Transient decline in hippocampal theta activity during the acquisition process of the negative patterning task

Modulation of recognition memory performance by light and its relationship with cortical EEG theta and gamma activities

Acute exposure to light exerts widespread effects on physiology, in addition to its key role in photoentrainment. Although the modulatory effect of light on physiological arousal is well demonstrated in mice, its effect on memory performance is inconclusive, as the direction of the effect depends on the nature of the behavioural task employed and/or the type of stimulus utilised. Moreover, in all rodent studies that reported significant effects of light on performance, brain activity was not assessed during the task and thus it is unclear how brain activity was modulated by light or the exact relationship between light-modulated brain activity and performance. Here we examine the modulatory effects of light of varying intensities on recognition memory performance and frontoparietal waking electroencephalography (EEG) in mice using the spontaneous recognition memory task. We report a light-intensity-dependent disruptive effect on recognition memory performance at the group level, but inspection of individual-level data indicates that light-intensity-dependent facilitation is observed in some cases. Using linear mixed-effects models, we then demonstrate that EEG fast theta (θ) activity at the time of encoding negatively predicts recognition memory performance, whereas slow gamma (γ) activity at the time of retrieval positively predicts performance. These relationships between θ/γ activity and performance are strengthened by increasing light intensity. Thus, light modulates θ and γ band activities involved in attentional and mnemonic processes, thereby affecting recognition memory performance. However, extraneous factors including the phase of the internal clock at which light is presented and homeostatic sleep pressure may determine how photic input is translated into behavioural performance.

The role of the hippocampal theta rhythm in non-spatial discrimination and associative learning task

The configural association theory and the conflict resolution model propose that hippocampal function is involved in learning negative patterning tasks (A+, B+, AB-). The first theory suggests a critical role of the hippocampus in the formation of configural representations of compound stimuli, in which stimuli A and B are presented simultaneously. The second theory hypothesizes that the hippocampus is important for inhibiting the response to a stimulus that is in conflict with response tendencies. Although these theories propose different interpretations of the link between hippocampal function and non-spatial discrimination tasks, they both predict that the hippocampus is involved in the information processing of compound stimuli in negative patterning tasks. Recently, our electrophysiological approach has shown that the hippocampal theta power correlate with response inhibition in a negative patterning task, positive patterning, simultaneous/serial feature negative task. These findings provide strong support for the assumption of the conflict resolution model that the role of the hippocampus in learning is to inhibit responses to conflicting stimuli during non-spatial stimulus discrimination tasks.

Change in hippocampal theta oscillation associated with multiple lever presses in a bimanual two-lever choice task for robot control in rats

Hippocampal theta oscillations have been implicated in working memory and attentional process, which might be useful for the brain-machine interface (BMI). To further elucidate the properties of the hippocampal theta oscillations that can be used in BMI, we investigated hippocampal theta oscillations during a two-lever choice task. During the task body-restrained rats were trained with a food reward to move an e-puck robot towards them by pressing the correct lever, ipsilateral to the robot several times, using the ipsilateral forelimb. The robot carried food and moved along a semicircle track set in front of the rat. We demonstrated that the power of hippocampal theta oscillations gradually increased during a 6-s preparatory period before the start of multiple lever pressing, irrespective of whether the correct lever choice or forelimb side were used. In addition, there was a significant difference in the theta power after the first choice, between correct and incorrect trials. During the correct trials the theta power was highest during the first lever-releasing period, whereas in the incorrect trials it occurred during the second correct lever-pressing period. We also analyzed the hippocampal theta oscillations at the termination of multiple lever pressing during the correct trials. Irrespective of whether the correct forelimb side was used, the power of hippocampal theta oscillations gradually decreased with the termination of multiple lever pressing. The frequency of theta oscillation also demonstrated an increase and decrease, before and after multiple lever pressing, respectively. There was a transient increase in frequency after the first lever press during the incorrect trials, while no such increase was observed during the correct trials. These results suggested that hippocampal theta oscillations reflect some aspects of preparatory and cognitive neural activities during the robot controlling task, which could be used for BMI.

The functional role of human right hippocampal/parahippocampal theta rhythm in environmental encoding during virtual spatial navigation

Low frequency theta band oscillations (4-8 Hz) are thought to provide a timing mechanism for hippocampal place cell firing and to mediate the formation of spatial memory. In rodents, hippocampal theta has been shown to play an important role in encoding a new environment during spatial navigation, but a similar functional role of hippocampal theta in humans has not been firmly established. To investigate this question, we recorded healthy participants' brain responses with a 160-channel whole-head MEG system as they performed two training sets of a virtual Morris water maze task. Environment layouts (except for platform locations) of the two sets were kept constant to measure theta activity during spatial learning in new and familiar environments. In line with previous findings, left hippocampal/parahippocampal theta showed more activation navigating to a hidden platform relative to random swimming. Consistent with our hypothesis, right hippocampal/parahippocampal theta was stronger during the first training set compared to the second one. Notably, theta in this region during the first training set correlated with spatial navigation performance across individuals in both training sets. These results strongly argue for the functional importance of right hippocampal theta in initial encoding of configural properties of an environment during spatial navigation. Our findings provide important evidence that right hippocampal/parahippocampal theta activity is associated with environmental encoding in the human brain. Hum Brain Mapp 38:1347-1361, 2017. © 2016 Wiley Periodicals, Inc.

