Humans have a big neural investment in processing visual information. The cortical regions devoted to processing information from vision and hearing. This investment in vision is part of our “inheritance” as primates, who have evolved to devote as much as 50% of their brains to visual processing (Barton, 1998). The enormous investment underlies the human ability to see the world.
This is vividly demonstrated by individuals with damage to certain brain regions who are not blind but are unable to recognize anything visually, a condition called visual agnosia. This condition results from neural damage. One case of visual agnosia involved a soldier who suffered brain damage resulting from accidental carbon monoxide poisoning. He could recognize objects by their feel, smell, or sound, but he was unable to distinguish a picture of a circle from that of a square or to recognize faces or letters (Benson & Greenberg, 1969).
On the other hand, he was able to discern light intensities and colors and to tell in what direction an object was moving. Thus, his sen- sory system was still able to register visual information, but the damage to his brain resulted in a loss of the ability to transform visual information into per- ceptual experience. This case shows that perception is much more than simply the registering of sensory information.
Generally, visual agnosia is classified as either apperceptive agnosia or associative agnosia (for a review, read Farah, 1990). Patients with apperceptive agnosia, like the soldier just described, are unable to recognize simple shapes such as circles or triangles, or to draw shapes they are shown. Patients with associative agnosia, in contrast, are able to recognize simple shapes and can successfully copy drawings, even of complex objects. However, they are unable to recognize the complex objects. Despite being able to produce a relatively accurate drawing, the patient could not recognize this object as an anchor (he called it an umbrella). Patients with ap- perceptive agnosia are generally believed to have problems with early processing of information in the visual system. In contrast, patients with associative agnosia are thought to have intact early processing but to have difficulties with pattern recogni- tion, which occurs later. This chapter will first discuss the early processing of infor- mation in the visual stream and then the later processing of this information.
This is vividly demonstrated by individuals with damage to certain brain regions who are not blind but are unable to recognize anything visually, a condition called visual agnosia. This condition results from neural damage. One case of visual agnosia involved a soldier who suffered brain damage resulting from accidental carbon monoxide poisoning. He could recognize objects by their feel, smell, or sound, but he was unable to distinguish a picture of a circle from that of a square or to recognize faces or letters (Benson & Greenberg, 1969).
On the other hand, he was able to discern light intensities and colors and to tell in what direction an object was moving. Thus, his sen- sory system was still able to register visual information, but the damage to his brain resulted in a loss of the ability to transform visual information into per- ceptual experience. This case shows that perception is much more than simply the registering of sensory information.
Generally, visual agnosia is classified as either apperceptive agnosia or associative agnosia (for a review, read Farah, 1990). Patients with apperceptive agnosia, like the soldier just described, are unable to recognize simple shapes such as circles or triangles, or to draw shapes they are shown. Patients with associative agnosia, in contrast, are able to recognize simple shapes and can successfully copy drawings, even of complex objects. However, they are unable to recognize the complex objects. Despite being able to produce a relatively accurate drawing, the patient could not recognize this object as an anchor (he called it an umbrella). Patients with ap- perceptive agnosia are generally believed to have problems with early processing of information in the visual system. In contrast, patients with associative agnosia are thought to have intact early processing but to have difficulties with pattern recogni- tion, which occurs later. This chapter will first discuss the early processing of infor- mation in the visual stream and then the later processing of this information.
■ Visual perception can be divided into an early phase, in which shapes and objects are extracted from the visual scene, and a later phase, in which the shapes and objects are recognized.
Early vision information processing
Early visual information processing begins in the eye. Light passes through the lens and the vitre- ous humor and falls on the retina at the back of the eye. The retina contains the photoreceptor cells, which are made up of light-sensitive molecules that undergo structural changes when exposed to light. Light is scattered slightly in passing through the vitreous humor, so the image that falls on the back of the retina is not perfectly sharp. One of the functions of early visual processing is to sharpen that image.
