Contribution of human short-wave cones to luminance and motion detection

Temporal dynamics of the neural representation of hue and luminance polarity

Hue and luminance contrast are basic visual features. Here we use multivariate analyses of magnetoencephalography data to investigate the timing of the neural computations that extract them, and whether they depend on common neural circuits. We show that hue and luminance-contrast polarity can be decoded from MEG data and, with lower accuracy, both features can be decoded across changes in the other feature. These results are consistent with the existence of both common and separable neural mechanisms. The decoding time course is earlier and more temporally precise for luminance polarity than hue, a result that does not depend on task, suggesting that luminance contrast is an updating signal that separates visual events. Meanwhile, cross-temporal generalization is slightly greater for representations of hue compared to luminance polarity, providing a neural correlate of the preeminence of hue in perceptual grouping and memory. Finally, decoding of luminance polarity varies depending on the hues used to obtain training and testing data. The pattern of results is consistent with observations that luminance contrast is mediated by both L-M and S cone sub-cortical mechanisms.

Predicting color matches from luminance matches

Color vision and spectral sensitivity vary among individuals with normal color vision; thus, for many applications, it is important to measure and correct for an observer's sensitivity. Full correction would require measuring color and luminance matches and is rarely implemented. However, luminance matches (equiluminance settings) are routinely measured and simple to conduct. We modeled how well an observer's color matches could be approximated by measuring only luminance sensitivity, since both depend on a common set of factors. We show that lens and macular pigment density and $L/M$L/M cone ratios alter equiluminance settings in different ways and can therefore be estimated from the settings. In turn, the density variations can account for a large proportion of the normal variation in color matching. Thus, luminance matches may provide a simple method to at least partially predict an observer's color matches without requiring more complex tasks or equipment.

Colorful glares: Effects of colors on brightness illusions measured with pupillometry

We hypothesized that pupil constrictions to the glare illusion, where converging luminance gradients subjectively enhance the perception of brightness, would be stronger for 'blue' than for other colors. Such an expectation was based on reflections about the ecology of vision, where the experience of dazzling light is common when one happens to look directly at sunlight through some occluders. Thus, we hypothesized that pupil constrictions to 'blue' reflect an ecologically-based expectation of the visual system from the experience of sky's light and color, which also leads to interpret the blue gradients of illusory glare to act as effective cues to impending probable intense light. We therefore manipulated the gradients color of glare illusions and measured changes in subjective brightness of identical shape stimuli. We confirmed that the blue resulted in what was subjectively evaluated as the brightest condition, despite all colored stimuli were equiluminant. This enhanced brightness effect was observed both in a psychophysical adjustment task and in changes in pupil size, where the maximum pupil constriction peak was observed with the 'blue' converging gradients over and above to the pupil response to blue in other conditions (i.e., diverging gradients and homogeneous patches). Moreover, glare-related pupil constrictions for each participant were correlated to each individual's subjective brightness adjustments. Homogenous blue hues also constricted the pupil more than other hues, which represents a pupillometric analog of the Helmholtz-Kohlrausch effect on brightness perception. Together, these findings show that pupillometry constitutes an easy tool to assess individual differences in color brightness perception.

S-cone photoreceptors in the primate retina are functionally distinct from L and M cones

Daylight vision starts with signals in three classes of cone photoreceptors sensitive to short (S), middle (M), and long (L) wavelengths. Psychophysical studies show that perceptual sensitivity to rapidly varying inputs differs for signals originating in S cones versus L and M cones; notably, S-cone signals appear perceptually delayed relative to L- and M-cone signals. These differences could originate in the cones themselves or in the post-cone circuitry. To determine if the cones could contribute to these and related perceptual phenomena, we compared the light responses of primate S, M, and L cones. We found that S cones generate slower light responses than L and M cones, show much smaller changes in response kinetics as background-light levels increase, and are noisier than L and M cones. It will be important to incorporate these differences into descriptions of how cone signaling shapes human visual perception.

Delayed S-cone sensitivity losses following the onset of intense yellow backgrounds linked to the lifetime of a photobleaching product?

Thirty years ago, Mollon, Stockman, & Polden (1987) reported that after the onset of intense yellow 581-nm backgrounds, S-cone threshold rose unexpectedly for several seconds before recovering to the light-adapted steady-state value-an effect they called: "transient-tritanopia of the second kind" (TT2). Given that 581-nm lights have little direct effect on S-cones, TT2 must arise indirectly from the backgrounds' effects on the L- and M-cones. We attribute the phenomenon to the action of an unknown L- and M-cone photobleaching product, X, which acts at their outputs like an "equivalent" background light that then inhibits S-cones at a cone-opponent, second-site. The time-course of TT2 is similar in form to the lifetime of X in a two-stage, first-order biochemical reaction A→X→C with successive best-fitting time-constants of 3.09 ± 0.35 and 7.73 ± 0.70 s. Alternatively, with an additional slowly recovering exponential "restoring-force" with a best-fitting time-constant 23.94 ± 1.42 s, the two-stage best-fitting time-constants become 4.15 ± 0.62 and 6.79 ± 1.00 s. Because the time-constants are roughly independent of the background illumination, and thus the rate of photoisomerization, A→X is likely to be a reaction subsidiary to the retinoid cycle, perhaps acting as a buffer when the bleaching rate is too high. X seems to be logarithmically related to S-cone threshold, which may result from the logarithmic cone-opponent, second-site response compression after multiplicative first-site adaptation. The restoring-force may be the same cone-opponent force that sets the rate of S-cone recovery following the unusual threshold increase following the offset of dimmer yellow backgrounds, an effect known as "transient-tritanopia" (TT1).

