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The topography of high-order human object areas

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Abstract

Cortical topography is one of the most fundamental organizing principles of cortical areas. One such topography – eccentricity mapping – is present even in high-order, ventral stream visual areas. Within these areas, different object categories have specific eccentricity biases. In particular, faces, letters and words appear to be associated with central visual-field bias, whereas buildings are associated with a peripheral one. We propose that resolution needs are an important factor in organizing object representations: objects whose recognition depends on analysis of fine detail will be associated with central-biased representations, whereas objects whose recognition entails large-scale integration will be more peripherally biased.

Section snippets

The basics of visual cortical topography

The topographic transformation from the retina to the cortical surface has been amply documented both in non-human primates (e.g. [4]) and, more recently, in human early visual areas, using functional magnetic resonance imaging (fMRI) 5., 6., 7.. A consistent finding in all these studies is that the mapping principle involves a topographic transformation from a Euclidean coordinate system in the retina to polar coordinates in the visual cortex.

In this transformation, each of the early visual

The organization of human object representations

In the past few years, neuroimaging techniques of human visual cortex, in particular positron emission tomography and fMRI, have begun to dissect out not only the organization of early visual areas, but the more high-order and less topographically organized parts of visual cortex. These studies have revealed a rich and complex specialization pattern within occipito–temporal cortex. Particularly relevant for the present discussion are the early findings that consistently revealed a large

Topography of occipito–temporal, object-related regions

All of the putative organization principles (Box 1) fail to account for the strikingly consistent neuroanatomical relationship between the different object-related activation patterns. Thus, an underlying organization rule that will put the different patterns of activation within the context of an overall principle is requisite.

Most previous attempts at deciphering the putative organizing principles of object representations focused on the functional properties of these regions. This article

Eccentricity map in high-order object representations

Early studies have mapped the organization of early visual areas using a variety of texture patterns 5., 6., 7., 30.. Such patterns were presented either as rotating wedges, to map the polar angle, or as contracting and expanding rings, to map the eccentricity. Although these stimuli are very effective in activating early visual areas, they are much less so for object-related cortex. In an attempt to reveal topographic organization principles within higher order object regions, visual stimuli

The link between objects and eccentricity

The natural question that emerges from this analysis is ‘is there a general principle that associates a specific eccentricity profile with each object category?’ This question has been addressed by examining one of the best-studied examples of category-specific specialization: the differential activation in response to images of houses and faces. Recall that this activation pattern shows a consistent, and unexplained, medial-to-lateral segregation (Fig. 2). Within the context of the

Eccentricity is tightly linked to acuity demands

At this point, we can only speculate about the significance of this new topography. It certainly highlights the fact that object recognition is not a uniform task: it engages different, often conflicting, processes. In particular, there are processes that require close inspection of fine detail; such processes include identifying subtle individual variations within a category, identifying gaze direction and emotional expression in faces, etc. It will be natural to process these aspects, at

A new scheme for the organization of occipito–temporal object areas

The fact that face and building-related regions now appear to belong to a single eccentricity map has important implications regarding the organization of human occipito–temporal cortex. Thus, all anterior ventral object selective regions, which extend from the parahippocampal gyrus and collateral sulcus (anterior to V4–V8) medially, to the pFs subdivision of the LOC and further laterally into the occipito–temporal sulcus, can be considered as specialized subdivisions of a single entity. We

Resolution-based topography and holistic representations

Although not explicitly stated, the hypothesis that specific object categories segregate according to acuity demands has strong implications related to the nature of object representations. More specifically, for the linking between specific objects and eccentricity to be tangible, one has to assume that the neuronal representation can differentiate explicitly between object categories. Such a link cannot be established if the object representation is based solely on local, simple, object

Eccentricity and visual perception

Within the perspective of eccentricity specialization, specific well-known visual phenomena take on a new meaning. A striking psychophysical illustration of the sharp dividing line between central and peripheral visual recognition processes is provided by the phenomenon of ‘crowding’, in which peripheral integration of a group of items actually inhibits the identification of individual items. Interestingly, it has been demonstrated recently that, although the information about individual items

Eccentricity organization and visual experience

Finally, these findings are relevant to a major issue that is of current central interest: to what extent the organization of object areas can be ascribed to innate factors, and to what extent it is laid out and modified by postnatal visual experience? Examining the eccentricity map in high-order visual areas gives a strong impression that it is an organization that smoothly continues from early visual areas to high-order ones. Furthermore, the gross layout of this map is highly consistent

Conclusions

This article proposes a new organizing principle of human high-order object areas that is based on an orderly layout of visual field eccentricity. The proposed scheme stems from the finding of a topography in high-order object areas, in which eccentricity bias, magnification factor and specific object shapes are linked in an orderly manner. Two such eccentricity maps are proposed: a posterior dorsal one, LO, located in lateral occipital cortex; and a ventral anterior one, VOT, in the ventral

Questions for future research

  • Are all eccentricities mapped uniformly in the high-order eccentricity representation – or are central and peripheral representations perhaps overly emphasized [35]?

  • Can the association between object category and eccentricity be modified by visual experience?

  • What precise recognition processes are associated with high and low acuity computations?

  • How is information from disparate regions of the eccentricity map integrated to form a unified object representation?

  • What are the additional

Acknowledgements

This study was funded by JSMF 99-28 CN-QUA.05 and Israel Academy 8009 grants. We thank M. Harel for the brain flattening, E. Okon for technical help, D. Ben Bashat and Y. Assaf for their help in the fMRI setup, and G. Avidan and T. Hendler in conducting experiments.

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