Photoreceptor cells in the retina contain light-sensitive molecules that undergo structural changes when exposed to light, initiating a photochemical process that converts light into neural signals. There are two distinct types of photoreceptors in the eye: cones and rods. Cones are involved in color vision and produce high reso- lution and acuity. Less light energy is required to trigger a response in the rods, but they produce poorer resolution. As a consequence, they are principally responsible for the less acute, black-and-white vision we experience at night. Cones are espe- cially concentrated in a small area of the retina called the fovea. When we focus on an object, we move our eyes so that the image of the object falls on the fovea, which enables us to take full advantage of the high resolution of the cones in perceiving the object. Foveal vision detects fine details, whereas the rest of the visual field—the periphery—detects more global information, including movement.
The receptor cells synapse onto bipolar cells and these onto ganglion cells, whose axons leave the eye and form the optic nerve, which goes to the brain. Altogether there are about 800,000 ganglion cells in the optic nerve of each eye. Each ganglion cell encodes information from a small region of the retina called the cell’s receptive field. Typically, the amount of light stimulation in that region of the retina is encoded by the neural firing rate on the ganglion cell’s axon.
the neural pathways from the eyes to the brain. The optic nerves from both eyes meet at the optic chiasma, where the nerves from the inside of the retina (the side nearest the nose) cross over and go to the opposite side of the brain. The nerves from the outside of the retina continue to the same side of the brain as the eye. This means that the right halves of both eyes are connected to the right hemisphere. The lens focuses the light so that the left side of the visual field falls on the right half of each eye. Thus, information about the left side of the visual field goes to the right brain, and information about the right side of the vis- ual field goes to the left brain. This is one instance of the general fact, discussed in Chapter 1, that the left hemisphere processes information about the right part of the world and the right hemisphere processes information about the left part.
Once inside the brain, the fibers from the ganglion cells synapse onto cells in various subcorti- cal structures. (“Subcortical” means that the struc- tures are located below the cortex.) These subcortical structures (such as the lateral geniculate nucleus) are connected to the primary visual cortex (Brodmann area 17 in Color Plate 1.1). The primary visual cortex is the first cortical area to receive visual input, but there are many other visual areas.
From the primary visual cortex, information tends to follow two path- ways, a “what” pathway and a “where” pathway. The “what” pathway goes to regions of the temporal cortex that are specialized for identifying objects. The “where” pathway goes to parietal regions of the brain that are specialized for representing spatial information and for coordinating vision with action. Monkeys with lesions in the “where” pathway have dif- ficulty learning to identify specific locations, whereas monkeys with lesions in the “what” pathway have difficulty learning to identify objects (Pohl, 1973; Ungerleider & Brody, 1977). Other researchers (e.g., Milner & Goodale, 1995) have argued that the “where” pathway is really a pathway specialized for action. They point out that patients with agnosia because of damage to the temporal lobe, but with intact parietal lobes, can often take actions appropriate to objects they cannot recognize. For instance, one patient (see Goodale, Milner, Jakobson, & Carey, 1991) could correctly reach out and grasp a door handle that she could not recognize.
■ A photochemical process converts light energy into neural activity. Visual information progresses by various neural tracks to the visual cortex. From the visual cortex it progresses along “what” and “where” pathways through the beain.
Early visual information processing begins in the eye. Light passes through the lens and the vitre- ous humor and falls on the retina at the back of the eye. The retina contains the photoreceptor cells, which are made up of light-sensitive molecules that undergo structural changes when exposed to light. Light is scattered slightly in passing through the vitreous humor, so the image that falls on the back of the retina is not perfectly sharp. One of the functions of early visual processing is to sharpen that image.
Photoreceptor cells in the retina contain light-sensitive molecules that undergo structural changes when exposed to light, initiating a photochemical process that converts light into neural signals. There are two distinct types of photoreceptors in the eye: cones and rods. Cones are involved in color vision and produce high reso- lution and acuity. Less light energy is required to trigger a response in the rods, but they produce poorer resolution. As a consequence, they are principally responsible for the less acute, black-and-white vision we experience at night. Cones are espe- cially concentrated in a small area of the retina called the fovea. When we focus on an object, we move our eyes so that the image of the object falls on the fovea, which enables us to take full advantage of the high resolution of the cones in perceiving the object. Foveal vision detects fine details, whereas the rest of the visual field—the periphery—detects more global information, including movement.
