Premotor theory of attention
|Giacomo Rizzolatti and Laila Craighero (2010), Scholarpedia, 5(1):6311.||doi:10.4249/scholarpedia.6311||revision #91668 [link to/cite this article]|
Spatial attention is the capacity to improve the processing of sensory information coming from a specific part of the space surrounding the observer. Classically, spatial attention was thought of as a dedicated supramodal control mechanism, anatomically distinct from the circuits underlying sensorimotor processing (see Posner and Dehaene, 1994). In the late eighties Rizzolatti et al. (1987) challenged this view. On the basis of some behavioral experiments (see below) they argued that there is no need to postulate two control mechanisms, one for action and one for attention. According to them spatial attention does not result from a dedicated control mechanism, but derives from a weaker activation of the same frontal-parietal circuits that, in other conditions, determine motor behavior toward specific spatial locations. This theory, known as the “premotor theory of attention”, has received in these last years a tremendous support from electrophysiological and brain imaging studies and has been extended from spatial attention to attention directed to objects (Rizzolatti and Craighero, 1998).
Premotor theory of spatial attention
Psychological evidence in favor of the premotor theory of spatial attention
A first set of data that challenged the classical theory of attention came from behavioral experiments that used a variant of the Posner paradigm. In these experiments, unexpected imperative stimuli were located either in the same or in the opposite hemifield with respect to the attended location (see Figure 1). The results showed that when unexpected imperative stimuli were located in the hemifield contralateral to where attention was located, reaction times (RTs) were longer than when the imperative stimuli and attention were deployed in the same hemifield, even when the distance from the unattended stimuli and the cued location was the same (Rizzolatti et al., 1994).
This reaction time delay (the so called meridian effect) hardly can fit the notion that attention is a control system independent of basic anatomical and physiological circuits. In fact, there is no reason whatsoever why an anatomical landmark such as the meridian of the visual field could affect the function of a supramodal control mechanism. On the contrary, the meridian effect can be easily accounted for by assuming that attention derives from preparation to move the eyes towards the cued location (premotor theory of attention). When a cue indicates the location of the imperative stimulus, an eye movement program is prepared toward the expected location. This program specifies the direction and the amplitude of the saccade. If a target does not appear in the cued location, a new eye movement program has to be prepared. This requires first a selection of the saccade direction and then the setting of the saccade amplitude. Thus, changes in saccade direction require a radical modification in the oculomotor program, while changes in saccade amplitude imply only a readjustment of an existing program.
The same process occurs with attention deployment in the absence of eye movements. When an unexpected imperative stimulus appears in the same hemifield where the attention is deployed, only the amplitude of attention program has to be modified. In contrast, when the unexpected imperative stimulus occurs in the hemifield opposite to the cued one, the direction of attention has to be modified. Because the direction change involves the construction of a new ocular motor program, this process is more time-consuming.
Strong evidence in favor of the premotor theory of attention came from a series of experiments that used vertical saccades as the measured variables, and visual or acoustic stimuli as attention cues. The results showed that when participants paid attention to a given spatial location, the trajectory of a saccade triggered by an imperative stimulus deviated contralateral to the attention site ( Figure 2, Sheliga et al., 1995). This finding indicates that allocating attention to a given position necessarily activates the eye movement system, even if no ocular movement is required. Experiments showing that the discrimination accuracy is higher when the discrimination target is presented at the saccade target than when it is presented at adjacent positions (Deubel and Schneider, 1996) are in line with this conclusion.
Finally, compelling evidence for a causal relation between eye movements and attention was provided by experiments in which individuals could not perform a saccade towards the cued position either because a constrained extreme eye deviation or as a consequence of a peripheral palsy. The rational underlying these experiments was the following. If the involvement of the oculomotor system during spatial attention tasks reflects a causal relationship between eye movement programming and attention, the modifications in eye movement abilities should be paralleled by a modification in the ability to orient visuospatial attention, while, if not, the eye position in the orbits should be irrelevant. The results showed that when the eyes cannot move towards a certain location also the attention cannot move (Craighero et al., 2001; Craighero et al., 2004).
