Reaction times to chromatic stimuli

Cortical synchronization and perceptual framing

How does the brain group together different parts of an object into a coherent visual object representation? Different parts of an object may be processed by the brain at different rates and may thus become desynchronized. Perceptual framing is a process that resynchronizes cortical activities corresponding to the same retinal object. A neural network model is presented that is able to rapidly resynchronize desynchronized neural activities. The model provides a link between perceptual and brain data. Model properties quantitatively simulate perceptual framing data, including psychophysical data about temporal order judgments and the reduction of threshold contrast as a function of stimulus length. Such a model has earlier been used to explain data about illusory contour formation, texture segregation, shape-from-shading, 3-D vision, and cortical receptive fields. The model hereby shows how many data may be understood as manifestations of a cortical grouping process that can rapidly resynchronize image parts that belong together in visual object representations. The model exhibits better synchronization in the presence of noise than without noise, a type of stochastic resonance, and synchronizes robustly when cells that represent different stimulus orientations compete. These properties arise when fast long-range cooperation and slow short-range competition interact via nonlinear feedback interactions with cells that obey shunting equations.

Influence of daytime running lamps on visual reaction time of pedestrians when detecting turn indicators

INTRODUCTION

This article describes one experiment that studied the influence of Daytime Running Lamps (DRL) on pedestrian detection of turn indicators.

METHOD

An experimental device including one DRL and one turn indicator was used in order to determine Visual Reaction Times (VRT) of 148 observers in different situations involving turn indicator activation. Such situations were combinations of three main variables: color of DRL, separation between DRL and Turn Indicator, and observation angle.

RESULTS

Significant changes in VRT were found depending on the configurations above, especially the observation angle and the color of DRL. This second result demonstrates that amber DRLs inhibit the detection of Turn Indicators.

IMPACT ON INDUSTRY

One of the main targets of this paper is to recommend that carmakers introduce only white DRLs on new vehicles. We also intend to advise regulatory bodies working on automotive regulation about the consequences of allowing amber DRLs and also about the danger of introducing constrains on the distance between DRL and Turn Indicator without further experimental evidences.

The effect of chromatic and luminance information on reaction times

We present a series of experiments exploring the effect of chromaticity on reaction time (RT) for a variety of stimulus conditions, including chromatic and luminance contrast, luminance, and size. The chromaticity of these stimuli was varied along a series of vectors in color space that included the two chromatic-opponent-cone axes, a red-green (L-M) axis and a blue-yellow [S - (L + M)] axis, and intermediate noncardinal orientations, as well as the luminance axis (L + M). For Weber luminance contrasts above 10-20%, RTs tend to the same asymptote, irrespective of chromatic direction. At lower luminance contrast, the addition of chromatic information shortens the RT. RTs are strongly influenced by stimulus size when the chromatic stimulus is modulated along the [S - (L + M)] pathway and by stimulus size and adaptation luminance for the (L-M) pathway. RTs are independent of stimulus size for stimuli larger than 0.5 deg. Data are modeled with a modified version of Pieron's formula with an exponent close to 2, in which the stimulus intensity term is replaced by a factor that considers the relative effects of chromatic and achromatic information, as indexed by the RMS (square-root of the cone contrast) value at isoluminance and the Weber luminance contrast, respectively. The parameters of the model reveal how RT is linked to stimulus size, chromatic channels, and adaptation luminance and how they can be interpreted in terms of two chromatic mechanisms. This equation predicts that, for isoluminance, RTs for a stimulus lying on the S-cone pathway are higher than those for a stimulus lying on the L-M-cone pathway, for a given RMS cone contrast. The equation also predicts an asymptotic trend to the RT for an achromatic stimulus when the luminance contrast is sufficiently large.

