Discovering the Brain (1992) / Chapter Skim
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7 From Perception to Attention
7 From Perception to Attention
Pages 104-122
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From page 104... ...
According to David Hubel, professor of neurobiology at Harvard Medical School, the present state of knowledge about some parts of the brain is like that of a visitor from, say, another civilization who sets out to understand everything there is to know about a television set. Such a person may have learned all about transmitters, capacitors, conductors, and properties of resistancebut he is still at a loss, because not only is he unaware of large areas of the circuitry, he still does not know what the television set as a whole is used for.
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From page 105... ...
Vision, the best understood of the perceptive systems, can be explained with confidence well beyond the action of light on the rods and cones of the retina. And some of the mechanisms of this system turn out to be active in other aspects of our conscious behavior as well namely, in spatial perception and the specific state of mind known as attention.
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From page 106... ...
The optic nerve in turn supplies a large amount of pooled information to the lateral geniculate nucleus, which then relays signals to the primary visual cortex. It is in the primary visual cortex, located in the occipital lobes at the back of the head, that the brain first begins to assemble something that looks like an image to our conscious awareness.
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From page 107... ...
The patterns of responsiveness among retinal ganglion ceils can vary strikingly, depending on which aspect of vision is being handled. One major pattern is that of center and surrounding area: the retinal cell is stimulated when a small central portion of its receptive field receives light but is inhibited from sending signals when all the area surrounding the center receives light.
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From page 108... ...
It includes two subsystems, which correspond to two distinct types of cell in the lateral geniculate (the relay center from the optic nerve to the visual cortex)
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From page 109... ...
(The cortex is made up of six layers of cells, as described in Chapter 2; those of most interest here are layers 2 and 4, the neuron-rich granular layers that make synaptic connections within the same locaTized region.) Signals from the optic nerve pass through the lateral geniculate to the intermediate layers of the cortex, where any given cell receives impulses from either the right or the left eye.
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From page 110... ...
The compactness of information in this system is a marvel of biological engineering, and it overwhelms standard approaches to the investigation of physiology even those that are extremely precise and that focus on highly localized functions. David Hubel, for example, feels that researchers eventually will need to observe the workings of single nerve cells and to identify the functions of each one, if neuroscience is ever to give a full account of any of the human sensory systems.
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From page 111... ...
THE BRAIN'S SYSTEM FOR SPATIAL PERCEPTION in addition to the neural circuitry that serves the five primary senses, the human brain has numerous other systems for making sense of external stimuli and regulating the body's ability to function in the world. Although they may produce no obvious perceptions, as does vision or the sense of smell, such systems are often highly sophisticated, drawing on several specialized areas of the brain.
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From page 112... ...
forms, such as the inability to pour a liquid accurately into a glass, have traditionally been collected under the term "parietal lobe syndrome." But a more current view, held, for example, by Vernon Mountcastle and his research team at the Johns Hopkins Medical School, is that this is not a disorder of one discrete lobe of the brain but of a system. Mountcastle, who studies the brain's means of spatial perception, has gathered evidence from numerous research projects that the parietal lobe forms part of a complex distributed system by which the brain actually "constructs" reality.
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From page 113... ...
in fact, extraordinarily dense connections have been found between two areas of the parietal lobe and parts of the frontal lobe involved in mental representations, planning, and cognition (see Chapter 8~. MAKING SENSE OF MOVEMENT Studying this kind of perception is a subtle endeavor for the researcher, because although visual stimuli are involved, the point of interest is not the visual system itself but something more like peripheral vision, or spatial awareness.
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From page 114... ...
Electrical recordings from single cells have enabled MountcastIe and his colleagues to work out the pattern by which parietal visual neurons are organized a pattern quite differ~ 1 1 ent trom that of the visual cortex itself. in the parietal lobe, a visual neuron will respond, within its receptive field, to a light moving in any of several directions depending on whether the light is moving toward or away from the center of the field.
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From page 115... ...
In "state control" systems, such as the angle of gaze or attention versus inattention, the "state" exerts an effect on all the information brought together by the system. For example, when we gaze alertly at the road in front of the steering wheel of a car, the parietal lobe's system for peripheral vision creates a "halo" of heightened sensitivity all around the center of our attention, which provides for safe ~ · .
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From page 116... ...
In PET imaging studies carried out by Posner and Marcus Raichie (discussed in part in Chapter 3) , subjects were shown groups of letters that conformed to English rules of construction but did not form a word in English; these nonwords, as well as authentic English words tended to activate a portion of the left occipital lobe that does not respond to mere strings of consonants or to strings of graphic forms that resemble letters.
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From page 117... ...
(in some other forms of attention, such as planning or mental representation, a number of studies have shown that the frontal lobes play a more important role.) Computer models are being developed to explain how information may be exchanged between the visual system and the posterior attention system.
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From page 118... ...
in an experiment in which subjects were asked to attend to these kinds of cues, PET scans showed a distinct pattern of activity, outside the primary visual cortex, associated with each kind of cue. When the target was uncertain, the scans revealed activity in the middle area of the frontal lobe, suggesting the existence of an anterior attention system as well.
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From page 119... ...
, they acquire an ability to disengage their attention and are freer to shift their eyes from one object of interest to another. Working from the adult state back to early development, Michael Posner and his colleagues have traced the origin of another useful element in the posterior attention system: a capacity that inhibits one's eyes from returning to a previous location.
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From page 120... ...
The area of interest here is a little further forward than the site of the posterior attention system, in the middle part of the frontal lobes at a ridge in the brain called the cingulate gyros. This portion of the brain shows high levels of activity, for example, on PET scans, when subjects in experiments are presented with written words and asked not merely to recognize them but to make some active response, such as saying the words aloud.
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From page 121... ...
Both these signs suggest a dysfunction in the left hemisphere, in an area that is also involved with the processing of language not that schizophrenic patients as a group show difficulties with language per se, but it is possible that some impairment in the way the brain handles language stimuli could contribute to the disturbances of thought that are characteristic of schizophrenia. Another familiar feature of schizophrenia is the "alien hand sign," in which the patient believes that his hand, although still attached to him, is controlled by an alien power; this is reminiscent of the "neglect" syndrome (discussed earlier)
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From page 122... ...
122 DISCOVERING THE BRAIN and neural circuitry that underlie attention in all its forms, researchers will ultimately be able to resolve a question that curious minds have pondered for a long time: just what goes on in the brain, at a physical level, to account for the subjective experiences of the perceiving mind. ACKNOWLEDGMENTS Chapter 7 is based on presentations by David Hubel, Vernon Mountcastle, and Michael Posner.
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