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Development of Vision in Infancy

The limited behavioral repertoire of the infant and the impossibility of instructing the test subject have made it necessary for vision scientists interested in human visual development to adapt the classical methods of psychophysics and electrophysiology for use with infants and pre-verbal children. First these methodological adaptations and their interpretation in the context of a hierarchical model of visual processing are considered. Within this framework, the…

Temporal Properties of Vision

Our visual perception arises from the interpretation of light information, which varies in space, wavelength, and time. It is the latter of these attributes that is explored in this chapter. Subjectively, the world appears to be stable despite continuous changes in the visual scene. How does the visual system respond to and interpret light variations that occur as a function of time? The duration of a…

Binocular Vision

Introduction Humans and other animals with frontally located eyes attain binocular vision from the two retinal images through a series of sensory and motor processes that culminate in the perception of singleness and stereoscopic depth. Keen stereopsis is considered the paramount consequence of binocular vision because optimal performance depends on the normal functioning of all of the underlying vision processes, including central fixation with normal visual…

The Visual Field

Introduction Perimetry and visual field testing have been used as diagnostic procedures for evaluation of visual function for nearly 200 years, and a historical review of these techniques may be found in several publications. During this period of time, a variety of new techniques have been developed, although the standard clinical procedure of detecting a small white target superimposed on a uniform background at different peripheral…

Color Vision

The ability to perceive color is a highly valued sensory capacity, and it has been a subject of experimental inquiry for over 200 years. Prior to the development of modern biological techniques, breakthroughs in color vision research stemmed from careful consideration of perceptual experiences. From color-mixing and matching experiments it was deduced that three different receptors in the eye, each maximally sensitive to a different region…

Visual Acuity

Visual acuity is a measure of the keenness of sight. The Egyptians used the ability to distinguish double stars as a measure of visual acuity more than 5000 years ago. Over the centuries visual acuity has been studied, measured and analyzed because it represents a fundamental limit in our ability to see. Consequently, visual acuity has been used as a criterion for military service and various…

Early Processing of Spatial Form

Introduction Vision is our most developed sense and unsurprisingly a substantial amount of brain processing is devoted to it, with over half the primate brain involved in vision-related processing. A first step in understanding the nature of this processing involves an appreciation of the capabilities and specialization that define human vision. Such an understanding of our overall visual strengths and weaknesses provides a framework within which…

Extrastriate Visual Cortex

What is extrastriate visual cortex? The term “extrastriate” refers to all visually responsive cortex other than primary visual (striate) cortex, and does not receive strong direct projections from the lateral geniculate nucleus (LGN) (see Chapter 30 ). A central hypothesis in studying extrastriate cortex is that it is composed of discrete cortical areas which can be identified by histology, retinotopic mapping, patterns of connections with other…

Processing in the Primary Visual Cortex

Overview: The primary visual cortex constructs local image features The visual system provides a description of the location and identification of objects that have survival value to the species. This description must be made in a dynamic world in which our gaze is constantly shifting and in which objects move. To be useful, the description must select items of importance depending on context and memory; in…

Processing in the Lateral Geniculate Nucleus (LGN)

The lateral geniculate nucleus: the gateway to conscious visual perception Conscious perception requires the visual information that passes through the dorsal lateral geniculate nucleus (LGN) in primates. Although the retina sends axons to many subcortical nuclei, only the pathway from the retina to LGN to cortex is critical to visual awareness. The LGN is a distinctively layered structure and is located at the posterior lateral margin…

Optic Nerve

Introduction The two optic nerves carry the axons of retinal ganglion cells (RGCs), and along these axons transmit all of the visual information from the inner retina ( Chapter 23 ) to the brain ( Chapter 29, Chapter 30, Chapter 31 ). Thus, diseases that affect the optic nerve commonly cause vision loss. The retina and optic nerve are developmentally an outgrowth of the forebrain, and,…

Ganglion-Cell Photoreceptors and Non-Image-Forming Vision

Overview In the past decade, compelling evidence has emerged for a novel class of photoreceptors in the mammalian retina. These neurons are ganglion cells that express the photopigment melanopsin and respond autonomously to bright light with a sustained depolarization and increase in spike frequency. These intrinsically photosensitive retinal ganglion cells (ipRGCs) differ radically in form and function from the classical rod and cone photoreceptors. In this…

Regulation of Light through the Pupil

The pupillary opening appears to occupy a central location, but if carefully measured, it is actually situated slightly inferior and nasal to the center of the cornea. Thus, the pupil center may not correspond exactly to the optical axis defined by a target viewed in the distance and the fovea. The major functions of the pupil are outlined in Figure 25.1 and are summarized as follows.…

Electroretinogram of Human, Monkey and Mouse

Introduction The electroretinogram (ERG) is a useful tool for objective, non-invasive assessment of retinal function both in the clinic and the laboratory. It is a mass electrical potential that represents the summed response of all the cells in the retina to a change in illumination. Recordings can be made in vivo under physiological or nearly physiological conditions using electrodes placed on the corneal surface. For standard…

Signal Processing in the Inner Retina

The inner plexiform layer (IPL) is the second synaptic layer of the retina ( Fig. 23.1 ) and the final stage for processing visual information before it leaves the eye. Visual signals from rod and cone photoreceptors are first processed in the outer plexiform layer (OPL; Fig. 23.1 ), where horizontal cells modulate their signaling to bipolar cells. Bipolar cells then transmit these signals to the…

Signal Processing in the Outer Retina

Light-evoked hyperpolarizing signals in rods and cones are transmitted to and processed by neurons in the rest of the retina through a complex, but highly organized network of electrical and chemical synapses. In the outer retina, photoreceptors are electrically coupled to one another, and rods and cones send output signals to second-order retinal cells, the horizontal cells (HCs) and bipolar cells (BCs), via chemical synapses. HCs…

The Synaptic Organization of the Retina

The basic architecture, signal flow, and neurochemistry of signaling through the vertebrate retina is well-understood: photoreceptors, bipolar cells (BCs), and ganglion cells (GCs) are all thought to be glutamatergic neurons and the fundamental synaptic chain that serves vision is photoreceptor → BC → GC. But, our understanding of detailed signaling is far from adequate and a complete description of synaptic interactions or signaling mechanisms is lacking…

Light Adaptation in Photoreceptors

1 Vision from starlight to sunlight The human visual system operates effectively over an enormously wide range of intensities, of at least a billion-fold, from around 10 −4 cd m −2 under starlight conditions to around 10 5 cd m −2 under intense sunlight. Changes in pupil area account for only about 1 log unit of this 9 log unit range, since the pupil diameter changes from a…

Photoresponses of Rods and Cones

Retinal rods and cones are highly specialized neurons that respond to light with an electrical signal (see Chapter 18 ) and provide the sensory input for vision. In contrast to most other neurons, rods and cones maintain a relatively depolarized membrane potential at rest (in darkness) and when stimulated (by light) decrease Na + entry by closing ion channels. In turn, the resultant hyperpolarization closes Ca…