Object-centered not scene-based visual neglect

The attentive homunculus: now you see it, now you don't

The nature of the neural system that directs our attention toward selective items in the extrapersonal world is a longstanding and interesting puzzle. The ability to image the human brain at work non-invasively using positron-emission tomography or functional magnetic resonance has provided the means to investigate this issue. In this article, I review the contributions of brain imaging toward the characterization of attentional control in the human brain. The majority of experiments to date have investigated visual spatial orienting. A consistent pattern of brain areas has been revealed, comprising most notably the posterior parietal cortex around the intraparietal sulcus and frontal regions including the frontal eye fields. The brain areas implicated in the control of visual spatial attention were noted to resemble those involved in the control of eye movements, and direct experimental comparisons supported a tight link between the two systems. The findings suggested a sensible view of the attentional 'homunculus' as a distributed neural system related to the control of eye movements. Eye movements form perhaps the most basic orienting response, and can be shifted rapidly and efficiently based on multiple frames of reference. Some attention experiments using objects and features instead of spatial locations as the target of selection also obtained similar patterns of parietal-frontal activations, rendering further support to this view of the attentional control system. Some recent experiments, however, have cautioned against a premature conclusion regarding the ubiquity of the attentional control system revealed by studies of visual spatial attention. Different parietal and frontal regions become engaged when attention is shifted along non-spatial dimensions, such as when attention is directed toward a particular motor act or toward a specific point in time. In these cases, the neural system resembles those involved in the control of limb movements. The attentional homunculus thus begins to dissolve. The alternative view suggested is that attentional control may be a property of specialized parietal-frontal systems that transform perception into action. Future studies will be needed to validate this view of attention, or to provide a more mature understanding of its true nature.

Simulating a lesion in a basis function model of spatial representations: comparison with hemineglect

The basis function theory of spatial representations explains how neurons in the parietal cortex can perform nonlinear transformations from sensory to motor coordinates. The authors present computer simulations showing that unilateral parietal lesions leading to a neuronal gradient in basis function maps can account for the behavior of patients with hemineglect, including (a) neglect in line cancellation and line bisection experiments; (b) neglect in multiple frames of reference simultaneously; (c) relative neglect, a form of what is sometime called object-centered neglect; and (d) neglect without optic ataxia. Contralateral neglect arises in the model because the lesion produces an imbalance in the salience of stimuli that is modulated by the orientation of the body in space. These results strongly support the basis function theory for spatial representations in humans and provide a computational model of hemineglect at the single-cell level.

Objects and their locations in exogenous cuing

Anatomical, physiological, and behavioral studies provide support for separate object- and location-based components of visual attention. Although studies of object-based components have usually involved voluntary attention, more recent evidence has suggested that objects may play an independent role in reflexive exogenous orienting, at least at long stimulus onset asynchronies (SOAs). In the present experiments, the role of objects in reflexive attentional orienting was investigated by developing a task in which location and object cuing could be separately examined for both short and long SOAs. Typical location cuing effects were obtained, indicating facilitation at short cue-target intervals and inhibition of return (IOR) at longer intervals. In contrast, object cuing resulted in facilitation for cued objects at long cue-target intervals and no object-based IOR. Interestingly, object cuing primarily affected targets at cued locations, and not those at uncued locations. Together, the experiments examine the interactive nature of objects and locations in exogenous orienting and seem most consistent with a location-mediated view of object-based orienting.

Paradox and cross purposes in recent work on consciousness

Functionalists about consciousness identify consciousness with a role; physicalists identify consciousness with an implementer of that role. The global workspace theory of consciousness fits the functionalist perspective, but the physicalist sees consciousness as a biological phenomenon that implements global accessibility.

