Conscious Experience of Stimulus Presence and Absence Is Actively Encoded by Neurons in the Crow Brain.

The emergence of consciousness from brain activity constitutes one of the great riddles in biology. It is commonly assumed that only the conscious perception of the presence of a stimulus elicits neuronal activation to signify a "neural correlate of consciousness," whereas the subjective experience of the absence of a stimulus is associated with a neuronal resting state. Here, we demonstrate that the two subjective states "stimulus present" and "stimulus absent" are represented by two specialized neuron populations in crows, corvid birds. We recorded single-neuron activity from the nidopallium caudolaterale of crows trained to report the presence or absence of images presented near the visual threshold. Because of the task design, neuronal activity tracking the conscious "present" versus "absent" percept was dissociated from that involved in planning a motor response. Distinct neuron populations signaled the subjective percepts of "present" and "absent" by increases in activation. The response selectivity of these two neuron populations was similar in strength and time course. This suggests a balanced code for subjective "presence" versus "absence" experiences, which might be beneficial when both conscious states need to be maintained active in the service of goal-directed behavior.

Convergent Circuit Computation for Categorization in the Brains of Primates and Songbirds.

Categorization is crucial for behavioral flexibility because it enables animals to group stimuli into meaningful classes that can easily be generalized to new circumstances. A most abstract quantitative category is set size, the number of elements in a set. This review explores how categorical number representations are realized by the operations of excitatory and inhibitory neurons in associative telencephalic microcircuits in primates and songbirds. Despite the independent evolution of the primate prefrontal cortex and the avian nidopallium caudolaterale, the neuronal computations of these associative pallial circuits show surprising correspondence. Comparing cellular functions in distantly related taxa can inform about the evolutionary principles of circuit computations for cognition in distinctly but convergently realized brain structures.

Brain regions controlling courtship behavior in the bluehead wrasse.

Bluehead wrasses (Thalassoma bifasciatum) follow a socially controlled mechanism of sex determination. A socially dominant initial-phase (IP) female is able to transform into a new terminal-phase (TP) male if the resident TP male is no longer present. TP males display an elaborate array of courtship behaviors, including both color changes and motor behaviors. Little is known concerning the neural circuits that control male-typical courtship behaviors. This study used glutamate iontophoresis to identify regions that may be involved in courtship. Stimulation of the following brain regions elicited diverse types of color change responses, many of which appear similar to courtship color changes: the ventral telencephalon (supracommissural nucleus of the ventral telencephalon [Vs], lateral nucleus of the ventral telencephalon [Vl], ventral nucleus of the ventral telencephalon [Vv], and dorsal nucleus of the ventral telencephalon [Vd]), parts of the preoptic area (NPOmg and NPOpc), entopeduncular nucleus, habenular nucleus, and pretectal nuclei (PSi and PSm). Stimulation of two regions in the posterior thalamus (central posterior thalamic [CP] and dorsal posterior thalamic [DP]) caused movements of the pectoral fins that are similar to courtship fluttering and vibrations. Furthermore, these responses were elicited in female IP fish, indicating that circuits for sexual behaviors typical of TP males exist in females. Immunohistochemistry results revealed regions that are more active in fish that are not courting: interpeduncular nucleus, red nucleus, and ventrolateral thalamus (VL). Taken together, we propose that the telencephalic-habenular-interpeduncular pathway plays an important role in controlling and regulating courtship behaviors in TP males; in this model, in response to telencephalic input, the habenular nucleus inhibits the interpeduncular nucleus, thereby dis-inhibiting forebrain regions and promoting the expression of courtship behaviors.

Neuroscience of cognitive control in crows.

Crows, a group of corvid songbird species, show superb behavioral flexibility largely stemming from their advanced cognitive control functions. These functions mainly originate from the associative avian pallium that evolved independently from the mammalian cerebral cortex. This article presents a brief overview of cognitive control functions and their neuronal foundation in crows.

Effects of the depletion of neural progenitors by focal X-ray irradiation on song production and perception in canaries.