Latency characteristics of the short-wavelength-sensitive cones and their associated pathways

There are many distinct types of retinal ganglion and LGN cells that have opponent cone inputs and which may carry chromatic information. Of interest are the asymmetries in those LGN cells that carry S-cone signals: in S-ON cells, S+ signals are opposed by (L + M) whereas, in many S-OFF cells, L+ signals are opposed by (S + M), giving -S + L - M (C. Tailby, S. G. Solomon, & P. Lennie, 2008). However, the S-opponent pathway is traditionally modeled as +/-[S - (L + M)]. A phase lag of the S-cone signal has been inferred from psychophysical thresholds for discriminating combinations of simultaneous sinusoidal modulations along +/-[L - M] and +/-[S - (L + M)] directions (C. F. Stromeyer, R. T. Eskew, R. E. Kronauer, & L. Spillmann, 1991). We extend this experiment, measuring discrimination thresholds as a function of the phase delay between pairs of orthogonal component modulations. When one of the components isolates the tritan axis, there are phase delays at which discrimination is impossible; when neither component is aligned with the tritan axis, discrimination is possible at all delays. The data imply that the S-cone signal is delayed by approximately 12 ms relative to (L - M) responses. Given that post-receptoral mechanisms show diverse tuning around the tritan axis, we suggest that the delay arises before the S-opponent channels are constructed, possibly in the S-cones themselves.

Characterising mesopic spectral sensitivity from reaction times

The spectral sensitivity of the eye was investigated using reaction times to broadband chromatic stimuli over a range of background luminances. Relative sensitivity was determined from the nonlinear reaction time curve by converting reaction times to a linear measure that was independent of spectral sensitivity. Two models for mesopic spectral sensitivity were compared. The first was a linear combination of V(lambda) and V'(lambda), and the second included input from the L-M colour-opponent mechanism and the S-cones. The second model produced a significantly better fit to the data. The chromatic mechanisms appear to contribute to reaction time when there is an appreciable chromatic signal but luminance contrast is low.

Changes in reaction time and search time with background luminance in the mesopic range

Vision relies on both rod and cone signals over a large range of ambient illumination that encompasses a number of common situations. It is important, therefore, to understand how performance changes with light level in functional visual tasks. We measured reaction times and search times using achromatic targets to examine the relationship between latency and luminance contrast as a function of background luminance. Visual search was more robust to changes in luminance than reaction time; search performance could be made invariant by scaling the effects of contrast, but the range of reaction time changed significantly over the mesopic range. We also investigated the extent to which two mesopic visual performance models described the dependence of reaction time and search time on stimulus spectra, using coloured stimuli. The 'effective contrast' model that we examined described the spectral dependence of both reaction time and search time well. A model for mesopic luminous efficiency based on reaction times described the spectral dependence of each response only in conditions where there was little influence of chromatic signals.

Postreceptoral chromatic-adaptation mechanisms in the red-green and blue-yellow systems using simple reaction times

Simple visual-reaction times (VRT) were measured for a variety of stimuli selected along red-green (L-M axis) and blue-yellow [S-(L + M) axis] directions in the isoluminant plane under different adaptation stimuli. Data were plotted in terms of the RMS cone contrast in contrast-threshold units. For each opponent system, a modified Piéron function was fitted in each experimental configuration and on all adaptation stimuli. A single function did not account for all the data, confirming the existence of separate postreceptoral adaptation mechanisms in each opponent system under suprathreshold conditions. The analysis of the VRT-hazard functions suggested that both color-opponent mechanisms present a well-defined, transient-sustained structure at marked suprathreshold conditions. The influence of signal polarity and chromatic adaptation on each color axis proves the existence of asymmetries in the integrated hazard functions, suggesting separate detection mechanisms for each pole (red, green, blue, and yellow detectors).

