Simon J. Cropper

The cone inputs to the unique-hue mechanisms

Our aim was to characterise the chromatic mechanisms that yield the four unique hues: red, green, yellow and blue. We measured the null planes for all four unique hues and report the following two main results. (1) We confirm that three chromatic mechanisms are required to account for the four unique hues. These three chromatic mechanisms do not coincide with the chromatic tuning found in parvocellular LGN neurones, i.e., neurones tuned to L-M and S-(L+M); these subcortical chromatic mechanisms are hence not the neural substrate of the perceptual unique hues and further higher-order colour mechanisms need to be postulated. Our results are consistent with the idea that the two higher-order colour mechanisms that yield unique red and unique green respectively combine the incremental and decremental responses of the subcortical chromatic mechanisms with different weights. In contrast, unique yellow and unique blue can be explained by postulating a single higher-order chromatic mechanism that combines the incremental and decremental subcortical chromatic responses with similar weights. (2) The variability between observers is small when expressed in terms of perceptual errors, which is consistent with the hypothesis that the colour vision system in adult humans is able to recalibrate itself based on prior visual experience.

The perception of motion in chromatic stimuli.

The issue of whether there is a motion mechanism sensitive to purely chromatic stimuli has been pertinent for the past 30 or more years. The aim of this review is to examine why such different conclusions have been drawn in the literature and to reach some reconciliation. The review critically examines the behavioral evidence and concludes that there is a purely chromatic motion mechanism but that it is limited to the fovea. Examination of motion performance for chromatic and luminance stimuli provides convincing evidence that there are at least two different mechanisms for the two kinds of stimuli. The authors further argue that the chromatic mechanism may be at a particular disadvantage when the integration of multiple local motion signals is required. Finally, the authors present a descriptive model that may go some way toward explaining the reasons for the differences in collected data outlined in this article.

Absence of linear subthreshold summation between red-green and luminance mechanisms over a wide range of spatio-temporal conditions

We have tested the independence of red-green chromatic and luminance mechanisms at detection threshold using a method of subthreshold summation. Stimuli were isoluminant red-green gratings and yellow-black luminance gratings that uniquely activate the red-green color and luminance mechanisms, respectively. Stimuli were Gaussian enveloped 0.25, 1 or 4 cpd sinewave gratings, counter-phase flickered at 0, 5 or 9 Hz. The threshold detection of red-green color contrast was measured in the presence of a subthreshold amount of luminance contrast, and vice versa. The results allow a model of linear summation between the color and luminance mechanisms to be rejected, but are well fitted by a model, assuming that these mechanisms are independent but combine to determine detection by probability summation, with a high summation index (median value = 4). We conclude that there are independent red-green chromatic mechanism and luminance detection mechanisms over this range of spatio-temporal conditions.

Motion Coherence Across Different Chromatic Axes

It has been reported that equiluminant plaid patterns constructed from component gratings modulated along different axes of a cardinal colour space fail to create a coherent impression of two-dimensional motion Krauskopf and Farell (1990). Nature, 348, 328–331. In this paper we assess whether this lack of interaction between cardinal axes is a general finding or is instead dependent upon specific stimulus parameters. Type I and Type II plaids were made from sinusoidal components (1 cpd) each modulated along axes in a cardinal colour space and presented at equivalent perceived contrasts. The spatial angular difference between the two components was varied from 5 to 90 deg whilst keeping the Intersection of Constraints (I.O.C.) solution of the pattern constant. Observers were required to indicate the perceived direction of motion of the pattern in a single interval direction-identification task. We find that: (i) When plaids were made from components modulated along the same cardinal axis, coherent “pattern” motion was perceived at all angular differences. As the angular difference between the components decreased in a Type II plaid, the perceived direction of motion moved closer to the I.O.C. solution and away from that predicted by the vector sum. (ii) A plaid made from components modulated along red-green and blue-yellow cardinal axes (cross-cardinal axis) did not cohere at high angular differences (>30 deg) but had a perceived direction of the fastest moving component. At lower angular differences, however, pattern motion was detected and approached the I.O.C. solution in much the same way as a same-cardinal axis Type II plaid. (iii) A plaid made from a luminance grating and a cardinal chromatic grating (red-green or blue-yellow) failed to cohere under all conditions, demonstrating that there is no interaction between luminance and chromatic cardinal axes. These results indicate that there are conditions under which red-green and blue-yellow cardinal components interact for the purposes of motion detection. Copyright

Summation of Visual Motion across Eye Movements Reflects a Nonspatial Decision Mechanism

Human vision remains perceptually stable even though retinal inputs change rapidly with each eye movement. Although the neural basis of visual stability remains unknown, a recent psychophysical study pointed to the existence of visual feature-representations anchored in environmental rather than retinal coordinates (e.g., “spatiotopic” receptive fields; Melcher and Morrone, 2003). In that study, sensitivity to a moving stimulus presented after a saccadic eye movement was enhanced when preceded by another moving stimulus at the same spatial location before the saccade. The finding is consistent with spatiotopic sensory integration, but it could also have arisen from a probabilistic improvement in performance due to the presence of more than one motion signal for the perceptual decision. Here we show that this statistical advantage accounts completely for summation effects in this task. We first demonstrate that measurements of summation are confounded by noise related to an observers uncertainty about motion onset times. When this uncertainty is minimized, comparable summation is observed regardless of whether two motion signals occupy the same or different locations in space, and whether they contain the same or opposite directions of motion. These results are incompatible with the tuning properties of motion-sensitive sensory neurons and provide no evidence for a spatiotopic representation of visual motion. Instead, summation in this context reflects a decision mechanism that uses abstract representations of sensory events to optimize choice behavior.

