Concept: Theta rhythm
Several experiments have demonstrated an intimate relationship between hippocampal theta rhythm (4-12 Hz) and memory. Lesioning the medial septum or fimbria-fornix, a fiber track connecting the hippocampus and the medial septum, abolishes the theta rhythm and results in a severe impairment in declarative memory. To assess whether there is a causal relationship between hippocampal theta and memory formation we investigated whether restoration of hippocampal theta by electrical stimulation during the encoding phase also restores fimbria-fornix lesion induced memory deficit in rats in the fear conditioning paradigm. Male Wistar rats underwent sham or fimbria-fornix lesion operation. Stimulation electrodes were implanted in the ventral hippocampal commissure and recording electrodes in the septal hippocampus. Artificial theta stimulation of 8 Hz was delivered during 3-min free exploration of the test cage in half of the rats before aversive conditioning with three foot shocks during 2 min. Memory was assessed by total freezing time in the same environment 24 h and 28 h after fear conditioning, and in an intervening test session in a different context. As expected, fimbria-fornix lesion impaired fear memory and dramatically attenuated hippocampal theta power. Artificial theta stimulation produced continuous theta oscillations that were almost similar to endogenous theta rhythm in amplitude and frequency. However, contrary to our predictions, artificial theta stimulation impaired conditioned fear response in both sham and fimbria-fornix lesioned animals. These data suggest that restoration of theta oscillation per se is not sufficient to support memory encoding after fimbria-fornix lesion and that universal theta oscillation in the hippocampus with a fixed frequency may actually impair memory.
IN THE RAPIDLY CHANGING CIRCUMSTANCES OF OUR INCREASINGLY DIGITAL WORLD, READING IS ALSO BECOMING AN INCREASINGLY DIGITAL EXPERIENCE: electronic books (e-books) are now outselling print books in the United States and the United Kingdom. Nevertheless, many readers still view e-books as less readable than print books. The present study thus used combined EEG and eyetracking measures in order to test whether reading from digital media requires higher cognitive effort than reading conventional books. Young and elderly adults read short texts on three different reading devices: a paper page, an e-reader and a tablet computer and answered comprehension questions about them while their eye movements and EEG were recorded. The results of a debriefing questionnaire replicated previous findings in that participants overwhelmingly chose the paper page over the two electronic devices as their preferred reading medium. Online measures, by contrast, showed shorter mean fixation durations and lower EEG theta band voltage density - known to covary with memory encoding and retrieval - for the older adults when reading from a tablet computer in comparison to the other two devices. Young adults showed comparable fixation durations and theta activity for all three devices. Comprehension accuracy did not differ across the three media for either group. We argue that these results can be explained in terms of the better text discriminability (higher contrast) produced by the backlit display of the tablet computer. Contrast sensitivity decreases with age and degraded contrast conditions lead to longer reading times, thus supporting the conclusion that older readers may benefit particularly from the enhanced contrast of the tablet. Our findings thus indicate that people’s subjective evaluation of digital reading media must be dissociated from the cognitive and neural effort expended in online information processing while reading from such devices.
Chemotherapy, especially if prolonged, disrupts attention, working memory and speed of processing in humans. Most cancer drugs that cross the blood-brain barrier also decrease adult neurogenesis. Because new neurons are generated in the hippocampus, this decrease may contribute to the deficits in working memory and related thought processes. The neurophysiological mechanisms that underlie these deficits are generally unknown. A possible mediator is hippocampal oscillatory activity within the theta range (3-12 Hz). Theta activity predicts and promotes efficient learning in healthy animals and humans. Here, we hypothesised that chemotherapy disrupts learning via decreases in hippocampal adult neurogenesis and theta activity. Temozolomide was administered to adult male Sprague-Dawley rats in a cyclic manner for several weeks. Treatment was followed by training with different types of eyeblink classical conditioning, a form of associative learning. Chemotherapy reduced both neurogenesis and endogenous theta activity, as well as disrupted learning and related theta-band responses to the conditioned stimulus. The detrimental effects of temozolomide only occurred after several weeks of treatment, and only on a task that requires the association of events across a temporal gap and not during training with temporally overlapping stimuli. Chemotherapy did not disrupt the memory for previously learned associations, a memory independent of (new neurons in) the hippocampus. In conclusion, prolonged systemic chemotherapy is associated with a decrease in hippocampal adult neurogenesis and theta activity that may explain the selective deficits in processes of learning that describe the ‘chemobrain’.
