As 2024 comes to a triumphant close, December ushers in the perfect opportunity to reflect on all the innovative progress made throughout the year. Research can be a long and gruelling process with many checks and challenges along the way, so publications deserve to be recognized and celebrated. This year’s studies have delved into fascinating topics, ranging from sound production in baleen whales to the role of Krause corpuscles in sexual behaviours, and even the crystallization of volatile working memory representations. Continuing our annual tradition, the following publication review highlights the standout studies of the year.

Featured image (photo by Nick Fewings on Unsplash with figures adapted from ©Bellafard et al (2024), licensed under CC BY 4.0) depicting two mice interacting, along with the effects of optogenetic inhibition on working memory task performance. A) The experimental setup, B) the delayed-association working memory task trial types, C) learning progress across eight sessions, by percentage of correct responses, D) an example of a learning session, with licks marked, E) the effect of photoinhibition during different task epochs on percent performance change, and F) photoinhibition of the secondary motor cortex during the last 2 seconds of the delay period across the first 7 days of training.

Evolutionary novelties underlie sound production in baleen whales

The vocalization mechanisms of baleen whales (mysticetes) have been a subject of speculation due to their unique laryngeal structure, which differs from the nasal vocal organs of toothed whales (odontocetes). While mysticetes are known to use their larynx for sound production, how the larynx can produce the diverse sounds that mysticetes are known for remains unknown. To address this, Elemans et al (2024) sought to explore how mysticetes evolved these specialized laryngeal structures.

Larynges from three whale species were dissected and analyzed: a sei whale, a humpback whale, and a minke whale, all of which were studied for their laryngeal structures and phonation mechanisms. High-speed cameras, endoscopy, electroglottographs, and specialized flow and pressure sensors were employed to evaluate the properties of the larynges. To further measure mechanical properties, such as Young’s modulus, Aurora Scientific’s 300C-I Dual-Mode Indenter was used with the 1-mm-diameter indenter mounted on an extension tube.

Upon analysis, two distinct phonation mechanisms were identified: the vibration of unique structures (CC and TAF mucosa) for low-frequency sounds and the use of thicker TAFs in certain species for higher-frequency vocalizations. These mechanisms, particularly in humpback and Balaenidae whales, allow for a broader vocal range. Furthermore, the depth at which mysticetes can vocalize is constrained by physiological limits, highlighting the potential impact of anthropogenic noise on their communication, especially in the low-frequency range used for long-distance communication. As scientists continue to investigate anthropogenic drivers in climate change and animal endangerment, this research further underscores the complexity of mysticete vocalization and its vulnerability to human-induced noise.

Krause corpuscles are genital vibrotactile sensors for sexual behaviours

Krause corpuscles, first described in the 1850s, are specialized sensory structures found in the genitalia and mucocutaneous tissues. Despite ongoing advancements in the field, their physiological properties and functions have remained unclear. Therefore, Qi et al (2024) sought to clarify the role of Krause corpuscles in sexual behavior by investigating their anatomical and physiological properties in the mouse clitoris and penis.

Adult male mice were anesthetized and prepared for surgery by securing the spinal column and exposing the L6 dorsal root ganglion (DRG). A 32-channel silicon probe was inserted into the DRG for electrophysiological recordings, while mechanical and optogenetic stimuli were applied to the glans penis and perineal skin to identify penis-innervating neurons. The neuronal responses to mechanical indentation and vibration stimuli were recorded using Aurora Scientific’s 300C-I Dual-Mode Indenter with a custom 200 μm diameter tip. The data was then analyzed for conduction velocities and mechanical thresholds using custom software for data acquisition and spike sorting.

The results revealed that Krause corpuscle afferents in the mouse genitalia are low-threshold, rapidly adapting mechanoreceptors, sensitive to 40–80 Hz vibrations. These afferents transmit vibrotactile signals to the DGC region of the spinal cord, which may relay genital sensory signals to the spinal ejaculation generator (SEG) involved in sexual reflexes. Additionally, Krause corpuscles were present in both male and female genitalia, with a higher density in the clitoris, suggesting sexually dimorphic patterns of innervation. The findings highlight the crucial role of Krause corpuscle afferents in mediating sexual function, providing new insights into the spinal circuits that underlie sexual behavior.

Volatile working memory representations crystallize with practice

Working memory (WM) can be defined as the process of temporarily storing and manipulating information for complex cognitive tasks. Understanding the mechanisms behind the generation of WM neuronal representations over time is of incredible interest in the neuroscience field, particularly during learning and task expertise. While the role of persistent neural activity in WM is well-established, the mechanisms behind its generation and maintenance during learning remain unclear. To investigate this, Bellafard et al (2024) explored how WM representations evolve in the secondary motor cortex (M2) during task learning and how these representations influence task performance.

To explore this, mice were surgically implanted with head-bars, cranial windows, and optical fibers for imaging and optogenetic manipulation. Viruses were used to express optogenetic channels in neurons, and calcium imaging was performed using two-photon microscopy to monitor neuronal activity. Behavioral training involved water-restriction and head fixation while mice learned a delayed-association task. Aurora Scientific’s 200C miniPID was used at the beginning of every training session to monitor the airflow and odor delivery.

Interestingly, late delay WM representations in the M2 initially fluctuated during early expertise but stabilized after continued practice. Moreover, M2 activity disruptions in early stages did not affect the task performance, suggesting that early-stage activity does not drive persistent attractors. On the other hand, disruptions in late-delay and choice epochs impaired performance, highlighting the importance of M2 interactions with other brain regions. These findings suggest that neural representations in M2 evolve with learning, and the variability in neural engagement during early expertise may allow for greater flexibility and adaptability in task performance.

Conclusions

These studies by Elemans et al (2024), Qi et al (2024), and Bellafard et al (2024) explore distinct but interrelated aspects of animal behavior and physiology, from the vocalization mechanisms in baleen whales, the role of Krause corpuscles in sexual behaviors, to the crystallization of volatile working memory representations in the brain. Collectively, these studies provide valuable insights into the complexity of communication, sensory processing, and memory mechanisms across species, emphasizing the connections between biological systems and their roles in adaptive behavior. As we head towards the New Year, the spirit of these intricate investigations lay the foundation for continued scientific advancements.

Follow Aurora Scientific’s journey over the years: Checkout out last year’s end-of-year publication review, ‘Best of 2023: Across Countries and Applications’.