Neurons in monkey visual cortex detect lines defined by coherent motion of dots

Gestalt neurons and emergent properties in visual perception: A novel concept for the transformation from local to global processing

Gestalten in visual perception are defined by emergent properties of the whole, which cannot be predicted from the sum of its parts; rather, they arise by virtue of inherent principles, the Laws of Seeing. This review attempts to assign neurophysiological correlates to select emergent properties in motion and contour perception and proposes parallels to the processing of local versus global attributes by classical versus contextual receptive fields. The aim is to identify Gestalt neurons in the visual system to account for the Laws of Seeing in causal terms and to explain "Why do things look as they do" (Koffka, 1935, p. 76).

Processing of motion boundary orientation in macaque V2

Human and nonhuman primates are good at identifying an object based on its motion, a task that is believed to be carried out by the ventral visual pathway. However, the neural mechanisms underlying such ability remains unclear. We trained macaque monkeys to do orientation discrimination for motion boundaries (MBs) and recorded neuronal response in area V2 with microelectrode arrays. We found 10.9% of V2 neurons exhibited robust orientation selectivity to MBs, and their responses correlated with monkeys' orientation-discrimination performances. Furthermore, the responses of V2 direction-selective neurons recorded at the same time showed correlated activity with MB neurons for particular MB stimuli, suggesting that these motion-sensitive neurons made specific functional contributions to MB discrimination tasks. Our findings support the view that V2 plays a critical role in MB analysis and may achieve this through a neural circuit within area V2.

Neuronal correlates of motion-defined shape perception in primate dorsal and ventral streams

Human and non-human primates can readily perceive the shape of objects using visual motion. Classically, shape, and motion are considered to be separately processed via ventral and dorsal cortical pathways, respectively. However, many lines of anatomical and physiological evidence have indicated that these two pathways are likely to be interconnected at some stage. For motion-defined shape perception, these two pathways should interact with each other because the ventral pathway must utilize motion, which the dorsal pathway processes, to extract shape signal. However, it is unknown how interactions between cortical pathways are involved in neural mechanisms underlying motion-defined shape perception. We review evidence from psychophysical, lesion, neuroimaging and physiological research on motion-defined shape perception and then discuss the effects of behavioral demands on neural activity in ventral and dorsal cortical areas. Further, we discuss functions of two candidate sets of levels: early and higher-order cortical areas. The extrastriate area V4 and middle temporal (MT) area, which are reciprocally connected, at the early level are plausible areas for extracting the shape and/or constituent parts of shape from motion cues because neural dynamics are different from those during luminance-defined shape perception. On the other hand, among other higher-order visual areas, the anterior superior temporal sulcus likely contributes to the processing of cue-invariant shape recognition rather than cue-dependent shape processing. We suggest that sharing information about motion and shape between the early visual areas in the dorsal and ventral pathways is dependent on visual cues and behavioral requirements, indicating the interplay between the pathways.

Neural changes in the primate brain correlated with the evolution of complex motor skills

Complex motor skills of eventual benefit can be learned after considerable trial and error. What do structural brain changes that accompany such effortful long-term learning tell us about the mechanisms for developing innovative behavior? Using MRI, we monitored brain structure before, during and after four marmosets learnt to use a rake, over a long period of 10-13 months. Throughout learning, improvements in dexterity and visuo-motor co-ordination correlated with increased volume in the lateral extrastriate cortex. During late learning, when the most complex behavior was maintained by sustained motivation to acquire the skill, the volume of the nucleus accumbens increased. These findings reflect the motivational state required to learn, and show accelerated function in higher visual cortex that is consistent with neurocognitive divergence across a spectrum of primate species.

The neural representation of objects formed through the spatiotemporal integration of visual transients

Oftentimes, objects are only partially and transiently visible as parts of them become occluded during observer or object motion. The visual system can integrate such object fragments across space and time into perceptual wholes or spatiotemporal objects. This integrative and dynamic process may involve both ventral and dorsal visual processing pathways, along which shape and spatial representations are thought to arise. We measured fMRI BOLD response to spatiotemporal objects and used multi-voxel pattern analysis (MVPA) to decode shape information across 20 topographic regions of visual cortex. Object identity could be decoded throughout visual cortex, including intermediate (V3A, V3B, hV4, LO1-2,) and dorsal (TO1-2, and IPS0-1) visual areas. Shape-specific information, therefore, may not be limited to early and ventral visual areas, particularly when it is dynamic and must be integrated. Contrary to the classic view that the representation of objects is the purview of the ventral stream, intermediate and dorsal areas may play a distinct and critical role in the construction of object representations across space and time.