Velocity discrimination in chromatic gratings and beats.

It is commonly assumed that the ability to discriminate velocity in a stimulus directly reflects the properties of the underlying directionally-selective mechanism. The results presented here show that this assumption is not always correct. Speed discrimination tasks over a range of base velocities were carried out for luminance gratings, chromatic gratings and contrast (beat) gratings of equivalent periodicity and contrasts. At low contrasts (0.5 log units above detection threshold), speed discrimination in luminance gratings was at least twice as good (when expressed as a Weber fraction), than in either chromatic gratings or beats. This is similar to the situation seen for tasks of direction discrimination using these stimuli [e.g. Cropper and Derrington (1990) Perception, 19, A31]. When the stimulus contrasts were increased to 1.5 log units above detection threshold, the ability to discriminate speed in both chromatic and beat stimuli improved to a performance level comparable to that shown for luminance gratings at all contrasts. This effect is not seen for tasks of direction discrimination when the same increase in stimulus contrast has little effect on the lower threshold of motion (LTM) measured for beat patterns. These results indicate that the ability to discriminate velocity in a stimulus does not necessarily directly reflect the characteristics of the ability to discriminate the direction of motion of that stimulus.

On the role of second-order signals in the perceived direction of motion of type II plaid patterns

Second-order Type I and Type II plaids were constructed by combining two random-dot gratings. Each component consisted of a dynamic random-dot field, the contrast of which was modulated by a drifting sinusoidal grating. Orienting the two components suitably and interleaving at 120 Hz allowed us to produce a two-dimensional plaid pattern made from one-dimensional second-order components. The perceived direction of motion of both Type I and Type II plaids was measured as a function of stimulus duration. Type I plaids had a perceived direction close to the intersection of constraints/vector sum solution (which only coincide for these patterns) at all durations. Type II plaids had a perceived direction that moved away from the vector sum and toward the intersection of constraints solution as the duration of presentation increased. These results are similar in form to those found for plaids made from first-order (luminance-defined) components [Yo & Wilson (1992), Vision Research, 32, 135-147]. This suggests that a delay which operates specifically on second-order signals cannot be the sole cause for the change in perceived direction of Type II plaids made from first-order components [Wilson, Ferrera & Yo (1992), Visual Neuroscience, 9, 79-97].

Insight Is Not in the Problem: Investigating Insight in Problem Solving across Task Types

The feeling of insight in problem solving is typically associated with the sudden realization of a solution that appears obviously correct (Kounios et al., 2006). Salvi et al. (2016) found that a solution accompanied with sudden insight is more likely to be correct than a problem solved through conscious and incremental steps. However, Metcalfe (1986) indicated that participants would often present an inelegant but plausible (wrong) answer as correct with a high feeling of warmth (a subjective measure of closeness to solution). This discrepancy may be due to the use of different tasks or due to different methods in the measurement of insight (i.e., using a binary vs. continuous scale). In three experiments, we investigated both findings, using many different problem tasks (e.g., Compound Remote Associates, so-called classic insight problems, and non-insight problems). Participants rated insight-related affect (feelings of Aha-experience, confidence, surprise, impasse, and pleasure) on continuous scales. As expected we found that, for problems designed to elicit insight, correct solutions elicited higher proportions of reported insight in the solution compared to non-insight solutions; further, correct solutions elicited stronger feelings of insight compared to incorrect solutions.

Adaptation to motion of a second-order pattern: the motion aftereffect is not a general result

It has become apparent from recent work that the spatial frequency and orientation content of the first-order (luminance) carrier is very important in determining the properties of a second-order (contrast) modulation of that carrier. In light of this we examined whether there was any evidence for a motion aftereffect in one-dimensional second-order patterns containing only two sinusoidal luminance components: a spatial beat. The stimuli were either 1 cpd luminance sinusoids or 1 cpd luminance beats modulating a carrier sinusoid of 5 cpd. The magnitude of any motion aftereffect, or any directionally specific effect of adaptation, was measured for all combinations of first and second-order test and adapting patterns. Both flickering and non-flickering stimuli were used. The results indicate that a motion aftereffect is only induced by first-order adapting stimuli, and likewise, is only measurable in first-order test stimuli. We find no evidence for any directionally specific effect of adaptation in second-order stimuli, whether the test is counterphased or otherwise. These results apparently conflict with recent reports of a second-order induced motion aftereffect, but are consistent with many other findings which show differences between the detection of motion for first and second-order stimuli. We conclude that the induction of a motion aftereffect for second-order stimuli is not a general result and is critically dependent upon (amongst other things) the local properties of the stimulus, including the spatial frequency and orientation content of the first-order carrier.

The Categorisation of Non-Categorical Colours: A Novel Paradigm in Colour Perception

In this paper, we investigate a new paradigm for studying the development of the colour ‘signal’ by having observers discriminate and categorize the same set of controlled and calibrated cardinal coloured stimuli. Notably, in both tasks, each observer was free to decide whether two pairs of colors were the same or belonged to the same category. The use of the same stimulus set for both tasks provides, we argue, an incremental behavioural measure of colour processing from detection through discrimination to categorisation. The measured data spaces are different for the two tasks, and furthermore the categorisation data is unique to each observer. In addition, we develop a model which assumes that the principal difference between the tasks is the degree of similarity between the stimuli which has different constraints for the categorisation task compared to the discrimination task. This approach not only makes sense of the current (and associated) data but links the processes of discrimination and categorisation in a novel way and, by implication, expands upon the previous research linking categorisation to other tasks not limited to colour perception.