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attended location, because it takes time to reorient attention to the new (unattended) stimulus location. Second, attention could more directly influence visual processes, for instance, by enhancing their sensitivity at the attended location. This would explain how attention also improves sensory thresholds (6).

When recording brain activity either at the whole brain level or at the level of single neurons, different types of signals will correspond to the activation of the attentional mechanism (“source” signals) and its interaction with the visual system (“site” signals). For example, a source signal would be associated with a shift of attention to a location and would be recorded in areas that implement the attentional mechanism and/or in visual areas responsible for stimulus analysis. In visual areas, a source signal may prime visual processes to a more efficient response. Once a stimulus is presented, stimulus analysis may be enhanced by attention. This would produce a modulation of visual processing (“site” signal) that marks the site of the interaction between source attentional signals and visual processes. Whereas source signals provide information on the organization of attention systems, site signals provide information on how sensory (or motor or cognitive) systems are affected by attention.

Overt Orienting. The discovery of a mechanism for covertly (without eye movements) directing attention to locations raises the question of its relationship to mechanisms responsible for saccadic generation. In normal conditions, attention and eye movements move synchronously and select common targets in the visual field. Following Shepherd and colleagues (8), this relationship can take three forms. At one extreme attention and eye movement generation can involve entirely different mechanisms (independence hypothesis). For example, locations could be simultaneously computed in separate spatial maps by attentional and oculomotor systems. An implication of this view is that it should be possible to operate simultaneous shifts of attention and eye movements in opposite directions. At the other extreme, attention and eye movement generation involve the same mechanisms (identity hypothesis). A location is encoded by the attentional mechanism in a set of motor coordinates that specify direction and amplitude and that are also used for planning a saccadic eye movement (9). In dual-task conditions, in which different locations are selected respectively by attentional and eye movement mechanisms, one would predict that locations selected by eye movement mechanisms should control behavior. An intermediate position is that attention and eye movement processes share resources or computations at some stage (interdependence hypothesis). For example, both attention and eye movement systems may depend on an early sensory visual representation. When both systems select the same location on the representation, their performance is optimal; when different locations are to be selected by each system, their performance is impaired.

Early papers provided conflicting evidence on whether preparing an eye movement toward a location enhanced the visual processing of stimuli presented at the same location and, vice versa, whether a shift of attention facilitated oculomotor execution (10, 11). Furthermore, under certain conditions attention could move in the opposite direction of an eye movement (3). Altogether these results indicated that attention and eye movements were either independent processes (11) or separate but functionally related processes (3), such that they could be recruited in isolation or in concert depending on task demands.

More recent work, however, has established that attention and eye movements are more tightly related. Shepherd and colleagues (8) manipulated spatial attention by varying the probability that peripheral probe stimuli would appear in different positions, and eye movements by cueing saccades with a central arrow cue. They found that the preparation of a saccadic eye movement enhanced the manual detection of stimuli presented at the saccadic target location, irrespective of the direction of attention. That is, even when attention and eye movements were cued to opposite locations, stimuli at the location of the saccade were always detected more rapidly. The latency of the saccades was also uninfluenced by the direction of attention. Hoffman and Subramaniam (12) confirmed in a dual-task situation that target detection is superior at the saccade location regardless of the direction of attention. In this experiment, saccadic latencies were slowest when attention and saccades were directed toward opposite locations. Klein (11) suggested that processing facilitation at the saccade location is induced by saccadic execution, but not saccadic programming.

The current view is that attention and eye movement systems are tightly related. During the preparation of a saccade, the selection of a location is controlled by the oculomotor system, even when attention is directed elsewhere through cognitive manipulations. This supports an identity view in which attention shifts are organized in oculomotor coordinates. Because the direction of attention is dissociable from eye position during fixation, an additional veto-going signal has been postulated to prevent breakdowns of fixation (9). It is still under discussion whether attentional processes are separate when a saccade is planned but not performed, or when the eyes are fixated (13, 14). Finally, these findings are not inconsistent with the notion that attention and eye movement systems may be separate but share resources. For example, the slowing of saccadic latencies in Hoffman and Subramaniam (12) is consistent with some sharing of common resources. However, the prevalent control of saccades on location would suggest that the eye movement system has preferential access to those resources.

The next section considers functional anatomical data recorded in normal human volunteers with various methods including positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). These experiments indicate that during visual orienting a network of frontal and parietal regions is consistently activated in the human brain. These frontoparietal regions are the source of a location bias in ventral occipital regions involved in object analysis. Hence, ventral occipital regions are the site of the spatial modulation. Finally, the anatomical overlap between brain regions involved in covert and overt orienting is discussed to assess whether attention and eye movements share common or separate neural representations.

Functional Anatomy of Visual Orienting in the Human Brain

Covert Orienting. Functional neuroimaging methods like PET and fMRI record in the living human brain local changes in blood flow and oxygenation, respectively. These metabolic parameters are indirectly related to the level of neuronal activity. Functional neuroimaging methods are used to image brain regions active during sensory, motor, and cognitive processing (15). The greatest strength of neuroimaging is the capacity to visualize the whole brain with a spatial precision of about 1 cm for PET and 2–3 mm for fMRI. Further, the possibility to test human volunteers allows the use of sophisticated experimental protocols that can be compared directly with those employed in psychological studies. The greatest weakness of neuroimaging is the poor temporal resolution, about 40 sec for PET and 2–4 sec for fMRI, far above the millisecond scale of neuronal activity. This limitation prevents any meaningful analysis of the temporal sequence of task-related activations.

Several studies have investigated the functional anatomy of covert visual orienting to simple unstructured peripheral stimuli. These studies have shown that a specific set of frontal and parietal regions is consistently recruited during visual orienting. Corbetta et al. (16) asked subjects to voluntarily shift attention along a series of locations positioned in left or right visual field to detect brief visual stimuli with a speeded key-press response (shifting-attention task). This paradigm involves endogenous spatial cueing, and, as expected, stimuli at attended locations were detected faster than stimuli at unattended locations. Areas involved in covert orienting were localized by subtracting PET activity recorded during the shifting-attention task from activity recorded during a central-detection task. In the central-detection task subjects attended to and manually responded to stimuli in the

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