Effective contrast of colored stimuli in the mesopic range: a metric for perceived contrast based on achromatic luminance contrast

Little is known about how color signals and cone- and rod-based luminance signals contribute to perceived contrast in the mesopic range. In this study the perceived contrast of colored, mesopic stimuli was matched with that of spatially equivalent achromatic stimuli. The objective was to develop a metric for perceived contrast in the mesopic range in terms of an equivalent achromatic luminance contrast, referred to here as effective contrast. Stimulus photopic luminance contrast, scotopic luminance contrast, and chromatic difference from the background all contributed to effective contrast over the mid-mesopic range, but their contributions were not independent and varied markedly with background luminance. Surprisingly, color made a significant contribution to effective contrast from 10 to approximately 0.003 cd m(-2). A model describing this relationship is introduced (R2 = 0.89) and compared with predictions of mesopic luminance contrast obtained from a number of models proposed as systems of mesopic photometry.

Chromatic contrast sensitivity: the role of absolute threshold and gain constant in differences between the fovea and the periphery

A model of foveal achromatic and chromatic sensitivity [Vision Res. 36, 1597 (1996)] was extended to the peripheral visual field. Threshold-versus-illuminance functions were analyzed to determine effects of eccentricity on absolute thresholds and gain constants of chromatic and luminance mechanisms. The resulting peripheral model successfully predicted peripheral contrast sensitivity as a function of wavelength, for both white and 500-nm backgrounds. We conclude that the short-wavelength-sensitive cone opponent mechanism may mediate thresholds in Sloan's notch in the normal periphery and that interpretation of reduced chromatic sensitivity in the periphery requires an explicit model of how eccentricity affects both the gain constant and the absolute threshold.

Global-motion perception: interaction of chromatic and luminance signals

A global dot-motion stimulus was employed in order to investigate the interaction between luminance and chromatic signals in motion processing. Thresholds are determined by measuring the minimum number of dots which need to move in a coherent fashion in a field of randomly moving dots in order for the observers to be able to determine the direction of coherent motion. We found that: (1) observers could not track an achromatic signal-dot which changes its luminance polarity between frame transitions. The addition of a consistent chromatic signal allowed observers to track such a dot when the dot contained low- (8%) luminance contrast but this ability was impaired as the luminance contrast was increased; (2) the addition of chromatic contrast to a dot which contained consistent low-luminance contrast could result in threshold elevation. For fixed contrast chromatic and luminance signals, the presence and degree of threshold elevation depended upon the spatiotemporal properties of the dot motion; (3) the ability of observers to extract a global-motion signal carried by a group of dots of one colour was impaired by the addition of a number of additional-noise dots of a different colour. These results are interpreted as indicating that: (1) the motion-selective cells that are sensitive to chromatic signals are also sensitive to luminance signals; (2) the combined chromatic and luminance and purely luminance motion cells are pooled to form a single pathway prior to global-motion extraction; and (3) the negative interaction observed between the chromatic and luminance signals is likely to be due to the differences in the processing speeds of the combined luminance and chromatic and the purely luminance sensitive motion cells.

The effects of luminance and chromatic background flicker on the human visual evoked potential

Previous studies report that background luminance flicker, which is asynchronous with signal averaging, reduces the amplitude and increases the latency of the pattern-onset visual evoked potential (VEP). This effect has been attributed to saturation of the magnocellular (m-) pathway by the flicker stimulus. In the current study, we evaluate this hypothesis and further characterize this effect. We found that flicker had similar effects on the pattern-onset and pattern-reversal VEP, suggesting that the reversal and onset responses have similar generators. Chromatic flicker decreased latency of the chromatic VEP whereas luminance flicker increased peak latency to luminance targets. This result indicates that luminance flicker saturates a rapidly conducting m-pathway whereas chromatic flicker saturates a more slowly conducting parvocellular (p-) pathway. Finally, evoked potentials to chromatic and luminance stimuli were recorded from 34 electrodes over the scalp in the presence of static and asynchronously modulated backgrounds. An equivalent dipole model was used to assess occipital, parietal, and temporal lobe components of the surface response topography. Results showed that chromatic flicker reduced activity to a greater extent in the ventral visual pathway whereas luminance flicker reduced activity to a greater extent in the dorsal visual pathway to parietal lobe. We conclude that the VEP to isoluminant color and luminance stimuli contains both m- and p-pathway components. Asynchronous flicker can be used to selectively reduce the contribution of these pathways to the surface recorded VEP. Our results provide evidence of parallel pathways in the human visual system, with a dorsal luminance channel projecting predominantly to the posterior parietal lobe and a ventral color channel projecting predominantly to inferior temporal lobe.