The song control nucleus HVC of songbirds has emerged as a widespread model system to study adult neurogenesis and the factors that modulate the incorporation of new neurons, including seasonal state, sex differences or sex steroid hormone concentrations. However, the specific function of these new neurons born in adulthood remains poorly understood. We implemented a new procedure based on focal X-ray irradiation to deplete neural progenitors in the ventricular zone adjacent to HVC and study the functional consequences. A 23 Gy dose depleted by more than 50 percent the incorporation of BrdU in neural progenitors, a depletion that was confirmed by a significant decrease in doublecortin positive neurons. This depletion of neurogenesis significantly increased the variability of testosterone-induced songs in females and decreased their bandwidth. Expression of the immediate early gene ZENK in secondary auditory areas of the telencephalon that respond to song was also inhibited. These data provide evidence that new neurons in HVC play a role in both song production and perception and that X-ray focal irradiation represents an excellent tool to advance our understanding of adult neurogenesis.

Categorising: Inside the crow's brain.

Crows can learn to categorise line lengths into experimenter-defined categories. The crow's brain codes and recodes line lengths in individual neurons in the nidopallium caudolaterale, part of the telencephalon.

Categorical representation of abstract spatial magnitudes in the executive telencephalon of crows.

The ability to group abstract continuous magnitudes into meaningful categories is cognitively demanding but key to intelligent behavior. To explore its neuronal mechanisms, we trained carrion crows to categorize lines of variable lengths into arbitrary "short" and "long" categories. Single-neuron activity in the nidopallium caudolaterale (NCL) of behaving crows reflected the learned length categories of visual stimuli. The length categories could be reliably decoded from neuronal population activity to predict the crows' conceptual decisions. NCL activity changed with learning when a crow was retrained with the same stimuli assigned to more categories with new boundaries ("short", "medium," and "long"). Categorical neuronal representations emerged dynamically so that sensory length information at the beginning of the trial was transformed into behaviorally relevant categorical representations shortly before the crows' decision making. Our data show malleable categorization capabilities for abstract spatial magnitudes mediated by the flexible networks of the crow NCL.

Revisiting a Telencephalic Extent of the Ascending Reticular Activating System.

Is the cerebrum involved in its own activation to states of attention or arousal? "Telencephalon" is a term borrowed from embryology to identify not only the cerebral hemispheres of the forebrain, but also the basal forebrain. We review a generally undercited literature that describes nucleus basalis of Meynert, located within the substantia innominata of the ventrobasal forebrain, as a telencephalic extension of the ascending reticular activating formation. Although that formation's precise anatomical definition and localization have proven elusive over more than 70 years, a careful reading of sources reveals that there are histological features common to certain brainstem neurons and those of the nucleus basalis, and that a largely common dendritic architecture may be a morphological aspect that helps to define non-telencephalic structures of the ascending reticular activating formation (e.g., in brainstem) as well as those parts of the formation that are telencephalic and themselves responsible for cortical activation. We draw attention to a pattern of dendritic arborization described as "isodendritic," a uniform (isos-) branching in which distal dendrite branches are significantly longer than proximal ones. Isodendritic neurons also differ from other morphological types based on their heterogeneous, rather than specific afferentation. References reviewed here are consistent in their descriptions of histology, particularly in studies of locales rich in cholinergic neurons. We discuss the therapeutic implications of a basal forebrain site that may activate cortex. Interventions that specifically target nucleus basalis and, especially, the survival of its constituent neurons may benefit afflictions in which higher cortical function is compromised due to disturbed arousal or attentiveness, including not only coma and related syndromes, but also conditions colloquially described as states of cognitive "fog" or of "long-haul" mental compromise.

Neural correlates of cognitively controlled vocalizations in a corvid songbird.

The neuronal basis of the songbird's song system is well understood. However, little is known about the neuronal correlates of the executive control of songbird vocalizations. Here, we record single-unit activity from the pallial endbrain region "nidopallium caudolaterale" (NCL) of crows that vocalize to the presentation of a visual go-cue but refrain from vocalizing during trials without a go-cue. We find that the preparatory activity of single vocalization-correlated neurons, but also of the entire population of NCL neurons, before vocal onset predicts whether or not the crows will produce an instructed vocalization. Fluctuations in baseline neuronal activity prior to the go-cue influence the premotor activity of such vocalization-correlated neurons and seemingly bias the crows' decision to vocalize. Neuronal response modulation significantly differs between volitional and task-unrelated vocalizations. This suggests that the NCL can take control over the vocal motor network during the production of volitional vocalizations in a corvid songbird.

Number selective sensorimotor neurons in the crow translate perceived numerosity into number of actions.

