The Superior Colliculus and Visual Thalamus

Abstract

Understanding how the brain uses the information it receives from the eye has been a primary subject of inquiry for a host of laboratories. We now know that the impulses generated in the retina are transmitted to a variety of subcortical nuclei, each designed to evaluate a specific aspect of the visual information present within a visual scene. For example, some of these nuclei sense levels of ambient light to control reflex responses and diurnal rhythms. But the neural computations that give rise to what we normally think of as “seeing,” including the ability to detect, orient to, and analyze elements of a visual scene, are carried out by numerous brain regions dedicated to various aspects of these functions. Chief among them are the superior colliculus, a midbrain structure that transforms sensory (including non-visual) signals into the motor commands for orienting, and the lateral geniculate nucleus, a thalamic structure that serves as the entry point into the cascade of areas dedicated to processing visual features. What has been learned about these two areas, and about how they are specialized to perform their distinct sensory functions, is discussed in depth.

While the eye has long been understood to be the basic organ for seeing, an understanding of how the brain processes the information it receives from the eye is comparatively new. The “seeing” part of the eye, the retina, transforms light into neural impulses. These impulses are transmitted to several subcortical sites that specialize in different features of the visual scene. For example, some sites include nuclei that sense the presence of light. These include the pretectum, which controls the pupillary reflex on the basis of ambient light, and the hypothalamus, which uses light signals to set the phases of day–night (i.e., circadian) rhythms.

Two other nuclei are involved in a more detailed analysis of visual information, as well as in the further encoding of information for use by the visual cortex. This chapter will consider each of these in turn. The superior colliculus (SC) primarily regulates eye and head movements and is a site for the merging of information from different senses to best perform this function. The lateral geniculate nucleus (LGN) (sometimes referred to as the lateral geniculate body, LGB) is the thalamic portion of the “primary” visual pathway that accesses the primary visual cortex (also referred to as striate cortex) for the detailed analysis of the visual scene. While both the SC and LGN are involved in visual perception, each is specialized for a unique function.

The visual process begins at the level of the photoreceptors in the retina but is finalized in the retina by the ganglion cells. These are the retina’s output cells. Their specialized properties are evident in their center/surround receptive field organization, with the circular surround working in opposition to the center, which is of the opposing sign, a function referred to as “opponent.” Either the center or the surround is sensitive to light onset and the other to light offset (see section on Retina).

Apart from the major “On–Off” center-surround opponent organization, the retinas of both primates and carnivores (i.e., the monkey and cat are two major models of visual system function) have at least three different cell classes, with unique functional properties. These are called by different names in monkey and in cat, but there are similarities between them that are useful to bear in mind as we consider the next stages of visual processing. In monkeys, these cell types include the P cells, M cells, and K cells. P cells (for parvocellular, or small cell) contribute to the analysis of visual form and (like X cells in the cat) show sustained responses to a visual stimulus applied to the receptive field of the neuron. M (for magnocellular, or large cell) cells (like their Y cell counterparts in cat) respond well to rapidly moving stimuli and have large receptive fields. Koniocellular (K) cells (like W cells in cat) possess complex receptive field properties that are not yet fully understood. Finally, color is represented in primate systems by color opponent center-surround mechanisms that are particularly strong in P cells. In contrast, the cat retina is far less sensitive to color.

These physiologically distinctive classes of retinal cells have their own patterns of central projections. Generally, W- and Y-like cells in the cat, and their primate counterparts, project to different layers of the SC, while all three classes send information to the LGN for more detailed analysis. We will first examine unique features of each subcortical structure, followed by a consideration of how they interact to underlie normal vision.