Dependence of visual latency on wavelength: predictions of a neural counting model

Simple reaction time (RT) was determined as a function of wavelength for equally visible, near-threshold stimuli. The stimuli were 5-deg spectral onsets of 1,000-ms duration presented on a 100-Td spatially coincident white background. All three subjects manifested the same result: RTs were fastest in the region of 590 nm. These data were analyzed in the context of a counting model of visual latency. This model predicts that for equally visible stimuli a transient detector will result in shorter visual latencies than will a more sustained detector. On the basis of this analysis, it is concluded that although most long duration, near-threshold, spectral step onsets are detected by the sustained parvocellular pathway, an exception occurs in the region of 590 nm: these stimuli are detected by a relatively transient pathway, presumably the magnocellular pathway.

Psychophysical evidence of differential latencies of colour inputs to motion perception

A novel psychophysical observation allows the determination of the relative latencies with which long, middle, and short cone signals provide input to motion perception. It is known that when two spatially displaced isoluminant stimuli in spectrally different colours are simultaneously presented, any temporal lag between the perception of the two will, due to the spatial displacement, cause the perception of apparent motion. The illusion reported here occurs through the inadvertent production of spatial displacement; peripheral observation of the boundary between two differently-coloured neighbouring areas which alternately interchange colours leads, due to transverse chromatic aberration caused by the eye's optics, to the formation of a double boundary on the retina, the serial perceptions of which create the sensation of motion. By offsetting the relative temporal phases of any two colours we have determined the relative magnitude of the latencies with which they provide input to motion perception. In all subjects motion of blue is perceived after that of red, and green is perceived after that of blue. The origins of these latencies are unclear.

Effect of sawtooth polarity on chromatic and luminance detection

Psychophysical studies have documented that many observers show lower thresholds for rapid-off than for rapid-on sawtooth luminance modulation. This finding, together with physiological findings from chromatically opponent ganglion cells of the macaque monkey, prompted a search for a similar bias in psychophysical detection of chromatic increments and decrements of light. Using a luminance pedestal in conjunction with a luminance background to favor detection by chromatic mechanisms, we measured spectral sensitivity for rapid-on and rapid-off sawtooth stimuli presented spatially coextensive with the pedestal. There were two different pedestal chromaticities: one broadband, and the second composed only of long-wavelength light to enhance short-wavelength-sensitive, cone-mediated detection. Spectral-sensitivity measurements for different wavelength stimuli revealed no systematic differences across the visible spectrum as a function of sawtooth waveform polarity or pedestal chromaticity. Similarly, temporal contrast-sensitivity functions for hetero-chromatically modulated red-green sawtooth stimuli did not reveal an asymmetry in sensitivity for rapid-red and rapid-green chromatic change. Some of the observers showed a higher sensitivity for luminance modulated rapid-off sawtooth stimuli, as also noted in previous studies. This asymmetry was not found when a white luminance pedestal and background was used. These results suggest that the cone inputs to chromatically opponent ON- and OFF-center cells are sufficiently balanced to provide equivalent psychophysical thresholds for chromatic increments and decrements of light.

Reaction time distributions and their relationship to the transient/sustained nature of the neural discharge

Reaction time distributions (RTDs) were determined in response to near-threshold increments of long duration. Stimulus parameters were selected to isolate the chromatic and achromatic systems. The RTDs for the achromatic system peak sooner and are more narrow than those obtained for the chromatic system. These results are analyzed in terms of the neural discharge pattern of parvo and magno pathways.