Translating a perceived number into a matching number of self-generated actions is a hallmark of numerical reasoning in humans and animals alike. To explore this sensorimotor transformation, we trained crows to judge numerical values in displays and to flexibly plan and perform a matching number of pecks. We report number selective sensorimotor neurons in the crow telencephalon that signaled the impending number of self-generated actions. Neuronal population activity during the sensorimotor transformation period predicted whether the crows mistakenly planned fewer or more pecks than instructed. During sensorimotor transformation, both a static neuronal code characterized by persistently number-selective neurons and a dynamic code originating from neurons carrying rapidly changing numerical information emerged. The findings indicate there are distinct functions of abstract neuronal codes supporting the sensorimotor number system.

Single-cell Stereo-seq reveals induced progenitor cells involved in axolotl brain regeneration.

The molecular mechanism underlying brain regeneration in vertebrates remains elusive. We performed spatial enhanced resolution omics sequencing (Stereo-seq) to capture spatially resolved single-cell transcriptomes of axolotl telencephalon sections during development and regeneration. Annotated cell types exhibited distinct spatial distribution, molecular features, and functions. We identified an injury-induced ependymoglial cell cluster at the wound site as a progenitor cell population for the potential replenishment of lost neurons, through a cell state transition process resembling neurogenesis during development. Transcriptome comparisons indicated that these induced cells may originate from local resident ependymoglial cells. We further uncovered spatially defined neurons at the lesion site that may regress to an immature neuron-like state. Our work establishes spatial transcriptome profiles of an anamniote tetrapod brain and decodes potential neurogenesis from ependymoglial cells for development and regeneration, thus providing mechanistic insights into vertebrate brain regeneration.

Single-cell analyses of axolotl telencephalon organization, neurogenesis, and regeneration.

Salamanders are tetrapod models to study brain organization and regeneration; however, the identity and evolutionary conservation of brain cell types are largely unknown. We delineated the cell populations in the axolotl telencephalon during homeostasis and regeneration using single-cell genomic profiling. We identified glutamatergic neurons with similarities to amniote neurons of hippocampus, dorsal and lateral cortex, and conserved γ-aminobutyric acid-releasing (GABAergic) neuron classes. We inferred transcriptional dynamics and gene regulatory relationships of postembryonic, region-specific neurogenesis and unraveled conserved differentiation signatures. After brain injury, ependymoglia activate an injury-specific state before reestablishing lost neuron populations and axonal connections. Together, our analyses yield insights into the organization, evolution, and regeneration of a tetrapod nervous system.

Cell-type profiling in salamanders identifies innovations in vertebrate forebrain evolution.

The evolution of advanced cognition in vertebrates is associated with two independent innovations in the forebrain: the six-layered neocortex in mammals and the dorsal ventricular ridge (DVR) in sauropsids (reptiles and birds). How these innovations arose in vertebrate ancestors remains unclear. To reconstruct forebrain evolution in tetrapods, we built a cell-type atlas of the telencephalon of the salamander Pleurodeles waltl. Our molecular, developmental, and connectivity data indicate that parts of the sauropsid DVR trace back to tetrapod ancestors. By contrast, the salamander dorsal pallium is devoid of cellular and molecular characteristics of the mammalian neocortex yet shares similarities with the entorhinal cortex and subiculum. Our findings chart the series of innovations that resulted in the emergence of the mammalian six-layered neocortex and the sauropsid DVR.

Neuronal satellitosis is a common finding in the avian brain.

Perineuronal or neuronal satellitosis is the term describing the presence of glial cells in the satellite space surrounding the neuronal perikaryon. Confusingly, this finding has been described both as a physiologic and pathologic condition in humans and animals. In animals, neuronal satellitosis has been described in mammals, as well as in avian species. For the latter, the authors wondered whether neuronal satellitosis is expressed in the normal telencephalon of different avian orders and families and whether this pattern in different species shows a specific brain-region association. For these aims, this study explored the presence of neuronal satellitosis in the major areas of the healthy telencephalon in wild and domestic avian species of different orders and families, evaluating its grade in different brain regions. Neuronal satellitosis was seen in the hyperpallium and mesopallium as areas with the highest grade. Passeriformes showed the highest grade of neuronal satellitosis compared to diurnal or nocturnal raptors, and Charadriiformes. To clarify the exact role of neuronal satellitosis in animals without neurological disease, further studies are needed.RESEARCH HIGHLIGHTSNeuronal satellitosis is a common finding in the healthy avian telencephalon.Neuronal satellitosis is a species- and brain-region-associated finding in birds.Passeriformes have the highest grade of neuronal satellitosis.

