Foveal

About: Foveal is a research topic. Over the lifetime, 2652 publications have been published within this topic receiving 94120 citations.

The Organization and Operation of Inferior Temporal Cortex

Abstract: Inferior temporal cortex (IT) is a key part of the ventral visual pathway implicated in object, face, and scene perception. But how does IT work? Here, I describe an organizational scheme that marries form and function and provides a framework for future research. The scheme consists of a series of stages arranged along the posterior-anterior axis of IT, defined by anatomical connections and functional responses. Each stage comprises a complement of subregions that have a systematic spatial relationship. The organization of each stage is governed by an eccentricity template, and corresponding eccentricity representations across stages are interconnected. Foveal representations take on a role in high-acuity object vision (including face recognition); intermediate representations compute other aspects of object vision such as behavioral valence (using color and surface cues); and peripheral representations encode information about scenes. This multistage, parallel-processing model invokes an innately determined organization refined by visual experience that is consistent with principles of cortical development. The model is also consistent with principles of evolution, which suggest that visual cortex expanded through replication of retinotopic areas. Finally, the model predicts that the most extensively studied network within IT-the face patches-is not unique but rather one manifestation of a canonical set of operations that reveal general principles of how IT works.

Differential distributions of red-green and blue-yellow cone opponency across the visual field.

Abstract: The color vision of Old World primates and humans uses two cone-opponent systems; one differences the outputs of L and M cones forming a red-green (RG) system, and the other differences S cones with a combination of L and M cones forming a blue-yellow (BY) system. In this paper, we show that in human vision these two systems have a differential distribution across the visual field. Cone contrast sensitivities for sine-wave grating stimuli (smoothly enveloped in space and time) were measured for the two color systems (RG & BY) and the achromatic (Ach) system at a range of eccentricities in the nasal field (0-25 deg). We spatially scaled our stimuli independently for each system (RG, BY, & Ach) in order to activate that system optimally at each eccentricity. This controlled for any differential variations in spatial scale with eccentricity and provided a comparison between the three systems under equivalent conditions. We find that while red-green cone opponency has a steep decline away from the fovea, the loss in blue-yellow cone opponency is more gradual, showing a similar loss to that found for achromatic vision. Thus only red-green opponency, and not blue-yellow opponency, can be considered a foveal specialization of primate vision with an overrepresentation at the fovea. In addition, statistical calculations of the level of chance cone opponency in the two systems indicate that selective S cone connections to postreceptoral neurons are essential to maintain peripheral blue-yellow sensitivity in human vision. In the red-green system, an assumption of cone selectivity is not required to account for losses in peripheral sensitivity. Overall, these results provide behavioral evidence for functionally distinct neuro-architectural origins of the two color systems in human vision, supporting recent physiological results in primates.

Topographic studies on visual neurons in the dorsolateral prefrontal cortex of the monkey.

Abstract: The topographic distribution and organization of visual neurons in the prefrontal cortex was examined in alert monkeys. The animal was trained to fixate straight ahead onto a tinty, dim light spot. While he was fixating, we presented a stationary second light spot (RF spot) at various locations in the visual field and examined unit responses of the prefrontal neurons to the RF-spot stimulus. Many prefrontal neurons, especially those located in the relatively superficial layers of the cortex, responded with a phasic and/or tonic activation to the RF spot illuminating a limited extent of the visual field, a receptive field (RF) being so determined. The visual neurons were found to be widely distributed in the prearcuate and inferior dorsolateral areas. One hemisphere mainly represented the contralateral visual field. According to the location of the neurons in these areas, their visual properties varied with respect to RF eccentricity from the fovea and in size. The neurons located in the lateral part of the areas and close to the inferior arcuate sulcus had relatively small RFs representing the foveal and parafoveal regions. When the recording site was moved medially, the RFs became eccentric from the fovea and were larger. Then, the neurons located between the caudal end of the principal sulcus and the arcuate sulcus had RFs with a considerable eccentricity. The size of the RF became progressively larger for anteriorly located neurons and this occurred generally without a change in RF eccentricity. The visual neurons were not organized on a regular pattern in the cortex with regard to their RF direction (vector angle) from the foveal region. From these observations, we conclude, first, that the prearcuate and inferior dorsolateral areas of the prefrontal cortex are functionally differentiated so that the lateral area's function is related to central vision, while that of the medial area to ambient vision. Second, the RF representation on the cortex with loss of the vector relation may generate an interaction between separate objects in visual space and may subserve the control of attention performance.

Grouping, pooling, and when bigger is better in visual crowding

Abstract: In crowding, perception of a target is strongly deteriorated by nearby elements. Crowding is often explained by pooling models predicting that adding flankers increases crowding. In contrast, the centroid hypothesis proposes that adding flankers decreases crowding--"bigger is better." In foveal vision, we have recently shown that adding flankers can increase or decrease crowding depending on whether the target groups or ungroups from the flankers. We have further shown how configural effects, such as good and global Gestalt, determine crowding. Foveal and peripheral crowding do not always reveal the same characteristics. Here, we show that the very same grouping and Gestalt results of foveal vision are also found in the periphery. These results can neither be explained by simple pooling nor by centroid models. We discuss when bigger is better and how grouping might shape crowding.

V1 Projection Zone Signals in Human Macular Degeneration Depend on Task, not Stimulus

Abstract: We used functional magnetic resonance imaging to assess abnormal cortical signals in humans with juvenile macular degeneration (JMD). These signals have been interpreted as indicating large-scale cortical reorganization. Subjects viewed a stimulus passively or performed a task; the task was either related or unrelated to the stimulus. During passive viewing, or while performing tasks unrelated to the stimulus, there were large unresponsive V1 regions. These regions included the foveal projection zone, and we refer to them as the lesion projection zone (LPZ). In 3 JMD subjects, we observed highly significant responses in the LPZ while they performed stimulus-related judgments. In control subjects, where we presented the stimulus only within the peripheral visual field, there was no V1 response in the foveal projection zone in any condition. The difference between JMD and control responses can be explained by hypotheses that have very different implications for V1 reorganization. In controls retinal afferents carry signals indicating the presence of a uniform (zero-contrast) region of the visual field. Deletion of retinal input may 1) spur the formation of new cortical pathways that carry task-dependent signals (reorganization), or 2) unmask preexisting task-dependent cortical signals that ordinarily are suppressed by the deleted signals (no reorganization).