Linking brain to behavior for the visual perception of figures and objects

The dissociation of a figure from its background is an essential feat of visual perception, as it allows us to detect, recognize, and interact with shapes and objects in our environment. In order to understand how the human brain gives rise to the perception of figures, we here review experiments that explore the links between activity in visual cortex and performance of perceptual tasks related to figure perception. We organize our review according to a proposed model that attempts to contextualize figure processing within the more general framework of object processing in the brain. Overall, the current literature provides us with individual linking hypotheses as to cortical regions that are necessary for particular tasks related to figure perception. Attempts to reach a more complete understanding of how the brain instantiates figure and object perception, however, will have to consider the temporal interaction between the many regions involved, the details of which may vary widely across different tasks.

Untangling perceptual memory: hysteresis and adaptation map into separate cortical networks

Perception is an active inferential process in which prior knowledge is combined with sensory input, the result of which determines the contents of awareness. Accordingly, previous experience is known to help the brain "decide" what to perceive. However, a critical aspect that has not been addressed is that previous experience can exert 2 opposing effects on perception: An attractive effect, sensitizing the brain to perceive the same again (hysteresis), or a repulsive effect, making it more likely to perceive something else (adaptation). We used functional magnetic resonance imaging and modeling to elucidate how the brain entertains these 2 opposing processes, and what determines the direction of such experience-dependent perceptual effects. We found that although affecting our perception concurrently, hysteresis and adaptation map into distinct cortical networks: a widespread network of higher-order visual and fronto-parietal areas was involved in perceptual stabilization, while adaptation was confined to early visual areas. This areal and hierarchical segregation may explain how the brain maintains the balance between exploiting redundancies and staying sensitive to new information. We provide a Bayesian model that accounts for the coexistence of hysteresis and adaptation by separating their causes into 2 distinct terms: Hysteresis alters the prior, whereas adaptation changes the sensory evidence (the likelihood function).

Phenomenology and neurophysiological correlations: two approaches to perception research

This article argues that phenomenological description and neurophysiological correlation complement each other in perception research. Whilst phenomena constitute the material, neuronal mechanisms are indispensable for their explanation. Numerous examples of neurophysiological correlates show that the correlation of phenomenology and neurophysiology is fruitful. Phenomena for which neuronal mechanism have been found include: (in area V1) filling-in of real and artificial scotomata, contour integration, figure-ground segregation by orientation contrast, amodal completion, and motion transparency; (in V2) modal completion, border ownership, surface transparency, and cyclopean perception; (in V3) alignment in dotted contours, and filling-in with dynamic texture; (in V4) colour constancy; (in MT) shape by accretion/deletion, grouping by coherent motion, apparent motion in motion quartets, motion in apertures, and biological motion. Results suggest that in monkey visual cortex, occlusion cues, including stereo depth, are predominantly processed in lower areas, whereas mechanisms for grouping and motion are primarily represented in higher areas. More correlations are likely to emerge as neuroscientists strive for a better understanding of visual perception. The paper concludes with a review of major achievements in visual neuroscience pertinent to the study of the phenomena under consideration.

Rotating dotted ellipses: motion perception driven by grouped figural rather than local dot motion signals

Unlike the motion of a continuous contour, the motion of a single dot is unambiguous and immune to the aperture problem. Here we exploit this fact to explore the conditions under which unambiguous local motion signals are used to drive global percepts of an ellipse undergoing rotation. In previous work, we have shown that a thin, high aspect ratio ellipse will appear to rotate faster than a lower aspect ratio ellipse even when the two in fact rotate at the same angular velocity [Caplovitz, G. P., Hsieh, P. -J., & Tse, P. U. (2006) Mechanisms underlying the perceived angular velocity of a rigidly rotating object. Vision Research, 46(18), 2877-2893]. In this study we examined the perceived speed of rotation of ellipses defined by a virtual contour made up of evenly spaced dots.

RESULTS

Ellipses defined by closely spaced dots exhibit the speed illusion observed with continuous contours. That is, thin dotted ellipses appear to rotate faster than fat dotted ellipses when both rotate at the same angular velocity. This illusion is not observed if the dots defining the ellipse are spaced too widely apart. A control experiment ruled out low spatial frequency "blurring" as the source of the illusory percept.

CONCLUSION

Even in the presence of local motion signals that are immune to the aperture problem, the global percept of an ellipse undergoing rotation can be driven by potentially ambiguous motion signals arising from the non-local form of the grouped ellipse itself. Here motion perception is driven by emergent motion signals such as those of virtual contours constructed by grouping procedures. Neither these contours nor their emergent motion signals are present in the image.

From perceptive fields to Gestalt

Studies on visual psychophysics and perception conducted in the Freiburg psychophysics laboratory during the last 35 years are reviewed. Many of these were inspired by single-cell neurophysiology in cat and monkey. The aim was to correlate perceptual phenomena and their effects to possible neuronal mechanisms from retina to visual cortex and beyond. Topics discussed include perceptive field organization, figure-ground segregation and grouping, fading and filling-in, and long-range color interaction. While some of these studies succeeded in linking perception to neuronal response patterns, others require further investigation. The task of probing the human brain with perceptual phenomena continues to be a challenge for the future.