Retinotopic organization of head-orienting vectors in the pigeon ventral lateral geniculate nucleus

Abstract

The ventral lateral geniculate nucleus (GLv) is a conserved retinorecipient structure positioned between visual and premotor systems, yet its role in visuomotor transformations remains poorly understood. Here, we examined whether the pigeon GLv contains a spatially organized motor representation linking visual-field locations to orienting movements. Anatomical tracing identified descending projections from neurons in the GLv inner lamina (GLv-li) through pretectal and tegmental regions toward pontine targets. GLv-li projection neurons are GABAergic, yet electrical microstimulation of GLv-li in awake pigeons evoked rapid head movements followed by smaller eye movements. Each stimulation locus produced a reproducible characteristic motor vector whose direction and duration were largely insensitive to stimulation parameters, whereas movement amplitude and velocity approached site-specific plateaus. Movements initiated from different head positions were better explained by repetition of a fixed displacement than by convergence toward a common spatial goal. Critically, motor-vector direction was systematically related to the direction of the visual receptive field represented at the same GLv locus, while motor-vector amplitude covaried with receptive-field eccentricity. These defining features persisted after optic tectum ablation. Together, these findings reveal a retinotopically-organized map of fixed-vector head movements in the avian GLv and identify a tectum-independent route through which visual spatial representations can access descending orienting circuitry. SIGNIFICANCE Orienting toward an object requires the brain to translate visual locations into movements. This transformation is classically associated with the optic tectum, or superior colliculus in mammals, but how other visual structures contribute remains less clear. We show that the pigeon ventral lateral geniculate nucleus contains a map of head movements aligned with its established visual map. Head movements evoked by stimulating this nucleus persist after tectal removal, revealing an additional route through which visual spatial representations can access orienting circuits. These findings establish a spatially organized motor role for a conserved inhibitory visual thalamic structure and broaden our understanding of how the brain controls gaze.