Naming rotated pictures and the riddle of object-centred neglect

In this study eight patients with left neglect were asked to name chimerical pictures of objects and animals with different spatial orientation: standard upright position, rotated 180 degrees, rotated 90 degrees to the right, and rotated 90 degrees to the left. All patients showed the typical pattern of egocentric neglect. They omitted the left part of the normally upright pictures and the right part of the inverted stimuli, now falling in the left space. When the pictures were tilted 90 degrees to the right, they reported the two component objects with the same level of accuracy. However, at variance with egocentric neglect, when the chimerical pictures were rotated 90 degrees to the left, the patients omitted the left half of the stimulus more often than the right half. We propose that since in the latter condition the less informative lower part of the pictures was available in the non-neglected space, the patients mentally rotated the perceived stimulus and aligned it with its upright orientation before naming its component parts. In our interpretation, the mental orientation and normalisation of rotated stimuli might underlie all the reported evidence of object-centred neglect for non-orthographic stimuli.

Shifting attention between objects

Experiment 1 used a modified spatial cueing paradigm that was introduced by Egly et al. [J. Exp. Psychol. Gen. 123 (1994) 161] to investigate the cost incurred in shifting attention within an object as opposed to shifting attention between objects. Subjects were presented with two outline rectangles and had to detect a target (a luminance increment) that could appear in the cued location (valid trials), in an uncued location inside the cued rectangle (inside-invalid trials), or in an uncued location inside the uncued rectangle (outside-invalid trials). Valid trials were faster than invalid trials, and inside-invalid trials were faster than outside-invalid trials. In Experiment 2, the two rectangles were joined to form a unitary object. Here, no difference was found between outside-invalid trials and inside-invalid trials. Experiment 3 showed that the delayed response on outside-invalid trials in Experiment 1 was not due to attention needing to cross the figural borders in order to re-orient to the uncued rectangle. The results were interpreted as showing that an extra cost is incurred for shifting attention between different objects.

Modulation of unilateral neglect as a function of direction of object motion

Unilateral neglect is a disorder involving difficulty in attending to the side of space contralesional to brain injury. Two recent experiments have shown that task-irrelevant background motion reduces neglect on line bisection tasks; however, task-relevant motion has not been assessed. We investigated the effect of task-relevant object motion on left neglect using a moving cube presented on a computer screen. Subjects responded to cued corners of the cube as it moved across the screen. Direction of cube motion had a significant impact on the magnitude of neglect. Responses to left hemispace targets appearing on a leftward moving cube were equal to patients' fastest responses. In contrast, responses to left hemispace targets appearing on a rightward moving cube were the slowest of all responses. These results demonstrate that contralesional object motion is capable of normalising neglect patients' detection of contralesional targets.

Object-based visual selective attention and perceptual organization

We report the results of four experiments that were conducted to examine both the representations that provide candidate entities available for object-based attentional selection and the influence of bottom-up factors (i.e., geometric and surface characteristics of objects) and top-down factors (i.e., context and expectancies) on the selection process. Subjects performed the same task in each of the experiments. They were asked to determine whether two target properties, a bent end and an open end of a wrench, appeared in a brief display of two wrenches. In each experiment, the target properties could occur on a single wrench or one property could occur on each of two wrenches. The question of central interest was whether a same-object effect (faster and/or more accurate performance when the target properties appeared on one vs. two wrenches) would be observed in different experimental conditions. Several interesting results were obtained. First, depending on the geometric (i.e., concave discontinuities on object contours) and surface characteristics (i.e., homogeneous regions of color and texture) of the stimuli, attention was preferentially directed to one of three representational levels, as indicated by the presence or absence of the same-object effect. Second, although geometric and surface characteristics defined the candidate objects available for attentional selection, top-down factors were quite influential in determining which representational level would be selected. Third, the results suggest that uniform connectedness plays an important role in defining the entities available for attention selection. These results are discussed in terms of the manner in which attention selects objects in the visual environment.

Scene-based and object-centered inhibition of return: evidence for dual orienting mechanisms

We investigated whether inhibition of return (IOR) could be observed in location-based, scene-based, and object-centered frames of reference. IOR was found to move both with a separate cued object (scene-based) and with a location within a single rotating object (object-centered). Importantly, however, IOR was also associated with the environmental location cued when cuing was of a separate object (scene-based), whereas facilitation of the cued location was found when cuing was of a component within an object. These results suggest that location is of central importance to scene-based representations of separate objects, which appear to be encoded in viewer-centered coordinates, whereas environmental locus is of little relevance when attention orients within a single object. The results also provide further evidence for the coexistence of both excitation and inhibition associated with uninformative exogenous cues.