High associative neuron numbers could drive cognitive performance in corvid species.

Corvids possess cognitive skills, matching those of nonhuman primates. However, how these species with their small brains achieve such feats remains elusive. Recent studies suggest that cognitive capabilities could be based on the total numbers of telencephalic neurons. Here we extend this hypothesis further and posit that especially high neuron counts in associative pallial areas drive flexible, complex cognition. If true, avian species like corvids should specifically accumulate neurons in the avian associative areas meso- and nidopallium. To test the hypothesis, we analyzed the neuronal composition of telencephalic areas in corvids and noncorvids (chicken, pigeons, and ostriches-the species with the largest bird brain). The overall number of pallial neurons in corvids was much higher than in chicken and pigeons and comparable to those of ostriches. However, neuron numbers in the associative mesopallium and nidopallium were twice as high in corvids and, in correlation with these associative areas, the corvid subpallium also contained high neuron numbers. These findings support our hypothesis that large absolute numbers of associative pallial neurons contribute to cognitive flexibility and complexity and are key to explain why crows are smart. Since meso-/nidopallial and subpallial areas scale jointly, it is conceivable that associative pallio-striatal loops play a similar role in executive decision making as described in primates.

Neural oscillations are locked to birdsong rhythms in canaries.

How vocal communication signals are represented in the cortex is a major challenge for behavioural neuroscience. Beyond a descriptive code, it is relevant to unveil the dynamical mechanism responsible for the neural representation of auditory stimuli. In this work, we report evidence of synchronous neural activity in nucleus HVC, a telencephalic area of canaries (Serinus canaria), in response to auditory playback of the bird's own song. The rhythmic features of canary song allowed us to show that this large-scale synchronization was locked to defined features of the behaviour. We recorded neural activity in a brain region where sensorimotor integration occurs, showing the presence of well-defined oscillations in the local field potentials, which are locked to song rhythm. We also show a correspondence between local field potentials, multiunit activity and single unit activity within the same brain region. Overall, our results show that the rhythmic features of the vocal behaviour are represented in a telencephalic region of canaries.

Behavioral and Neuronal Representation of Numerosity Zero in the Crow.

Different species of animals can discriminate numerosity, the countable number of objects in a set. The representations of countable numerosities have been deciphered down to the level of single neurons. However, despite its importance for human number theory, a special numerical quantity, the empty set (numerosity zero), has remained largely unexplored. We explored the behavioral and neuronal representation of the empty set in carrion crows. Crows were trained to discriminate small numerosities including the empty set. Performance data showed a numerical distance effect for the empty set in one crow, suggesting that the empty set and countable numerosities are represented along the crows' "mental number line." Single-cell recordings in the endbrain region nidopallium caudolaterale (NCL) showed a considerable proportion of NCL neurons tuned to the preferred numerosity zero. As evidenced by neuronal distance and size effects, NCL neurons integrated the empty set in the neural number line. A subsequent neuronal population analysis using a statistical classifier approach showed that the neuronal numerical representations were predictive of the crows' success in the task. These behavioral and neuronal data suggests that the conception of the empty set as a cognitive precursor of a zero-like number concept is not an exclusive property of the cerebral cortex of primates. Zero as a quantitative category cannot only be implemented in the layered neocortex of primates, but also in the anatomically distinct endbrain circuitries of birds that evolved based on convergent evolution.SIGNIFICANCE STATEMENT The conception of "nothing" as number "zero" is celebrated as one of the greatest achievements in mathematics. To explore whether precursors of zero-like concepts can be found in vertebrates with a cerebrum that anatomically differs starkly from our primate brain, we investigated this in carrion crows. We show that crows can grasp the empty set as a null numerical quantity that is mentally represented next to number one. Moreover, we show that single neurons in an associative avian cerebral region specifically respond to the empty set and show the same physiological characteristics as for countable quantities. This suggests that zero as a quantitative category can also be implemented in the anatomically distinct endbrain circuitries of birds that evolved based on convergent evolution.

A role for sustained MAPK activity in the mouse ventral telencephalon.

