Unconscious retinotopic motion processing affects non-retinotopic motion perception

Unconscious visual stimuli can affect conscious perception: For example, an invisible prime can affect responses to a subsequent target. The invisible interpretation of an ambiguous figure can have similar effects. Invisibility in these situations is typically explained by stimulus-suppression in early, retinotopic brain areas. We have previously argued that invisibility is closely linked to Gestalt ("object") organization principles. For example, motion is typically perceived in non-retinotopic, object-centered, and not in retinotopic coordinates. Such is the case for a bicycle-reflector that is perceived as circling, although its retinotopic trajectory is cycloidal. Here, we used a modified Ternus-Pikler display in which, just as in everyday vision, the retinotopic motion is invisible and the non-retinotopic motion is perceived. Nevertheless, the invisible retinotopic motion, can strongly degrade the conscious non-retinotopic motion percept. This effect cannot be explained by inhibition at a retinotopic processing stage.

EEG Correlates of Relative Motion Encoding

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.

Putting low-level vision into global context: Why vision cannot be reduced to basic circuits

To cope with the complexity of vision, most models in neuroscience and computer vision are of hierarchical and feedforward nature. Low-level vision, such as edge and motion detection, is explained by basic low-level neural circuits, whose outputs serve as building blocks for more complex circuits computing higher level features such as shape and entire objects. There is an isomorphism between states of the outer world, neural circuits, and perception, inspired by the positivistic philosophy of the mind. Here, we show that although such an approach is conceptually and mathematically appealing, it fails to explain many phenomena including crowding, visual masking, and non-retinotopic processing.

Differential motion processing between species facing Ternus-Pikler display: non-retinotopic humans versus retinotopic pigeons

Retinotopic encoding is preserved in primate visual cortex. However, several physiological and psychophysical studies have revealed that visual processes can be disengaged from retinotopic coordinates. We examined whether this non-retinotopic processing is common to humans and pigeons, two visually dominant vertebrate species with similar retinotopic organizations in their brains. We used a variant of Ternus-Pikler stimulus as a litmus test for non-retinotopic processing. Six humans and four pigeons were required to discriminate the rotational direction of a target disk placed among linearly arranged non-rotating disks. When all disks flickered in synchrony (a blank screen was inserted between the stimulus presentations) and moved in tandem back and forth, target localization was hampered in humans but not pigeons (Experiment 1). The duration of the blank screen (Experiment 2) and the connection between the disks (Experiment 3) did not affect the pigeons' performance. These results suggest that non-retinotopic processing in human vision is not a feature of pigeon vision, which is instead strictly retinotopic in case of motion. This may reflect the different mechanisms for stimulus selection in both species, in which local motion signals were pooled at later stages of visual processing in humans, but the signals were selected at early stages in pigeons.

Object-centered representations support flexible exogenous visual attention across translation and reflection

Visual attention can be deployed to stimuli based on our willful, top-down goal (endogenous attention) or on their intrinsic saliency against the background (exogenous attention). Flexibility is thought to be a hallmark of endogenous attention, whereas decades of research show that exogenous attention is attracted to the retinotopic locations of the salient stimuli. However, to the extent that salient stimuli in the natural environment usually form specific spatial relations with the surrounding context and are dynamic, exogenous attention, to be adaptive, should embrace these structural regularities. Here we test a non-retinotopic, object-centered mechanism in exogenous attention, in which exogenous attention is dynamically attracted to a relative, object-centered location. Using a moving frame configuration, we presented two frames in succession, forming either apparent translational motion or in mirror reflection, with a completely uninformative, transient cue presented at one of the item locations in the first frame. Despite that the cue is presented in a spatially separate frame, in both translation and mirror reflection, behavioralperformance in visual search is enhanced when the target in the second frame appears at the same relative location as the cue location than at other locations. These results provide unambiguous evidence for non-retinotopic exogenous attention and further reveal an object-centered mechanism supporting flexible exogenous attention. Moreover, attentional generalization across mirror reflection may constitute an attentional correlate of perceptual generalization across lateral mirror images, supporting an adaptive, functional account of mirror images confusion.