Cortical summation and attentional modulation of combined chromatic and luminance signals

Cortical networks that process colour and luminance signals are often studied separately, although colour appearance depends on both colour and luminance. In fact, objects in everyday life are very rarely defined by only colour or only luminance, necessitating an investigation into combined processing of these signals. We used steady-state visual evoked potentials (SSVEPs) to investigate (1) cortical summation of luminance and chromatic contrast and (2) attentional modulation of neural activity driven by competing stimuli that differ in chromoluminant content. Our stimuli combined fixed amounts of chromatic contrast from either of the two cone-opponent mechanisms (bluish and yellowish; reddish and greenish) with two different levels of positive luminance contrast. Our experiments found evidence of non-linear processing of combined colour and luminance signals, which most likely originates in V1-V3 neurons tuned to both colour and luminance. Differences between luminance contrast of stimuli were found to be a key determinant for the size of feature-based voluntary attentional effects in SSVEPs, with colours of lower contrast than the colour they were presented with receiving the highest level of attentional modulation. Our results indicate that colour and luminance contrast are processed interdependently, both in terms of perception and in terms of attentional selection, with a potential candidate mediating their link being stimulus appearance, which depends on both chromaticity and luminance.

Melanopsin- and L-cone-induced pupil constriction is inhibited by S- and M-cones in humans

The human retina contains five photoreceptor types: rods; short (S)-, mid (M)-, and long (L)-wavelength-sensitive cones; and melanopsin-expressing ganglion cells. Recently, it has been shown that selective increments in M-cone activation are paradoxically perceived as brightness decrements, as opposed to L-cone increments. Here we show that similar effects are also observed in the pupillary light response, whereby M-cone or S-cone increments lead to pupil dilation whereas L-cone or melanopic illuminance increments resulted in pupil constriction. Additionally, intermittent photoreceptor activation increased pupil constriction over a 30-min interval. Modulation of L-cone or melanopic illuminance within the 0.25-4-Hz frequency range resulted in more sustained pupillary constriction than light of constant intensity. Opposite results were found for S-cone and M-cone modulations (2 Hz), mirroring the dichotomy observed in the transient responses. The transient and sustained pupillary light responses therefore suggest that S- and M-cones provide inhibitory input to the pupillary control system when selectively activated, whereas L-cones and melanopsin response fulfill an excitatory role. These findings provide insight into functional networks in the human retina and the effect of color-coding in nonvisual responses to light, and imply that nonvisual and visual brightness discrimination may share a common pathway that starts in the retina.

The Peripheral Flicker Illusion

A new illusion is reported. A visual object suddenly appearing on a red background sometimes causes an impression of flicker or double flash. In Experiment 1, a red, green, or blue object was presented on a red, green, blue, or gray background. Participants evaluated the illusion strength in reference to the physical flicker of a gray object presented in central vision. The results show that the green or blue object presented on the red background caused the illusion. In Experiment 2, the effect of retinal eccentricity on the illusion was tested. The results showed that the illusion was weak in central vision but became stronger as the retinal eccentricity of the objects' presentation increased. In Experiment 3, optimal luminance conditions for the illusion were explored with the green and blue objects. The illusion was strong when object luminance was lower than background luminance and the optimal luminance for the blue object was lower than that for the green object. We propose a tentative theory for the illusion and discuss its cause.

The Verriest Lecture: Short-wave-sensitive cone pathways across the life span

Structurally and functionally, the short-wave-sensitive (S) cone pathways are thought to decline more rapidly with normal aging than the middle- and long-wave-sensitive cone pathways. This would explain the celebrated results by Verriest and others demonstrating that the largest age-related color discrimination losses occur for stimuli on a tritan axis. Here, we challenge convention, arguing from psychophysical data that selective S-cone pathway losses do not cause declines in color discrimination. We show substantial declines in chromatic detection and discrimination, as well as in temporal and spatial vision tasks, that are mediated by S-cone pathways. These functional losses are not, however, unique to S-cone pathways. Finally, despite reduced photon capture by S cones, their postreceptoral pathways provide robust signals for the visual system to renormalize itself to maintain nearly stable color perception across the life span.

Effects of luminance contrast on the color selectivity of neurons in the macaque area v4 and inferior temporal cortex

Appearance of a color stimulus is significantly affected by the contrast between its luminance and the luminance of the background. In the present study, we used stimuli evenly distributed on the CIE-xy chromaticity diagram to examine how luminance contrast affects neural representation of color in V4 and the anterior inferior temporal (AITC) and posterior inferior temporal (PITC) color areas (Banno et al., 2011). The activities of single neurons were recorded from monkeys performing a visual fixation task, and the effects of luminance contrast on the color selectivity of individual neurons and their population responses were systematically examined by comparing responses to color stimuli that were brighter or darker than the background. We found that the effects of luminance contrast differed considerably across V4 and the PITC and AITC. In both V4 and the PITC, the effects of luminance contrast on the population responses of color-selective neurons depended on color. In V4, the size of the effect was largest for blue and cyan, whereas in the PITC, the effect gradually increased as the saturation of the color stimulus was reduced, and was especially large with neutral colors (white, gray, black). The pattern observed in the PITC resembles the effect of luminance contrast on color appearance, suggesting PITC neurons are closely involved in the formation of the perceived appearance of color. By contrast, the color selectivities of AITC neurons were little affected by luminance contrast, indicating that hue and saturation of color stimuli are represented independently of luminance contrast in the AITC.