Sustained and transient properties of chromatic and luminance systems

Reaction times were measured to 450 and 650 mm test increments to examine the temporal behavior of the chromatic and luminance systems. A response-terminated random foreperiod paradigm was employed. Stimuli consisted of chromatic test increments upon backgrounds of varying spatial structure. Conditions were chosen which may preferentially favor the reaction time response being mediated by the chromatic or luminance systems. The temporal properties of the chromatic and luminance systems were demonstrated by the shape of the estimated hazard functions of the reaction time distributions. When the white background was spatially coincident with the test field, the hazard functions showed a relatively small peak. As white sectors were added to the annulus surround (introducing spatial transients between test and background fields), however, the hazard functions became more and more peaked. The hazard functions of the luminance system were estimated by assuming that the chromatic and luminance systems function in parallel. We concluded from the results that the chromatic system may be characterized as a quasi-sustained mechanism and the luminance system as a transient mechanism.

Analysis of simple reaction time to a sinusoidal grating by means of a linear filter model of the detection process

Simple reaction time (RT) to a sinusoidal grating was analyzed in terms of a linear filter model of the detection process. First, RT contrast functions were determined over a wide range of spatial frequencies and retinal illuminances. Second, calculating the time course of the linear filter's response, theoretical visual latency contrast functions were derived for the same conditions of spatial frequency and retinal illuminance as those in the RT measurements. Comparison of the two functions showed that the contrast dependence of the RT functions was much larger than that of the visual latency functions. The discrepancy between the two functions was satisfactorily described as a power function of the slope of the filter's response at threshold level. On the basis of these results, we propose a model of the RT process. According to the model, the RT process is mediated by a cascade that consists of a level detector, which includes a linear filter followed by a threshold device, and a differentiator of the filter's response.

Response pooling between chromatic and luminance systems

Two experiments were designed to examine interactions of the responses of the chromatic and luminance systems to suprathreshold stimuli. We measured simple reaction times (RT) to eight photometrically matched (1 cd/m2) wavelengths between 448 and 658 nm. These chromatic test stimuli were incrementally presented on either a spatially coextensive 1.2 or a larger 2 degrees steady white background. Sectors of the outer annulus region (between 1.2 and 2 degrees) could be removed to allow systematic variation of the extent of the spatial contour between test and background fields. When the white background was spatially coincident with the test field, RTs showed trichromatic saturation-like wavelength dependence, with a maximum RT at 572 nm. As white sectors were added to the annulus (introducing spatial transients between test and background fields), RTs became less and less wavelength-dependent, and were nearly wavelength-independent when a full annulus was used. The data were analysed in terms of a two-system RT model for processing chromaticity and luminance.

Rod-cone dependence of saccadic eye-movement latency in a foveating task

This study examines the relations between some well known oculomotor functions (saccades) and well known retinal physiology (dark adaptation): it deals with the overall latency versus target luminance functions, with the underlying rod and cone latency-luminance functions, and with the synergistic interaction between these latency functions for mesopic targets. Saccadic latency was measured to small lit targets presented at 10 deg retinal eccentricity in complete darkness. Target luminance and wavelength were varied. Additional measurements were made during dark adaptation or on backgrounds, or at different retinal eccentricities. Luminance matched stimuli and Palmer's (1968) equivalent luminance transformation were also used. Latency is determined by an achromatic luminance mechanism that receives substantial rod inputs above the cone threshold. Latencies for pure rod or pure cone inputs increase rapidly as target luminance decreases. For the rods this latency increase appears to represent the waiting time for the 140 or so photons (lambda = 507 nm) that are required for a saccade. Errors in direction occur at scotopic luminances, or at low photopic luminances when only cones are functioning.