Optic Glomeruli and Their Inputs inDrosophilaShare an Organizational Ground Pattern with the Antennal Lobes

Abstract

Studying the insect visual system provides important data on the basic neural mechanisms underlying visual processing. Similar to vertebrates, the first step of visual processing in insects is through a series of retinotopic neurons. Recent studies on flies have found that these converge onto assemblies of columnar neurons in the lobula, the axons of which segregate to project to discrete optic glomeruli in the lateral protocerebrum. This arrangement is much like the fly’s olfactory system, in which afferents target uniquely identifiable olfactory glomeruli. Here, whole-cell patch recordings show that even though visual primitives are unreliably encoded by single lobula output neurons due to high synaptic noise, they are reliably encoded by the ensemble of outputs. At a glomerulus, local interneurons reliably code visual primitives, as do projection neurons conveying information centrally from the glomerulus. These observations demonstrate that in Drosophila, as in other dipterans, optic glomeruli are involved in further reconstructing the fly’s visual world. Optic glomeruli and antennal lobe glomeruli share the same ancestral anatomical and functional ground pattern enabling reliable responses to be extracted from converging sensory inputs.

Responses ofDrosophilagiant descending neurons to visual and mechanical stimuli

“…1A). Their labeling, together with that of the GDN, is consistent with previous results in Drosophila (Phelan et al, 1996;Mu et al, 2012a) and Musca (Bacon and Strausfeld, 1986).…”

“…In our setup, the looming stimulus expands at a constant speed and could therefore be considered as the last moment of an approaching object. As reported previously (Mu et al, 2012a), an expanding black-square stimulus initiated a depolarization in the GDN, which, in different preparations, showed variable adaptation (Fig. 3A).…”

“…Although in an approximation of the natural sensory environment, our looming stimuli did not elicit a spiking response by the GDN, the neuron could, however, be activated at subthreshold levels by dark expanding stimuli (Mu et al, 2012a). This activation is almost certainly not a response to looming, but to luminance decrease, likely mediated by the same pathway as that for light-off responses.…”

“…Possible candidates are other members of the GDN cluster that comprise a system of at least eight parallel channels, which with the GDN likely share at least the same lobula-derived inputs (Strausfeld and Bacon, 1983;Milde and Strausfeld, 1990). More recent studies on the Drosophila GDN have shown that looming visual stimuli elicit subthreshold responses (Mu et al, 2012a), as do acoustic stimuli detected by the antenna's Johnston's organ (Tootoonian et al, 2012) or direct mechanical stimulation of the antenna (Lehnert et al, 2013). These subthreshold GDN responses were elicited in restrained flies, but in a preparation in which the mesothoracic legs were left free to elicit the escape jump, a looming stimulus did initiate single GDN spikes on some occasions (Von Reyn and Card, 2012).…”

A single-cell level and connectome-derived computational model of the Drosophila brain

“…Adopting a multi-compartmental model will also help to improve the accuracy of the simulations (Günay et al 2015). Moreover, some neurons in the visual system conduct signals by graded potentials or by mixed graded and action potentials (Mu et al 2012;Baden et al 2013). Although in the current study we only investigated the resting state activity of the model brain without visual stimulus, it is important to identify those non-spiking neurons in our sample and choose models that correctly represent their response properties in the future study which involves visual responses of the brain.…”