The transient decline in hippocampal theta power during response inhibition in a positive patterning task

It is believed that a transient decline in hippocampal theta power is induced by behavioral inhibition during a go/no-go stimulus discrimination task. In a previously reported positive patterning (PP) task, rats learn to lever press when a compound stimulus, both tone and light, is presented and inhibit their lever press when a single stimulus, tone or light, is presented. In this task, rats were required to inhibit their response to the single stimulus in a task where both compound and single stimuli were presented with an overlapping element. Thus, we hypothesized that there would be a transient decline in hippocampal theta power induced by behavioral inhibition to the presence of a single stimuli in the PP task. The result of this study showed that a decline in hippocampal theta power occurred during response inhibition to the presence of a single tone stimulus in the PP task, supporting our hypothesis. However, we did not observe any decline in hippocampal theta power during response inhibition to the presence of a single light stimulus. We found that the error response rate for the tone stimulus was slightly lower than that for light stimulus in the PP task. Thus, we proposed that the decline in hippocampal theta power related to more accurate response inhibition to the stimulus that had an overlapping element.

Behavioral inhibition during a conflict state elicits a transient decline in hippocampal theta power

Although it has been shown that hippocampal theta power transiently declines during response inhibition in a simultaneous feature negative (FN: A+, AB-) task, observations of additional changes after this initial decline have been inconsistent across subjects. We hypothesized that the cause of these inconsistencies might be that variations in the learning speed for the FN task differentially affect the changes in hippocampal theta activity observed during the task. In this study, we classified rats into three groups (fast, intermediate, and slow FN-learning groups) based on the number of sessions required to complete learning of the FN task. We then examined whether there was a difference in hippocampal theta power among the fast, intermediate, and slow FN-learning groups, and rats that learned a simple discrimination task (SD group). We observed that compared to the SD group, the slow FN-learning group, but not the fast FN-learning group, showed an increase in hippocampal theta power. In addition, a transient decline of hippocampal theta power occurred in the fast FN-learning group, but not in the slow FN-learning group. These results indicate that the hippocampal theta activity during response inhibition in the FN task differed between fast- and slow-learning rats. Thus, we propose that a difference in learning speed affected hippocampal theta activity during response inhibition under a conflict state.

Change in hippocampal theta activity during behavioral inhibition for a stimulus having an overlapping element

It is believed that a decline in hippocampal theta power is induced by response inhibition for a conflict stimulus having an overlapping element. This study used a simultaneous feature positive (simul FP: A-, AX+) task and a serial FP (A-, X→A+) task. In these tasks, the compound and single stimuli have an overlapping element, and rats are required to exhibit response inhibition for the single stimulus A. We examined hippocampal theta activity during simul FP (A-, AX+), serial FP (A-, X→A+), and simple discrimination (SD; A-, X+) tasks and revealed that the transient decrease in hippocampal theta power occurred during response inhibition for the single stimulus A in simul FP tasks, which provides evidence that a transient decline in hippocampal theta power is induced by behavioral inhibition of conflict stimuli having an overlapping element. Thus, we concluded that the transient decline in hippocampal theta power was induced by behavioral inhibition for the conflict stimulus having an overlapping element.

Hippocampal theta activity during behavioral inhibition for conflicting stimuli

A recent behavioral inhibitory theory proposed that the hippocampus plays an important role in response inhibition to conflicting stimuli composed of simple inhibitory associations between events embedded in concurrent simple excitatory associations. In addition, the theory states that a serial feature negative (FN) task is a hippocampal-dependent task requiring the formation of a simple inhibitory association; on the other hand, a simple discrimination (SD) task is a typical hippocampus-independent task. In the present study, we recorded hippocampal theta activity from rats during FN and SD tasks to identify any potential differences. In the FN (A+, B→A-) task used in this study, rats were required to press a lever to present stimulus A (A+) and avoid pressing a lever to present a serial compound stimulus (B→A-). In the simple discrimination task (A+, B-), rats were required to press a lever to present stimulus A (A+) and avoid pressing a lever to present stimulus B (B-). We observed a transient decline of hippocampal theta power during response inhibition for a serial compound stimulus in the FN task. Thus, we conclude that the transient decline in hippocampal theta power reflects response inhibition for a conflicting stimulus. The results of the present study strongly support the behavioral inhibition theory.

Change in hippocampal theta activity with transfer from simple discrimination tasks to a simultaneous feature-negative task

It was showed that solving a simple discrimination task (A+, B−) and a simultaneous feature-negative (FN) task (A+, AB−) used the hippocampal-independent strategy. Recently, we showed that the number of sessions required for a rat to completely learn a task differed between the FN and simple discrimination tasks, and there was a difference in hippocampal theta activity between these tasks. These results suggested that solving the FN task relied on a different strategy than the simple discrimination task. In this study, we provided supportive evidence that solving the FN and simple discrimination tasks involved different strategies by examining changes in performance and hippocampal theta activity in the FN task after transfer from the simple discrimination task (A+, B− → A+, AB−). The results of this study showed that performance on the FN task was impaired and there was a difference in hippocampal theta activity between the simple discrimination task and FN task. Thus, we concluded that solving the FN task uses a different strategy than the simple discrimination task.

The decline in rat hippocampal theta activity during response inhibition for the compound stimulus of negative patterning and simultaneous feature-negative tasks

In experiment 1 of this study, we compared hippocampal theta activity between negative patterning and simple discrimination tasks. Our results demonstrated a transient decline in theta activity during response inhibition for a compound stimulus in the negative patterning task. In experiment 2 of this study, we compared hippocampal theta activity among simultaneous feature-negative, compound stimulus discrimination, and simple discrimination tasks in order to determine the cause of the decline in hippocampal theta activity during negative patterning tasks. Our results revealed that the decline in hippocampal theta activity occurred during the response inhibition for a compound stimulus in the simultaneous feature-negative task but not during the compound stimulus discrimination or simple discrimination tasks. Thus, we conclude that the transient decline in hippocampal theta activity is related to the inhibition in response to a compound stimulus that has an element that overlaps with a single stimulus.