- Experimental brain research. Experimentelle Hirnforschung. Experimentation cerebrale
- Published about 6 years ago
This study examined hippocampal theta power during configural and non-configural tasks in rats. Experiment 1 compared hippocampal theta power during a negative patterning task (A+, B+, AB-) to a configural task and a simple discrimination task (A+, B-) as a non-configural task. The results showed that hippocampal theta power during the non-reinforcement trial (non-RFT) of the negative patterning task was higher than that during the simple discrimination task. However, this hippocampal power may reflect sensory processing for compound stimuli that have cross-modality features (the non-RFT of the negative patterning task was presented together with visual and auditory stimuli, but the non-RFT of the simple discrimination task was presented with visual or auditory stimulus alone). Thus, in experiment 2, we examined whether the experiment 1 results were attributable to sensory processing of a compound stimulus by comparing hippocampal theta power during negative patterning (A+, B+, AB-), simultaneous feature-negative (A+, AB-), and simple discrimination tasks (A+, B-). Experiment 2 showed that hippocampal theta activity during the non-RFT in the negative patterning task was higher than that in the simultaneous feature-negative and simple discrimination tasks. Thus, we showed that hippocampal theta activity increased during configural tasks but not during non-configural tasks.
Rapid eye movement sleep (REMS) has been linked with spatial and emotional memory consolidation. However, establishing direct causality between neural activity during REMS and memory consolidation has proven difficult because of the transient nature of REMS and significant caveats associated with REMS deprivation techniques. In mice, we optogenetically silenced medial septum γ-aminobutyric acid-releasing (MS(GABA)) neurons, allowing for temporally precise attenuation of the memory-associated theta rhythm during REMS without disturbing sleeping behavior. REMS-specific optogenetic silencing of MS(GABA) neurons selectively during a REMS critical window after learning erased subsequent novel object place recognition and impaired fear-conditioned contextual memory. Silencing MS(GABA) neurons for similar durations outside REMS episodes had no effect on memory. These results demonstrate that MS(GABA) neuronal activity specifically during REMS is required for normal memory consolidation.
- The Journal of neuroscience : the official journal of the Society for Neuroscience
- Published almost 6 years ago
Hippocampal theta oscillations are postulated to support mnemonic processes in humans and rodents. Theta oscillations facilitate encoding and spatial navigation, but to date, it has been difficult to dissociate the effects of volitional movement from the cognitive demands of a task. Therefore, we examined whether volitional movement or cognitive demands exerted a greater modulating factor over theta oscillations during decision-making. Given the anatomical, electrophysiological, and functional dissociations along the dorsal-ventral axis, theta oscillations were simultaneously recorded in the dorsal and ventral hippocampus in rats trained to switch between place and motor-response strategies. Stark differences in theta characteristics were found between the dorsal and ventral hippocampus in frequency, power, and coherence. Theta power increased in the dorsal, but decreased in the ventral hippocampus, during the decision-making epoch. Interestingly, the relationship between running speed and theta power was uncoupled during the decision-making epoch, a phenomenon limited to the dorsal hippocampus. Theta frequency increased in both the dorsal and ventral hippocampus during the decision epoch, although this effect was greater in the dorsal hippocampus. Despite these differences, ventral hippocampal theta was responsive to the navigation task; theta frequency, power, and coherence were all affected by cognitive demands. Theta coherence increased within the dorsal hippocampus during the decision-making epoch on all three tasks. However, coherence selectively increased throughout the hippocampus (dorsal to ventral) on the task with new hippocampal learning. Interestingly, most results were consistent across tasks, regardless of hippocampal-dependent learning. These data indicate increased integration and cooperation throughout the hippocampus during information processing.
