Shifting attention in viewer- and object-based reference frames after unilateral brain injury

The aims of the present study were to investigate the respective roles that object- and viewer-based reference frames play in reorienting visual attention, and to assess their influence after unilateral brain injury. To do so, we studied 16 right hemisphere injured (RHI) and 13 left hemisphere injured (LHI) patients. We used a cueing design that manipulates the location of cues and targets relative to a display comprised of two rectangles (i.e., objects). Unlike previous studies with patients, we presented all cues at midline rather than in the left or right visual fields. Thus, in the critical conditions in which targets were presented laterally, reorienting of attention was always from a midline cue. Performance was measured for lateralized target detection as a function of viewer-based (contra- and ipsilesional sides) and object-based (requiring reorienting within or between objects) reference frames. As expected, contralesional detection was slower than ipsilesional detection for the patients. More importantly, objects influenced target detection differently in the contralesional and ipsilesional fields. Contralesionally, reorienting to a target within the cued object took longer than reorienting to a target in the same location but in the uncued object. This finding is consistent with object-based neglect. Ipsilesionally, the means were in the opposite direction. Furthermore, no significant difference was found in object-based influences between the patient groups (RHI vs. LHI). These findings are discussed in the context of reference frames used in reorienting attention for target detection

The Role of Spatial Frequency Selection in Local versus Global Perception

The aim of this research is to investigate the extent to which selective attention to spatial frequency (SF) mediates local versus global perception in general and in the context of face perception. Previous research has suggested a relationship between processing high versus low SFs and local versus global perception, respectively, but the nature of this relationship is debated. The experiments reported here demonstrate that attention to local and global aspects of a hierarchical display biases the flexible selection of relatively higher and relatively lower SFs during image processing. Moreover, the attentional selection of relative SF mediates the perceptual integration of the identity of elements in a hierarchical display with the level (local/global) at which they occur. Finally, the attentional selection of SF is shown to modulate early stages of face perception reflected in the N170-effect, a neurobiological index of face categorization that is particularly sensitive to face features. The N170-effect is found to be equally robust in response to the selection of both HSFs and LSFs in face-related stimuli, but the reliance on one SF scale or another is contingent upon the nature of the attended face-related stimulus, and whether its configuration is intact. Taken together, this investigation provides clear evidence that the flexible, top-down selection of low-level SF channels mediates the perception of local and global elements of visual displays, both for tightly controlled experimental stimuli as well as faces, a natural and frequently viewed hierarchical object category

The Role of Spatial Frequency Selection in Local versus Global Perception

The aim of this research is to investigate the extent to which selective attention to spatial frequency (SF) mediates local versus global perception in general and in the context of face perception. Previous research has suggested a relationship between processing high versus low SFs and local versus global perception, respectively, but the nature of this relationship is debated. The experiments reported here demonstrate that attention to local and global aspects of a hierarchical display biases the flexible selection of relatively higher and relatively lower SFs during image processing. Moreover, the attentional selection of relative SF mediates the perceptual integration of the identity of elements in a hierarchical display with the level (local/global) at which they occur. Finally, the attentional selection of SF is shown to modulate early stages of face perception reflected in the N170-effect, a neurobiological index of face categorization that is particularly sensitive to face features. The N170-effect is found to be equally robust in response to the selection of both HSFs and LSFs in face-related stimuli, but the reliance on one SF scale or another is contingent upon the nature of the attended face-related stimulus, and whether its configuration is intact. Taken together, this investigation provides clear evidence that the flexible, top-down selection of low-level SF channels mediates the perception of local and global elements of visual displays, both for tightly controlled experimental stimuli as well as faces, a natural and frequently viewed hierarchical object category

Orientation-specific surround suppression in the primary visual cortex varies as a function of autistic tendency

Individuals with autism spectrum disorder (ASD) exhibit superior performance on tasks that rely on local details in an image, and they exhibit deficits in tasks that require integration of local elements into a unified whole. These perceptual abnormalities have been proposed to underlie many of the characteristic features of ASD, but the underlying neural mechanisms are poorly understood. Here, we investigated the degree to which orientation-specific surround suppression, a well-known form of contextual modulation in visual cortex, is associated with autistic tendency in neurotypical individuals. Surround suppression refers to the phenomenon that the response to a stimulus in the receptive field of a neuron is suppressed when it is surrounded by stimuli just outside the receptive field. The suppression is greatest when the center and surrounding stimuli share perceptual features such as orientation. Surround suppression underlies a number of fundamental perceptual processes that are known to be atypical in individuals with ASD, including perceptual grouping and perceptual pop-out. However, whether surround suppression in the primary visual cortex (V1) is related to autistic traits has not been directly tested before. We used fMRI to measure the neural response to a center Gabor when it was surrounded by Gabors having the same or orthogonal orientation, and calculated a suppression index (SI) for each participant that denoted the magnitude of suppression in the same versus orthogonal conditions. SI was positively correlated with degree of autistic tendency in each individual, as measured by the Autism Quotient (AQ) scale, a questionnaire designed to assess autistic traits in the general population. Age also correlated with SI and with autistic tendency in our sample, but did not account for the correlation between SI and autistic tendency. These results suggest a reduction in orientation-specific surround suppression in V1 with increasing autistic tendency

Visual hemispatial neglect, re-assessed

Increased computer use in clinical settings offers an opportunity to develop new neuropsychological tests that exploit the control computers have over stimulus dimensions and timing. However, before adopting new tools, empirical validation is necessary. In the current study, our aims were twofold: to describe a computerized adaptive procedure with broad potential for neuropsychological investigations, and to demonstrate its implementation in testing for visual hemispatial neglect. Visual search results from adaptive psychophysical procedures are reported from 12 healthy individuals and 23 individuals with unilateral brain injury. Healthy individuals reveal spatially symmetric performance on adaptive search measures. In patients, psychophysical outcomes (as well as those from standard paper-and-pencil search tasks) reveal visual hemispatial neglect. Consistent with previous empirical studies of hemispatial neglect, lateralized impairments in adaptive conjunction search are greater than in adaptive feature search tasks. Furthermore, those with right hemisphere damage show greater lateralized deficits in conjunction search than do those with left hemisphere damage. We argue that adaptive tests, which automatically adjust to each individual's performance level, are efficient methods for both clinical evaluations and neuropsychological investigations and have the potential to detect subtle deficits even in chronic stages, when flagrant clinical signs have frequently resolved

Data from: Suppression and facilitation of human neural responses

Efficient neural processing depends on regulating responses through suppression and facilitation of neural activity. Utilizing a well-known visual motion paradigm that evokes behavioral suppression and facilitation, and combining 5 different methodologies (behavioral psychophysics, computational modeling, functional MRI, pharmacology, and magnetic resonance spectroscopy), we provide evidence that challenges commonly held assumptions about the neural processes underlying suppression and facilitation. We show that: 1) both suppression and facilitation can emerge from a single, computational principle - divisive normalization; there is no need to invoke separate neural mechanisms, 2) neural suppression and facilitation in the motion-selective area MT mirror perception, but strong suppression also occurs in earlier visual areas, and 3) suppression is not primarily driven by GABA-mediated inhibition. Thus, while commonly used spatial suppression paradigms may provide insight into neural response magnitudes in visual areas, they should not be used to infer neural inhibition