Properties of the receptive ¢elds of simple cells in macaque cortex were compared with properties of independent component ¢lters generated by independent component analysis (ICA) on a large set of natural images. Histograms of spatial frequency bandwidth, orientation tuning bandwidth, aspect ratio and length of the receptive ¢elds match well. This indicates that simple cells are well tuned to the expected statistics of natural stimuli. There is no match, however, in calculated and measured distributions for the peak of the spatial frequency response: the ¢lters produced by ICA do not vary their spatial scale as much as simple cells do, but are ¢xed to scales close to the ¢nest ones allowed by the sampling lattice. Possible ways to resolve this discrepancy are discussed.

Power spectra of an extensive set of natural images were analysed. Both the total power in a spectrum (corresponding to image contrast) and its dependence on spatial frequency vary considerably between images, and also within images when considered as functions of orientation. A series of probabilistic models for power spectra enabled calculating the information obtained from prior knowledge of parameters describing spectra. Most information is gained from contrast, 1/f2 spatial frequency behaviour, and contrast as a function of orientation. Variations in spatial frequency behaviour are relatively unimportant. For oriented contrast, a bandwidth of 10-30 deg is sufficient to obtain most information.

Simple cells in the primary visual cortex process incoming visual information with receptive ¢elds localized in space and time, bandpass in spatial and temporal frequency, tuned in orientation, and commonly selective for the direction of movement. It is shown that performing independent component analysis (ICA) on video sequences of natural scenes produces results with qualitatively similar spatio-temporal properties. Whereas the independent components of video resemble moving edges or bars, the independent component ¢lters, i.e. the analogues of receptive ¢elds, resemble moving sinusoids windowed by steady Gaussian envelopes. Contrary to earlier ICA results on static images, which gave only ¢lters at the ¢nest possible spatial scale, the spatio-temporal analysis yields ¢lters at a range of spatial and temporal scales. Filters centred at low spatial frequencies are generally tuned to faster movement than those at high spatial frequencies.

A major problem that a visual system faces is how to fit the large intensity variation of natural image streams into the limited dynamic range of its neurons. One of the means to accomplish this is through the use of gain control. In order to investigate this, natural time series of intensities were measured, as well as the responses of blowfly photoreceptors and Large Monopolar Cells (LMCs) to these time series. Time series representative of what each photoreceptor of a real visual system would normally receive were measured with an optical system measuring the light intensity of a spot comparable with the field of view of single human foveal cones. This system was worn on a headband by a freely walking person. Resulting time series have rms-contrasts ranging from an average of 0.45 for 1-sec segments to 1.39 for 100-sec segments (both when limited to frequencies up to 100 Hz). Power spectra behave approximately as 1/f (f: temporal frequency). Measured time series were subsequently presented to fly photoreceptors and LMCs by playing them back on an LED. The results show that fast gain controls indeed keep the response within the dynamic range of the cells and that a large part of this range is actually used for packing the information in natural time series.

A theory is developed on the assumption that early sensory processing aims at maximizing the information rate in the channels connecting the sensory system to more central parts of the brain, where it is assumed that these channels are noisy and have a limited dynamic range. Given a stimulus power spectrum, the theory enables the computation of filters accomplishing this maximizing of information. Resulting filters are band-pass or high-pass at high signal-to-noise ratios, and low-pass at low signal-to-noise ratios. In spatial vision this corresponds to lateral inhibition and pooling, respectively. The filters comply with Weber's law over a considerable range of signal-to-noise ratios.

1. A theory is presented that utilizes the structure of natural images, and how they change in time, to produce spatiotemporal filters that maximize information flow through a noisy channel of limited dynamic range. For low signal-to-noise ratios (SNRs) the filter has low-pass, and for high SNRs band-pass characteristics, both in space and time.2. Theoretical impulse responses are compared to measurements in Large Monopolar Cells (LMCs) in the fly (Calliphora vicina) brain. Two different spatial stimuli (point source and wide field) were given at background intensities over a 5.5 log unit wide range. Theory and experiment correspond well, and they share the following properties: impulse responses get much faster and more biphasic with increasing background intensity (SNR); they show larger off-transients for wide field stimuli than for point sources; the half-width of the spatial receptive field Changes only slightly with increased intensity, and lateral inhibition increases; contrast efficiency increases with intensity.

A model for the sensitivity regulation in the primate outer retina is developed and validated using horizontal cell measurements from the literature. The main conclusion is that the phototransduction of the cones is the key factor regulating sensitivity. The model consists of a nonlinearity cascaded with three feedback control loops. The nonlinearity is caused by the hydrolysis of cGMP by activated phosphodiesterase. The first feedback loop is divisive, with calcium regulating the photocurrent in the cone outer segment. The second feedback loop is also divisive, with voltage-sensitive channels regulating the membrane voltage of the cone inner segment. The final feedback loop is subtractive, where the membrane voltage of the horizontal cell is subtracted from that of the cone before the cone drives the horizontal and bipolar cells. The model describes adequately the major characteristics of the horizontal cell responses to wide field, spectrally white stimuli. In particular, it shows (1) sensitivity and bandwidth control as a function of background intensity; (2) the major nonlinearities observed in the horizontal cells; and (3) the transition from linear responses toward contrast constancy (Weber's law) for background illuminances ranging from 1-1000 td.

Sensing is often implicitly assumed to be the passive acquisition of information. However, part of the sensory information is generated actively when animals move. For instance, humans shift their gaze actively in a sequence of saccades towards interesting locations in a scene. Likewise, many insects shift their gaze by saccadic turns of body and head, keeping their gaze fixed between saccades. Here we employ a novel panoramic virtual reality stimulator and show that motion computation in a blowfly visual interneuron is tuned to make efficient use of the characteristic dynamics of retinal image flow. The neuron is able to extract information about the spatial layout of the environment by utilizing intervals of stable vision resulting from the saccadic viewing strategy. The extraction is possible because the retinal image flow evoked by translation, containing information about object distances, is confined to low frequencies. This flow component can be derived from the total optic flow between saccades because the residual intersaccadic head rotations are small and encoded at higher frequencies. Information about the spatial layout of the environment can thus be extracted by the neuron in a computationally parsimonious way. These results on neuronal function based on naturalistic, behaviourally generated optic flow are in stark contrast to conclusions based on conventional visual stimuli that the neuron primarily represents a detector for yaw rotations of the animal.