S-cone psychophysics

We review the features of the S-cone system that appeal to the psychophysicist and summarize the celebrated characteristics of S-cone mediated vision. Two factors are emphasized: First, the fine stimulus control that is required to isolate putative visual mechanisms and second, the relationship between physiological data and psychophysical approaches. We review convergent findings from physiology and psychophysics with respect to asymmetries in the retinal wiring of S-ON and S-OFF visual pathways, and the associated treatment of increments and decrements in the S-cone system. Beyond the retina, we consider the lack of S-cone projections to superior colliculus and the use of S-cone stimuli in experimental psychology, for example to address questions about the mechanisms of visually driven attention. Careful selection of stimulus parameters enables psychophysicists to produce entirely reversible, temporary, "lesions," and to assess behavior in the absence of specific neural subsystems.

Opponent melanopsin and S-cone signals in the human pupillary light response

In the human, cone photoreceptors (L, M, and S) and the melanopsin-containing, intrinsically photosensitive retinal ganglion cells (ipRGCs) are active at daytime light intensities. Signals from cones are combined both additively and in opposition to create the perception of overall light and color. Similar mechanisms seem to be at work in the control of the pupil's response to light. Uncharacterized however, is the relative contribution of melanopsin and S cones, with their overlapping, short-wavelength spectral sensitivities. We measured the response of the human pupil to the separate stimulation of the cones and melanopsin at a range of temporal frequencies under photopic conditions. The S-cone and melanopsin photoreceptor channels were found to be low-pass, in contrast to a band-pass response of the pupil to L- and M-cone signals. An examination of the phase relationships of the evoked responses revealed that melanopsin signals add with signals from L and M cones but are opposed by signals from S cones in control of the pupil. The opposition of the S cones is revealed in a seemingly paradoxical dilation of the pupil to greater S-cone photon capture. This surprising result is explained by the neurophysiological properties of ipRGCs found in animal studies.

Color selectivity of the spatial congruency effect: evidence from the focused attention paradigm

The multisensory response enhancement (MRE), occurring when the response to a visual target integrated with a spatially congruent sound is stronger than the response to the visual target alone, is believed to be mediated by the superior colliculus (SC) (Stein & Meredith, 1993). Here, we used a focused attention paradigm to show that the spatial congruency effect occurs with red (SC-effective) but not blue (SC-ineffective) visual stimuli, when presented with spatially congruent sounds. To isolate the chromatic component of SC-ineffective targets and to demonstrate the selectivity of the spatial congruency effect we used the random luminance modulation technique (Experiment 1) and the tritanopic technique (Experiment 2). Our results indicate that the spatial congruency effect does not require the distribution of attention over different sensory modalities and provide correlational evidence that the SC mediates the effect.

Do S cones contribute to color-motion feature binding?

Wu et al. [Nature 429, 262 (2004)] describe a visual illusion in which color and motion are incorrectly bound: green dots moving downward and red dots moving upward are seen as green dots going up and red dots going down. The present study determined whether S cones contribute to color-motion feature-binding errors, in order to assess the neural representation of color at the level of binding. The specific experimental question is whether binding errors depend on S-cone responses from the objects perceived to have an illusory direction of motion. Alternatively, only L and M cones may determine the neural representation of color that regulates color-motion feature binding. In two experiments, the chromatic difference was manipulated between central objects, which induce color-motion binding errors, and peripheral objects, where color-motion binding errors occur. The chromaticity difference was varied along only the L/M-cone axis or only the S-cone axis. As in Wu et al. [Nature 429, 262 (2004)], color-motion binding was frequently observed in the periphery when there were no central versus peripheral chromatic differences. Further, the results showed that the frequency of color-motion binding errors in the periphery depended on the difference in S-cone excitation between center and periphery, thereby demonstrating that the neural representation of color at the level of feature binding depends on signals from not only L and M cones but also S cones.

Tool manipulation knowledge is retrieved by way of the ventral visual object processing pathway

Using functional Magnetic Resonance Imaging (fMRI), we find that object manipulation knowledge is accessed by way of the ventral object processing pathway. We exploit the fact that parvocellular channels project to the ventral but not the dorsal stream, and show that increased neural responses for tool stimuli are observed in the inferior parietal lobule when those stimuli are visible only to the ventral object processing stream. In a control condition, tool-preferences were observed in a superior and posterior parietal region for stimuli titrated so as to be visible by the dorsal visual pathway. Functional connectivity analyses confirm the dissociation between sub-regions of parietal cortex according to whether their principal afferent input is via the ventral or dorsal visual pathway. These results challenge the 'Embodied Hypothesis of Tool Recognition', according to which tool identification critically depends on simulation of object manipulation knowledge. Instead, these data indicate that retrieval of object-associated manipulation knowledge is contingent on accessing the identity of the object, a process that is subserved by the ventral visual pathway.

The representation of S-cone signals in primary visual cortex

Recent studies of middle-wavelength-sensitive and long-wavelength-sensitive cone responses in primate primary visual cortex (V1) have challenged the view that color and form are represented by distinct neuronal populations. Individual V1 neurons exhibit hallmarks of both color and form processing (cone opponency and orientation selectivity), and many display cone interactions that do not fit classic chromatic/achromatic classifications. Comparable analysis of short-wavelength-sensitive (S) cone responses has yet to be achieved and is of considerable interest because S-cones are the basis for the primordial mammalian chromatic pathway. Using intrinsic and two-photon imaging techniques in the tree shrew, we assessed the properties of V1 layer 2/3 neurons responsive to S-cone stimulation. These responses were orientation selective, exhibited distinct spatiotemporal properties, and reflected integration of S-cone inputs via opponent, summing, and intermediate configurations. Our observations support a common framework for the representation of cone signals in V1, one that endows orientation-selective neurons with a range of chromatic, achromatic, and mixed response properties.

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.

