast, lesions that also encompassed a larger portion of the ventral hippocampus caused a moderate deficit in the olfactory task. These results are consistent with the role of the dorsal hippocampus in long-term spatial episodic memory, and support the involvement of larger portions of the hippocampus on the encoding of non-spatial olfactory representations.

A comparison of discrimination and reversal learning for olfactory and visual stimuli in aged rats

The present study investigated age-related differences in discrimination and reversal learning for olfactory and visual stimuli in 6-month and 24-month-old rats. Rats were trained to discriminate between two pseudo-randomly selected odors or objects. Once each animal reached a criterion on discrimination trials, the reward contingencies were reversed. Young and aged rats acquired the olfactory and visual discrimination tasks at similar rates. However, on reversal trials, aged rats required significantly more trials to reach the learning criterion on both the olfactory and visual reversal tasks than young rats. The deficit in reversal learning was comparable for odors and objects. Furthermore, the results showed that rats acquired the olfactory task more readily than the visual task. The present study represents the first examination of age-related differences in reversal learning using the same paradigm for odors and objects to facilitate cross-modal comparisons. The results may have important implications for the selection of memory paradigms for future research studies on aging.

Rats depend on habit memory for discrimination learning and retention

We explored the circumstances in which rats engage either declarative memory (and the hippocampus) or habit memory (and the dorsal striatum). Rats with damage to the hippocampus or dorsal striatum were given three different two-choice discrimination tasks (odor, object, and pattern). These tasks differed in the number of trials required for learning (approximately 10, 60, and 220 trials). Dorsal striatum lesions impaired discrimination performance to a greater extent than hippocampal lesions. Strikingly, performance on the task learned most rapidly (the odor discrimination) was severely impaired by dorsal striatum lesions and entirely spared by hippocampal lesions. These findings suggest that discrimination learning in the rat is primarily supported by the dorsal striatum (and habit memory) and that rats engage a habit-based memory system even for a task that takes only a few trials to acquire. Considered together with related studies of humans and nonhuman primates, the findings suggest that different species will approach the same task in different ways.

The right hippocampus participates in short-term memory maintenance of object–location associations

Doubts have been cast on the strict dissociation between short- and long-term memory systems. Specifically, several neuroimaging studies have shown that the medial temporal lobe, a region almost invariably associated with long-term memory, is involved in active short-term memory maintenance. Furthermore, a recent study in hippocampally lesioned patients has shown that the hippocampus is critically involved in associating objects and their locations, even when the delay period lasts only 8 s. However, the critical feature that causes the medial temporal lobe, and in particular the hippocampus, to participate in active maintenance is still unknown. This study was designed in order to explore hippocampal involvement in active maintenance of spatial and non-spatial associations. Eighteen participants performed a delayed-match-to-sample task in which they had to maintain either object-location associations, color-number association, single colors, or single locations. Whole-brain activity was measured using event-related functional magnetic resonance imaging and analyzed using a random effects model. Right lateralized hippocampal activity was evident when participants had to maintain object-location associations, but not when they had to maintain object-color associations or single items. The present results suggest a hippocampal involvement in active maintenance when feature combinations that include spatial information have to be maintained online.

Mnemonic functions of the hippocampus: A comparison between animals and humans

This review summarizes a series of experiments aimed at answering the question whether the hippocampus in rats and humans performs parallel functions focusing on studies that assess spatial and temporal pattern separation, sequential learning, spatial and temporal pattern association, spatial and temporal pattern completion, and short-term and intermediate-term memory. It is recognized that a comparison of the functions of the rat hippocampus with human hippocampus is difficult, because of differences in methodology, differences in complexity of life experiences, and differences in the degree of hippocampal damage as well as damage to interconnected brain regions. Yet, in general the data support the idea that with respect to spatial and temporal pattern separation, sequential learning, spatial and temporal pattern associations, spatial and temporal pattern completion, and short-term and intermediate-term memory, similar functions are observed in rats and humans with hippocampal damage using analogous tasks. These data provide support for evolutionary continuity in cognitive function assigned to the hippocampus of rats and humans.