The hippocampus is important in learning during a discrimination-reversal task. In this task, animals first learn to emit the go response to one stimulus and the no-go response to another stimulus (S1+, S2-) during the discrimination phase, and then they learn to reverse these relationships between stimulus and response during the reversal phase (S1-, S2+). To emit a no-go response for non-reinforced trial during the reversal phase, animals needed to inhibit the previously learned response pattern. This study examined the relationship between the reversal phase of the discrimination-reversal task and hippocampal electric activity in operant conditioning. The results revealed that hippocampal theta power transiently declined during the non-reinforced trial in the reversal phase compared with that during the discrimination phase. This decrease was observed during the 400- to 600-ms epoch after the onset of stimulus presentation. This study suggested that transient decline in hippocampal theta power is related to negative memory retrieval.
- The Journal of neuroscience : the official journal of the Society for Neuroscience
- Published about 6 years ago
Recent reports converge to the idea that high-frequency oscillations in local field potentials (LFPs) represent multiunit activity. In particular, the amplitude of LFP activity above 100 Hz-commonly referred to as “high-gamma” or “epsilon” band-was found to correlate with firing rate. However, other studies suggest the existence of true LFP oscillations at this frequency range that are different from the well established ripple oscillations. Using multisite recordings of the hippocampus of freely moving rats, we show here that high-frequency LFP oscillations can represent either the spectral leakage of spiking activity or a genuine rhythm, depending on recording location. Both spike-leaked, spurious activity and true fast oscillations couple to theta phase; however, the two phenomena can be clearly distinguished by other key features, such as preferred coupling phase and spectral signatures. Our results argue against the idea that all high-frequency LFP activity stems from spike contamination and suggest avoiding defining brain rhythms solely based on frequency range.
Sleep is ubiquitous in animals and humans, but its function remains to be further determined. The synaptic homeostasis hypothesis of sleep-wake regulation proposes a homeostatic increase in net synaptic strength and cortical excitability along with decreased inducibility of associative synaptic long-term potentiation (LTP) due to saturation after sleep deprivation. Here we use electrophysiological, behavioural and molecular indices to non-invasively study net synaptic strength and LTP-like plasticity in humans after sleep and sleep deprivation. We demonstrate indices of increased net synaptic strength (TMS intensity to elicit a predefined amplitude of motor-evoked potential and EEG theta activity) and decreased LTP-like plasticity (paired associative stimulation induced change in motor-evoked potential and memory formation) after sleep deprivation. Changes in plasma BDNF are identified as a potential mechanism. Our study indicates that sleep recalibrates homeostatic and associative synaptic plasticity, believed to be the neural basis for adaptive behaviour, in humans.
Previous, albeit correlative, findings have shown that the neural mechanisms underlying working memory critically require cross-structural and cross-frequency coupling mechanisms between theta and gamma neural oscillations. However, the direct causality between cross-frequency coupling and working memory performance remains to be demonstrated. Here we externally modulated the interaction of theta and gamma rhythms in the prefrontal cortex using novel cross-frequency protocols of transcranial alternating current stimulation to affect spatial working memory performance in humans. Enhancement of working memory performance and increase of global neocortical connectivity were observed when bursts of high gamma oscillations (80-100 Hz) coincided with the peaks of the theta waves, whereas superimposition on the trough of the theta wave and low gamma frequency protocols were ineffective. Thus, our results demonstrate the sensitivity of working memory performance and global neocortical connectivity to the phase and rhythm of the externally driven theta-gamma cross-frequency synchronization.