Ben J. Jennings

Luminance and color inputs to mid-level and high-level vision.

We investigated the interdependence of activity within the luminance (L + M) and opponent chromatic (L - M and S - [L + M]) postreceptoral mechanisms in mid-level and high-level vision. Mid-level processes extract contours and perform figure-background organization whereas high-level processes depend on additional semantic input, such as object knowledge. We collected mid-level (good/poor continuation) and high-level (object/nonobject) two-alternative forced-choice discrimination threshold data over a range of conditions that isolate mechanisms or simultaneously stimulate them. The L - M mechanism drove discrimination in the presence of very low luminance inputs. Contrast-dependent interactions between the luminance and L - M as well as combined L - M and S - (L + M) inputs were also found, but S - (L + M) signals, on their own, did not interact with luminance. Mean mid-level and high-level thresholds were related, with luminance providing inputs capable of sustaining performance over a broader, linearly corresponding range of contrasts when compared to L - M signals. The observed interactions are likely to be driven by L - M signals and relatively low luminance signals (approximately 0.05-0.09 L + M contrast) facilitating each other. The results are consistent with previous findings on low-level interactions between chromatic and luminance signals and demonstrate that functional interdependence between the geniculate mechanisms extends to the highest stages of the visual hierarchy.

Colour detection thresholds as a function of chromatic adaptation and light level

Colour threshold discrimination ellipses were measured for a number of states of chromatic adaptation and a range of luminance levels using the Colour Assessment and Diagnosis (CAD) test. An analysis of these results was carried out by examining the cone excitation signals along the cardinal axes that correspond to detection thresholds in the +L‐M (reddish), −L+M (greenish), +S (bluish) and −S (yellowish) colour directions. The results reveal a strong linear relationship between the excitations induced by the adapting background field in each cone class and the corresponding changes needed for threshold detection. These findings suggest that the cone excitation change for threshold detection of colour signals is always the same for a given background excitation level (in any cone class), independent of the excitations generated in the other cone classes. These observations have been used to develop a model to predict colour detection thresholds for any specified background luminance and chromaticity within the range of values investigated in this study (e.g., luminances in the range 0.3 to 31 cd.m−2 and chromaticities within the gamut of typical CRT displays). Predicted colour thresholds were found to be in close agreement with measured values with errors that do not, in general, exceed the measured within‐subject variability.

Detection of between-eye differences in color: Interactions with luminance.

Between-eye differences in color or luminance result in the appearance of luster, which provides a cue for detecting between-eye differences. We measured thresholds for detecting between-eye differences in both hue and chromatic contrast (saturation) in dichoptically superimposed color patches. Sensitivity was found to be highest at isoluminance and decreased with the addition of task-irrelevant, spatially coextensive, binocular (i.e., same in both eyes) luminance contrast. However, when the members of each dichoptic pair were presented side by side on the screen and viewed with the same eye, the added luminance contrast had no effect on the detection of their differences. If the effect of the luminance contrast was simply to dilute or desaturate the chromatic signals, we would expect thresholds to increase for the within-eye and not just the between-eye (dichoptic) conditions. We suggest that the presence of binocular luminance contrast reduces the interocular suppression between the dichoptic colors, causing the dichoptic color pairs to blend, thus rendering their differences harder to detect.

Shape recognition: convexities, concavities and things in between.

Visual objects are effortlessly recognized from their outlines, largely irrespective of viewpoint. Previous studies have drawn different conclusions regarding the importance to shape recognition of specific shape features such as convexities and concavities. However, most studies employed familiar objects, or shapes without curves, and did not measure shape recognition across changes in scale and position. We present a novel set of random shapes with well-defined convexities, concavities and inflections (intermediate points), segmented to isolate each feature type. Observers matched the segmented reference shapes to one of two subsequently presented whole-contour shapes (target or distractor) that were re-scaled and re-positioned. For very short segment lengths, performance was significantly higher for convexities than for concavities or intermediate points and for convexities remained constant with increasing segment length. For concavities and intermediate points, performance improved with increasing segment length, reaching convexity performance only for long segments. No significant differences between concavities and intermediates were found. These results show for the first time that closed curvilinear shapes are encoded using the positions of convexities, rather than concavities or intermediate regions. A shape-template model with no free parameters gave an excellent account of the data.

