Role of a cholinergic nucleus in processing spatial information in the barn owl midbrain

There is an abundance of sensory information in the world, and a fundamental task of the brain is to preferentially allocate processing resources to the most important stimuli. The modulation of neural processing based on stimulus importance occurs on short-timescales, by enhancing neural responses for stimuli to which an animal is attending, as well as on long-timescales, by expanding neural representations of behaviorally relevant stimuli. This thesis studies the isthmotectal system of the barn owl to investigate how this modulation of neural responses occurs.;The optic tectum (OT) is a structure in the midbrain that functions as a spatial processor. It contains aligned maps of auditory, visual, and motor space, enabling the animal to see or hear a stimulus and generate a movement to orient toward the stimulus. The nucleus isthmi pars parvocellularis (Ipc) shares spatially precise, reciprocal connections with the OT, and is therefore well positioned to modulate OT responses. Further, the Ipc uses acetylcholine, a neurotransmitter that has been implicated in regulating neural responses and plasticity in other systems. The experiments presented here investigate the role of the Ipc in modulating sensory processing in the barn owl OT.;The first study characterized the properties of the Ipc in barn owls. We recorded the activity of neurons in the Ipc, using in vivo electrophysiology, while presenting auditory and visual stimuli from different spatial positions. Visual stimuli were presented on a calibrated screen, and auditory stimuli were presented dichotically through earphones to simulate sounds from different spatial positions. The Ipc has previously been described as a visual processor, but we found that Ipc neurons respond to both auditory and visual stimuli from restricted regions of space. The representations of auditory and visual space are aligned to form a bimodal space map, with neurons in the rostral Ipc responding to auditory or visual stimuli from frontal space, and neurons in the caudal Ipc to stimuli from contralateral space Ipc neurons were tuned for auditory localization cues, but not for tone frequency. We also made tracer injections into the Ipc and OT to study the patterns of connectivity between these structures. We found that the Ipc and OT share reciprocal, spatially-precise, topographic projections, and that the Ipc projection to the OT contacts all layers within a narrow column. Taken together, the electrophysiological and anatomical results indicate that the Ipc encodes stimulus location rather than stimulus features or modality, and activates the OT with a spatially precise signal.;The second study investigated the role of the Ipc in long-timescale modulation of neural representations. It has previously been demonstrated that when owls are chronically exposed to prisms that displace the visual field, the brain compensates for this displacement by shifting auditory tuning to bring auditory and visual representations of space back into mutual alignment. To study the role of the Ipc in this plasticity, we unilaterally lesioned the Ipc in 5 juvenile owls before fitting them with prismatic spectacles. After 8 weeks of prism exposure, we assessed neural plasticity by comparing auditory and visual spatial tuning at sites in the OT. We found that auditory spatial timing shifted adaptively on the lesioned side of the OT in these animals in response to the prisms, indicating that the Ipc is not necessary for this plasticity.;The third study explored the role of the Ipc in short-timescale modulation of neural responses. We briefly activated the Ipc through electrical microstimulation while recording OT responses to auditory stimuli. We found that Ipc microstimulation could increase OT auditory responses at some sites and decrease auditory responses at others. However, the direction of modulation could not be predicted from the relative alignment of the Ipc and OT sites. Further, microstimulation of the Ipc did not systematically affect the timecourse or reliability of OT responses. We conclude that this method of Ipc activation does not modulate OT responses in a consistent manner.;In these three studies, we have seen that the Ipc processes spatial information independent of modality, and is positioned to regulate OT responses. The Ipc is not necessary for plasticity in response to prism exposure, and electrical activation of the Ipc does not modulate OT responses in a predictable manner. Given the complex patterns of cellular connectivity between the Ipc and the OT, we suggest that future studies should study the effects of Ipc inactivation on distinct neuron classes in the OT. There is evidence from a number of species that the nucleus isthmi is involved in representing the location of salient and behaviorally relevant stimuli. Few studies have recorded isthmic activity in awake, behaving animals, and it is likely that much will be learned from identifying specific behavioral tasks that are dependent on Ipc activity.