A computational theory of hippocampal function, and empirical tests of the theory

The main aim of the paper is to present an up-to-date computational theory of hippocampal function and the predictions it makes about the different subregions (dentate gyrus, CA3 and CA1), and to examine behavioral and electrophysiological data that address the functions of the hippocampus and particularly its subregions. Based on the computational proposal that the dentate gyrus produces sparse representations by competitive learning and via the mossy fiber pathway forces new representations on the CA3 during learning (encoding), it has been shown behaviorally that the dentate gyrus supports spatial pattern separation during learning. Based on the computational proposal that CA3-CA3 autoassociative networks are important for episodic memory, it has been shown behaviorally that the CA3 supports spatial rapid one-trial learning, learning of arbitrary associations where space is a component, pattern completion, spatial short-term memory, and sequence learning by associations formed between successive items. The concept that the CA1 recodes information from CA3 and sets up associatively learned backprojections to neocortex to allow subsequent retrieval of information to neocortex, is consistent with findings on consolidation. Behaviorally, the CA1 is implicated in processing temporal information as shown by investigations requiring temporal order pattern separation and associations across time; computationally this could involve temporal decay memory, and temporal sequence memory which might also require CA3. The perforant path input to DG is implicated in learning, to CA3 in retrieval from CA3, and to CA1 in retrieval after longer time intervals ("intermediate-term memory").

An olfactory biconditional discrimination in the mouse

Male CD-1 mice were given a biconditional discrimination task with four odors; A, B, C, and D. Mice were presented with odor compounds AC+, BD+, BC-, AD- for thirteen days. Pieces of odorized filter paper were placed in the bottom of odor pots and covered with bedding. On reinforced AC and BD trials, sugar was buried in the bedding, and on nonreinforced AD and BC trials no sugar was present. Following training, simultaneous nonreinforced tests were given between AD and AC, and between BC and BD. The mice spent more time digging in the previously reinforced odor compounds than in the previously nonreinforced compounds. In a second experiment, mice were conditioned to dig in AC+ and not BD-. In a subsequent test with the separate elements they dug more in A and C than in B and D, indicating that the biconditional discrimination had not been solved on the basis of complete perceptual blending. The data demonstrate that mice are capable of olfactory configural learning when solving a biconditional discrimination.

Rapid learning and flexible memory in “habit” tasks in rats trained with brain stimulation reward

Two groups of rats, one rewarded with sweetened food and the other rewarded with medial forebrain bundle (MFB) stimulation, were trained to home in on and dig for a buried object coated with a target odor. After each group had 15 training trials, MFB rats searched with greater accuracy and speed than food-rewarded rats. MFB rats were subsequently tested (1) after 6 weeks with no additional practice; (2) with food or non-food distractor odors, and (3) with major spatial alterations to the search environment, and in all cases searched with the same high accuracy, short search time, and low level of distractibility as in baseline. These results suggest that the high motivation provided by MFB reward engenders rapidly formed, long-lasting, and surprisingly flexibly deployable "habit" memories.

The Role of CA1 in the Acquisition of an Object-Trace-Odor Paired Associate Task

This experiment was designed to determine whether adding a temporal component to an object-odor association task would recruit the hippocampus. The rats were given CA1, CA3, or control lesions prior to learning the object-trace-odor task. Rats were presented with an object for 10 s, after which the object was removed, followed by a 10-s trace period, followed by the presentation of an odor 50 cm away. If the odor and the object were paired, rats were to dig in the odor cup for a reward. If unpaired, rats were to refrain from digging. Rats that had CA1 lesions were unable to make the association, whereas rats that had CA3 lesions performed as well as controls. These results support the idea that the hippocampus is involved in forming arbitrary associations that do not necessarily involve space as long as they involve a temporal component.