Probability, not linear summation, mediates the detection of concentric orientation-defined textures

Previous studies investigating signal integration in circular Glass patterns have concluded that the information in these patterns is linearly summed across the entire display for detection. Here we test whether an alternative form of summation, probability summation (PS), modeled under the assumptions of Signal Detection Theory (SDT), can be rejected as a model of Glass pattern detection. PS under SDT alone predicts that the exponent β of the Quick- (or Weibull-) fitted psychometric function should decrease with increasing signal area. We measured spatial integration in circular, radial, spiral, and parallel Glass patterns, as well as comparable patterns composed of Gabors instead of dot pairs. We measured the signal-to-noise ratio required for detection as a function of the size of the area containing signal, with the remaining area containing dot-pair or Gabor-orientation noise. Contrary to some previous studies, we found that the strength of summation never reached values close to linear summation for any stimuli. More importantly, the exponent β systematically decreased with signal area, as predicted by PS under SDT. We applied a model for PS under SDT and found that it gave a good account of the data. We conclude that probability summation is the most likely basis for the detection of circular, radial, spiral, and parallel orientation-defined textures.

Low-level and high-level modulations of fixational saccades and high frequency oscillatory brain activity in a visual object classification task

Until recently induced gamma-band activity (GBA) was considered a neural marker of cortical object representation. However, induced GBA in the electroencephalogram (EEG) is susceptible to artifacts caused by miniature fixational saccades. Recent studies have demonstrated that fixational saccades also reflect high-level representational processes. Do high-level as opposed to low-level factors influence fixational saccades? What is the effect of these factors on artifact-free GBA? To investigate this, we conducted separate eye tracking and EEG experiments using identical designs. Participants classified line drawings as objects or non-objects. To introduce low-level differences, contours were defined along different directions in cardinal color space: S-cone-isolating, intermediate isoluminant, or a full-color stimulus, the latter containing an additional achromatic component. Prior to the classification task, object discrimination thresholds were measured and stimuli were scaled to matching suprathreshold levels for each participant. In both experiments, behavioral performance was best for full-color stimuli and worst for S-cone isolating stimuli. Saccade rates 200–700 ms after stimulus onset were modulated independently by low and high-level factors, being higher for full-color stimuli than for S-cone isolating stimuli and higher for objects. Low-amplitude evoked GBA and total GBA were observed in very few conditions, showing that paradigms with isoluminant stimuli may not be ideal for eliciting such responses. We conclude that cortical loops involved in the processing of objects are preferentially excited by stimuli that contain achromatic information. Their activation can lead to relatively early exploratory eye movements even for foveally-presented stimuli.

Searching for Radial Symmetry

Symmetry is ubiquitous in the natural world. Numerous investigations, dating back over one hundred years, have explored the visual processing of symmetry. However, these studies have been concerned with mirror symmetry, overlooking radial (or rotational) symmetry, which is also prevalent in nature. Using a visual search paradigm, which approximates the everyday task of searching for an object embedded in background clutter, we have measured how quickly and how accurately human observers detect radially symmetric dot patterns. Performance was compared with mirror symmetry. We found that with orders of radial symmetry greater than 5, radial symmetry can be detected more easily than mirror symmetry, revealing for the first time that radial symmetry is a salient property of objects for human vision.

Chromatic contrast in luminance-defined images affects performance and neural activity during a shape classification task.