The receptor cells synapse onto bipolar cells and these onto ganglion cells, whose axons leave the eye and form the optic nerve, which goes to the brain. Altogether there are about 800,000 ganglion cells in the optic nerve of each eye. Each ganglion cell encodes information from a small region of the retina called the cell’s receptive field. Typically, the amount of light stimulation in that region of the retina is encoded by the neural firing rate on the ganglion cell’s axon.
the neural pathways from the eyes to the brain. The optic nerves from both eyes meet at the optic chiasma, where the nerves from the inside of the retina (the side nearest the nose) cross over and go to the opposite side of the brain. The nerves from the outside of the retina continue to the same side of the brain as the eye. This means that the right halves of both eyes are connected to the right hemisphere. The lens focuses the light so that the left side of the visual field falls on the right half of each eye. Thus, information about the left side of the visual field goes to the right brain, and information about the right side of the vis- ual field goes to the left brain. This is one instance of the general fact, discussed in Chapter 1, that the left hemisphere processes information about the right part of the world and the right hemisphere processes information about the left part.
Once inside the brain, the fibers from the ganglion cells synapse onto cells in various subcorti- cal structures. (“Subcortical” means that the struc- tures are located below the cortex.) These subcortical structures (such as the lateral geniculate nucleus) are connected to the primary visual cortex (Brodmann area 17 in Color Plate 1.1). The primary visual cortex is the first cortical area to receive visual input, but there are many other visual areas.
From the primary visual cortex, information tends to follow two path- ways, a “what” pathway and a “where” pathway. The “what” pathway goes to regions of the temporal cortex that are specialized for identifying objects. The “where” pathway goes to parietal regions of the brain that are specialized for representing spatial information and for coordinating vision with action. Monkeys with lesions in the “where” pathway have dif- ficulty learning to identify specific locations, whereas monkeys with lesions in the “what” pathway have difficulty learning to identify objects (Pohl, 1973; Ungerleider & Brody, 1977). Other researchers (e.g., Milner & Goodale, 1995) have argued that the “where” pathway is really a pathway specialized for action. They point out that patients with agnosia because of damage to the temporal lobe, but with intact parietal lobes, can often take actions appropriate to objects they cannot recognize. For instance, one patient (see Goodale, Milner, Jakobson, & Carey, 1991) could correctly reach out and grasp a door handle that she could not recognize.
■ A photochemical process converts light energy into neural activity. Visual information progresses by various neural tracks to the visual cortex. From the visual cortex it progresses along “what” and “where” pathways through the beain.
Information coding in visual cells
Kuffler’s (1953) research showed how information is encoded by the ganglion cells. These cells generally fire at some spontaneous rate even when the eyes are not receiving any light. For some ganglion cells, if light falls on a small region of the retina at the center of the cell’s receptive field, their spontaneous rates of firing will increase. If light falls in the region just around this sensitive center, however, the spontaneous rate of firing will decrease. Light farther from the center elicits no change from the spontaneous firing rate—neither an increase nor a decrease. Ganglion cells that respond in this way are known as on-off cells. There are also off-on ganglion cells: Light at the center decreases the spontaneous rate of firing, and light in the surrounding areas increases that rate. Cells in the lateral geniculate nucleus respond in the same way.
The ganglion cells encode the visual field by means of on-off and off-on cells, which are combined by higher visual processing to form various features.
Kuffler’s (1953) research showed how information is encoded by the ganglion cells. These cells generally fire at some spontaneous rate even when the eyes are not receiving any light. For some ganglion cells, if light falls on a small region of the retina at the center of the cell’s receptive field, their spontaneous rates of firing will increase. If light falls in the region just around this sensitive center, however, the spontaneous rate of firing will decrease. Light farther from the center elicits no change from the spontaneous firing rate—neither an increase nor a decrease. Ganglion cells that respond in this way are known as on-off cells. There are also off-on ganglion cells: Light at the center decreases the spontaneous rate of firing, and light in the surrounding areas increases that rate. Cells in the lateral geniculate nucleus respond in the same way.
The ganglion cells encode the visual field by means of on-off and off-on cells, which are combined by higher visual processing to form various features.
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