Emergent organization of multiple visuotopic maps without a feature hierarchy

The primate visual system is comprised of multiple visual areas. Despite their foundational relevance, there are no normative accounts for why there are multiple areas nor why they have their signature “mirrored map” topography. Here I show that the stereotyped cortical organization of multiple mirrored areas naturally emerges in simulated cortex, in which self-organizing processes are used to map a multi-scale representation of visual space smoothly onto a two-dimensional cortical sheet. Predominant accounts of multiple areas emphasize hierarchical processing, where each area extends and elaborates on the previous areas’ representation. Here, no explicit hierarchical relationships were required to manifest this multi-areal organization, suggesting that feature hierarchies may be the derived rather than the driving force of this organization. This modeling work thus provides a simple computational explanation for the hallmark features of early visual topography, and the presence of multiple areas, as emergent from a single functional goal — to smoothly represent the visual field at multiple spatial scales.One Sentence SummaryThis work presents a formal model of simulated cortex with multiple visual areas, where purely spatial relationships underlie the large-scale motifs of visual cortex.

Regarding the shipwreck of others: for a critical visual topography of Mediterranean migration

This article reflects on the relationship between photography and migration from a cultural geography perspective, with particular reference to the visual construction of the Mediterranean border regime. The contemporary aesthetics of Mediterranean migration is one of the domains in which visual and social norms are most closely intertwined. My main purpose is to unveil these processes of mutual (re)production by analysing how photographs forge our everyday perceptions of migration and affect our very ability to produce ethical and political responses to the events they portray. Drawing on a range of fields and approaches, including cultural geography, visual culture studies and postcolonial theories, I attempt here to develop a critical topography of looking, mapping out some of the performances and places involved in looking at Mediterranean migration. In the final part of the article, I seek to reinforce my theoretical statements by focusing on two photographic images of people being rescued along the Mediterranean route, in order to illustrate how photography can actually interfere with our ability to create spaces of cultural and political responsibility.

The functional logic of cortico–pulvinar connections

The pulvinar is an ‘associative’ thalamic nucleus, meaning that most of its input and output relationships are formed with the cerebral cortex. The function of this circuitry is little understood and its anatomy, though much investigated, is notably recondite. This is because pulvinar connection patterns disrespect the architectural subunits (anterior, medial, lateral and inferior pulvinar nuclei) that have been the traditional reference system. This article presents a simplified, global model of the organization of cortico–pulvinar connections so as to pursue their structure–function relationships. Connections between the cortex and pulvinar are topographically organized, and as a result the pulvinar contains a ‘map’ of the cortical sheet. However, the topography is very blurred. Hence the pulvinar connection zones of nearby cortical areas overlap, allowing indirect transcortical communication via the pulvinar. A general observation is that indirect cortico–pulvino–cortical circuits tend to mimic direct cortico–cortical pathways: this is termed ‘the replication principle’. It is equally apt for certain pairs (or groups) of nearby cortical areas that happen not to connect with each other. The ‘replication’ of this non–connection is achieved by discontinuities and dislocations of the cortical topography within the pulvinar, such that the associated pair of connection zones do not overlap. Certain of these deformations can be used to divide the global cortical topography into specific sub–domains, which form the natural units of a connectional subdivision of the pulvinar. A substantial part of the pulvinar also expresses visual topography, reflecting visual maps in occipital cortex. There are just two well–ordered visual maps in the pulvinar, that both receive projections from area V1, and several other occipital areas; the resulting duplication of cortical topography means that each visual map also acts as a separate connection domain. In summary, the model identifies four topographically ordered connection domains, and reconciles the coexistence of visual and cortical maps in two of them. The replication principle operates at and below the level of domain structure. It is argued that cortico–pulvinar circuitry replicates the pattern of cortical circuitry but not its function, playing a more regulatory role instead. Thalamic neurons differ from cortical neurons in their inherent rhythmicity, and the pattern of cortico–thalamic connections must govern the formation of specific resonant circuits. The broad implication is that the pulvinar acts to coordinate cortical information processing by facilitating and sustaining the formation of synchronized trans–areal assemblies; a more pointed suggestion is that, owing to the considerable blurring of cortical topography in the pulvinar, rival cortical assemblies may be in competition to recruit thalamic elements in order to outlast each other in activity.

The functional organization of area V2, II: The impact of stripes on visual topography

We have examined the visuotopic organization of area V2 of macaque monkeys in relation to its modular construction, comprising repetitive cycles of stripes running perpendicular to the border with area V1. Receptive fields were plotted in anesthetised animals, mainly using long penetrations parallel to the V1 border crossing several stripes in dorsal V2 within the representation of paracentral, inferior visual field. We confirm that each set of modules (thick, thin, and interstripes) mounts an unbroken coverage of the visual field, since there is almost invariably some overlap between the aggregate fields recorded in successive stripes of the same class, at intervals of one cycle. Also as expected, penetrations perpendicular to the stripes record changes in eccentricity along an isopolar visual meridian. We measured the size of the point image along such an isopolar meridian in nine cases, and showed that on average it exceeds the length of a typical cycle; again, this implies that no point in space escapes analysis by any of the functional modules. The representation of eccentricity across a cycle of stripes resembles a “ratchet” model, in which the gradient of eccentricity across a single stripe exceeds the gradient across the full cycle, leading to discontinuities (“switchbacks”) at the borders between stripes. The shift in eccentricity across the width of a stripe is sufficient to maintain a virtually continuous map across successive stripes of the same class; when coupled to receptive field scatter about the mean trend, this creates the overlap of aggregate fields.