Human medial occipital cortex comprises multiple visual areas, each with a distinct retinotopic representation of visual environment. We measured spatial frequency (SF) tuning curves with functional magnetic resonance imaging (fMRI) and found consistent differences between these areas. Areas V1, V2, VP, V3, V4v, and V3A were all band-pass tuned, with progressively lower SF optima in V1, V2, and V3A. In VP and V3, the SF optima were similar to optima in V2, whereas V4v showed more individual variation and scattered SF representations on the cortical surface. Area V5+ showed low-pass SF tuning. In each area, the SF optimum declined with increasing eccentricity. After accounting for the cortical magnification, the cortical extent of the optimal spatial wavelengths was approximately constant across eccentricity in V1, which suggests an anatomical constraint for the optimal SF, and this extent is actually comparable to the extent of horizontal connections within primate V1. The optimal spatial wavelengths in the visual field are also of similar extent to the spatial summation fields of macaque V1. The progressive decline in the SF tuning from V1 to V2 and V3A is compatible with the view that these areas represent visual information at different spatial scales.

Proper understanding of processes underlying visual perception requires information on the activation order of distinct brain areas. We measured dynamics of cortical signals with magnetoencephalography while human subjects viewed stimuli at four visual quadrants. The signals were analyzed with minimum current estimates at the individual and group level. Activation emerged 55-70 ms after stimulus onset both in the primary posterior visual areas and in the anteromedial part of the cuneus. Other cortical areas were active after this initial dual activation. Comparison of data between species suggests that the anteromedial cuneus either comprises a homologue of the monkey area V6 or is an area unique to humans. Our results show that visual stimuli activate two cortical areas right from the beginning of the cortical response. The anteromedial cuneus has the temporal position needed to interact with the primary visual cortex V1 and thereby to modify information transferred via V1 to extrastriate cortices.A natomy of connections between primate visual cortices suggests a hierarchical organization of signal processing (1). However, the order of processes at different functional areas cannot be directly deduced from the anatomical hierarchy without relevant timing information. In monkeys, several areas become active immediately after the primary visual cortex (V1), while another set of areas is active at clearly longer latencies (2, 3). The areas showing early responses, such as V5͞middle temporal area (MT), medial superior temporal area, and frontal eye field, are specialized in analyzing dynamical visual information and in visuomotor transformation (for reviews, see refs. 4 and 5). The areas with longer latencies, such as V4, are sensitive to object form and color and participate in object recognition (6, 7). The dissimilarities in activation latencies and functional properties of these areas suggest diversity in the type and amount of necessary input and preprocessing before activation. Given the differences between monkey and human visual cortices (for a review, see ref. 8), we aimed to explore the dynamics and distribution of early cortical activation in humans. Neuromagnetic signals were recorded while the subjects viewed pattern reversal or luminance stimuli at the four visual quadrants. The signals were analyzed with minimum current estimates (MCEs), which require minimal human intervention for determining the location and orientation of the cortical currents (9). The individual three-dimensional estimates were aligned with a nonlinear transformation according to individual brain shapes (10, 11), and both the individual and group average MCEs were compared with the existing maps of human visual cortices (12-16). Materials and MethodsSubjects and Stimuli. We studied five female and five male subjects (mean age 27 years, range 20-42 years). The stimuli were generated with a Macintosh computer and presented with a dataprojector (VistaPro, Electrohome Ltd., Ontario, Canada) on a back projection screen, with viewing d...

In primary visual cortex (V1) neuronal responses to stimuli inside the receptive field (RF) are usually suppressed by stimuli in the RF surround. This suppression is orientation-specific. Similarly, in human vision surround stimuli can suppress perceived contrast of a central stimulus in an orientation-dependent manner. The surround consists of two regions likely generated by different circuits: a near-surround generated predominantly by geniculocortical and intra-V1 horizontal connections, and a far-surround generated exclusively by inter-areal feedback. Using stimuli confined to the near- or far-surround of V1 neurons, and similar stimuli in human psychophysics, we find that near-surround suppression is more sharply orientation-tuned than far-surround suppression in both macaque V1 and human perception. These results point to a similarity between surround suppression in macaque V1 and human vision, and suggest that feedback circuits are less orientation-biased than horizontal circuits. We find sharpest tuning of near-surround suppression in V1 layers (3, 4B, 4Cα) with patterned and orientation-specific horizontal connections. Sharpest tuning of far-surround suppression occurs in layer 4B, suggesting greater orientation-specificity of feedback to this layer. Different orientation-tuning of near and far surround suppression may reflect a statistical bias in natural images, whereby nearby edges have higher probability than distant edges of being co-oriented and belonging to the same contour. Surround suppression would, thus, increase the coding efficiency of frequently co-occurring contours and the saliency of less frequent ones. Such saliency increase can help detect small orientation differences in nearby edges (for contour completion), but large orientation differences in distant edges (for directing saccades/attention).

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