Abstract
During natural viewing, the eyes are never still. Even during fixation, miniature movements of the eyes move the retinal image across tens of foveal photoreceptors. Most theories of vision implicitly assume that the visual system ignores these movements and somehow overcomes the resulting smearing. However, evidence has accumulated to indicate that fixational eye movements cannot be ignored by the visual system if fine spatial details are to be resolved. We argue that the only way the visual system can achieve its high resolution given its fixational movements is by seeing via these movements. Seeing via eye movements also eliminates the instability of the image, which would be induced by them otherwise. Here we present a hypothesis for vision, in which coarse details are spatially encoded in gaze-related coordinates, and fine spatial details are temporally encoded in relative retinal coordinates. The temporal encoding presented here achieves its highest resolution by encoding along the elongated axes of simple-cell receptive fields and not across these axes as suggested by spatial models of vision. According to our hypothesis, fine details of shape are encoded by inter-receptor temporal phases, texture by instantaneous intra-burst rates of individual receptors, and motion by inter-burst temporal frequencies. We further describe the ability of the visual system to readout the encoded information and recode it internally. We show how reading out of retinal signals can be facilitated by neuronal phase-locked loops (NPLLs), which lock to the retinal jitter; this locking enables recoding of motion information and temporal framing of shape and texture processing. A possible implementation of this locking-and-recoding process by specific thalamocortical loops is suggested. Overall it is suggested that high-acuity vision is based primarily on temporal mechanisms of the sort presented here and low-acuity vision is based primarily on spatial mechanisms.
Highlights
During fixation, the eyes move across several arcminutes with amplitudes that fall off with the scanning frequency (Findlay, 1971; Eizenman et al, 1985)
We show how reading out of retinal signals can be facilitated by neuronal phase-locked loops (NPLLs), which lock to the retinal jitter; this locking enables recoding of motion information and temporal framing of shape and texture processing
We suggest that information obtained across the elongated axes of subfield of a simple receptive field (sRF) is used for coarse image analysis, while fine details are encoded along the elongated axes of the sRFs (Figure 2, red, horizontal sRFs)
Summary
The eyes move across several arcminutes with amplitudes that fall off with the scanning frequency (Findlay, 1971; Eizenman et al, 1985). In every response, occurring upon an illumination transient, the timing of the first spike in a burst is accurate and reliable while the number and rate of following spikes is variable and not consistent (Reich et al, 1997) It takes a great deal of justification for the visual system to ignore the most informative code and use the less informative one when processing fine spatial details (Van Rullen and Thorpe, 2001; see A Simple Quantitative Account Against Spatial Retinal Coding of Fine Spatial Details in Appendix 2, and Meister and Berry, 1999). We do claim that the mechanism presented here is a plausible and efficient way to process retinal outputs and that it is consistent with a large body of data
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