Long-range suppressive interactions between S-cone and luminance channels

Surround suppression (SS) refers to a reduction in the effective stimulus contrast in one visual location produced by a stimulus presented in an adjacent location. This type of suppression is tuned for orientation and spatial frequency and is thought to be a cortical process. In this paper we used psychophysical measurements to determine whether S-cone-driven signals are affected by surround suppression and, if so, whether S-cone and achromatic signals interact at spatially-remote locations. Our results revealed three important aspects of surround suppression. Firstly, we show that S-cone probes are suppressed by simultaneous S-cone contrast surrounds and that this suppression has the characteristics of a cortical mechanism. Secondly, we show that when probes and surrounds are presented simultaneously, there are no suppressive interactions between S-cone and luminance stimuli. Finally, we demonstrate that this apparent independence is an artifact of signal timing: when the S-cone components of the stimuli precede the luminance components by approximately 40 ms, we find a significant interaction between the two pathways. The amplitude of this interaction depends critically upon the relative onset times of the two components. These results indicate that some component of surround suppression depends on neural computations that occur after the S- and luminance pathways are combined in striate cortex. In addition, the strong dependence of the magnitude of surround suppression on temporal ordering suggests that much of the effect is driven by transient signals.

The perception of speed based on L-M and S-(L+M) cone opponent processing

We have measured perceived speed and speed discrimination thresholds for stimuli that selectively activate the L-M, S-(L+M) cone opponent and L+M (luminance) post-receptoral pathways. For low speeds and low contrasts speed discrimination thresholds for L-M and S-(L+M) are similar but are higher and have a greater dependency upon contrast than those for luminance motion. These differences between chromatic and luminance speed perception can be eliminated when stimuli are equated with respect to their individual motion detection thresholds (MDTs). For fast moving gratings speed perception based upon L-M, S-(L+M) and L+M signals is similar in terms of threshold performance and contrast dependency. These results are consistent with the view that there are separate mechanisms for the analysis of chromatic and luminance motion, the relative contributions of which may change as a function of stimulus contrast and speed. The similarity in performance for S-(L+M) and L+M chromatic stimuli across a range of stimulus parameters suggests that signals derived from the two cone opponent pathways can be used equally well. Our results argue against the idea that speed perception is compromised when it is based upon information derived from the S-(L+M) cone opponent pathway.

S-cone contributions to linear and non-linear motion processing

We investigated the characteristics of mechanisms mediating motion discrimination of S-cone isolating stimuli and found a double dissociation between the effects of luminance noise, which masks linear but not non-linear motion, and chromatic noise, which masks non-linear but not linear motion. We conclude that S-cones contribute to motion via two different pathways: a non-linear motion mechanism via a chromatic pathway and a linear motion mechanism via a luminance pathway. Additionally, motion discrimination and detection thresholds for drifting, S-cone isolating Gabors are unaffected by luminance noise, indicating that grating motion is mediated via chromatic mechanisms and based on higher-order motion processing.

Impulse response of an S-cone pathway in the aging visual system

Age-related changes in the temporal properties of an S-cone pathway were characterized by the psychophysical impulse-response function (IRF). Participants included 49 color-normal observers ranging in age from 16.8 to 86.3 years. A double-pulse method was used to measure the IRF with S-cone modulation at constant luminance. Stimuli were presented as a Gaussian patch (+/-1SD = 2.3 degrees ) in one of four quadrants around a central fixation cross on a CRT screen. The test stimulus was modulated from the equal-energy white of the background toward the short-wave spectrum locus. Each of the two pulses (6.67 ms) was separated by an interstimulus interval (ISI) from 20 to 720 ms. Chromatic detection thresholds were determined by a four-alternative forced-choice method with staircases for each ISI in one session. IRFs were calculated from the threshold data using a model with four parameters of an exponentially damped sine wave. S-cone IRFs have only an excitatory phase and a much longer time course compared with IRFs for luminance modulation measured with the same apparatus. The results demonstrated significant age-related losses in IRF amplitude, but the latency (time to peak) of the IRF was stable with age.

Spectrally opponent inputs to the human luminance pathway: slow +M and -L cone inputs revealed by intense long-wavelength adaptation

The nature of the inputs to achromatic luminance flicker perception was explored psychophysically by measuring middle- (M-) and long-wavelength-sensitive (L-) cone modulation sensitivities, M- and L-cone phase delays, and spectral sensitivities as a function of temporal frequency. Under intense long-wavelength adaptation, the existence of multiple luminance inputs was revealed by substantial frequency-dependent changes in all three types of measure. Fast (f) and slow (s) M-cone input signals of the same polarity (+sM and +fM) sum at low frequencies, but then destructively interfere near 16 Hz because of the delay between them. In contrast, fast and slow L-cone input signals of opposite polarity (-sL and +fL) cancel at low frequencies, but then constructively interfere near 16 Hz. Although these slow, spectrally opponent luminance inputs (+sM and -sL) would usually be characterized as chromatic, and the fast, non-opponent inputs (+fM and +fL) as achromatic, both contribute to flicker photometric nulls without producing visible colour variation. Although its output produces an achromatic percept, the luminance channel has slow, spectrally opponent inputs in addition to the expected non-opponent ones. Consequently, it is not possible in general to silence this channel with pairs of 'equiluminant' alternating stimuli, since stimuli equated for the non-opponent luminance mechanism (+fM and +fL) may still generate spectrally opponent signals (+sM and +sL).

Color vision

Color vision starts with the absorption of light in the retinal cone photoreceptors, which transduce electromagnetic energy into electrical voltages. These voltages are transformed into action potentials by a complicated network of cells in the retina. The information is sent to the visual cortex via the lateral geniculate nucleus (LGN) in three separate color-opponent channels that have been characterized psychophysically, physiologically, and computationally. The properties of cells in the retina and LGN account for a surprisingly large body of psychophysical literature. This suggests that several fundamental computations involved in color perception occur at early levels of processing. In the cortex, information from the three retino-geniculate channels is combined to enable perception of a large variety of different hues. Furthermore, recent evidence suggests that color analysis and coding cannot be separated from the analysis and coding of other visual attributes such as form and motion. Though there are some brain areas that are more sensitive to color than others, color vision emerges through the combined activity of neurons in many different areas.

