Demonstration of nondeclarative sequence learning in mice: development of an animal analog of the human serial reaction time task

Differential effects of bilateral hippocampal CA3 damage on the implicit learning and recognition of complex event sequences

Learning regularities in the environment is a fundament of human cognition, which is supported by a network of brain regions that include the hippocampus. In two experiments, we assessed the effects of selective bilateral damage to human hippocampal subregion CA3, which was associated with autobiographical episodic amnesia extending ~50 years prior to the damage, on the ability to recognize complex, deterministic event sequences presented either in a spatial or a non-spatial configuration. In contrast to findings from related paradigms, modalities, and homologue species, hippocampal damage did not preclude recognition memory for an event sequence studied and tested at four spatial locations, whereas recognition memory for an event sequence presented at a single location was at chance. In two additional experiments, recognition memory for novel single-items was intact, whereas the ability to recognize novel single-items in a different location from that presented at study was at chance. The results are at variance with a general role of the hippocampus in the learning and recognition of complex event sequences based on non-adjacent spatial and temporal dependencies. We discuss the impact of the results on established theoretical accounts of the hippocampal contributions to implicit sequence learning and episodic memory.

Behavioral evidence for two distinct memory systems in rats

Serial reaction time tasks, in which subjects have to match a target to a cue, are used to explore whether non-human animals have multiple memory systems. Predictable sub-sequences embedded in the sequence of cues are responded to faster, demonstrating incidental learning, often considered implicit. Here, we used the serial implicit learning task (SILT) to determine whether rats' memory shows similar effects. In SILT, subjects must nose-poke into a sequence of two lit apertures, S1 and S2. Some S1 are always followed by the same S2, creating predictable sequences (PS). Across groups, we varied the proportion of PS trials, from 10 to 80%, and show that rats with more PS experience do better on them than on unpredictable sequences, and better than rats with less experience. We then introduced test trials in which no S2 was cued. Rats with more PS experience did better on test trials. Finally, we reversed some sequences (from predictable to unpredictable and vice versa) and changed others. We find that rats with more PS experience perseverate on old (now incorrect) responses more than those with less PS experience. Overall, we find a discontinuity in performance as the proportion of PS increases, suggesting a switch in behavioral strategies or memory systems, which we confirm using a Process Dissociation Procedure analysis. Our data suggest that rats have at least two distinct memory systems, one of which appears to be analogous to human implicit memory and is differentially activated by varying the proportion of PS in our task.

Implicit sequence learning in ring-tailed lemurs (Lemur catta)

Implicit learning involves picking up information from the environment without explicit instruction or conscious awareness of the learning process. In nonhuman animals, conscious awareness is impossible to assess, so we define implicit learning as occurring when animals acquire information beyond what is required for successful task performance. While implicit learning has been documented in some nonhuman species, it has not been explored in prosimian primates. Here we ask whether ring-tailed lemurs (Lemur catta) learn sequential information implicitly. We tested lemurs in a modified version of the serial reaction time task on a touch screen computer. Lemurs were required to respond to any picture within a 2 × 2 grid of pictures immediately after its surrounding border flickered. Over 20 training sessions, both the locations and the identities of the images remained constant and response times gradually decreased. Subsequently, the locations and/or the identities of the images were disrupted. Response times indicated that the lemurs had learned the physical location sequence required in original training but did not learn the identity of the images. Our results reveal that ring-tailed lemurs can implicitly learn spatial sequences, and raise questions about which scenarios and evolutionary pressures give rise to perceptual versus motor-implicit sequence learning.

Implicit learning in cotton-top tamarins (Saguinus oedipus) and pigeons (Columba livia)

There is considerable interest in the conditions under which human subjects learn patterned information without explicit instructions to learn that information. This form of learning, termed implicit or incidental learning, can be approximated in nonhumans by exposing subjects to patterned information but delivering reinforcement randomly, thereby not requiring the subjects to learn the information in order to be reinforced. Following acquisition, nonhuman subjects are queried as to what they have learned about the patterned information. In the present experiment, we extended the study of implicit learning in nonhumans by comparing two species, cotton-top tamarins (Saguinus oedipus) and pigeons (Columba livia), on an implicit learning task that used an artificial grammar to generate the patterned elements for training. We equated the conditions of training and testing as much as possible between the two species. The results indicated that both species demonstrated approximately the same magnitude of implicit learning, judged both by a random test and by choice tests between pairs of training elements. This finding suggests that the ability to extract patterned information from situations in which such learning is not demanded is of longstanding origin.

