Distinct neural circuits underlie assessment of a diversity of natural dangers by American crows

Abstract

Social animals encountering natural dangers face decisions such as whether to freeze, flee or harass the threat. The American crow, Corvus brachyrhynchos, conspicuously mobs dangers. We used positron emission tomography to test the hypothesis that distinct neuronal substrates underlie the crow's consistent behavioural response to different dangers. We found that crows activated brain regions associated with attention and arousal (nucleus isthmo-opticus/locus coeruleus), and with motor response (arcopallium), as they fixed their gaze on a threat. However, despite this consistent behavioural and neural response, the sight of a person who previously captured the crow, a person holding a dead crow and a taxidermy-mounted hawk activated distinct forebrain regions (amygdala, hippocampus and portion of the caudal nidopallium, respectively). We suggest that aspects of mobbing behaviour are guided by unique neural circuits that respond to differences in mental processing-learning, memory formation and multisensory discrimination-required to appropriately nuance a risky behaviour to specific dangers.

Keywords: fear; memory; whole brain image.

Figure 1.

Sequence of steps in the experimental protocol. Crows learned two faces during their approximately one month long tenure in captivity: the face of the person who captured them (threatening face) and the face of the person who fed and cared for them (caring face). During stimulation, as crows metabolized a previous interperitoneal injection of (F-18) fluorodeoxyglucose (FDG), they saw these faces, another person never seen before (novel) holding or not holding a taxidermy-mounted crow in prone (dead) position, or a taxidermy-mounted red-tailed hawk whose head moved. Rubber masks molded from actual people were used to create faces so that the same face could be randomly assigned as either threatening, caring or novel for each crow. Each of the 25 crows only saw one of the six possible stimuli (n = 5 threatening face, n = 4 caring face, n = 5 novel face, n = 4 novel face holding dead crow, n = 4 hawk, n = 3 room without any person or taxidermy mount). After stimulation, crows were anaesthetized, and the distribution and relative concentration of FDG throughout the whole brain was assessed. After sufficient radioactive decay (24 h), crows were returned to the wild.

Figure 2.

A fixed stare (reduced blinking), one behavioural response to dangerous situations observed in nature and in the laboratory, was associated with activation of three brain regions. (a) Voxel-wise correlations converted to Z-score maps are superimposed to a composite (n = 4 birds) structural MRI of the crow brain. Voxels with Z > 1.64 are coloured; those with Z > 3.8 (3.6 for small structures hypothesized a priori to be activated by stimulus) are considered significant with associated structures as indicated (BS: dorsal brainstem area possibly, including nucleus isthmo-opticus and locus coeruleus, Z = 4.3; N/M: nidopallium/mesopallium, Z = 3.8; A: arcopallium, Z = 3.7). (b–d) Individual values for normalized (global) uptake in each structure that met the threshold for statistical significance on Z-score voxel-wise mapping. The scatterplots we present describe the individual covariation between neural activity and blinking by all individual crows. Symbol type further describes covariation within and between treatments. Correlation coefficients describe the magnitude of covariation (effect size) between neural activity and behaviour at the brain location where the effect was greatest. (b) Differential activation of the brainstem (BS, r = −0.82) related to subject rate of blinking in each experiment (type is indicated by symbol) where blinking could be observed. (c) Differential activation of the nidopallium/mesopallium (N/M, r = −0.76) in relation to blinking by individual crows. (d) Differential activation of the arcopallium (A, r = −0.74) in relation to blinking by individual crows.

Figure 3.

Differential brain activation patterns of crows show dangerous versus neutral stimuli. (a) Voxel-wise subtractions converted to Z-score maps are superimposed to a composite (n = 4 birds) structural MRI of the crow brain. Top row shows the activation pattern of crows viewing a threatening face previously associated with capture versus a group shown an empty room; second row indicates crows viewing a never before seen person holding a taxidermy mount of a dead crow versus a new person and no crow (electronic supplementary material, figure S1 confirms that regions activated differentially by a person holding a dead crow relative to empty room were the same as those indicated here); bottom row indicates the activation pattern of crows shown a taxidermy-mounted red-tailed hawk versus empty room. Coronal slices, from anterior to posterior, with voxels coloured (Z > 1.64) to indicate heightened activation; those with Z > 3.8 (3.6 for small structures hypothesized a priori to be activated by stimulus) are considered significant with associated structures as indicated. Abbreviations are as defined for (b–d) below. (b–d) Individual values for normalized (global) uptake in each structure that met the threshold for statistical significance on Z-score voxel-wise mapping. Horizontal lines indicate group mean. Individual data points describe variation within and between treatments at VOIs centred on peak activation coordinates. Z-values describe magnitude of the largest differences between treatment and baseline conditions determined during voxel-wise mapping. Percentage increase at each peak in activation describes the greatest average differences in FDG uptake (effect size) between crows that viewed the stimulus and those that viewed the appropriate baseline. (b) Activated structures for threatening face: A/TnA: arcopallium/nucleus taeniae of the amygdala, 11% increased Z = 4.4; BS: dorsal brainstem including nucleus isthmo-opticus and locus coeruleus, 5.9% increased, Z = 4.3; N/M: nidopallium/mesopallium, 12.9% increased, Z = 4.1; N: nidopallium, 10.6% increased, Z = 3.9. (c) Activated structures for new person and dead crow versus new person and no crow: M/HA: mesopallium/apical part of the hyperpallium, 8% increased, Z = 4.0; TeO: optic tectum, 19% increased Z = 3.9; Cb: cerebellum, 7% increased, Z = 3.8; Hp/NCM: hippocampus/caudomedial nidopallium, 13% increased, Z = 3.6 (borderline significance based on a priori prediction). (d) Activated structures for red-tailed hawk: N/M, 7% increased, Z = 4.1; HA, 9% increased, Z = 4.0; NC: caudal nidopallium, 17% increased, Z = 4.2; NCL: caudolateral nidopallium, 7% increased, Z = 4.1.

Figure 4.

Sagittal view of difference in crow brain activation at the sight of a novel person holding a dead crow versus a novel person without a dead crow. This view illustrates the distinct activation of the region, including the hippocampus and the caudomedial nidopallium (Hp/NCM) and the cerebellum (Cb), that is less apparent in the coronal sections of figure 3a, middle row.

Figure 5.

Possible visual input routes to structures in the brain of a crow stimulated by the sight of three dangers: (a) a person who has been learned as threatening, (b) a novel person holding a dead crow, and (c) a hawk that frequently preys on adult and nestling crows. A, arcopallium; BG, basal ganglia; BS, brain stem; Cb, cerebellum; DLG, dorsal lateral geniculate nucleus of the thalamus; E, entopallium; HA, hyperpallium apicale; Hp, hippocampus; M, mesopallium; N, nidopallium; NC, caudal nidopallium; NCL, caudolateral nidopallium; Rt, nucleus rotundus of the thalamus; TeO, optic tectum. (Online version in colour.)