We review evidence for partially segregated networks of brain areas that carry out different attentional functions. One system, which includes parts of the intraparietal cortex and superior frontal cortex, is involved in preparing and applying goal-directed (top-down) selection for stimuli and responses. This system is also modulated by the detection of stimuli. The other system, which includes the temporoparietal cortex and inferior frontal cortex, and is largely lateralized to the right hemisphere, is not involved in top-down selection. Instead, this system is specialized for the detection of behaviourally relevant stimuli, particularly when they are salient or unexpected. This ventral frontoparietal network works as a 'circuit breaker' for the dorsal system, directing attention to salient events. Both attentional systems interact during normal vision, and both are disrupted in unilateral spatial neglect.

Control of goal-directed and stimulus-driven attention in the brain

This paper presents a spreading-acti vation theory of human semantic processing, which can be applied to a wide range of recent experimental results The theory is based on Quillian's theory of semantic memory search and semantic preparation, or priming In conjunction with this, several of the miscondeptions concerning Qullian's theory are discussed A number of additional assumptions are proposed for his theory in order to apply it to recent experiments The present paper shows how the extended theory can account for results of several production experiments by Loftus, Juola and Atkinson's multiple-category experiment, Conrad's sentence-verification experiments, and several categorization experiments on the effect of semantic relatedness and typicality by Holyoak and Glass, Rips, Shoben, and Smith, and Rosch The paper also provides a critique of the Smith, Shoben, and Rips model for categorization judgments Some years ago, Quillian1 (1962, 1967) proposed a spreading-acti vation theory of human semantic processing that he tried to implement in computer simulations of memory search (Quillian, 1966) and comprehension (Quillian, 1969) The theory viewed memory search as activation spreading from two or more concept nodes in a semantic network until an intersection was found The effects of preparation (or priming) in semantic memory were also explained in terms of spreading activation from the node of the primed concept Rather than a theory to explain data, it was a theory designed to show how to build human semantic structure and processing into a computer

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A spreading-activation theory of semantic processing

: The concept of attention as central to human performance extends back to the start of experimental psychology, yet even a few years ago, it would not have been possible to outline in even a preliminary form a functional anatomy of the human attentional system. New developments in neuroscience have opened the study of higher cognition to physiological analysis, and have revealed a system of anatomical areas that appear to be basic to the selection of information for focal (conscious) processing. The importance of attention is its unique role in connecting the mental level of description of processes used in cognitive science with the anatomical level common in neuroscience. Sperry describes the central role that mental concepts play in understanding brain function. As is the case for sensory and motor systems of the brain, our knowledge of the anatomy of attention is incomplete. Nevertheless, we can now begin to identify some principles of organization that allow attention to function as a unified system for the control of mental processing. Although many of our points are still speculative and controversial, we believe they constitute a basis for more detailed studies of attention from a cognitive-neuroscience viewpoint. Perhaps even more important for furthering future studies, multiple methods of mental chronometry, brain lesions, electrophysiology, and several types of neuro-imaging have converged on common findings.

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The Attention System of the Human Brain

Recent studies of eye movements in reading and other information processing tasks, such as music reading, typing, visual search, and scene perception, are reviewed. The major emphasis of the review is on reading as a specific example of cognitive processing. Basic topics discussed with respect to reading are (a) the characteristics of eye movements, (b) the perceptual span, (c) integration of information across saccades, (d) eye movement control, and (e) individual differences (including dyslexia). Similar topics are discussed with respect to the other tasks examined. The basic theme of the review is that eye movement data reflect moment-to-moment cognitive processes in the various tasks examined. Theoretical and practical considerations concerning the use of eye movement data are also discussed.

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Eye movements in reading and information processing: 20 years of research.

Standard alphabetical procedures for organizing lexical information put together words that are spelled alike and scatter words with similar or related meanings haphazardly through the list. Unfortunately, there is no obvious alternative, no other simple way for lexicographers to keep track of what has been done or for readers to find the word they are looking for. But a frequent objection to this solution is that finding things on an alphabetical list can be tedious and time-consuming. Many people who would like to refer to a dictionary decide not to bother with it because finding the information would interrupt their work and break their train of thought.