The influence of color on the perception of luminance motion

We examined the role of color in the processing of motion of a luminance-varying pattern by alternating the color of a moving pattern and measuring the luminance contrast required for accurate discrimination of the motion direction. We report that the contrast threshold for perceiving the direction of motion of luminance-varying patterns is greatly elevated when the mean chromaticity of the moving luminance pattern alternates between two hues. Thus, color plays a critical role in the discrimination of luminance motion direction. The magnitude of the threshold elevation is directly related to the magnitude of the LM opponent color contrast produced by the color alternation. S-cone contrast produces little or no effect. The interference produced by color alternation was greatly reduced in the retinal periphery. Our results indicate that first-order luminance motion mechanisms are sensitive to the color of moving objects as coded by a differencing of the outputs of L and M cones. Contrary to the widely accepted notion that luminance-defined motion is processed primarily in the spectrally broadband magnocellular (M) pathway, our results suggest that the hue-selective parvocellular (P) mechanisms provide input to first-order motion detectors.

S cone contributions to the magnocellular visual pathway in macaque monkey

The magnocellular visual pathway is believed to receive input from long (L) and middle (M), but not short (S), wavelength-sensitive cones. Recording from neurons in magnocellular layers of lateral geniculate nucleus (LGN) in macaque monkeys, we found that magnocellular neurons were unequivocally responsive to S cone-isolating stimuli. A quantitative analysis suggests that S cones provided about 10% of the input to these cells, on average, while L:M ratios were far more variable. S cone signals influenced responses with the same sign as L and M cone inputs (i.e., no color opponency). Magnocellular afferent recordings following inactivation of primary visual cortex demonstrated that S cone signals were feedforward in nature and did not arise from cortical feedback to LGN

Chromatic Discrimination in a Cortically Colour Blind Observer

We tested the ability of a subject with cerebral achromatopsia to discriminate between colours and to detect chromatic borders. He was unable to identify colours or to arrange them in an orderly series or choose the odd colour out of an array or even to pick out a colour embedded in an array of greys. Nevertheless, he could select the odd colour when the colours were contiguous, even when they were isoluminant, and could discriminate an ordered from a disordered chromatic series as long as the colours in each row abutted one other. His verbal replies showed that he did so by detecting an edge between two stimuli that were, to him, perceptually identical. Introducing a narrow isoluminant grey stripe between adjacent colours abolished or greatly impaired this ability. As long as isoluminant colours were contiguous the patient could identify the orientation of the chromatic borders. Photopic spectral sensitivity showed evidence both for activity of three cone channels and for chromatic opponent processing, indicating that postreceptoral chromatic processing is occurring despite the absence of any conscious awareness of colour. The results indicate that both parvocellular colour opponent and magnocellular broad-band channels are active and that the cortical brain damage has selectively disrupted the appreciation of colour but not the ability to detect even isoluminant chromatic borders, which would be invisible to a retinal achromat. The subject's performance on non-colour tasks involving the discrimination of shape, texture, greyness and position was excellent. His disorder is therefore not like that of macaque monkeys in which cortical area V4 has been removed, and which are much more severely impaired at discriminating shape than colour.

The influence of stimulus chromaticity on the isoluminant motion-onset VEP

Motion-onset visual evoked potentials (VEPs) were elicited by low spatial frequency chromatic isoluminant gratings presented in a central 7 degrees circular field. The chromatic composition of the stimuli was varied so as to modulate along different axes in colour space. For slow speeds (<5 degrees/s) changing the chromatic axis induced large response differences between the S- and L/M-cone VEPs. At faster speeds (5-12 degrees/s) the effects were not as marked. A dichotomy between the slow and fast responses was also shown to exist in terms of their contrast dependencies, the former exhibiting a stronger dependency on contrast than the latter. These findings suggest that neural substrates with chromatic sensitivity are involved in the generation of S- and L/M-cone mediated motion-onset VEPs at low velocities. At higher velocities, responses are generated by different mechanisms that possess little or no chromatic sensitivity.

Seeing with S cones

The S cone is highly conserved across mammalian species, sampling the retinal image with less spatial frequency than other cone photoreceptors. In human and monkey retina, the S cone represents typically 5-10% of the cone mosaic and distributes in a quasi-regular fashion over most of the retina. In the fovea, the S cone mosaic recedes from a central "S-free" zone whose size depends on the optics of the eye for a particular primate species: the smaller the eye, the less extreme the blurring of short wavelengths, and the smaller the zone. In the human retina, the density of the S mosaic predicts well the spatial acuity for S-isolating targets across the retina. This acuity is likely supported by a bistratified retinal ganglion cell whose spatial density is about that of the S cone. The dendrites of this cell collect a depolarizing signal from S cones that opposes a summed signal from M and L cones. The source of this depolarizing signal is a specialized circuit that begins with expression of the L-AP4 or mGluR6 glutamate receptor at the S cone-->bipolar cell synapse. The pre-synaptic circuitry of this bistratified ganglion cell is consistent with its S-ON/(M+L)-OFF physiological receptive field and with a role for the ganglion cell in blue/yellow color discrimination. The S cone also provides synapses to other types of retinal circuit that may underlie a contribution to the cortical areas involved with motion discrimination.