Implicit chaining in cotton-top tamarins (Saguinus oedipus) with elements equated for probability of reinforcement

Three experiments examined the implicit learning of sequences under conditions in which the elements comprising a sequence were equated in terms of reinforcement probability. In Experiment 1 cotton-top tamarins (Saguinus oedipus) experienced a five-element sequence displayed serially on a touch screen in which reinforcement probability was equated across elements at .16 per element. Tamarins demonstrated learning of this sequence with higher latencies during a random test as compared to baseline sequence training. In Experiments 2 and 3, manipulations of the procedure used in the first experiment were undertaken to rule out a confound owing to the fact that the elements in Experiment 1 bore different temporal relations to the intertrial interval (ITI), an inhibitory period. The results of Experiments 2 and 3 indicated that the implicit learning observed in Experiment 1 was not due to temporal proximity between some elements and the inhibitory ITI. The results taken together support two conclusion: First that tamarins engaged in sequence learning whether or not there was contingent reinforcement for learning the sequence, and second that this learning was not due to subtle differences in associative strength between the elements of the sequence.

Implicit chaining in cotton-top tamarins (Saguinus oedipus) with elements equated for probability of reinforcement

Three experiments examined the implicit learning of sequences under conditions in which the elements comprising a sequence were equated in terms of reinforcement probability. In Experiment 1 cotton-top tamarins (Saguinus oedipus) experienced a five-element sequence displayed serially on a touch screen in which reinforcement probability was equated across elements at .16 per element. Tamarins demonstrated learning of this sequence with higher latencies during a random test as compared to baseline sequence training. In Experiments 2 and 3, manipulations of the procedure used in the first experiment were undertaken to rule out a confound owing to the fact that the elements in Experiment 1 bore different temporal relations to the intertrial interval (ITI), an inhibitory period. The results of Experiments 2 and 3 indicated that the implicit learning observed in Experiment 1 was not due to temporal proximity between some elements and the inhibitory ITI. The results taken together support two conclusion: First that tamarins engaged in sequence learning whether or not there was contingent reinforcement for learning the sequence, and second that this learning was not due to subtle differences in associative strength between the elements of the sequence.

Unaltered instrumental learning and attenuated body-weight gain in rats during non-rotating simulated shiftwork

Exposure to shiftwork has been associated with multiple health disorders and cognitive impairments in humans. We tested if we could replicate metabolic and cognitive consequences of shiftwork, as reported in humans, in a rat model comparable to 5 wks of non-rotating night shifts. The following hypotheses were addressed: (i) shiftwork enhances body-weight gain, which would indicate metabolic effects; and (ii) shiftwork negatively affects learning of a simple goal-directed behavior, i.e., the association of lever pressing with food reward (instrumental learning), which would indicate cognitive effects. We used a novel method of forced locomotion to model work during the animals' normal resting period. We first show that Wistar rats, indeed, are active throughout a shiftwork protocol. In contrast with previous findings, the shiftwork protocol attenuated the normal weight gain to 76 ± 8 g in 5 wks as compared to 123 ± 15 g in the control group. The discrepancy with previous work may be explained by the concurrent observation that with our shiftwork protocol rats did not adjust their between-work circadian activity pattern. They maintained a normal level of activity during the "off-work" periods. In the control experiment, rats were kept active during the dark period, normally dominated by activity. This demonstrated that forced activity, per se, did not affect body-weight gain (mean ± SEM: 85 ± 11 g over 5 wks as compared to 84 ± 11 g in the control group). Rats were trained on an instrumental learning paradigm during the fifth week of the protocol. All groups showed equivalent increases in lever pressing from the first (3.8 ± .7) to the sixth (21.3 ± 2.4) session, and needed a similar amount of sessions (5.1 ± .3) to reach a learning criterion (≥ 27 out of 30 lever presses). These results suggest that while on prolonged non-rotating shiftwork, not fully reversing the circadian rhythm might actually be beneficial to prevent body-weight gain and cognitive impairments.

Sequential learning and rule abstraction in Bengalese finches

The Bengalese finch (Lonchura striata var. domestica) is a species of songbird. Males sing courtship songs with complex note-to-note transition rules, while females discriminate these songs when choosing their mate. The present study uses serial reaction time (RT) to examine the characteristics of the Bengalese finches’ sequential behaviours beyond song production. The birds were trained to produce the sequence with an “A–B–A” structure. After the RT to each key position was determined to be stable, we tested the acquisition of the trained sequential response by presenting novel and random three-term sequences (random test). We also examined whether they could abstract the embedded rule in the trained sequence and apply it to the novel test sequence (abstract test). Additionally, we examined rule abstraction through example training by increasing the number of examples in baseline training from 1 to 5. When considered as (gender) groups, training with 5 examples resulted in no statistically significant differences in the abstract tests, while statistically significant differences were observed in the random tests, suggesting that the male birds learned the trained sequences and transferred the abstract structure they had learned during the training trials. Individual data indicated that males, as opposed to females, were likely to learn the motor pattern of the sequence. The results are consistent with observations that males learn to produce songs with complex sequential rules, whereas females do not.

