Cortical Magnification Factors in Human Primary Visual Cortex Correlate with Acuity Thresholds.
     We show that visual acuity in humans is limited by the amount of primary visual cortex (V1) devoted to a region of the visual field, called the linear cortical magnification factor (M). We used fMRI to measure M in V1, and two psychophysical tasks to measure acuity (Vernier and grating) in the same ten observers. Across observers, the decrease in M with increasing eccentricity predicts the corresponding decrease in acuity for both tasks. Furthermore, observers with lower grating acuity thresholds, measured with laser interferometry, had a significantly greater overall M. These results establish faithful estimates of cortical limits to visual acuity.

Mechanisms of attention
     Spatial attention can be thought of as the process that enables us to scrutinize one region of visual space at the expense of others. Shockingly little is known about the neural mechanisms of attention. Our lab uses fMRI technology to investigate the effects of spatial attention in several early visual areas (V1, V2, V3, V3A, V4, VP, MT, and the LGN). First, we intend to localize and further quantify the effects of spatial attention observed in these early visual areas. We will map the retinotopic organization of attentional effects and determine whether the pattern and magnitude of activity observed in the brain correlates with the location and degree to which spatial attention is allocated. Second, we seek to localize the regions of extrastriate visual cortex that respond when attention is differentially allocated to specific object features. Third, we hope to establish a quantitative and predictable relationship between neural activity and behavioral responses during tasks that tax attentional resources.

Physiology of motion perception
     Visual motion can be represented in terms of the dynamic visual features in the retinal image or in terms of the moving surfaces in the environment that give rise to these features. For natural images, the two types of representation are necessarily quite different as many moving features are only spuriously related to the motion of surfaces in the visual scene. Such "extrinsic" features arise at occlusion boundaries and may be detected by virtue of the depth-ordering cues that exist at those boundaries. Though a number of studies have provided evidence of the impact of depth ordering on the perception of visual motion, few attempts have been made to identify the neuronal substrate of this interaction. To address this issue, we devised a simple contextual manipulation that decouples surface motion from the motions of visual image features. By altering the depth ordering between a moving pattern and abutting static regions, the perceived direction of motion changes dramatically while image motion remains constant. When stimulated with these displays, many neurons in the primate middle temporal visual area (MT) represent the implied surface motion rather than the motion of retinal image features. These neurons thus use contextual depth-ordering information to achieve a representation of the visual scene consistent with perceptual experience.

Disparity-based surface segmentation influences perceived motion and oculomotor responses.
     A correspondence between eye movement direction and direction of perceived motion was previously demonstrated using stimuli defined by color (Dobkins et al. 1992). We have extended the results of Dobkins et al. using image cues for perceptual transparency/occlusion (Duncan et al. 1994). We found that perceived direction of motion co-varies with eye movement direction in the absence of changes in the stimulus. Hence, tracking eye movement signals could not be entirely dependent on an "early" mechanism that is ignorant of contextual information. Our results also provide evidence for the claim that oculomotor/perceptual correlates generalize across different cues for image segmentation. Several lines of evidence have converged to suggest that motion signal integration and image segmentation involve area MT of the rhesus monkey; this physiological mechanism may be a crucial step in the perception of motion and the generation of eye movements. The observed correspondence between perceptual state and movements of the eyes implies that the former can be inferred from the later. This relationship has afforded two significant advantages: First, it has allowed us to confirm that the neurophysiological response variations we have previously observed in monkeys parallel changes in perceptual state (Stoner & Albright, 1992). Second, it has allowed us to begin to explore the neural correlates of foreground/background assignment and perceptual metastability in image segmentation.