| Abstract |
Purpose: The use of natural scenes in psychophysics studies constitutes an important tool for investigating the functional characteristics of the visual system and the way the information is neurally-represented. Previous studies suggested that the responses of visual pathway neurons to external stimuli are encoding information representative of the human’s natural environment.
The first part of this work focused on the analysis of the statistical characteristics of natural scenes, at high and low photopic light levels. Particular emphasis was given on the selectivity of optical information distribution at concrete orientations. In the second part, psychophysical measurements were performed, in order to test the neurophysiological evidence of orientation and spatial frequency tuning in the visual system.
Methods: Natural scenes were captured at different ambient light levels, using a calibrated camera. Then, appropriate algorithms were developed in Matlab for the analysis of their statistical characteristics, such as energy, RMS, Michelson and bandwidth-limited contrast and orientation selectivity.
For studying neuronal adaptation, simple reaction times (RTs) were measured to the onset of a Gaussian windowed vertical sinusoidal grating, displayed on a calibrated CRT monitor. Adaptation was tested by the presence of a 20% contrast test grating whoch was presented for 10 seconds prior to each RT recording. Two subjects were tested for a wide range of orientations (0, 2, 5, 10, 22.5, 45, 90 deg angles either side to the testing grating) and spatial frequency (0.5, 1, 2 octaves difference either side to the testing grating) combinations.
Results: The analysis of natural scenes statistics confirmed the ascertainment that they have more energy at low spatial frequencies and that the energy is decreased exponentially with spatial frequency. Moreover, energy is distributed randomly with orientation, depending on the content of each scene. The reduction of ambient light decreases the Fast Fourier amplitude respectively, without influence the slope of the amplitude vs. spatial frequency function or its distribution per orientation. The carpentered scenes showed similar characteristics, except that they had more energy in vertical and horizontal orientations, regardless of their content.
The effect of adaptation on RTs was more pronounced when the adaptor and the test grating had the same orientation and spatial frequency. When the test grating was orthogonal to the test, the adaptation was minimal. The orientation tuning functions were modelled using Gaussian and exponential functions. The bandwidth of functions found to be from 12.80 to 19.60. The spatial frequency tuning curves revealed a bandwidth of 1.15-1.57 octaves.
Conclusion: The reduction of ambient illumination decreases the energy spectra of the images. This agrees with the psychophysical findings of decreased visual performance at low light levels. Visual RTs form supra-threshold behavioural responses that could lead to indicative values of selectivity tuning in orientation and spatial frequency channels, using contrast adaptation paradigms. They offer the advantage over threshold testings in terms of accuracy in timing, which is very important due to the time course of contrast adaptation.
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