A strong support for the premotor theory of attention came from neuroimaging studies. These studies clearly showed that visuospatial attention and eye movements share the same cortical neuronal network (Corbetta et al., 1998; Nobre et al., 2000). No system of distinct cortical areas was activated exclusively by covert attention or by a saccade task ( Figure 3). The network found in those studies includes the putative human homologue of monkey frontal eye fields (FEF) and the lateral intraparietal area (LIP), both areas known to be involved in voluntary control of eye movements.
A recent event-related fMRI experiment on congenitally blind individuals (Garg et al., 2007) supported the notion that eye movement and attention planning cannot be separated. In a covert orienting task, with endogenous verbal cues and lateralized auditory targets, these authors found a robust stimulus-locked FEF activation in congenitally blind individuals, similar to that observed in sighted controls with eyes closed.
Further support to the premotor theory of attention came from physiological studies in monkeys. Kustov and Robinson (1996) recorded neuron activity from the monkey superior colliculus, a center crucially involved in eye movements. The results showed an increase in the excitability of this structure when a monkey paid attention to a given location in space. Particularly striking was the observation that the collicular excitation also changed when the monkey was instructed to make a manual response and to keep the eyes still after imperative stimulus presentation.
In a very brilliant electrophysiological experiment Moore and Fallah (2001) reported that it is possible to enhance spatial perception by altering oculomotor signals within the brain. The authors trained two monkeys to make manual responses at the detection of a transient dimming of a peripheral visual target and tested the effects of FEF microstimulation on monkeys’ performance. The results showed that subthreshold stimulation of a particular subregion of FEF determined a decrease in the psychophysical threshold for stimulus detection, but only when the target stimulus was positioned in the motor field corresponding to the stimulation point. This finding provided evidence of a direct effect of eye movement control on the allocation of spatial attention ( Figure 4).
Subsequently, Moore and Armstrong (2003) also showed that a subthreshold microstimulation of the FEF enhanced visual responses in V4 neurons located at corresponding spatial locations. Similar results have been also obtained by Ekstrom et al. (2008) by combining fMRI and chronic electrical microstimulation in awake, behaving monkeys. Stimulation of subregions in the FEF determined a strong activation of higher-order visual areas, as those obtained during shifts in attention. These findings suggest that the gain of visual responses in extrastriate cortex is modulated by the same activity that elicits a saccade to a particular location. This provides evidence for a mechanism modulating visual responses at the attended locations, when saccades are planned, but not executed. The results establishing a causal effect of FEF stimulation on visual cortex have been recently confirmed in humans by Ruff and colleagues (Ruff et al., 2006). In an experiment combining fMRI with transcranial magnetic stimulation (TMS) they showed that stimulating human FEF produces systematic effects on fMRI signal in early human visual cortex, including even area V1.
In everyday life most of our actions in space are preceded by foveation and this gives the eye movement system a special central position in spatial attention. There are, however, some conditions in which we do not use, or do not use primarily, eye movements to select stimuli in space. In these cases spatial attention should depend on circuits other than those related to eye movements. Probably the best documented evidence in favor of spatial attention not related to eye movements is that deriving from experiments conducted by Tipper et al. (1992). They studied, in normal participants, the effect of an irrelevant stimulus (distractor) located in or out of the arm trajectory necessary to execute a pointing response. The result showed that an interference effect was present only when the distractor was located in the trajectory necessary to execute a pointing response. Control experiments suggested that the effect was not due to a purely visual representation of the stimuli or to spatial attention related to eye movements. Rather, the organization of the arm-hand movement determined a change in the attentional relevance of stimuli close to the hand or far from it.
