During binocular rivalry, two incompatible monocular images compete for perceptual dominance, with one pattern temporarily suppressed from conscious awareness. We measured fMRI signals in early visual cortex while subjects viewed rival dichoptic images of two different contrasts; the contrast difference served as a 'tag' for the neuronal representations of the two monocular images. Activity in primary visual cortex (V1) increased when subjects perceived the higher contrast pattern and decreased when subjects perceived the lower contrast pattern. These fluctuations in V1 activity during rivalry were about 55% as large as those evoked by alternately presenting the two monocular images without rivalry. The rivalry-related fluctuations in V1 activity were roughly equal to those observed in other visual areas (V2, V3, V3a and V4v). These results challenge the view that the neuronal mechanisms responsible for binocular rivalry occur primarily in later visual areas.

Shifting attention away from a visual stimulus reduces, but does not abolish, visual discrimination performance. This residual vision with 'poor' attention can be compared to normal vision with 'full' attention to reveal how attention alters visual perception. We report large differences between residual and normal visual thresholds for discriminating the orientation or spatial frequency of simple patterns, and smaller differences for discriminating contrast. A computational model, in which attention activates a winner-take-all competition among overlapping visual filters, quantitatively accounts for all observations. Our model predicts that the effects of attention on visual cortical neurons include increased contrast gain as well as sharper tuning to orientation and spatial frequency.

The neural basis of pattern recognition is a central problem in visual neuroscience. Responses of single cells were recorded in area V4 of macaque monkey to three classes of periodic stimuli that are based on spatial derivative operators: polar (concentric and radial), hyperbolic, and conventional sinusoidal (Cartesian) gratings. Of 118 cells tested, 16 percent responded significantly more to polar or hyperbolic (non-Cartesian) gratings than to Cartesian gratings and only 8 percent showed a significant preference for Cartesian gratings. Among cells selective for non-Cartesian gratings, those that preferred concentric gratings were most common. Cells selective for non-Cartesian gratings may constitute an important intermediate stage in pattern recognition and the representation of surface shape.

Gender differences in brain activation during working memory tasks were examined with fMRI. Seventeen right-handed subjects (nine males, eight females) were studied with four different verbal working memory tasks of varying difficulty using whole brain echo-planar fMRI. Consistent with prior studies, we observed activation of the lateral prefrontal cortices (LPFC), the parietal cortices (PC), and additionally, caudate activation in both sexes. The volume of activated brain tissue increased with increasing task difficulty. For all four tasks, the male subjects showed bilateral activation or right-sided dominance (LPFC, PC and caudate), whereas females showed activation predominantly in the left hemisphere. The task performance data demonstrated higher accuracy and slightly slower reaction times for the female subjects. Our results show a highly significant (p < 0.001) gender differences in the functional organization of the brain for working memory. These gender-specific differences in functional organization of the brain may be due to gender-differences in problem solving strategies or the neurodevelopment. Therefore, gender matching or stratification is required for studies of brain function using imaging techniques.

Injury to the neural substrate caused by HIV infection may necessitate greater attentional modulation of the neural circuits, hence a greater use of the brain reserve; additional activation of the frontal lobes is required to perform the more complex tasks. The task-dependent increased frontal activation in patients with HIV suggests that the neural correlate of attentional deficits may be excessive attentional modulation as a result of frontostriatal brain injury.

Weused a concurrent-task paradigm to investigate the attentional cost of simple visual tasks. As in earlier studies, we found that detecting a unique orientation in an array of oriented elements ("popout") carries little or no attentional cost. Surprisingly, this is true at all levels of performance and holds even when pop-out is barely discriminable. Wediscuss this finding in the context of our previous report that the attentional cost of stimulus detection is strongly influenced by the presence and nature of other stimuli in the display (Braun, 1994b). For discrimination tasks, we obtained a similarly mixed outcome: Discrimination of letter shape carried a high attentional cost whereas discrimination of color and orientation did not. Taken together, these findings lead us to modify our earlier position on the attentional costs of detection and discrimination tasks (Sagi & Julesz, 1985). We now believe that observers enjoy a significant degree of "ambient" visual awareness outside the focus of attention, permitting them to both detect and discriminate certain visual information. We hypothesize that the information in question is selected by a competition for saliency at the level of early vision.

To clarify the relation between attention and microsaccades, we monitored microsaccades while observers performed tasks with different attentional demand. In four high-demand conditions, observers shifted attention covertly to a peripheral location, or focused attention at fixation. Three corresponding low-demand conditions on physically identical displays provided a basis for comparison. Our results revealed two distinct effects of attentional load: higher loads were associated consistently with lower microsaccade rates, but also with increased directional selectivity (up to 98% congruent). In short, when microsaccades were most rare, the direction of microsaccades proved most informative about the location of the attention focus. The detailed time-courses of the two effects differed, however, suggesting that they may reflect independent processes.

We investigated the relationship between focal attention and a feature-gradient detection that is performed in a parallel manner. We found that a feature gradient can be detected without measurable impairment of performance even while a concurrent form-recognition task is carried out, in spite of the fact that the form-recognition task engages focal attention and thus removes attentive resources from the vicinity of the feature gradient. This outcome suggests strongly that certain perceptions concerning salient boundaries and singularities in a visual scene can be accomplished without the aid of resource-limited processes, such as focal attention, and, by implication, that there may exist two distinct perceptual faculties (one attentive, the other not) that are able to bring complementary kinds of visual information simultaneously to our awareness.In certain experimental situations, resources for the processing of visual information appear to become concentrated in one subregion of the field of view. For example, when a precue reveals where in visual space a test target is about to appear, visual performance improves in the cued region well above the performance level elsewhere in the field of view (Eriksen