Neuropsychology of Object Based Attention 1 Object Centered Neglect

As with most neuropsychological studies of spatial attention, investigations of object-based selection have focused on patients with neglect following damage to the parietal lobe areas. Two key findings from neglect patients have implications for the neural basis of object selection. The first finding is that some of these patients exhibit object-centered neglect. The second finding is a hemispheric difference between object and spatial attention in patients with neglect.

Some patients with right parietal lobe damage (left neglect) neglect not only the left side of space but also the left side of an object, even when that object is located in the good (ipsilesional) visual field. Left neglect of objects can be observed when patients are asked to compare two novel objects and determine if they are the same or different. If the two objects are identical on their right sides but different on their left sides, neglect patients will incorrectly report that the objects are the same. Neglect patients continue to show this error when both objects are rotated 45° clockwise to place the differing feature in the good right visual field. If neglect occurred for spatial coordinates only, rotating part of the objects into the nonneglected visual field should have allowed the patients to notice the difference between the two objects. Because the neglected region of the object follows the object as it is rotated, the neglect must be defined relative to a reference point on the object, such as the object's principal axis (i.e., midline).

Although object-centered neglect has been the focus of many recent studies, these effects are not present in all neglect patients, which raises two points. First, failure to find object-centered neglect in some patients may occur because visual neglect is not a unitary disorder. Second, and more interesting, object-centered neglect may be unobservable because of the stimulus objects used. Objects that do not posses a strong principal axis or do not have well-defined left and right sides (a "canonical handedness") may not have a single object-centered reference point, which could prevent object-centered neglect. For example, a symmetric letter such as "A", does not have a canonical handedness because the left and right sides are identical; there is no need for the visual system to represent the sides of a symmetric letter differently. Asymmetric letters (e.g., "F") have a canonical handedness, and the visual system must represent the differences between the two sides. Accordingly, object-centered neglect is less likely for symmetric letters than for asymmetric letters, suggesting that the object's handedness may influence the allocation of visual attention.

Finally, what type of object representation is involved in object-centered neglect—a grouped array or an object-centered representation? As noted previously, the common assumption is that object-centered neglect occurs within an object-centered reference frame. However, recent simulation results from connectionist models demonstrate that object-centered neglect could arise from a spatiotopic grouped array representation. These simulations indicate that perceptually organized input to a damaged attentional system may be sufficient to demonstrate object neglect; no object-centered coordinates, principle axis, or canonical handedness need to be computed to explain the findings from neglect patients.

2. Lateralization of Object and Spatial Attention

The second finding from neglect patients relevant to object attention is an apparent hemispheric difference between object-based attention and spatially based attention. Patients with damage to the right parietal lobe most often present with hemispatial neglect in which the left side of space is ignored. (Patients who demonstrate object-centered neglect also exhibit spatial neglect.) Left neglect can also be demonstrated in these patients by using object-based attention tasks. For example, in the task shown in Fig. 5, left neglect patients are slower to detect targets in the contrale-sional visual field than in the ipsilesional visual field. However, left neglect patients show preserved object-based attention: They detect targets in the cued rectangle faster than targets in the uncued rectangle, and this object-based effect is found in both the contralesional and ipsilesional visual fields. In contrast to patients with left neglect, patients with damage to the left parietal lobe (right neglect) appear to have deficits in object-based attention. Right neglect patients also show slower target detection in the con-tralesional visual field than in the ipsilesional field. However, these patients exhibit larger object attention effects in the contralesional field; it is more difficult for these patients to switch attention from the cued rectangle to the uncued rectangle in the contralesional field than in the ipsilesional field. These hemispheric differences between object-based and spatially based attention have been supported by research with a split-brain patient. In performing the cued detection task shown in Fig. 5, this patient had greater difficulty switching attention from the cued rectangle to the uncued rectangle when stimuli were presented in the right visual field (left hemisphere) than when stimuli were presented in the left visual field (right hemisphere).

3. Other Patient Groups

Finally, a few patient groups besides neglect patients have been studied to understand object-based attention. For example, studies with a visual form agnosic reported impaired object-based attention but intact spatial attention following diffuse damage to the occipital cortices. Patients with visual form agnosia fail to perceive objects because of damaged early level visual areas. Although these patients have intact sensory processes (e.g., acuity and color perception), they are unable to organize visual features using the gestalt principles. The damage to perceptual organization processes impairs the ability to form perceptual groups, which prevents any object-based component of visual selection. Despite having impaired object-based attention, spatial selection appears to be intact following diffuse occipital damage, suggesting that object grouping processes appear to be dissociable from spatial selection processes. Thus, the parietal lobe attention system is not the only cortical system involved in mediating object selection.

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