To explore the role of primary visual cortex in contour integration, we measured the contextual sensitivity of human contrast thresholds and of superficial layer complex cells in monkey V1. An observer's contrast detection was 40% improved by a second suprathreshold bar; the effect was decreased as the two bars were separated along their axis of orientation, were displaced from colinearity, and had their relative orientation changed. Recordings from V1 showed that 42% of complex cells demonstrated facilitation for a second bar outside their classical receptive fields with a similar dependency on relative location and orientation. Both effects were eliminated by an orthogonal line between the two iso-oriented lines. Multiple randomly placed and oriented lines in the receptive field surround often caused a reduction in a cell's response to an optimally oriented stimulus, but this inhibition could be eliminated by changing the orientation of a few of these elements to colinearity with the centrally located target.

To examine the role of primary visual cortex in visuospatial integration, we studied the spatial arrangement of contextual interactions in the response properties of neurons in primary visual cortex of alert monkeys and in human perception. We found a spatial segregation of opposing contextual interactions. At the level of cortical neurons, excitatory interactions were located along the ends of receptive fields, while inhibitory interactions were strongest along the orthogonal axis. Parallel psychophysical studies in human observers showed opposing contextual interactions surrounding a target line with a similar spatial distribution. The results suggest that V1 neurons can participate in multiple perceptual processes via spatially segregated and functionally distinct components of their receptive fields.

One of the fundamental tasks of the visual cortex is to integrate input from different parts of the retina, parsing an image into contours and surfaces, and then assembling these features into coherent representations of objects. To examine the role of the primary visual cortex in the integration of visual information, we measured the response properties of neurons under different stimulus conditions. Surprisingly, we found that even the most conventional measures of receptive field (RF) size were not fixed, but could vary depending on stimulus contrast and foregroundbackground relationships. On average, the length of the excitatory RF was 4-fold greater for a low-contrast stimulus than for a stimulus at high contrast. Embedding a high-contrast stimulus in a textured background tended to suppress neuronal responses and produced an enlargement in RF size similar to that observed by decreasing the contrast of an isolated stimulus. The results show that RF dimensions are regulated in a dynamic manner that depends both on local stimulus characteristics, such as contrast, and on global relationships between a stimulus and its surroundings.

Discrimination of simple visual attributes can improve significantly with practice. We have trained human observers to perform peripherally presented tasks involving the localization of short line segments and examined the specificity of the learning for the visual location, orientation, and geometric arrangement of the trained stimulus. Several weeks of training resulted in dramatic threshold reductions. The learning was specific for the orientation and location of the trained stimulus, indicating the involvement of the earliest cortical stages in the visual pathway where the orientation and location of stimuli are mapped with highest resolution. Furthermore, improvement was also specific for both the configuration of the trained stimulus and the attribute of the stimulus that was under scrutiny during training. This degree of specificity suggests that the learning cannot be achieved by cortical recruitment alone, as proposed in current models, but is likely to involve a refinement of lateral interactions within the cortex and possibly a gating of lower level changes by attentional mechanisms.