Isolated short-wavelength sensitive cones can mediate a reflex accommodation response

Both long- and middle-wavelength sensitive cones mediate the reflex accommodation signal but the contribution from the short-wavelength sensitive cones is unknown. A short-wavelength sensitive cone contribution could extend the range of the signed defocus signal from chromatic aberration. The aim was to determine whether isolated short-wavelength sensitive cones mediate reflex accommodation independently of long- and middle-wavelength sensitive cones. Accommodation was monitored continuously (eight subjects) to a sine-wave grating (3 cpd; 0.53 contrast) moving with a sum of sines motion in a Badal optometer. Two illumination conditions were used: a 'blue' condition that isolated short-wavelength sensitive cones, and a 'white' control condition that stimulated all three cone types. Of the eight subjects, two responded equally in the 'white' and 'blue' condition, four gave reduced responses in the 'blue' condition and two failed to respond in both conditions. The mean response in the 'blue' condition was reduced by 50% compared to the 'white' condition. Further analysis indicated that four of the eight subjects gave responses that were considerably greater than noise (S.D.>1.82) when short-wavelength sensitive cones were isolated. Some subjects can accommodate using only S-cones.

The spectral sensitivities of the middle- and long-wavelength-sensitive cones derived from measurements in observers of known genotype

The spectral sensitivities of middle- (M-) and long- (L-) wavelength-sensitive cones have been measured in dichromats of known genotype: M-cone sensitivities in nine protanopes, and L-cone sensitivities in 20 deuteranopes. We have used these dichromat cone spectral sensitivities, along with new luminous efficiency determinations, and existing spectral sensitivity and color matching data from normal trichromats, to derive estimates of the human M- and L-cone spectral sensitivities for 2 and 10 degrees dia. central targets, and an estimate of the photopic luminosity function [V(lambda)] for 2 degrees dia. targets, which we refer to as V(2)*(lambda). These new estimates are consistent with dichromatic and trichromatic spectral sensitivities and color matches.

Evidence for the contribution of S cones to the detection of flicker brightness and red-green

We were interested in the question of how cones contribute to the detection of brightness, red-green, and blue-yellow. The linear combination of cone signals contributing to flicker detection was determined by fitting a plane to 64 points (colors) of equal heterochromatic flicker brightness. A small S-cone contribution to flicker brightness of similar amplitude in all five subjects was identified. The ratio of L- to M-cone contribution was found to vary considerably among subjects (1.7-4.1). Chromatic detection thresholds were determined for small patches in the isoluminant plane defined by flicker brightness. These stimuli were presented at an eccentricity of 40 arc min. By using color naming at the detection threshold, one can attribute different segments of the resulting detection ellipses to different chromatic mechanisms. Linear approximation of these segments provided an estimate for the contribution of the different cone types to the detection of red-green and blue-yellow. The results are consistent with the hypothesis that S cones contribute to the red-green mechanism with the same sign as that of the contribution from L cones. The blue-yellow mechanism very probably subtracts S-cone contrast from luminance contrast. The detection ellipse can be mapped into a circle in cone difference space. The base of this canonical transformation is a set of three cone fundamentals that differs from previously published estimates. Projecting the circle onto the three cone difference axes produces sinusoidal changes within the respective excitations. We propose that simultaneous sinusoidal changes of equal increment in the three cone difference excitations generate stimuli differing by equal saliency.

Perceptual motion standstill in rapidly moving chromatic displays

In motion standstill, a quickly moving object appears to stand still, and its details are clearly visible. It is proposed that motion standstill can occur when the spatiotemporal resolution of the shape and color systems exceeds that of the motion systems. For moving red-green gratings, the first- and second-order motion systems fail when the grating is isoluminant. The third-order motion system fails when the green/red saturation ratio produces isosalience (equal distinctiveness of red and green). When a variety of high-contrast red-green gratings, with different spatial frequencies and speeds, were made isoluminant and isosalient, the perception of motion standstill was so complete that motion direction judgments were at chance levels. Speed ratings also indicated that, within a narrow range of luminance contrasts and green/red saturation ratios, moving stimuli were perceived as absolutely motionless. The results provide further evidence that isoluminant color motion is perceived only by the third-order motion system, and they have profound implications for the nature of shape and color perception.

Perceived speed of colored stimuli

The influence of contrast and color on perceived motion was measured using a speed-matching task. Observers adjusted the speed of an L cone contrast pattern to match that of a variety of colored test patterns. The dependence of speed on test contrast was the same for all test colors measured, differing only by a sensitivity factor. This result suggests that the reduced apparent speed of low contrast targets and certain colored targets is caused by a common cortical mechanism. The cone contrast levels that equate perceived speed differ substantially from those that equate visibility. This result suggests that the neural mechanisms governing speed perception and visibility differ. Perceived speed differences caused by variations in color can be explained by color responses that are characteristic of motion-selective cortex.

The mechanism of isoluminant chromatic motion perception

An isoluminant chromatic display is a color display in which the component colors have been so carefully equated in luminance that they stimulate only color-sensitive perceptual mechanisms and not luminance-sensitive mechanisms. The nature of the mechanism by which isoluminant chromatic motion is perceived is an important issue because color and motion processing historically have been associated with different neural pathways. Here we show that isoluminant chromatic motion (i) fails a pedestal test, (ii) has a temporal tuning function that declines to half-amplitude at 3-6 Hz, and (iii) is perceived equally well when the entire motion sequence is presented monocularly (entire motion sequence to one eye) versus interocularly (the frames of motion sequence alternate between eyes so that neither eye individually could perceive motion). These three characteristics indicate that chromatic motion is detected by the third-order motion system. Based on this theory, it was possible to take a moving isoluminant red-green grating and, by simply increasing the chromatic contrast of the green component, to generate the full gamut of motion percepts, from compelling smooth motion to motion standstill. The perception of motion standstill when the third-order mechanism is nullified indicates that there is no other motion computation available for purely chromatic motion. It follows that isoluminant chromatic motion is not computed by specialized chromatic motion mechanisms within a color pathway but by the third-order motion system at a brain level where binocular inputs of form, color, depth, and texture are simultaneously available and where selective attention can exert a major influence.

