Abstract: []
We quantified and compared the effect of element spacing on contour integration between the achromatic, red-green and blue-yellow mechanisms. The task requires the linking of orientation across space to detect a contour in a stimulus composed of randomly oriented Gabor elements (1.5 cpd, sigma = 0.17 deg), measured using a temporal 2AFC method. A contour of 10 elements was pasted into a 10 x 10 cells array, and background elements were randomly positioned within the available cells. The effect of element spacing was investigated by varying the mean inter-element distance between 2 and 6 times the period of the Gabor elements (lambda = 0.66 deg) while the total number of elements was fixed. Contour detection was measured as a function of its curvature for jagged contours and for closed contours. At all curvatures we found that performance for chromatic mechanisms declines more steeply with the increase in element separation than performance does for the achromatic mechanism. Averaged critical element separations were 4.6±0.7, 3.6±0.4, and 2.9±0.2 deg for Ach, BY and RG mechanisms, respectively. These results suggest that contour integration by the chromatic mechanisms relies more on short-range interactions in comparison to the achromatic mechanism. In a further experiment, we looked at the combined effect of element size and element separation in contour integration for the achromatic mechanism. We found that the critical separation decreases linearly with the spatial frequency, from about 5 deg at low spatial frequency (larger elements) to about 1 deg at high spatial frequency (smaller elements) suggesting a scale invariance in contour integration. In both experiments we also found no differences between closed and open jagged contours detection in terms of element separation. The neuro-anatomical implications of these findings relatively to area V1 are discussed.
Beaudot W.H.A. (2002) Role of onset asynchrony in contour integration. Vision Research 42(1):1-9
Abstract: [Download Full PDF Paper]
Evidence that visual grouping is facilitated when elements comprising a foreground figure are presented simultaneously, and are temporally separated from elements comprising the background, has suggested cortical synchronous oscillations as a possible neural substrate. Supporting this theory, Usher & Donnelly (1998) showed in one of their experiments that contour integration is
facilitated when path and background elements alternate with an asynchrony below the integration time of the visual system, suggesting that these flickering stimuli interact with this hypothetical binding mechanism. I replicated this experiment and report that the effect depends in fact on the
order of asynchrony between path and background elements in the first cycle of stimuli presented for more than 100ms: facilitation in visual grouping only occurs when path elements are presented before background elements. A second experiment, exploring the effect of onset delays between path and background elements, demonstrates a strong priming effect of path elements. I conclude
that Usher & Donnelly's result is likely due to the high sensitivity of the visual system to stimulus onset, and that simple flickering stimuli are inadequate for revealing the neural code for binding in figure-ground segregation without controlling for the effect of stimulus onset.
Beaudot W.H.A., Mullen K.T. (2001) Processing time of contour integration: the role of colour, contrast, and curvature. Perception 30(7):833-853
Abstract: [Download Full PDF Paper]
We investigated the temporal properties of the red-green, blue-yellow, and luminance mechanisms in a contour-integration task which required the
linking of orientation across space to detect a 'path'. Reaction times were obtained for simple detection of the stimulus regardless of the presence of a
path, and for path detection measured by a yes/no procedure with path and no-path stimuli randomly presented. Additional processing times for contour
integration were calculated as the difference between reaction times for simple stimulus detection and path detection, and were measured as a function of
stimulus contrast for straight and curved paths. We found that processing time shows effects not apparent in choice reaction-time measurements. (i)
Processing time for curved paths is longer than for straight paths. (ii) For straight paths, the achromatic mechanism is faster than the two chromatic
ones, with no difference between the red-green and blue-yellow mechanisms. For curved paths there is no difference in processing time between
mechanisms. (iii) The extra processing time required to detect curved compared to straight paths is longest for the achromatic mechanism, and similar
for the red-green and blue-yellow mechanisms, (iv) Detection of the absence of a path requires at least 50 ms of additional time independently of
chromaticity, contrast, and path curvature. The significance of these differences and similarities between postreceptoral mechanisms is discussed.
