Our research focus
We work in the field of cognitive neuroscience. Our goal is to elucidate the psychological and neural mechanisms underlying conscious and unconscious episodic learning, consolidation, and retrieval. To this aim, we examine healthy individuals and neurological patients with amnesia using behavioral methods, functional and anatomical and spectroscopic magnetic resonance imaging, positron emission tomography, electroencephalography, polysomnography, and transcranial direct current stimulation.
Our findings suggest that the human hippocampal formation specializes in the rapid establishment of new sensory, conceptual, spatial, and temporal relations between components of experiences (Henke et al., 1997, 1999). We study not only conscious episodic encoding but also unconscious episodic encoding in the waking state using subliminal (masked) visual learning material and in the unconscious state of deep sleep using acoustic learning material. Our findings indicate that the hippocampal formation mediates the rapid encoding of new relations between components of episodes both when encoding is conscious and when encoding is unconscious, both in the waking state (e.g., Reber et al., 2012; Schneider et al., 2021) and during deep sleep (Züst et al., 2019; Schmidig et al., 2023). Sleep-learning appears to be bound to peaks of slow oscillations recorded during deep sleep. Patients with hippocampal damage exhibit a decrease in both conscious and unconscious relational encoding and retrieval (Duss et al., 2014). An advantage of unconscious over conscious episodic memory formation is the larger storage capacity and no forgetting over time (Schneider et al., 2021; Pacozzi et al., 2022). An advantage of conscious over unconscious episodic memory formation is a greater strength of the formed memory representations and the hereby resulting increase in power to guide behavior. This research informs models of information processing, memory models and sleep theories. In particular, our research suggests that consciousness is no valid divider between memory systems (Henke, 2010).
In a large research project (SNSF Advanced Grant) financed by the Swiss National Science Foundation we currently explore the fate of consciously formed episodic memories over time in healthy participants and amnesic patients tracking memories and their neural memory traces with ultra high-field magnetic resonance imaging. Amnesia is a deficit of episodic memory, which is believed to result from a loss of episodic learning. Yet, research in a mouse model of amnesia suggests that episodic learning is still possible in amnesia because physical memory traces of newly formed memories were discovered. We explore whether the same is true in human amnesic patients. We use ultra high-field functional, anatomical, and spectroscopic magnetic resonance imaging in healthy and amnesic individuals to track the dynamic transitions of newly formed episodic memories and their underlying physical memory traces as these memories change their representational status over the 24 hours following learning. During this consolidation time, some memories shift from being consciously accessible to being only unconsciously accessible and to being inaccessible (forgotten). We find that apparently forgotten memories remain implemented in the episodic memory network, even if they lost the power to steer behavior (Willems et al., in preparation). Using noninvasive electrical brain-stimulation (transcranial direct current stimulation) we reinforce the physical memory traces of newly formed memories in healthy and amnesic individuals to bring unconscious memories to conscious access. We uncover the mechanism of this strengthening of memory traces by applying ultra high-field magnetic resonance spectroscopy that reveals changes in neurotransmitter levels. The discovery that amnesic patients continue to store experiences using spared hippocampal tissue will inform memory theories and instigate new diagnostic procedures and therapeutic tools aimed at strengthening memory trace formation and tissue regeneration.
References:
Schmidig, F.J., Ruch, S., Henke, K. (2023). Episodic long-term memory formation during slow-wave sleep. eLife 12:RP89601. https://doi.org/10.7554/eLife.89601.1.
Pacozzi, L., Knüsel, L., Ruch, S., Henke, K. (2022). Inverse forgetting in unconscious episodic memory. Scientific Reports, published online, https://rdcu.be/c0Fwe.
Schneider, E., Züst, M., Wuethrich, S., Kloeppel, S., Wiest, R., Schmidig, F., Ruch, S., Henke, K. (2021). Larger capacity for unconscious versus conscious episodic memory. Current Biology, 31, 3551–3563.
Züst, M.A., Ruch, S., Wiest, R., Henke, K. (2019). Implicit vocabulary learning during sleep is bound to slow-wave peaks. Current Biology, 29, 541–553.
Duss, S.B., Reber, T.P., Hänggi, J., Schwab, S., Wiest, R., Müri, R.M., Brugger, P., Gutbrod, K., Henke, K. (2014). Unconscious Relational Encoding Depends on Hippocampus. Brain, 137, 3355-3370.
Reber, T.P., Luechinger, R., Boesiger, P., Henke, K. (2012). Unconscious relational inference recruits the hippocampus. Journal of Neuroscience, 32, 6138-6148..
Henke, K., Weber, B., Kneifel, S., Wieser, H.G., & Buck, A. (1999). Human hippocampus associates information in memory. Proceedings of the National Academy of Sciences USA, 96, 5884-5889.
Henke, K., Buck, A., Weber, B., & Wieser, H.G. (1997). Human hippocampus establishes associations in memory. Hippocampus, 7, 249-256.
Techniques
- Behavioral techniques of experimental psychology
- Neuropsychological and experimental examination of neurological patients
- Functional and structural and spectroscopic magnetic resonance imaging (MRI)
- Positron emission tomography (PET)
- Electroencephalography (EEG)
- intracranial EEG recordings in patients with epilepsy
- Sleep polysomnography
- Auditory closed-loop stimulation of the sleep slow oscillation
- Transcranial direct current stimulation
Keywords
Memory, consciousness, memory consolidation, forgetting curve, sleep-wake, sleep-learning, subliminal processing and memory formation, hippocampus, neuroimaging, brain stimulation, amnesia.