Similar results are those by Eimer and colleagues on covert manual movement preparation (Eimer et al. 2005). The authors recorded ERPs in normal volunteers during the interval between a visual cue and a subsequent visual Go/Nogo signal. During this interval the volunteers were instructed to prepare to lift their left or right index finger. Results showed that somatosensory ERP components were enhanced when task-irrelevant tactile probes were delivered during response preparation to the hand involved in an anticipated response, indicating that covert manual movement preparation influences somatosensory processing.
Recently, a series of experiments tested whether the premotor theory of attention could be extended from orienting of attention to spatial locations to orienting of attention to graspable objects. This hypothesis came from neurophysiological studies of the monkey parietal (anterior intraparietal area, AIP) and premotor cortex (F5). These studies showed that many neurons located in these areas selectively discharge during the execution of object grasping and some also during object observation. The visual responses of the latter neurons were only present when there was a congruence between the intrinsic properties of the object (size and shape) and the coded grip (precision grip, whole hand prehension) (Rizzolatti and Luppino, 2001). These findings indicate that every time a graspable object is observed, these visuo-motor neurons are activated eliciting a potential specific motor act. On the basis of these observations, it is possible to draw a parallelism between spatial attention and object-related attention: as in the case of spatial attention eye movement preparation selects a given spatial location, the preparation of a grasping movement selects an object with specific intrinsic characteristics.
Tucker and Ellis (1998) provide evidence that a potential specific motor act is evoked every time a graspable object is presented. Normal human volunteers were presented with photographs of common graspable objects. The volunteers had to respond with their left or right hand. The results showed that the reaction times were faster when the response was executed with the hand most suited to grasp the presented object. Very recently, Adamo and Ferber (2009), using the attentional blink paradigm and event-related potentials, confirmed these findings demonstrating that the presentation of a tool leads to attentional enhancement towards subsequently presented objects that are consistent with the action afforded by the tool.
In a recent experiment it has been tested if the shape of the to-be-grasped object influences observer’s prediction about the way in which the agent will act onto that object (Craighero et al. 2008). To this purpose volunteers were requested to detect the instant at which the demonstrator’s hand touched the object during a grasping and lifting action. Two types of grasping were presented, differing for the type of fingers opposition space: in one case the grasping was the one more commonly used to grasp the presented object, in the other case it was a less appropriate one. In this way a conflict was present when the action evoked by object observation didn’t coincide with that executed by the experimenter.
Results showed that the response times were shorter for ‘suitable’ grasping trials than for ‘not suitable’ ones, indicating that the observed action is recognized faster when the corresponding action representation is already activated by the vision of the to-be-grasped object. This facilitation reflects a modification in the perceptual salience of others’ action, thus linking this effect to classical attentional phenomena.
In line with these findings is the observation that the preparation of a grasping movement to a given object determines the allocation of attention to the intrinsic characteristic of that object. Craighero et al., (1999) asked volunteers to be ready to grasp a bar and then to grasp it as fast as possible on the presentation of a visual rectangle differently oriented ( Figure 5). The results showed that grasping reaction times to rectangles congruently oriented were faster than reaction times to those incongruently oriented. These data indicate that the preparation to act on an object produces a faster visual processing of the stimuli congruent with that object.
Premotor theory of attention has been originally advanced on the basis of indirect evidence plus anatomical and physiological considerations on the organization of the nervous circuits involved in sensorimotor transformation. In these last years a large number of evidence coming from neurophysiological studies on monkeys, and behavioral and brain imaging studies in humans, clearly demonstrated the validity of the main tenet of the premotor theory of attention, that is that attention derives from the activity of the sensorimotor circuits, rather than from a specific center devoted to attention. Furthermore, while the initial version of the premotor theory of attention concerned only those aspects of attention that are linked to the oculomotor system, recent evidence enlarged the scope of the theory including attention effects deriving from motor programs related to reaching and grasping movements. This enlargement provides a unitary explanation of a variety of visual-related attentional phenomena.
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