Cortical processing of visual space : retinotopy or “perceptopy” ? – PERCEPTOPY

It is possible to separate the objective and subjective properties of visual space using contextual information. Contextual information can lead to distinct subjective interpretations of the same objective properties. For instance, an image of constant size on the retina can be interpreted as induced by a small or large object, as the context specifies a close or far distance for the object, or that his appearance causes a strong or small reward. Such dissociations are used to search correlates of objective and subjective performances in the brain : only the latter are affected by the manipulation of context. In this project, these representations and their dependence on context are measured through functional imaging techniques (fMRI) and cell recordings (electrophysiology) in both human and non-human primates.

The first part of the project focused mainly on characterizing how the manipulation of contextual information could modulate the perceived position or size of visual objects . We have demonstrated the contextual influence of spatial information collected in peripheral vision, as well as the lateralized nature of such information. We have also established that a visual object is perceived bigger when it is associated with a greater promise of reward. These data allow to study the correlates of these dissociations using functional imaging techniques.

This first part of the project allowed to characterize how contextual information (information collected by peripheral vision , or reward signals) modulate subjective interpretations of visual objects whose spatial properties are objectively constants. This knowledge allows to search for cerebral correlates of subjective changes of visual space induced by manipulating contextual signals.

Studies of this first phase of the project resulted in a published article (Camors, Jouffrais, Cottereau, Durand Vision Res , 2015 PMID : 25749676 ) and 2 articles in preparation, presented in conferences (Aedo-Jury, Cottereau, Trotter, Durand, 2014 Perception; Rima, Cottereau, Durand 2015 J Vis).

Numerous visual field maps have been documented in the visual cortex of human and non-human primates. These topographic cortical maps are generally called “retinotopic maps” because they are supposed to hold a strict topographic correspondence with retinal space. It is thus assumed that knowing the location and size of a stimulus on the retina, one can precisely predict the location and size of the stimulus-induced activation in these cortical maps. This organizational principle is thought to play a central role in visual processing. Yet, recent functional imaging studies have challenged this classical view by suggesting that even in the early visual cortex, visual space mapping might be much more dynamic and flexible than anticipated. The main finding of these studies is that in what is considered as the most prototypical example of retinotopic map, namely the primary visual area (V1), the size of stimuli-induced activations reflects the perceived size of those stimuli rather than their actual size on the retina. If this surprising finding was to be confirmed, current models of visual processing might well need major revision.
The aim of this project is to question this potentially major discovery: is there a strict retinotopic mapping of visual space in these cortical maps and/or a dynamic mapping reflecting how visual space is perceived. In order to clearly disentangle “retinotopy” from “perceptopy”, severe limitations of the above mentioned functional imaging studies must be overcome. The most severe limitation is that the fMRI signal gives limited information about visual space mapping at the level of individual neurons’ receptive fields. The size and position of neuronal receptive fields is thought to be rigidly determined by the fine spatial arrangement of anatomical connections, while these recent findings suggest that such basic receptive field properties might be dynamically modified. Another limitation is that the fMRI approach developed in these studies was not sensitive enough to address visual areas beyond V1. A more exhaustive view of visual space mapping across the visual cortex is needed to understand the general and particular properties of these cortical visual field maps.
In this project, we propose an integrative approach that will overcome these limitations. On the one hand, we will address three levels of analyses: perception – cortical visual field maps – individual neurons’ receptive fields, by combining psychophysics, fMRI and single cell recordings in non-human primates. The psychophysics and functional imaging approaches will be run in parallel in humans, so as to assess the validity of the animal model and to transfer more straightforwardly to humans the findings from the neuronal level. On the other hand, we will significantly improve the strength of the fMRI approach by adapting recent and powerful methodological approaches normally used for classical retinotopic mapping, which will allow addressing cortical maps well beyond V1. By linking perceptual, cortical and neuronal mapping of visual space, in both human and non-human primates and across larger cortical territories, this project should shed new lights on the cortical processing of visual space and will undoubtedly improve our current understanding of the neuronal operations by which sensory (retinal) space is transformed into cognitive (perceptual) space. Our results may well definitely confirm an unexpected topographic flexibility in these cortical maps, and thus lead to a major updating of our current models of visual processing.
The whole project will be run in Toulouse (Brain Institute), by a team holding scientific and methodological expertise in the domain and within a recent and unique scientific platform equipped with a 3T scanner dedicated to research in humans and monkeys, as well as several labs for psychophysics and single cell recordings.