Physiology of Object Based Attention

In addition to the focus on parietal lobe involvement in neuropsychological studies of object selection, physiological studies have implicated other cortical areas in object attention. Multineuron recordings from macaque primary visual cortex (area V1) have implicated this area in object-based attention. Specifically, attending to one of two objects in a display results in enhanced firing for neurons whose receptive fields contain features of the attended object. Monkeys viewed scenes containing two objects (simple curves); one of the objects was connected to the fixation point, and monkeys were trained to attend to this object. The monkeys' task was to make an eye movement from the fixation point to the opposite end of the attended curve. Segments of the curves fell within receptive fields of V1 neurons. The neuronal responses were larger when a receptive field contained a segment of the attended curve than a segment of the unattended curve. This object-based attentional modulation in area V1 is important because previous studies of spatial attention were equivocal in finding V1 atten-tional modulation. Neurons in V1 exhibit attentional modulation when an object can act as the recipient or focus of attention; neurons in V1 do not appear to exhibit attentional modulation with blank displays or nonorganized cluttered displays. These neurophysio-logical findings are consistent with results from visual form agnosia, in which diffuse damage to early cortical visual areas (possibly V1) impaired object-based attention. Thus, there is growing evidence to support a central role for early cortical areas in perceptual organization and object attention.

Neurophysiological results have also suggested that object selection can occur in the oculomotor system as well as in purely sensory areas. Neurons in the supplementary eye field (SEF), located on the dor-somedial surface of the frontal lobes, appear to represent object-centered spatial selectivity for the direction of eye movements. That is, these neurons seem to code for spatial positions within an object, such as the left side of the object. Macaque monkeys were trained to make eye movements to the onset of a target spot. The target appeared in one of three conditions: alone in an otherwise blank display, at the left end of an object (a rectangle), or at the right end of an object. The absolute direction of the eye movement was identical in all three conditions; that is, the monkeys' eyes moved in exactly the same direction and same distance across these conditions. Although the eyes moved identically, a subset of SEF neurons fired at higher rates when eye movements were executed to a specific region of the object, regardless of its absolute spatial location. For example, some neurons responded vigorously to eye movements to the right side of the object; the same eye movement that landed on the left side of the object resulted in a smaller neuronal response. Thus, SEF neurons code for locations within an object; how these locations are coded—in a spatial reference frame such as the grouped array or in an object-centered coordinate frame—is unknown.

Finally, ERP studies with humans have investigated object-based selection. When viewing displays containing two superimposed surfaces (two transparent surfaces of different colored dots that rotate in opposite directions), observers can selectively attend to one of the two surfaces despite their spatial superimposition. If observers are instructed to attend to one of the surfaces, changes to the attended surface will produce evoked potential components with larger amplitudes than stimuli presented on the unattended surface. Specifically, changes on the attended surface generate larger P1 and N1 components compared to changes on the unattended surface. The similarity of these effects to the spatial attention effects described previously suggests that some object-based effects may be generated by neural processes shared with spatial attention. Attentional selection may be occurring from a grouped array in which motion segregation cues allow the two dot surfaces to be separated from one another in depth.

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