Research in my lab investigates the neural basis of animal behavior.
More specifically, we focus on identifying the structure and function of neural circuits that control discrete behaviors in crayfish. Because the nervous system of crayfish features similar neurophysiological and neurochemical mechanisms compared to organisms of higher complexity, but allows relevant analysis at the cellular and circuit level, they are our primary model for neurobehavioral research.
Crayfish display easily quantifiable behavioral patterns controlled by a nervous system of tractable complexity with large "command-like" neurons that are accessible for a variety of experimental approaches (Sullivan & Herberholz 2013). We are investigating how these neurons and neural circuits orchestrate behavioral action in specific behavioral contexts. Below are two examples of ongoing research, which center on decision-making processes and behavioral choice.
1. Intraspecific aggression and social dominance
Most social animals, including crayfish and other crustaceans, compete aggressively for resources such as food, shelter and mates (Herberholz 2014). The early stages of an encounter between well-matched crayfish are marked by an aggressive escalation that can include strikes, aggressive posture, grappling with claws, and bouts of offensive tail-flips (Herberholz et al. 2001, Edwards & Herberholz 2005). At some point, fighting in crayfish is interrupted by an abrupt change in the agonistic behavior of one animal as it switches from aggressive to submissive behaviors. This switch marks the change in the relationship among the two animals and unambiguously identifies the new subordinate (Herberholz et al. 2001). Dominant and subordinate crayfish then show clear differences in their behavior. The dominant animal displays a dominant posture, initiates approaches and attacks, constructs a shelter and claims first access to most resources, while the subordinate displays a submissive posture, retreats and escapes from the dominant, and is left with unwanted resources (Herberholz et al. 2001, Edwards et al. 2003, Herberholz et al. 2003, Song et al. 2006, Herberholz et al. 2007).
We found that the stability of dominance relationships is highly context-dependent. A small change in the social environment, i.e. the brief introduction of another, larger crayfish to the pair quickly disrupts dominance relationships and facilitates reversals of social status (Graham & Herberholz 2009). After both animals (the former dominant and subordinate) are defeated by the intruder crayfish, and the original pair reforms its social relationship after removal of the intruder, half of all animals reversed social status. This shows that social status is a more transient condition than previously assumed, and (temporarily) accepting subordinate status might be beneficial under certain circumstances. More recently, we discovered that for dominance reversals to occur, both members of the pair (the dominant and subordinate) must be defeated by intruder in the presence of each other. If they are defeated in separate tanks and reunited afterwards, their dominance relationships remains stable. This is an interesting results because it suggests that the subordinate must witness the defeat of the dominant to challenge it in subsequent encounters (Herberholz et al. 2016).
In our attempt to understand the behavioral mechanisms underlying social dominance, we use video analysis to identify aggressive and submissive behaviors that are expressed during agonistic interactions in pairs and groups of crayfish. Along with our behavioral investigations, we are interested to understand the neural mechanisms underlying the decision to "give up", i.e. to terminate a fight and accept subordinate status, and we plan to apply neurophysiological and neurochemical methods to answer this important question.
2. Anti-predator behavior
When attacked by a natural predator (e.g., dragonfly nymphs), juvenile crayfish produce three different forms of escape tail-flips controlled by as many neural circuits (Herberholz et al. 2004a). Two of the tail-flips are mediated by giant neurons (medial giant and lateral giant) that evoke stereotyped, reflexive escape responses away from the stimulus. One is mediated by non-giant circuitry that produces tail-flips of less stereotyped forms with longer response latencies. The neural circuits that control the tail-flip behavior are only partially understood (Herberholz et al. 2002, Antonsen et al. 2005, Herberholz 2009). We are interested in how the different escape circuits integrate sensory signals and how they interact with each other to produce effective behavioral outputs (Liu & Herberholz 2010).
