Neural Processing to Visual Stimuli in a Three-Choice Reaction-Time Task

Some authors have suggested that certain components of the event-related potentials (ERPs) reflect underlying stages in the discrimination process. Previous studies have shown that in an auditive three-choice reaction-time task the discrimination is accomplished as a two-stage process, with the more frequently occurring stimulus discriminated at an earlier point than the rarer stimulus and the subjects could be classified as fast and slow responders in function of their response to the most frequent of the three tones. We continuously recorded the electrocerebral activity (EEG) from the scalp and the electromyographic activity (EMG) from the responding muscles in a visual three-choice reaction-time task in 10 strictly right-handed subjects. EEG and EMG responses were subsequently analyzed off-line by aligning them by the onset of either the stimulus (stimulus-synchronized) or the response (response-synchronized). The results suggest that processes of visual stimuli evaluation and response execution are continuously integrated. The discrimination and response systems to visual stimuli is accomplished as a three-stage process, one to frequent tone, another for rare 1, and the last for rare 2. The subjects were classified as fast and slow responders in function of their response to the most frequent of the three tones.     

Short-term memory reactivation of a weak CS–US association promotes long-term memory persistence in conditioned odor aversion

In conditioned odor aversion (COA), the association of a tasteless odorized solution (the conditioned stimulus [CS]) with an intraperitoneal injection of LiCl (the unconditioned stimulus [US[), which produces visceral malaise, results in its future avoidance. The strength of this associative memory is mainly dependent on two parameters, that is, the strength of the US and the interstimuli interval (ISI). In rats, COA has been observed only with ISIs of ≤15 min and LiCl (0.15 M) doses of 2.0% of bodyweight, when tested 48 h after acquisition (long-term memory [LTM]). However, we previously reported a robust aversion in rats trained with ISIs up to 60 min when tested 4 h after acquisition (short-term memory [STM]). Since memories get reactivated during retrieval, in the current study we hypothesized that testing for STM would reactivate this COA trace, strengthening its LTM. For this, we compared the LTM of rats trained with long ISIs or low doses of LiCl initially tested for STM with that of rats tested for LTM only. Interestingly, rats conditioned under parameters sufficient to produce STM, but not LTM, showed a reliable LTM when first tested for STM. These observations suggest that under suboptimal training conditions, such as long ISIs or low US intensities, a CS–US association is established but requires reactivation in the short-term in order to persist in the long-term.

The dynamic and malleable nature of memories is a well-studied phenomenon. Traditionally, for memory formation to occur, a set of processes collectively known as consolidation are thought to be needed in order to stabilize memories, making them susceptible to modification during this period (Dudai et al. 2015). More recently a slightly distinct theory, known as memory integration, was proposed according to which memories are rapidly formed during learning without the need for consolidation, but any relevant information around the event can be integrated modifying them (Gisquet-Verrier and Riccio 2019). Common to both theories though, is that memories alternate between an inactive and an active state and modifications can mostly occur during the active state, which lasts for some time after learning, or during its reactivation due to retrieval (Lee et al. 2017; Albo and Gräff 2018; Gisquet-Verrier and Riccio 2019). Thus, memory malleability is explained either because consolidation can be altered or because additional information can be integrated with the initial memory (Bailey et al. 1996; Dudai 2004; Wixted 2004; Alberini et al. 2006; Lee et al. 2008, 2017; McGaugh and Roozendaal 2009; Roesler and Schröder 2011; Dudai et al. 2015; Nader 2015; Crossley et al. 2019; Gisquet-Verrier and Riccio 2019).In conditioned odor aversion (COA), an odorized tasteless solution (conditioned stimulus, CS) whose ingestion is followed by gastrointestinal malaise (unconditioned stimulus, US) is rejected in future encounters (conditioned response, CR). In most COA studies, a robust aversion has been observed only when the interstimulus interval (ISI) is ∼5 min, and no significant aversion can be seen when the ISI is >15 min (Hankins et al. 1973; Palmerino et al. 1980; Ferry et al. 1995, 1996; Ferry and di Scala 1997; Ferry et al. 2006; Chapuis et al. 2007). This observation has been attributed to a short-lasting memory of the odor that becomes unavailable for its association with the US after ISI >15 min. However, in all these instances the CR was measured 48 h after conditioning (LTM test), leaving up the possibility that CS–US association was formed but somehow did not last till the long-term. In keeping with this possibility, we previously reported a significant aversion during a test performed 4 h after conditioning (i.e., STM test) in rats trained with ISIs up to 60 min, three times longer than previously described (Tovar-Díaz et al. 2011). The LTM, however, was not tested so no further insight was provided regarding its persistence due to STM reactivation.Thus, in the current paper we hypothesized that a STM test would reactivate the initial memory, allowing it to further consolidate/integrate the information and to persist in the LTM. To test this possibility, we trained independent groups of rats with reduced US intensities or prolonged ISIs in a standard two-bottle choice COA paradigm and tested them twice at 4 and 48 h after conditioning. Our findings suggest that COA takes place under milder US and longer ISIs than previously thought and reactivating this memory during the STM test promotes its persistence in the LTM test.