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

Bartlett viewed thinking as a high level skill exhibiting ballistic properties that he called its “point of no return”. This paper explores one aspect of cognition through the use of a simple model...

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Orienting of attention

The two basic phenomena that define the problem of visual attention can be illustrated in a simple example. Consider the arrays shown in each panel of Figure 1. In a typical experiment, before the arrays were presented, subjects would be asked to report letters appearing in one color (targets, here black letters), and to disregard letters in the other color (nontargets, here white letters). The array would then be briefly flashed, and the subjects, without any opportunity for eye movements, would give their report. The display mimics our. usual cluttered visual environment: It contains one or more objects that are relevant to current behavior, along with others that are irrelevant. The first basic phenomenon is limited capacity for processing information. At any given time, only a small amount of the information available on the retina can be processed and used in the control of behavior. Subjectively, giving attention to any one target leaves less available for others. In Figure 1, the probability of reporting the target letter N is much lower with two accompa­ nying targets (Figure la) than with none (Figure Ib). The second basic phenomenon is selectivity-the ability to filter out un­ wanted information. Subjectively, one is aware of attended stimuli and largely unaware of unattended ones. Correspondingly, accuracy in identifying an attended stimulus may be independent of the number of nontargets in a display (Figure la vs Ie) (see Bundesen 1990, Duncan 1980).

Neural Mechanisms of Selective Visual Attention

Separate visual pathways for perception and action.

Psychophysical and physiological evidence indicates that the visual system of primates and humans has evolved a specialized processing focus moving across the visual scene. This study addresses the question of how simple networks of neuron-like elements can account for a variety of phenomena associated with this shift of selective visual attention. Specifically, we propose the following: (1) A number of elementary features, such as color, orientation, direction of movement, disparity etc. are represented in parallel in different topographical maps, called the early representation. (2) There exists a selective mapping from the early topographic representation into a more central non-topographic representation, such that at any instant the central representation contains the properties of only a single location in the visual scene, the selected location. We suggest that this mapping is the principal expression of early selective visual attention. One function of selective attention is to fuse information from different maps into one coherent whole. (3) Certain selection rules determine which locations will be mapped into the central representation. The major rule, using the conspicuity of locations in the early representation, is implemented using a so-called Winner-Take-All network. Inhibiting the selected location in this network causes an automatic shift towards the next most conspicious location. Additional rules are proximity and similarity preferences. We discuss how these rules can be implemented in neuron-like networks and suggest a possible role for the extensive back-projection from the visual cortex to the LGN.

Shifts in selective visual attention: towards the underlying neural circuitry.

that area 7 of posterior parietal cortex plays a role in visual attention and eye movements. We have operationally defined attention as a stimulus-selection process independent of the specific movement used to respond to the stimulus. We trained monkeys to make various hand or eye movements and recorded from single neurons in area 7 while the monkeys were performing these tasks. 2. As demonstrated previously ( 19, 20, 46, 47, 63), many cells in area 7 respond to visual stimuli independent of any behavior. The discharge to a stimulus may be enhanced when the animal makes an eye movement to the stimulus. 3. We used two paradigms to study the modulation of visual responses when the animal used a stimulus without making an eye movement. The first was a peripheral-attention task in which the animal had to signal the occurrence of a peripheral stimulus without making an eye movement to it. Half of the cells studied gave an enhanced response in this task. In the second task, the animal had to reach out and touch a stimulus without making an eye movement to it. Visually responsive parietal neurons also yielded enhanced responses in this task. 4. The enhancements demonstrable in the saccade, peripheral-attention, and handreach tasks probably represent the same underlying process because their frequency of occurrence in our sample is almost identical, the intensities are quite similar, and cells that give an enhanced response on one task give an enhanced response in the others. 5. These results show that in posterior parietal cortex, the behavioral enhancement of a visual response is independent of the specific movement used to respond to the stimulus. The physiological mechanism of enhancement, which is movement independent and spatially selective, resembles the psychological phenomenon of selective spatial attention. We suggest that the role of area 7 in visual attention may be mediated by the enhancement of visual responses to selected stimuli.

D. L. Robinson

Papers

Behavioral enhancement of visual responses in monkey cerebral cortex. I. Modulation in posterior parietal cortex related to selective visual attention

Parietal association cortex in the primate: sensory mechanisms and behavioral modulations