Hess R.F., Beaudot W.H.A., Mullen K.T. (2001) Dynamics of contour integration. Vision Research 41(8):1023-1037
Abstract: [Download Full PDF Paper]
To determine the dynamics of contour integration the temporal properties of the individual contour elements were varied as well as those of the contour
they form. A temporal version of a contour integration paradigm (Field, D. J., Hayes, A., & Hess, R. F. (1993) Vision Research, 33, 173-193) was
used to assess these two temporal dynamics as a function of the contrast of individual elements and the curvature of the contour. The results show that
the dynamics of contour integration are good when the contrast of the individual elements is modulated in time (10-30 Hz), but are poor when contour
linking per se is temporally modulated (1-12 Hz). The dynamics of contour linking is not dependent on the absolute contrast of the linking elements, so
long as they are visible, but does vary with the curvature of the contour. For straight contours, temporal resolution is around 6-12 Hz but falls to
around 1-2 Hz for curved contours.
Beaudot W.H.A., Mullen K.T. (2000) Role of chromaticity, contrast, and local orientation cues in the perception of density. Perception 29(5):581-600
Abstract: [Download Full PDF Paper]
We compared the role of the red-green, blue-yellow, and luminance post-receptoral mechanisms in the perception of density. The task requires the
comparison of densities between two stimuli composed of oriented bandpass elements, pseudo-randomly scattered across an area of constant size. The
perception of density differences was measured by a temporal 2AFC procedure for all pairs of mechanisms and for four possible densities. We found
that stimuli of identical physical densities are not perceived equally: there is a consistent bias in favour of blue-yellow stimuli which are perceived as
significantly more dense than red-green and achromatic stimuli. We considered three factors that could have differentially affected the density perception
of blue-yellow stimuli: an increase in the perceived size of the individual blue-yellow elements, a perceived contrast difference, and the presence of local orientation cues. We found that the increased perceived density of the blue-yellow stimuli occurred despite the fact that there was no increase in
perceived size of the individual elements, and remained despite corrections for the two other factors. We conclude that the significant increase in
perceived density for the blue-yellow mechanism is a global effect, associated with a perceived colour 'melting' of the elements in the array. Our data
were fitted with the occupancy model of Allik and Tuulmets (1991, Perception & Psychophysics 49 303-314) and we found that blue-yellow stimuli
have a greater 'occupancy' than red-green or achromatic stimuli.
Mullen K.T., Beaudot W.H.A., McIlhagga W.H. (2000) Contour integration in color vision: a common process for the blue-yellow, red-green and luminance mechanisms?. Vision Research 40(6):639-655
Abstract: [Download Full PDF Paper]
We compare the performance of the red-green, blue-yellow and luminance postreceptoral mechanisms on a contour integration task requiring the linking
of oriented Gabor elements across space to extract a winding 'path' or contour. We first establish that for all three mechanisms curvature and contrast
are independent; losses in performance due to one cannot be compensated by changes in the other. We then compare contour integration by the three
mechanisms using a method that controls for their differences in cone contrast thresholds. Our results show that despite the poor orientation
discrimination thresholds and poor spatial sampling found for the blue-yellow mechanism, all three mechanisms perform similarly on contour
integration over a wide range of curvatures. Furthermore, all three mechanisms have the same dependence on path curvature. We also investigate the
effects of adding external orientation noise. Our results imply that the internal orientation noise for extracting 'aligned' path elements is similar in the three mechanisms and for all path curvatures, and the relative efficiencies are also similar for the three mechanisms. To account for our results, we
propose that the three postreceptoral mechanisms use a common contour integration process. This linking process, however, cannot be color-blind; our
last experiment shows that linking between different chromatic mechanisms or between opposite spatial phases disrupts contour integration. We thus
propose that the common integration process remains sensitive to the color contrast and phase of its inputs.
Beaudot W.H.A. (1996) Sensory coding in the vertebrate retina: Towards an adaptive control of visual sensitivity. Network: Computation in Neural Systems 7(2):317-323
Abstract: [Download Full PS Paper]
We propose a theoretical framework for the adaptive control of visual sensitivity in the vertebrate retina. The photoreceptor transfer function is modelled with a Michaelis-Menten law instead of a logarithmic function. This more plausible function has a biophysical correlate, and it allows consideration of the photoreceptor as the main locus of retinal adaptation. The retinal model suggests that the function of photoreceptors might be to control visual sensitivity, defined as the optimal transcoding of non-stationary visual information. This is done by using an adaptive transfer function whose parameters are spatiotemporally and locally estimated by the subsequent retinal circuit and fed back to the photoreceptors. The proposed model also supports the functional architecture of the vertebrate retina.
William H.A. Beaudot
Last modified: December 10th, 2002
