Michele Rucci

Contrast sensitivity reveals an oculomotor strategy for temporally encoding space

The contrast sensitivity function (CSF), how sensitivity varies with the spatial frequency of the stimulus, is a fundamental assessment of visual performance. The CSF is generally assumed to be determined by low-level sensory processes. However, the sensitivities of neurons in the early visual pathways, as measured in experiments with immobilized eyes, diverge from psychophysical CSF measurements in primates. Under natural viewing conditions, as in typical psychophysical measurements, humans continually move their eyes, drifting in a seemingly erratic manner even when looking at a fixed point. Here, we show that the resulting transformation of the visual scene into a spatiotemporal flow on the retina constitutes a processing stage that reconciles human CSF and the response characteristics of retinal ganglion cells under a broad range of conditions. Our findings suggest a fundamental integration between perception and action: eye movements work synergistically with the sensitivities of retinal neurons to encode spatial information.

Temporal Coding of Visual Space

Establishing a representation of space is a major goal of sensory systems. Spatial information, however, is not always explicit in the incoming sensory signals. In most modalities it needs to be actively extracted from cues embedded in the temporal flow of receptor activation. Vision, on the other hand, starts with a sophisticated optical imaging system that explicitly preserves spatial information on the retina. This may lead to the assumption that vision is predominantly a spatial process: all that is needed is to transmit the retinal image to the cortex, like uploading a digital photograph, to establish a spatial map of the world. However, this deceptively simple analogy is inconsistent with theoretical models and experiments that study visual processing in the context of normal motor behavior. We argue here that, as with other senses, vision relies heavily on temporal strategies and temporal neural codes to extract and represent spatial information.

Perspective: Can eye movements contribute to emmetropization?

During development, the eye tunes its size to its optics so that distant objects are in focus, a state known as emmetropia. Although multiple factors contribute to this process, a strong influence appears to be exerted by the visual input signals entering the eye. Much research has been dedicated to the possible roles of specific features of the retinal image, such as the magnitude of blur. However, in humans and other species, the input to the retina is not an image, but a spatiotemporal flow of luminance. Small eye movements occur incessantly during natural fixation, continually transforming the spatial scene into temporal modulations on the retina. An emerging body of evidence suggests that this space–time reformatting is crucial to many aspects of visual processing, including sensitivity to fine spatial detail. The resulting temporal modulations depend not only on ocular dynamics, but also on the optics and shape of the eye, and the spatial statistics of the visual scene. Here we examine the characteristics of these signals and suggest that they may play a role in emmetropization. A direct consequence of this viewpoint is that abnormal oculomotor behavior may contribute to the development of myopia and hyperopia.