S-cone contribution to pupillary responses evoked by chromatic flash offset

On a green or red background, the action spectrum of the pupillary responses evoked following the offset of chromatic test flashes shows a prominent short-wavelength lobe and suggests the contribution from photoreceptors other than the previously inferred M- and L-cones (Kimura & Young, Vision Research (1996). 36, 1543-1550), most likely from S-cones. Systematic changes in the shape of the intensity versus amplitude functions with test wavelengths and in the shape of the short-wavelength lobe with response amplitude criteria suggest an antagonistic interaction involving the short- and longer-wavelength photoreceptors.

Color from invisible flicker: a failure of the Talbot-Plateau law caused by an early 'hard' saturating nonlinearity used to partition the human short-wave cone pathway

The Talbot-Plateau law fails for flicker detected by the short-wavelength-sensitive (S) cones: a 30-40 Hz target, flickering too fast for the flicker to be resolved, looks more yellow than a steady target of the same average intensity. The color change, which is produced by distortion at an early compressive nonlinearity, was used to reveal a slightly bandpass S-cone temporal response before the distortion site and a lowpass response after it. The nonlinearity is probably a 'hard' nonlinearity that arises because the S-cone signal is limited by a response ceiling, which the mean signal level approaches and exceeds as the S-cone adaptation level increases. The nonlinearity precedes the combination of flicker signals from all three cone types.

Effects of known variations in photopigments on L/M cone ratios estimated from luminous efficiency functions

The extent to which known variations in photopigment lambda max and optical density may affect cone ratios estimated from the spectral luminous efficiency function (LEF) was examined. LEFs were generated using L- and M-cone fundamentals, one of which had been shifted in lambda max (+/- 1, 2, 4 or 6 nm) or varied in peak optical density (increased or decreased by 10, 25 or 50%). A curve-fitting program was then used to estimate the L/M cone ratios for the generated LEFs assuming standard L- and M-cone fundamentals. These modeling exercises indicate that L/M cone ratios estimated from LEFs are highly correlated with long-wave sensitivity and with known variations in L-cone lambda max. Variations in M-cone lambda max and photopigment optical density for both cone types are also correlated with L/M cone ratios, but have much less impact on the estimated ratios.

Motion detection on flashed, stationary pedestal gratings: evidence for an opponent-motion mechanism

Contrast thresholds were measured for discriminating left vs right motion of a vertical, 1 c/deg luminance grating lasting for one cycle of motion. This test was presented on a 1 c/deg stationary grating (pedestal) of twice-threshold, flashed for the duration of the test motion. Lu and Sperling [(1995). Vision Research, 35, 2697-2722] argue that the visual system detects the underlying, first-order motion of the test and is immune to the presence of the stationary pedestal (and the 'feature wobble' which it induces). On the contrary, we observe that the stationary pedestal has large effects on motion detection at 7 and 15 Hz, and smaller effects at 0.9-3.7 Hz, evidenced by a spatial phase dependency between the stationary pedestal and moving test. At 15 Hz the motion threshold drops as much as five-fold, with the stationary pedestal in the optimal spatial phase (i.e., pedestal and test spatially in phase at middle of motion), and the perceived direction of the test motion reverses with the pedestal in the opposite phase. Phase dependency was also explored using a very brief (approximately 1 msec) static pedestal presented with the moving test. The pedestal of Lu and Sperling (flashed for the duration of the test) has a broad spectrum of left and right moving components which interact with the moving test. The pedestal effects can be explained by the visual system's much higher sensitivity to the difference of the contrast of right vs left moving components than to either component alone.

Short-wave cone signal in the red-green detection mechanism

Previous work shows that the red-green (RG) detection mechanism is highly sensitive, responding to equal and opposite long-wave (L) and middle-wave (M) cone contrast signals. This mechanism mediates red-green hue judgements under many conditions. We show that the RG detection mechanism also receives a weak input from the short-wave (S) cones that supports the L signal and equally opposes M. This was demonstrated with a pedestal paradigm, in which weak S cone flicker facilitates discrimination and detection of red-green flicker. Also, a near-threshold +S cone flash facilitates detection of red flashes and inhibits green flashes, and a near-threshold -S cone flash facilitates detection of green flashes and inhibits red flashes. The S contrast weight in RG is small relative to the L and M contrast weights. However, a comparison of our results with other studies suggests that the strength of the absolute S cone contrast contribution to the RG detection mechanism is 1/4 to 1/3 the strength of the S contribution to the blue-yellow (BY) detection mechanism. Thus, the S weight in RG is a significant fraction of the S weight in BY. This has important implications for the 'cardinal' color mechanisms, for it predicts that for detection or discrimination, the mechanisms limiting performance do not lie on orthogonal M-L and S axes within the equiluminant color plane.

Motion minima for different directions in color space

We have used the minimum-motion stimulus of Cavanagh, MacLeod & Anstis [(1987) Journal of the Optical Society of America A, 4, 1428-1438] to examine how signals along different directions in color space interact in motion perception. Stimuli were pairs of counterphasing gratings combined 90 deg out of phase in both space and time and modulated along different color-luminance axes. The axis for one of the gratings was fixed, while the axis for the second was varied so as to null perceived motion in the stimulus. The motion nulls show that observers are sensitive to motion signals carried by each of the cardinal directions of color space [an achromatic axis and L-M and S-(L+M) chromatic axes], but that signals along different cardinal axes are not combined to yield a net direction of motion. Pairing an achromatic and chromatic grating resulted in a motion null regardless of the relative or overall contrast of the two gratings, while the null directions for intermediate axes shifted depending on contrast. This result points to the special status of the luminance and chromatic axes. However, our results do not reveal a special pair of axes within the equiluminant plane. When contrasts along the cardinal axes are scaled for equal multiples of their respective detection thresholds, the L-M and S chromatic contrasts contribute roughly equally to the perceived motion, but are many times weaker than luminance contrast. Moreover, sensitivity to luminance motion is little affected by the presence of chromatic contrast, whereas sensitivity to chromatic motion is strongly masked by either luminance or chromatic contrast. These asymmetric interactions suggest that the motion of the luminance and chromatic components is encoded in qualitatively different ways.

