Exploring the role of the pulvinar-cortical interactions in vision: a tale of maps, loops and gates

Abstract

The pulvinar is a dorsal thalamic nucleus strongly associated with the visual system, but of uncertain function. One of the central mysteries of the pulvinar is the direction and quality of the information sent from the cortex to the pulvinar and back along a set of projections called cortico-thalamo-cortical loops. Sherman and Guillery (1998) have proposed a methodology for classifying projections from one brain area to another. They suggest that glutamatergic projections may be classified as either drivers (fast-acting projections conveying the main message) or modulators (slow acting projections that only modify the message). In this thesis, the direction, content and organization of information transmitted through the primate pulvinar was investigated using the bush baby as a model species. Single and multiunit electrophysiological recordings were used to define areas in the pulvinar that contain visuotopic maps. Injections in these electrophysiologically defined areas were used to manipulate the activity of pulvinar cells or deliver neural tract tracers. Single and multiunit recordings were performed in the primary visual cortex (V1) while activity in connected neurons in pulvinar was silenced or increased. Immunohistochemical stains were performed on brain sections and used to in conjunction with light microscopy to analyze morphology in the thalamus and cortex. Our findings indicate that the bush baby visual pulvinar is organized into two complete retinotopic maps. Cells in these mapped areas project to the visual cortex including V1 and the secondary visual cortex (V2). Pulvinar projections to V1 have the anatomical features of modulators while pulvinar projections to V2 have the anatomical features of drivers. The pulvinar can exert a type of strong modulatory control over V1 layer II/III cells that suggests that the pulvinar may gate V1 output. Taken together, these data show that the pulvinar plays an active role in the flow of visual information between cortical areas.