Power spectra of the natural input to the visual system

Looking for Image Statistics: Active Vision With Avatars in a Naturalistic Virtual Environment

The efficient coding hypothesis posits that sensory systems are tuned to the regularities of their natural input. The statistics of natural image databases have been the topic of many studies, which have revealed biases in the distribution of orientations that are related to neural representations as well as behavior in psychophysical tasks. However, commonly used natural image databases contain images taken with a camera with a planar image sensor and limited field of view. Thus, these images do not incorporate the physical properties of the visual system and its active use reflecting body and eye movements. Here, we investigate quantitatively, whether the active use of the visual system influences image statistics across the visual field by simulating visual behaviors in an avatar in a naturalistic virtual environment. Images with a field of view of 120° were generated during exploration of a virtual forest environment both for a human and cat avatar. The physical properties of the visual system were taken into account by projecting the images onto idealized retinas according to models of the eyes' geometrical optics. Crucially, different active gaze behaviors were simulated to obtain image ensembles that allow investigating the consequences of active visual behaviors on the statistics of the input to the visual system. In the central visual field, the statistics of the virtual images matched photographic images regarding their power spectra and a bias in edge orientations toward cardinal directions. At larger eccentricities, the cardinal bias was superimposed with a gradually increasing radial bias. The strength of this effect depends on the active visual behavior and the physical properties of the eye. There were also significant differences between the upper and lower visual field, which became stronger depending on how the environment was actively sampled. Taken together, the results show that quantitatively relating natural image statistics to neural representations and psychophysical behavior requires not only to take the structure of the environment into account, but also the physical properties of the visual system, and its active use in behavior.

Temporal statistics of natural image sequences generated by movements with insect flight characteristics

Many flying insects, such as flies, wasps and bees, pursue a saccadic flight and gaze strategy. This behavioral strategy is thought to separate the translational and rotational components of self-motion and, thereby, to reduce the computational efforts to extract information about the environment from the retinal image flow. Because of the distinguishing dynamic features of this active flight and gaze strategy of insects, the present study analyzes systematically the spatiotemporal statistics of image sequences generated during saccades and intersaccadic intervals in cluttered natural environments. We show that, in general, rotational movements with saccade-like dynamics elicit fluctuations and overall changes in brightness, contrast and spatial frequency of up to two orders of magnitude larger than translational movements at velocities that are characteristic of insects. Distinct changes in image parameters during translations are only caused by nearby objects. Image analysis based on larger patches in the visual field reveals smaller fluctuations in brightness and spatial frequency composition compared to small patches. The temporal structure and extent of these changes in image parameters define the temporal constraints imposed on signal processing performed by the insect visual system under behavioral conditions in natural environments.

Representation of central and peripheral vision in the primate cerebral cortex: Insights from studies of the marmoset brain

How the visual field is represented by neurons in the cerebral cortex is one of the most basic questions in visual neuroscience. However, research to date has focused heavily on the small part of the visual field within, and immediately surrounding the fovea. Studies on the cortical representation of the full visual field in the primate brain are still scarce. We have been investigating this issue with electrophysiological and anatomical methods, taking advantage of the small and lissencephalic marmoset brain, which allows easy access to the representation of the full visual field in many cortical areas. This review summarizes our main findings to date, and relates the results to a broader question: is the peripheral visual field processed in a similar manner to the central visual field, but with lower spatial acuity? Given the organization of the visual cortex, the issue can be addressed by asking: (1) Is visual information processed in the same way within a single cortical area? and (2) Are different cortical areas specialized for different parts of the visual field? The electrophysiological data from the primary visual cortex indicate that many aspects of spatiotemporal computation are remarkably similar across the visual field, although subtle variations are detectable. Our anatomical and electrophysiological studies of the extrastriate cortex, on the other hand, suggest that visual processing in the far peripheral visual field is likely to involve a distinct network of specialized cortical areas, located in the depths of the calcarine sulcus and interhemispheric fissure.