Biomedical subjects

Gidon Felsen

Publications and source records attributed to Gidon Felsen.

5 recordsLinked to original sources

PubMed2005

Cortical sensitivity to visual features in natural scenes.

A central hypothesis concerning sensory processing is that the neuronal circuits are specifically adapted to represent natural stimuli efficiently. Here we show a novel effect in cortical coding of natural images. Using spike-triggered average or spike-triggered covariance analyses, we first identified the visual features selectively represented by each cortical neuron from its responses to natural images. We then measured the neuronal sensitivity to these features when they were present in either natural images or random stimuli. We found that in the responses of complex cells, but not of simple cells, the sensitivity was markedly higher for natural images than for random stimuli. Such elevated sensitivity leads to increased detectability of the visual features and thus an improved cortical representation of natural scenes. Interestingly, this effect is due not to the spatial power spectra of natural images, but to their phase regularities. These results point to a distinct visual-coding strategy that is mediated by contextual modulation of cortical responses tuned to the spatial-phase structure of natural scenes.

Animals↗

PubMed2005

Spatial structure of complex cell receptive fields measured with natural images.

Neuronal receptive fields (RFs) play crucial roles in visual processing. While the linear RFs of early neurons have been well studied, RFs of cortical complex cells are nonlinear and therefore difficult to characterize, especially in the context of natural stimuli. In this study, we used a nonlinear technique to compute the RFs of complex cells from their responses to natural images. We found that each RF is well described by a small number of subunits, which are oriented, localized, and bandpass. These subunits contribute to neuronal responses in a contrast-dependent, polarity-invariant manner, and they can largely predict the orientation and spatial frequency tuning of the cell. Although the RF structures measured with natural images were similar to those measured with random stimuli, natural images were more effective for driving complex cells, thus facilitating rapid identification of the subunits. The subunit RF model provides a useful basis for understanding cortical processing of natural stimuli.

Action Potentials↗

PubMed2005

A natural approach to studying vision.

An ultimate goal of systems neuroscience is to understand how sensory stimuli encountered in the natural environment are processed by neural circuits. Achieving this goal requires knowledge of both the characteristics of natural stimuli and the response properties of sensory neurons under natural stimulation. Most of our current notions of sensory processing have come from experiments using simple, parametric stimulus sets. However, a growing number of researchers have begun to question whether this approach alone is sufficient for understanding the real-life sensory tasks performed by the organism. Here, focusing on the early visual pathway, we argue that the use of natural stimuli is vital for advancing our understanding of sensory processing.

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PubMed2002

Dynamic modification of cortical orientation tuning mediated by recurrent connections.

Receptive field properties of visual cortical neurons depend on the spatiotemporal context within which the stimuli are presented. We have examined the temporal context dependence of cortical orientation tuning using dynamic visual stimuli with rapidly changing orientations. We found that tuning to the orientation of the test stimulus depended on a briefly presented preceding stimulus, with the preferred orientation shifting away from the preceding orientation. Analyses of the spatial-phase dependence of the shift showed that the effect cannot be explained by purely feedforward mechanisms, but can be accounted for by activity-dependent changes in the recurrent interactions between different orientation columns. Thus, short-term plasticity of the intracortical circuit can mediate dynamic modification of orientation tuning, which may be important for efficient visual coding.

Animals↗

PubMedDate not supplied

Contextual modulation of orientation tuning contributes to efficient processing of natural stimuli.

It has been proposed that sensory neurons are adapted to the statistical structure of the natural environment in order to encode natural stimuli efficiently. While spatiotemporal correlations in luminance signals may be decorrelated by neurons in early visual processing stages, higher-order correlations, such as those in the orientation domain, are likely to persist in the input representation until the cortical level. In this study, we first examine orientation correlations in natural stimuli across brief time intervals and across nearby regions of space, and find strong correlations in both domains. We then examine contextual modulation of orientation tuning. We find that both temporal and spatial contexts exert a common influence on orientation tuning, shifting tuning away from the orientation of either the adapting (temporal) or surrounding (spatial) grating. Finally, we incorporate this context-mediated repulsive shift in orientation tuning into a model of cortical responses. We find that a direct result of the shift is a reduction of the redundancy in the population responses evoked by the orientation configurations that are most common in natural stimuli. Thus, cortical neurons may be adapted to the statistics of orientation in natural stimuli in order to increase the efficiency of natural stimulus representation.

Animals↗