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Introduction to WordNet: An On-line Lexical Database

I. A consideration of methods for assessing contrast sensitivity leads to the conclusion that, for a clinical test, letters are more suitable than gratings. 2. A letter chart is described in which letters decrease in contrast but not in size. The letters are arranged in groups of three; successive groups decrease in contrast by a factor of IfJ2 from a very high contrast down to a contrast below the threshold of normal observers. A subject's threshold is taken to be the lowest contrast for which at least two letters in a group are correctly reported. 3. A mathematical model of the observer and the chart-testing procedure has been used to predict how the accuracy and repeatability of the test score depend on the parameters of the chart and observer. This reveals that even a low probability of misreporting supra threshold letters will seriously bias the test score if the passing criterion is strict, requiring correct report of all letters in each group, but will have little effect if the passing criterion is less strict. This effect of the passing criterion may explain Rubin's [Clin. Vision Sci. 2, No. I (1987)] finding that the new test, which uses a lenient criterion, has excellent test-retest reliability, much higher than the Ginsburg [Am. J. Optom. Physiol. Opt. 61,403-407 (1984)] chart with its strict criterion.

The design of a new letter chart for measuring contrast sensitivity

It is well established that generalized epileptiform EEG discharges unaccompanied by overt clinical change may nevertheless be associated with transitory cognitive impairment (TCI) detectable by appropriate psychological testing. However, the tests employed in most research studies of this phenomenon are of little use for routine clinical application. They are suitable for administration only over short periods of time and are therefore applicable only to patients with a high discharge rate, a serious limitation, as the discharges are generally depressed by the tests themselves. We have developed two short-term memory tasks, one using verbal, the other nonverbal material presented in the form of television games which patients are generally prepared to perform for up to an hour or longer. Forty-six patients with subclinical EEG discharges have been studied. They were screened by video monitoring before and during testing to exclude any with overt clinical changes during the discharges. Despite this rigorous selection, in 50 per cent TCI was demonstrable with a confidence level (within the individual patient) of 10 per cent. Discharges during stimulus presentation were most disruptive of performance and those confined to the period when the patient was responding were without demonstrable effect. A significant association was found between the laterality of focal or asymmetrical generalized discharges and impairment of one or other task, left-sided discharges being associated with errors in the verbal task and right-sided with impairment of the nonverbal test. Two case histories are cited illustrating patients who were clearly handicapped by TCI and whose functioning improved when the subclinical discharges were suppressed by medication. To determine how many patients suffer such disabilities or can be helped by appropriate medication, further prospective studies are required.

Selective cognitive impairment during focal and generalized epileptiform EEG activity

Frontal lesions and sustained attention.

Summary: Purpose: This report summarizes background material presented to a consensus conference on visually provoked seizures, convened by the Epilepsy Foundation of America.



Methods: A comprehensive review of literature was performed.



Results: Photosensitivity, an abnormal EEG response to light or pattern stimulation, occurs in ∼0.3–3% of the population. The estimated prevalence of seizures from light stimuli is ∼1 per 10,000, or 1 per 4,000 individuals age 5–24 years. People with epilepsy have a 2–14% chance of having seizures precipitated by light or pattern. In the Pokemon cartoon incident in Japan, 685 children visited a hospital in reaction to red–blue flashes on broadcast television (TV). Only 24% who had a seizure during the cartoon had previously experienced a seizure. Photic or pattern stimulation can provoke seizures in predisposed individuals, but such stimulation is not known to increase the chance of subsequent epilepsy. Intensities of 0.2–1.5 million candlepower are in the range to trigger seizures. Frequencies of 15–25 Hz are most provocative, but the range is 1–65 Hz. Light–dark borders can induce pattern-sensitive seizures, and red color also is a factor. Seizures can be provoked by certain TV shows, movie screen images, video games, natural stimuli (e.g, sun on water), public displays, and many other sources.



Conclusions: Recommendations on reducing risk of seizures have been developed by agencies in the United Kingdom, Japan, and the International Telecommunications Union, affiliated with the United Nations. The Epilepsy Foundation of America has developed a consensus of medical experts and scientists on this subject, reported in an accompanying work.

Photic- and pattern-induced seizures: a review for the Epilepsy Foundation of America Working Group.

Certain patterns of stripes are judged to be unpleasant to look at. They include illusions of colour, shape and motion that are sometimes perceived predominantly to one side of fixation. People who suffer frequent headaches tend to report more illusions, and if the pain consistently occurs on the samer side of the head the illusions tend to be lateralized. The parameters of the patterns that induce illusions (including their shape, spatial frequency, duty cycle, contrast and cortical representation) closely resemble those that epileptiform electroencephalographic abnormalities in patients with photosensitive epilepsy. The viewing conditions under which such abnormalities are likely to appear are also those under which more illusions are seen.

Arnold J. Wilkins

Papers

The design of a new letter chart for measuring contrast sensitivity

Selective cognitive impairment during focal and generalized epileptiform EEG activity

Frontal lesions and sustained attention.

Photic- and pattern-induced seizures: a review for the Epilepsy Foundation of America Working Group.

A neurological basis for visual discomfort.