By combining video- and electrophysiological recordings, we can measure behavioral and neural responses to simulated predatory attacks. We expose juvenile crayfish to moving shadows while the animals are searching for food. In response to approaching shadows, each crayfish produces one of two discrete behavioral outputs: it either tail-flips backwards by rapid flexion of its abdomen, or it immediately stops forward locomotion and freezes in place (Liden & Herberholz 2008).
By using a non-invasive technique to record neural activity (Herberholz et al. 2001, Herberholz et al. 2004a, Herberholz 2009) we were able to identify the "decisions"- neurons (the medial giant [MG] interneurons) that control the tail-flips produced in response to shadows. When exposed to different predator (shadow) signals combined with food incentives of different qualities, or when testing crayfish of different internal energy states (e.g., more or less hungry), we found that crayfish make suprisingly complex decisions by carefully balancing potential gain (finding a meal) and risk (becoming a meal); this identifies crayfish as a new and important model to study value-based decision making (Liden et al. 2010, Schadegg & Herberholz 2017). The different components of the MG circuit are accessible for intracellular electrophysiology, and we are planning to test how the threshold for this circuit is regulated. One possibility is that threshold changes are mediated through monoaminergic modulation (Herberholz & Marquart 2012). Crayfish share common neuromodulators such as serotonin, dopamine, and octopamine (the invertebrate homologue of noradrenalin) with other organisms, and these neurochemicals have been shown to affect the excitability of crayfish neurons and circuits (Herberholz 2013).
To identify discrete patterns of neural activity that are correlated with decision-making processes, we have use manganese-enhanced Magnetic Resonance Imaging (MEMRI). Manganese, a paramagnetic contrast agent and calcium analog, can highlight active brain areas. Preliminary experiments with live crayfish have shown that they are well suited for MEMRI studies because they have no blood-brain barrier and can easily be restrained within the imaging apparatus. Crayfish tolerate long imaging sessions in high magnetic fields, and they can be placed in small imaging coils which results in images of very high spatial resolution. We have previously demonstrated the feasibility of MEMRI for identification and reconstruction of neural structures in crayfish; thus, manganese can be used as a contrast agent for crayfish neural tissue (Herberholz et al. 2004b). Recently we found that MEMRI can also be used to visualize functional uptake of manganese into brain areas of live juvenile crayfish. Stimulation of one of the animals' antennae led to localized unilateral labeling in areas of the brain that received input from the stimulated antenna. This shows that MEMRI has the potential to allow identification of stimulus-evoked neural activity in live animals at near single-cell resolution (Herberholz et al. 2011).
3. Alcohol intoxication
Alcohol has devastating effects on human health and can lead to immediate and long-term changes in the nervous system. However, the cellular-molecular mechanisms underling alcohol intoxication are still poorly understood because alcohol (contrary to other drugs of abuse) interacts with multiple neurotransmitter systems. We found that juvenile crayfish are behaviorally sensitive to alcohol exposure and exhibit stages of hyperexcitability that are followed by sedation, which is very similar to other intoxicated organisms (see image below). One of the early stages of intoxication is characterized by spontaneous tail-flipping, which implies disinhibition of the tail-flip circuits. Interestingly, we found that socially housed juvenile crayfish have higher sensitivity to alcohol than socially isolated animals, and this difference can be observed behaviorally as well as on the level of individual neurons, the LG tail-flip interneurons (Swierzbinski et al. 2017). We have now begun to look into the underlying mechanisms for this intriguing interplay between social history, alcohol, and nervous system function.
4. Other ongoing projects
Besides the projects describe above, members of the lab study inhibitory mechanisms that suppress escape circuit activation. Most of this work is focused on the lateral giant (LG) neurons and types of fast auto-inhibition as well as the inhibitory processes between the two giant neuron systems. We are also interested in sensory-motor integration that takes place in (non-tail-flip) neural circuits that are activated during escape behavior.
Selected media reports
California Academy of SciencesThe Diamondback
National Science Foundation
Effects of alcohol
Maryland Student Researchers in the Maryland Center for Undergraduate
Junior and seniors can earn academic credit for research participation (maximum of 9 credits) through PSYC479 or BSCI399.