Neurophysiology of Control

As with neuroanatomical studies of attentional control, there has been a substantial amount of research on single-neuron recordings from a range of brain areas. Those regions relevant to the biased competition framework are regions in the parietal and frontal lobes; other important areas, such as the superior colliculus or pulvinar, will not be reviewed here.

Many neurophysiological studies investigate overt spatial attention, in which the eyes overtly move to an attended location, in contrast to covert spatial attention, in which the eyes do not move. One consequence of overt shifts of spatial attention is that stimuli in a visual scene occupy different retinal locations from one eye movement to the next. Covert spatial attention appears to shift before the overt eye movement, allowing the representation of a visual scene to be updated prior to the eye movement. Neurons in the parietal lobe appear to play a role in controlling the focus of spatial attention.

Evidence for parietal lobe involvement in covert shifts of spatial attention derives from single-unit recordings from the lateral intraparietal (LIP) area in monkeys. Neurons in LIP have topographically mapped receptive fields that respond to visual stimulation (Fig. 2a). When a monkey makes an eye movement to a new location, the receptive field of a LIP neuron will also fall in a new location (Fig. 2c). Prior to the eye movement, however, a LIP neuron will respond to visual stimuli at a location based on the planned eye movement that has not been executed (Fig. 2b). That is, the receptive fields of LIP neurons are remapped in anticipation of an eye movement. This shift of the LIP representation of space may provide the neural mechanism for covert shifts of attention that precede and anticipate overt shifts involving eye movements.

The ability of LIP neurons to update their representation of space implies that this area receives inputs regarding the intended eye movement. This input likely comes from the frontal eye fields (FEFs), suggesting that updating the spatial representation is based on endogenous, top-down factors. Neurons in LIP are also able to alter their firing based on exogenous, bottom-up factors, such as the abrupt appearance of a stimulus in a LIP neuron's receptive field. The appearance of a new stimulus inside a LIP neuron's receptive field results in a large increase in the neuron's firing rate. However, if an eye movement brings a stationary stimulus into the neuron's receptive field, only a weak neural response is produced. The abrupt appearance of a stimulus is the critical parameter for evoking a large neural response: If a stimulus appears shortly before an eye movement (approximately 400 ms), there is a large neural response when the eye movement brings the new stimulus into the LIP

Figure 2 Remapping of receptive fields in area LIP in response to an intended eye movement. (a) The center tree is fixated, and the sun falls within an LIP receptive field. (b) An eye movement to the other tree is planned, and the LIP neuron's receptive field is remapped in accordance with the intended movement. The cloud falls within the receptive field, even though the eyes remain fixated on the center tree. (c) The eye movement is performed, allowing the second tree to be fixated.

Figure 2 Remapping of receptive fields in area LIP in response to an intended eye movement. (a) The center tree is fixated, and the sun falls within an LIP receptive field. (b) An eye movement to the other tree is planned, and the LIP neuron's receptive field is remapped in accordance with the intended movement. The cloud falls within the receptive field, even though the eyes remain fixated on the center tree. (c) The eye movement is performed, allowing the second tree to be fixated.

neuron's receptive field. The ''newness'' or salience of the stimulus, indicated by the recency of its appearance, in part controls the response of LIP neurons and, presumably, covert spatial attention.

Finally, consistent with neuroimaging data, frontal lobe areas participate in the control of attention. In visual search, the FEF is involved in selecting the target to which an eye movement will be directed. Monkeys viewed displays that contained a target that differed from a field of distractors by one feature (Fig. 1a); they were trained to make an eye movement to this target. Prior to the saccade, FEF neurons discriminated target items from distractor items. If the target fell within a FEF neuron's receptive field, the neuron responded vigorously; if a distractor fell within the receptive field, the neuron responded weakly. Additional studies demonstrated that the enhanced firing of these FEF neurons was not in response to a bottom-up capture of attention by the odd item in the display. Other monkeys were trained to make eye movements to a target defined by color (e.g., make an eye movement to any white target). If a monkey trained to move to a white target viewed Fig. 1a, this monkey would respond by generating an eye movement to any of the white bars. Despite the presence of multiple targets in this situation, FEF neurons continue to show larger firing rates when targets fall in their receptive fields than when the single distractor falls within their receptive field.

In addition to FEF, other areas in prefrontal cortex participate in attentional selection. Many studies implicate dorsolateral prefrontal areas in selection; these studies examined search tasks in which the monkey first sees a cue object that depicts the target for which the monkey must search. Following the presentation of the cue, a display of objects appeared, and the monkey had to search for and remember the location of the target object. During the presentation of the search array, the activity of neurons in the dorsolateral prefrontal cortex was sensitive to the visual attributes of the target only; the distractors were effectively filtered out and did not influence the response of prefrontal neurons. Complementary studies have been performed in extrastriate regions such as the inferior temporal cortex with similar results. The selectivity to target attributes occurs earlier for pre-frontal neurons than for inferotemporal neurons, suggesting that target selection first occurs in frontal areas and provides the top-down target template that guides selection in extrastriate areas.

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