The effects of practice on object-based, location-based, and static-display inhibition of return

We report two experiments that examine the effects of practice on object-based, location-based, and static-display inhibition of return (IOR). The results are clear: All three effects get smaller with practice. These findings are at odds with the results of Müller and von Mühlenen (1996), who failed to observe object-based IOR and detected no effect of practice on static-display IOR. However, their subjects were more practiced than ours prior to data collection. We suggest, therefore, that the reducing effect of practice on IOR have occurred in their unrecorded practice sessions. We also discuss a two-process model in which IOR is seen as the net effect of underlying inhibitory and excitatory processes. In such models (e.g., Solomon & Corbit, 1974), practice often results in a reduction of the net effect of the two processes.

Body-centered representations for visually-guided action emerge during early infancy

The nature of the spatial representations that underlie simple visually guided actions early in life were investigated through the application of a 'double-step' saccade paradigm to 3- and 7-month-old infants. Saccades in the older infants, like those observed in adults, were executed within body-centered spatial coordinates that take into account the effects of intervening eye movements. In contrast, younger infants tended to respond according to the targets' retinocentric locations and did not incorporate the effects of displacements caused by previous saccades. These results indicate that contrary to prevailing views, body-centered representations for action are not present from birth but emerge, probably through experience, over the first few months of life.

A new view of hemineglect based on the response properties of parietal neurones

Lesion studies of the parietal cortex have led to a wide range of conclusions regarding the coordinate reference frame in which hemineglect is expressed. A model of spatial representation in the parietal cortex has recently been developed in which the position of an object is not encoded in a particular frame of reference, but instead involves neurones computing basis functions of sensory inputs. In this type of representation, a nonlinear sensorimotor transformation of an object is represented in a population of units having the response properties of neurones that are observed in the parietal cortex. A simulated lesion in a basis-function representation was found to replicate three of the most important aspects of hemineglect: (i) the model behaved like parietal patients in line-cancellation and line-bisection experiments; (ii) the deficit affected multiple frames of reference; and (iii) the deficit could be object-centred. These results support the basis-function hypothesis for spatial representations and provide a testable computational theory of hemineglect at the level of single cells.

Object-centered neglect for letters: do informational asymmetries play a role?

In a study of right-parietal patients, Behrmann and Moscovitch [Journal of Cognitive Neuroscience, Vol. 6, pp.1-16, 1994] found object-centered left neglect for asymmetrical but not symmetrical letters, leading them to attribute this neglect to the objects' canonical handedness or intrinsic asymmetry. Using a similar task, we find the same results in neurologically intact observers. However, right-sided bias was observed only when the letters were identified. Because asymmetrical letters are distinguished by features on their right sides, we propose that the apparent object-centered bias in normals and in right-parietal patients may arise from attending to identifying features rather than from representational differences between symmetrical and asymmetrical letters.

Representation of object-centered space in the primate frontal lobe

Object-centered spatial awareness--awareness of locations of parts relative to a an object--plays an important role in perception and action. Indirect evidence from psychological and neuropsychological studies has indicated that this form of spatial awareness may be served by a cortical system in which neurons encode specific object-centered locations. We set out to obtain direct evidence for object-centered spatial selectivity by recording from single neurons in the frontal cortex of monkeys trained to make eye movements to particular locations on reference objects. We found that neurons in the supplementary eye field (SEF) fire differentially as a function of the location on an object to which an eye movement is directed.

Brain representation of object-centered space

Object-centered spatial awareness underlies many important cognitive functions, including reading, which requires registering the locations of letters relative to a word, and pattern recognition, which requires registering the locations of features relative to a whole pattern. Recent studies have elucidated the nature of the brain mechanisms underlying this form of spatial awareness by showing the attention tends to focus on objects rather than on regions of space: by demonstrating that each hemisphere contributes selectively to awareness of the opposite half of object space, and by revealing that neurons in some cortical areas are selective for particular locations in object space. These results are concordant with the general idea that imagining or attending to an object is accompanied by projecting its image onto a neural map of object-centered space. An important aim for future studies will be to test and extend this 'object map' hypothesis.