Hippocampal ripple-contingent training accelerates trace eyeblink conditioning and retards extinction in rabbits

Abstract

There are at least two distinct oscillatory states of the hippocampus that are related to distinct behavioral patterns. Theta (4-12 Hz) oscillation has been suggested to indicate selective attention during which the animal concentrates on some features of the environment while suppressing reactivity to others. In contrast, sharp-wave ripples ( approximately 200 Hz) can be seen in a state in which the hippocampus is at its most responsive to any kind of afferent stimulation. In addition, external stimulation tends to evoke and reset theta oscillation, the phase of which has been shown to modulate synaptic plasticity in the hippocampus. Theoretically, training on a hippocampus-dependent learning task contingent upon ripples could enhance learning rate due to elevated responsiveness and enhanced phase locking of the theta oscillation. We used a brain-computer interface to detect hippocampal ripples in rabbits to deliver trace eyeblink conditioning and extinction trials selectively contingent upon them. A yoked control group was trained regardless of their ongoing neural state. Ripple-contingent training expedited acquisition of the conditioned response early in training and evoked stronger theta-band phase locking to the conditioned stimulus. Surprisingly, ripple-contingent training also resulted in slower extinction in well trained animals. We suggest that the ongoing oscillatory activity in the hippocampus determines the extent to which a stimulus can induce a phase reset of the theta oscillation, which in turn is the determining factor of learning rate in trace eyeblink conditioning.

Figure 1.

Trial presentation. Animals were trained in pairs, where one animal belonged to the R+ and the other to the YC group. The occurrence of a ripple event was detected from the hippocampal LFP of the subject assigned to the R+ group by means of a simple brain–computer interface. A trial was presented simultaneously to both animals contingent upon the occurrence of a ripple in the R+ animal. Thus, subjects in both groups received identical training. The minimum intertrial interval was set to 15 s. For the R+ example, both the raw (top) and the bandpass-filtered (80–250 Hz, middle) signals are shown. In addition, a dashed line indicates the threshold set for the ripples in this animal. For the YC example (bottom), only the raw signal is shown. R, Ripple detection time window.

Figure 2.

Location of the recording electrodes. All 28 subjects had at least one recording electrode correctly placed in the right dorsal hippocampus and showing ripple activity. A, Electrode locations by group (YC and R+) and learning during conditioning. A CR on 8 of 9 consecutive paired trials or >50% CRs during at least one conditioning session was used as the criterion for learning. B, Electrode locations by group and learning during extinction. Less than 30% CRs during at least one training session was used as a criterion for learning the extinction.

Figure 3.

Ripple-contingent training led to faster learning and slower extinction. A, CRs as a function of training block. Each block represents the average CR percentage during two consecutive daily 60-trial sessions. During conditioning, rabbits in the R+ group (n = 14) received paired presentations of the CS (tone) and the US (airpuff) contingent upon hippocampal ripples, while rabbits in the YC (n = 14) group received conditioning trials regardless of their neural state. A 500 ms trace period separated the CS and the US during each conditioning trial. Rabbits in both groups acquired a robust CR, although rabbits in the R+ group showed more CRs during the very early phase of conditioning (see left panel, first block). Also, more animals in the R+ group (12 of 14) reached asymptotic learning than in the YC group (7 of 14). Extinction training was administered only in rabbits showing robust learning at the end of conditioning (n = 19). These rabbits that learned well from the original R+ and YC groups were divided into R+ (n = 9) and YC (n = 10) groups in a counterbalancing manner. During extinction training, the tone-CS was always presented alone. Extinction training reduced the number of CRs in both groups, but more so in the YC group. That is, extinction was slower in rabbits in the R+ group. Block 0 in the right panel represents CR percentage during the last block of conditioning training. B, The cumulative number of CRs plotted as a function of trial number in the R+ and YC groups separately shows that there is no initial difference in responding. Vertical lines depict the SEM. C, The average response topographies of the two groups during the first two sessions (block 1) were notably similar. Both groups show correctly timed CRs that peak around US onset. Only trials during which an animal emitted a CR were included in these averages. D, Reflex facilitation was stronger in the R+ group. Asterisks refer to statistical significance: *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 4.

A, B, Change (percentage) in hippocampal ripple rate (A) and theta-band phase locking (B) across learning. The data were divided into three sections (before, during, and after learning) based on the session during which the fifth CR was seen (conditioning) or when the animal showed <30% conditioned responses (extinction). A, The presession versus postsession change (percentage) in ripple rate per minute showed no differences between the YC and the R+ group, nor did it show any changes across learning. However, the change between presession versus postsession recordings was significant early in conditioning (before learning) in the YC group (one-sample t test: p < 0.05). B, The CS-evoked theta-band PLV was higher in the R+ group early in conditioning, before the animals reached the criterion of 5 conditioned responses (fifth CR). After that, no statistically significant changes were found between the two groups. The CS-evoked theta-band PLV showed no changes across learning. C, The theta-band PLV across time during each phase of learning during conditioning (compare to B, left panel). Note that the width of the PLV peak clearly surpasses the duration of an evoked potential and that the PLV in the R+ group also remains significant for the duration of ISI, which is not the case in the YC group. The gray bars at the bottom indicate the timing of the CS and the unconditioned stimulus. The peak in the PLVs of the R+ group occurring immediately before the CS reflects the sharp-wave component of the SPW-ripple complex. Asterisks refer to statistical significance: *p < 0.05, **p < 0.01.