The MAPK pathway is a major growth signal that has been implicated during the development of progenitors, neurons, and glia in the embryonic brain. Here, we show that the MAPK pathway plays an important role in the generation of distinct cell types from progenitors in the ventral telencephalon. Our data reveal that phospho-p44/42 (called p-ERK1/2) and the ETS transcription factor Etv5, both downstream effectors in the MAPK pathway, show a regional bias in expression during ventral telencephalic development, with enriched expression in the dorsal region of the LGE and ventral region of the MGE at E13.5 and E15.5. Interestingly, expression of both factors becomes more uniform in ventricular zone (VZ) progenitors by E18.5. To gain insight into the role of MAPK activity during progenitor cell development, we used a cre inducible constitutively active MEK1 allele (RosaMEK1DD/+) in combination with a ventral telencephalon enriched cre (Gsx2e-cre) or a dorsal telencephalon enriched cre (Emx1cre/+). Sustained MEK/MAPK activity in the ventral telencephalon (Gsx2e-cre; RosaMEK1DD/+) expanded dorsal lateral ganglionic eminence (dLGE) enriched genes (Gsx2 and Sp8) and oligodendrocyte progenitor cell (OPC) markers (Olig2, Pdgfrα, and Sox10), and also reduced markers in the ventral (v) LGE domain (Isl1 and Foxp1). Activation of MEK/MAPK activity in the dorsal telencephalon (Emx1cre/+; RosaMEK1DD/+) did not initially activate the expression of dLGE or OPC genes at E15.5 but ectopic expression of Gsx2 and OPC markers were observed at E18.5. These results support the idea that MAPK activity as readout by p-ERK1/2 and Etv5 expression is enriched in distinct subdomains of ventral telencephalic progenitors during development. In addition, sustained activation of the MEK/MAPK pathway in the ventral or dorsal telencephalon influences dLGE and OPC identity from progenitors.

Neural Code of Motor Planning and Execution during Goal-Directed Movements in Crows.

The planning and execution of head-beak movements are vital components of bird behavior. They require integration of sensory input and internal processes with goal-directed motor output. Despite its relevance, the neurophysiological mechanisms underlying action planning and execution outside of the song system are largely unknown. We recorded single-neuron activity from the associative endbrain area nidopallium caudolaterale (NCL) of two male carrion crows (Corvus corone) trained to plan and execute head-beak movements in a spatial delayed response task. The crows were instructed to plan an impending movement toward one of eight possible targets on the left or right side of a touchscreen. In a fraction of trials, the crows were prompted to plan a movement toward a self-chosen target. NCL neurons signaled the impending motion direction in instructed trials. Tuned neuronal activity during motor planning categorically represented the target side, but also specific target locations. As a marker of intentional movement preparation, neuronal activity reliably predicted both target side and specific target location when the crows were free to select a target. In addition, NCL neurons were tuned to specific target locations during movement execution. A subset of neurons was tuned during both planning and execution period; these neurons experienced a sharpening of spatial tuning with the transition from planning to execution. These results show that the avian NCL not only represents high-level sensory and cognitive task components, but also transforms behaviorally-relevant information into dynamic action plans and motor execution during the volitional perception-action cycle of birds.SIGNIFICANCE STATEMENT Corvid songbirds have become exciting new models for understanding complex cognitive behavior. As a key neural underpinning, the endbrain area nidopallium caudolaterale (NCL) represents sensory and memory-related task components. How such representations are converted into goal-directed motor output remained unknown. In crows, we report that NCL neurons are involved in the planning and execution of goal-directed movements. NCL neurons prospectively signaled motion directions in instructed trials, but also when the crows were free to choose a target. NCL neurons showed a target-specific sharpening of tuning with the transition from the planning to the execution period. Thus, the avian NCL not only represents high-level sensory and cognitive task components, but also transforms relevant information into action plans and motor execution.

Olfaction in Lamprey Pallium Revisited-Dual Projections of Mitral and Tufted Cells.

The presence of two separate afferent channels from the olfactory glomeruli to different targets in the brain is unravelled in the lamprey. The mitral-like cells send axonal projections directly to the piriform cortex in the ventral part of pallium, whereas the smaller tufted-like cells project separately and exclusively to a relay nucleus called the dorsomedial telencephalic nucleus (dmtn). This nucleus, located at the interface between the olfactory bulb and pallium, in turn projects to a circumscribed area in the anteromedial, ventral part of pallium. The tufted-like cells are activated with short latency from the olfactory nerve and terminate with mossy fibers on the dmtn cells, wherein they elicit large unitary excitatory postsynaptic potentials (EPSPs). In all synapses along this tufted-like cell pathway, there is no concurrent inhibition, in contrast to the mitral-like cell pathway. This is similar to recent findings in rodents establishing two separate exclusive projection patterns, suggesting an evolutionarily conserved organization.