Models of object recognition generally emphasize the importance of luminance-defined shape. However, it is still not fully understood how color signals combine with luminance signals to affect object-related form processing. This electroencephalographic study aimed to examine the contribution of chromatic contrast by assessing its effects on the time course of shape-related processing. Participants classified Gaborized images of object shapes, nonobject shapes, and patches of pseudorandomly scattered Gabors. Stimuli excited (a) the luminance (L+M) channel alone, (b) luminance and L-M channels, or (c) luminance, L-M, and S-(L+M) channels and were presented either at mean discrimination threshold or at twice this mean threshold. As expected, classification accuracy was comparable at threshold, as were the attributes of the early, perceptual first negative (N1) component of the event-related potential (ERP). Differences emerged at suprathreshold: Objects defined by the full combination of channels were associated with the poorest performance and the lowest N1 amplitude. Shape sensitivity was not consistently observed in the N1 but was more evident in the late positive potential (LPP), a cognitive ERP component. Both the N1 and the LPP were affected by the amount and type of contrast in the image. While the effects of luminance and L-M contrast were similar, affecting the ERP selectively during the N1 and LPP period, S-(L+M) contrast elicited a sustained shift in amplitude. Our results demonstrate, for the first time using a combination of behavioral as well as early and late electrophysiological effects, that shape classification is determined by both the chromatic and the luminance content of the image.

Symmetry perception for patterns defined by colour and luminance

Perception of visual symmetry is fast and efficient and relies on both early low-level and late mid- and high-level neural mechanisms. To test for potential influences of early low-level mechanisms on symmetry perception, we used isoluminant, achromatic, and combined (color + luminance) patterns in a psychophysical and an event-related-potential (ERP) experiment. In the psychophysical experiment, pattern contrast was fixed at individual symmetry-discrimination threshold. Participants then judged whether a pattern was symmetric or random. Stimuli at isoluminance were associated with a large bias toward symmetry, achromatic stimuli introduced the opposite bias, and stimuli containing a balance of both color and luminance were perceived without bias. These findings are in line with distinct contrast sensitivity functions for color and luminance, with color providing low-frequency information useful for symmetry detection and luminance providing high-frequency information useful for detection of detail. The subsequent ERP experiment was run at high contrasts to assess processing of symmetry in suprathreshold conditions. Sustained posterior negativity, a symmetry-sensitive ERP component, was observed in all conditions and showed the expected dependence on symmetry. However, interactions between symmetry and contrast type were not observed. In conclusion, while our findings at threshold support models that propose an important contribution of low-level mechanisms to symmetry perception, at suprathreshold these low-level contributions do not persist. Therefore, under everyday viewing conditions, symmetry perception engages a relatively broad cortical network that is not constrained by low-level inputs.

Chromatic blur perception in the presence of luminance contrast

Hel-Or showed that blurring the chromatic but not the luminance layer of an image of a natural scene failed to elicit any impression of blur. Subsequent studies have suggested that this effect is due either to chromatic blur being masked by spatially contiguous luminance edges in the scene (Journal of Vision 13 (2013) 14), or to a relatively compressed transducer function for chromatic blur (Journal of Vision 15 (2015) 6). To test between the two explanations we conducted experiments using as stimuli both images of natural scenes as well as simple edges. First, we found that in color-and-luminance images of natural scenes more chromatic blur was needed to perceptually match a given level of blur in an isoluminant, i.e. colour-only scene. However, when the luminance layer in the scene was rotated relative to the chromatic layer, thus removing the colour-luminance edge correlations, the matched blur levels were near equal. Both results are consistent with Sharman et al.s explanation. Second, when observers matched the blurs of luminance-only with isoluminant scenes, the matched blurs were equal, against Kingdom et al.s prediction. Third, we measured the perceived blur in a square-wave as a function of (i) contrast (ii) number of luminance edges and (iii) the relative spatial phase between the colour and luminance edges. We found that the perceived chromatic blur was dependent on both relative phase and the number of luminance edges, or dependent on the luminance contrast if only a single edge is present. We conclude that this Hel-Or effect is largely due to masking of chromatic blur by spatially contiguous luminance edges.