A (fascinating) litmus test for human retino- vs. non-retinotopic processing

In human vision, the optics of the eye map neighboring points of the environment onto neighboring photoreceptors in the retina. This retinotopic encoding principle is preserved in the early visual areas. Under normal viewing conditions, due to the motion of objects and to eye movements, the retinotopic representation of the environment undergoes fast and drastic shifts. Yet, perceptually our environment appears stable suggesting the existence of non-retinotopic representations in addition to the well-known retinotopic ones. Here, we present a simple psychophysical test to determine whether a given visual process is accomplished in retino- or non-retinotopic coordinates. As examples, we show that visual search and motion perception can occur within a non-retinotopic frame of reference. These findings suggest that more mechanisms than previously thought operate non-retinotopically. Whether this is true for a given visual process can easily be found out with our "litmus test."

EEG Correlates of Relative Motion Encoding

Michael Herzog, Gijs Plomp, Yvonne Evelina Thunell

A large portion of the visual cortex is organized retinotopically, but perception is usually non-retinotopic. For example, a reflector on the spoke of a bicycle wheel appears to move on a circular or prolate cycloidal orbit as the bicycle moves forward, while in fact it traces out a curtate cycloidal trajectory. The moving bicycle serves as a non-retinotopic reference system to which the motion of the reflector is anchored. To study the neural correlates of non-retinotopic motion processing, we used the Ternus-Pikler display, where retinotopic processing in a stationary reference system is contrasted against non-retinotopic processing in a moving one. Using high-density EEG, we found similar brain responses for both retinotopic and non-retinotopic rotational apparent motion from the earliest evoked peak (around 120 ms) and throughout the rest of the visual processing, but only minor correlates of the motion of the reference system itself (mainly around 100-120 ms). We suggest that the visual system efficiently discounts the motion of the reference system from early on, allowing a largely reference system independent encoding of the motion of object parts.

Springer Verlag2016

Dorsal and ventral stream contributions to form-from-motion perception in a patient with form-from motion deficit: a case report

Olaf Blanke, Manuel Mercier

The main model of visual processing in primates proposes an anatomo-functional distinction between the dorsal stream, specialized in spatio-temporal information, and the ventral stream, processing essentially form information. However, these two pathways also communicate to share much visual information. These dorso-ventral interactions have been studied using form-from-motion (FfM) stimuli, revealing that FfM perception first activates dorsal regions (e.g., MT+/V5), followed by successive activations of ventral regions (e.g., LOC). However, relatively little is known about the implications of focal brain damage of visual areas on these dorso-ventral interactions. In the present case report, we investigated the dynamics of dorsal and ventral activations related to FfM perception (using topographical ERP analysis and electrical source imaging) in a patient suffering from a deficit in FfM perception due to right extrastriate brain damage in the ventral stream. Despite the patient's FfM impairment, both successful (observed for the highest level of FfM signal) and absent/failed FfM perception evoked the same temporal sequence of three processing states observed previously in healthy subjects. During the first period, brain source localization revealed cortical activations along the dorsal stream, currently associated with preserved elementary motion processing. During the latter two periods, the patterns of activity differed from normal subjects: activations were observed in the ventral stream (as reported for normal subjects), but also in the dorsal pathway, with the strongest and most sustained activity localized in the parieto-occipital regions. On the other hand, absent/failed FfM perception was characterized by weaker brain activity, restricted to the more lateral regions. This study shows that in the present case report, successful FfM perception, while following the same temporal sequence of processing steps as in normal subjects, evoked different patterns of brain activity. By revealing a brain circuit involving the most rostral part of the dorsal pathway, this study provides further support for neuro-imaging studies and brain lesion investigations that have suggested the existence of different brain circuits associated with different profiles of interaction between the dorsal and the ventral streams.

2017

A New Approach to Non-Retinotopic Processing

Marco Boi

At every moment, the image the world projects on our eyes undergoes rapid and dramatic changes due to the eye movements and to the motion of the objects. Surprisingly, we are not aware of these changes, experiencing the world as stable and constant. This apparent contradiction suggests that our visual system uses non-retinotopic representations of the objects in the visual scene to create a stable percept of the world. Which visual mechanisms make use of non-retinotopic representations is a question of fundamental importance in neuroscience. The study of visual stability and non-retinotopic processing has classically focused on the problem of eye movements and has been studied in this context. We have proposed a new view on the problem of non-retinotopic processing that allows explaining how a stable percept of the world is created both across the movements of the eyes and objects. Based on this view, we devised an experimental paradigm that allows testing whether a given mechanism makes use of non-retinotopic representations and we applied it to several different visual processes. Our results suggest that non-retinotopic representations are used in higher-level visual processes whereas low-level mechanisms seem to operate on retinotopic representations.

EPFL2010

Dorsal and ventral stream contributions to form-from-motion perception in a patient with form-from motion deficit: a case report

Olaf Blanke, Manuel Mercier

2017