Space reconstruction by primary visual cortex activity: a parallel, non-computational mechanism of object representation

Anatomical, physiological, and psychophysical data show that the nature of conscious perception is incompatible with the integrated information theory (IIT)

The integrated information theory (IIT) equates levels of consciousness with the amount of information integrated over the elements that constitute a system. Conscious visual perception provides two observations that contradict the IIT. First, objects are accurately perceived when presented for ≪100 ms during which time no neural integration is possible. Second, an object is seen as an integrated whole and, concurrently, all constituent elements are evident. Because integration destroys information about details, IIT cannot account for perceptual detail preservation.

Oculo-retinal dynamics can explain the perception of minimal recognizable configurations

Natural vision is a dynamic and continuous process. Under natural conditions, visual object recognition typically involves continuous interactions between ocular motion and visual contrasts, resulting in dynamic retinal activations. In order to identify the dynamic variables that participate in this process and are relevant for image recognition, we used a set of images that are just above and below the human recognition threshold and whose recognition typically requires >2 s of viewing. We recorded eye movements of participants while attempting to recognize these images within trials lasting 3 s. We then assessed the activation dynamics of retinal ganglion cells resulting from ocular dynamics using a computational model. We found that while the saccadic rate was similar between recognized and unrecognized trials, the fixational ocular speed was significantly larger for unrecognized trials. Interestingly, however, retinal activation level was significantly lower during these unrecognized trials. We used retinal activation patterns and oculomotor parameters of each fixation to train a binary classifier, classifying recognized from unrecognized trials. Only retinal activation patterns could predict recognition, reaching 80% correct classifications on the fourth fixation (on average, ∼2.5 s from trial onset). We thus conclude that the information that is relevant for visual perception is embedded in the dynamic interactions between the oculomotor sequence and the image. Hence, our results suggest that ocular dynamics play an important role in recognition and that understanding the dynamics of retinal activation is crucial for understanding natural vision.

Why Does Space Feel the Way it Does? Towards a Principled Account of Spatial Experience

There must be a reason why an experience feels the way it does. A good place to begin addressing this question is spatial experience, because it may be more penetrable by introspection than other qualities of consciousness such as color or pain. Moreover, much of experience is spatial, from that of our body to the visual world, which appears as if painted on an extended canvas in front of our eyes. Because it is ‘right there’, we usually take space for granted and overlook its qualitative properties. However, we should realize that a great number of phenomenal distinctions and relations are required for the canvas of space to feel ‘extended’. Here we argue that, to be experienced as extended, the canvas of space must be composed of countless spots, here and there, small and large, and these spots must be related to each other in a characteristic manner through connection, fusion, and inclusion. Other aspects of the structure of spatial experience follow from extendedness: every spot can be experienced as enclosing a particular region, with its particular location, size, boundary, and distance from other spots. We then propose an account of the phenomenal properties of spatial experiences based on integrated information theory (IIT). The theory provides a principled approach for characterizing both the quantity and quality of experience by unfolding the cause-effect structure of a physical substrate. Specifically, we show that a simple simulated substrate of units connected in a grid-like manner yields a cause-effect structure whose properties can account for the main properties of spatial experience. These results uphold the hypothesis that our experience of space is supported by brain areas whose units are linked by a grid-like connectivity. They also predict that changes in connectivity, even in the absence of changes in activity, should lead to a warping of experienced space. To the extent that this approach provides an initial account of phenomenal space, it may also serve as a starting point for investigating other aspects of the quality of experience and their physical correspondents.

Very small faces are easily discriminated under long and short exposure times

Acuity measures related to overall face size that can be perceived have not been studied quantitatively. Consequently, experimenters use a wide range of sizes (usually large) without always providing a rationale for their choices. I studied thresholds for face discrimination by presenting both long (500 ms)- and short (17, 33, 50 ms)-duration stimuli. Face width threshold for the long presentation was ~0.2°, and thresholds for the flashed stimuli ranged from ~0.3° for the 17-ms flash to ~0.23° for the 33- and 50-ms flashes. Such thresholds indicate that face stimuli used in physiological or psychophysical experiments are often too large to tap human fine spatial capabilities, and thus interpretations of such experiments should take into account face discrimination acuity. The 0.2° threshold found in this study is incompatible with the prevalent view that faces are represented by a population of specialized “face cells” because those cells do not respond to <1° stimuli and are optimally tuned to >4° faces. Also, the ability to discriminate small, high-spatial frequency flashed face stimuli is inconsistent with models suggesting that fixational drift transforms retinal spatial patterns into a temporal code. It seems therefore that the small image motions occurring during fixation do not disrupt our perception, because all relevant processing is over with before those motions can have significant effects.

NEW & NOTEWORTHY Although face perception is central to human behavior, the minimally perceived face size is not known. This study shows that humans can discriminate very small (~0.2°) faces. Furthermore, even when flashed for tens of milliseconds, ~0.25° faces can be discriminated. Such fine acuity should impact modeling of physiological mechanisms of face perception. The ability to discriminate flashed faces where there is almost no eye movement indicates that eye drift is not essential for visibility.

Human visual perceptual organization beats thinking on speed

What is the degree to which knowledge influences visual perceptual processes? This question, which is central to the seeing-versus-thinking debate in cognitive science, is often discussed using examples claimed to be proof of one stance or another. It has, however, also been muddled by the usage of different and unclear definitions of perception. Here, for the well-defined process of perceptual organization, I argue that including speed (or efficiency) into the equation opens a new perspective on the limits of top-down influences of thinking on seeing. While the input of the perceptual organization process may be modifiable and its output enrichable, the process itself seems so fast (or efficient) that thinking hardly has time to intrude and is effective mostly after the fact.

The anatomical and physiological properties of the visual cortex argue against cognitive penetration

We are consciously aware of visual objects together with the minute details that characterize each object. Those details are perceived instantaneously and in parallel. V1 is the only visual area with spatial resolution and topographical exactitude matching perceptual abilities. For cognition to penetrate perception, it needs to affect V1 image representation. That is unlikely because of the detailed parallel V1 organization and the nature of top-down connections, which can influence only large parts of the visual field.

Consciousness weaves our internal view of the outside world

Low-level consciousness is fundamental to our understanding of the world. Within the conscious field, the constantly changing external visual information is transformed into stable, object-based percepts. Remarkably, holistic objects are perceived while we are cognizant of all of the spatial details comprising the objects and of the relationship between individual elements. This parallel conscious association is unique to the brain. Conscious contributions to motor activity come after our understanding of the world has been established.

Reconstruction of shape contours from V1 activity at high resolution

The role of primary visual cortex (V1) in encoding physical stimulus features is well known, while stimulus categorization is mainly attributed to higher visual areas. However, visual experience is not stripped down to invariant, categorical-only "labels." Rather, visual experiences are remarkably rich with details resulting in high-resolution perception of objects. If V1 is involved in this process, high-resolution readout of shape contours should be possible from V1 activity. To test this, we presented various shapes to awake, fixating monkeys while recording V1 population activity using voltage-sensitive dye imaging. A simplified bottom-up model was constructed based on known cortical properties and without an image prior. Contours were reconstructed from single trials, in sub-degree resolution by applying the inverse model to neuronal responses. These novel reconstruction results suggest V1 can be an important constituent in the detailed internal representation of visual experiences.