Effects of frontal cortex lesions on action sequence learning in the rat

To understand the role of frontal cortex in motor sequence learning we compared the effects of motor (M1), premotor (M2) and midline frontal (MFr) cortical lesions on rats making nose-pokes guided by luminance cues. Organizational demands were manipulated by varying the number (1 vs. 5) and predictability (random vs. repeated) of nose-pokes in a response. Learning was studied by comparing sessions with random or repeated cues. All cortical lesions increased reaction time (RT) during response initiation. These effects were larger for nose-pokes initiating sequential responses but spared RT for nose-pokes completing them. Repetition learning had significant effects on the speed and accuracy of single nose-poke responses that were unaffected by any of the cortical lesions. Repetition learning had more complex effects on sequential responding. RTs increased for nose-pokes initiating sequences over several sessions of continuous repetition and then decreased or leveled off. RTs decreased incrementally across all repetition sessions for subsequent nose-pokes in repeated sequences, following a time-course consistent with habit learning. Lesions involving M2 and MFr cortex exacerbated the increase in RT during initiation without affecting the incremental decrease in RT for nose-pokes completing repeated sequences. These results were confirmed by analyses of interference effects when training shifted from repeated (learned) to random (novel) sequences or to a new repeated sequence. These results implicate dorsomedial frontal cortex in organizational aspects of sensory-guided responding and motor sequence learning reflected in RT during response initiation.

Sequential behavior in the rat: role of skill and attention

The serial reaction time task (SRTT) is a well-established experimental tool to study cognitive and neural mechanisms of sequential performance in humans. We have recently developed a rodent version of the human serial reaction time task, in which rats have to respond to visual stimuli by nose-poking into one of four spatial locations in order to obtain food reward. In this task, rats display superior performance under sequential as compared to random conditions of stimulus presentation. Specifically, the subjects are able to profit from sequential regularities in terms of faster reaction times and higher response accuracy. Here, we studied the effects of violating a single stimulus in rats, which had been intensively trained under sequential conditions, and we asked whether these subjects, when confronted with sequence violations, still attend to the actual stimulus order (that is, show correct responses), or whether their behavior has become fully automated (leading to specific incorrect responses to violated stimulus positions). In two independent experiments using partly differing instrumental set-ups, we found that the responses to non-cued violations of single stimulus positions were mostly correct, that is, the animals were apparently attending to the stimuli. Nevertheless, these reaction times were slowed, which probably reflects cognitive resources necessary to respond correctly to the unexpected irregularities. When quantifying the minority of responses, which were incorrect, we found that most of them were directed to the position, where the stimulus would have appeared if the sequence had not been violated. These responses were faster than the correct ones (to the violated stimulus), which indicates that sequential responding had become partly automated. Together, our data show that both, attention and skill play a role for sequential performance in our SRT task, and that they can be dissected by quantification of specific response types. In future work, the neural correlates underlying these functional mechanisms will have to be addressed.

A long list visuo-spatial sequential learning in mice

Sequential learning has been extensively studied in humans using the serial reaction time (SRT) paradigm, and has contributed significantly to the description of the neurobiological processes and substrates underlying different memory systems. More precisely, patients with basal ganglia, but not medial temporal lobe pathology exhibit selective deficits in this task, qualified as implicit learning, since this learning occurs without any conscious awareness of the subjects. While, the construction of transgenic mouse models of human neurological diseases has created a great need for developing mouse analogs of this or other types of human memory tasks, only a few studies exist in rodents, and more specifically in mice. The present study is aimed at examining a SRT protocol for mice using our new operant chamber designed to be polyvalent for different experimental conditions and uses. We provide data for learning by normal C57BL/6 mice of a repeating sequence of 12 nose poke responses, first, via the observation of increases in reaction times when repeated sequence is replaced by random sequence, and, second, by analysis of behavior during transfer trials in which one sequential element is discretely replaced by a new item. The potential of our protocol for dissecting the different neural systems of learning and memory is discussed as well as its usefulness for the validation of transgenic mouse models of human neurodegenerative diseases such as Huntington's disease and Alzheimer's disease.

Oculomotor and manual indexes of incidental and intentional spatial sequence learning during middle childhood and adolescence

The goal of this study was to examine incidental and intentional spatial sequence learning during middle childhood and adolescence. We tested four age groups (8-10 years, 11-13 years, 14-17 years, and young adults [18+ years]) on a serial reaction time task and used manual and oculomotor measures to examine incidental sequence learning. Participants were also administered a trial block in which they were explicitly instructed to learn a sequence. Replicating our previous study with adults, oculomotor anticipations and response times showed learning effects similar to those in the manual modality. There were few age-related differences in the sequence learning indexes during incidental learning, but intentional learning yielded differences on all indexes. Results indicate that the search for regularities and the ability to learn a sequence rapidly under incidental conditions are mature by 8 to 10 years of age. In contrast, the ability to learn a sequence intentionally, which requires cognitive resources and strategies, continues to develop through adolescence.