Early impairment of foveal magno- and parvocellular pathways in juxta chiasmal tumours

Foveal pathway visual function was assessed in 11 patients having tumours extending into the suprasellar region but without evidence of visual impairment as assessed by visual acuity and Bjerrum screen campimetry. Psychophysical and routine visual evoked potential (VEP) measurements were obtained from the eye ipsilateral to the maximal suprasellar extension. The sensitivity of luminance and chromatic pathways was assessed psychophysically by measuring increment thresholds for white and red flashes of light presented on a white adapting field. Temporal sensitivity was assessed psychophysically by measuring threshold modulation sensitivity for sinusoidally modulating stimuli (de Lange attenuation characteristic). The patient group showed approximately equal significant psychophysical losses in chromatic, luminance and temporal sensitivities relative to normal controls. Midline VEP P100 latencies of the patient group did not significantly differ from those of the normal control group. It is concluded that tumours extending into the suprasellar region can cause foveal pathway dysfunction affecting both magno- and parvocellular pathways, even in the presence of normal visual acuity and fields suggesting a more widespread and insidious abnormality of the visual pathways in this condition than previously thought.

Colour adaptation modifies the long-wave versus middle-wave cone weights and temporal phases in human luminance (but not red-green) mechanism

1. The human luminance (LUM) mechanism detects rapid flicker and motion, responding to a linear sum of contrast signals, L' and M', from the long-wave (L) and middle-wave (M) cones. The red-green mechanism detects hue variations, responding to a linear difference of L' and M' contrast signals. 2. The two detection mechanisms were isolated to assess how chromatic adaptation affects summation of L' and M' signals in each mechanism. On coloured background (from blue to red), we measured, as a function of temporal frequency, both the relative temporal phase of the L' and M' signals producing optimal summation and the relative L' and M' contrast weights of the signals (at the optimal phase for summation). 3. Within the red-green mechanism at 6 Hz, the phase shift between the L' and M' signals was negligible on each coloured field, and the L' and M' contrast weights were equal and of opposite sign. 4. Relative phase shifts between the L' and M' signals in the LUM mechanism were markedly affected by adapting field colour. For stimuli of 1 cycle deg-1 and 9 Hz, the temporal phase shift was zero on a green-yellow field (approximately 570 nm). On an orange field, the L' signal lagged M' by as much as 70 deg phase while on a green field M' lagged L' by as much as 70 deg. The asymmetric phase shift about yellow adaptation reveals a spectrally opponent process which controls the phase shift. The phase shift occurs at an early site, for colour adaptation of the other eye had no effect, and the phase shift measured monocularly was identical for flicker and motion, thus occurring before the motion signal is extracted (this requires an extra delay). 5. The L' versus M' phase shift in the LUM mechanism was generally greatest at intermediate temporal frequencies (4-12 Hz) and was small at high frequencies (20-25 Hz). The phase shift was greatest at low spatial frequencies and strongly reduced at high spatial frequencies (5 cycle deg-1), indicating that the receptive field surround of neurones is important for the phase shift. 6. These temporal phase shifts were confirmed by measuring motion contrast thresholds for drifting L cone and M cone gratings summed in different spatial phases. Owing to the large phase shifts on green or orange fields, the L and M components were detected about equally well by the LUM mechanism (at 1 cycle deg-1 and 9 Hz) when summed spatially in phase or in antiphase. Antiphase summation is typically thought to produce an equiluminant red-green grating. 7. At low spatial frequency, the relative L' and M' contrast weights in the LUM mechanism (assessed at the optimal phase for summation) changed strongly with field colour and temporal frequency. 8. The phase shifts and changing contrast weights were modelled with phasic retinal ganglion cells, with chromatic adaptation strongly modifying the receptive field surround. The cells summate L' and M' in their centre, while the surround L' and M' signals are both antagonistic to the centre for approximately 570 nm yellow adaptation. Green or orange adaptation is assumed to modify the L and M surround inputs, causing them to be opponent with respect to each other, but with reversed polarity on the green versus orange field (to explain the chromatic reversal of the phase shift). Large changes in the relative L' and M' weights on green versus orange fields indicate the clear presence of the spectrally opponent surround even at 20 Hz. The spectrally opponent surround appears sluggish, with a long delay (approximately 20 ms) relative to the centre.

Interaction of motion and color in the visual pathways

In recent years the idea of parallel and independent processing streams for different visual attributes has become a guiding principle for linking the organization, architecture and function of the visual system. Findings concerning the segregation of motion and color information have been at the forefront of the evidence in favor of the parallel processing scheme. A number of studies have shown that motion perception is impaired for isoluminant stimuli, which are thought to isolate the color system. However, there are now many studies, the results of which are incompatible with the simple idea of segregated pathways. We propose two processing streams for motion that differ mostly in their temporal characteristics. Although neither of the two motion streams is color-blind, as was originally suggested, they differ radically in the way they process color information. The view that we propose provides a framework that reconciles a number of seemingly contradictory results. Evidence to support the new framework comes from psychophysical, physiological and lesion studies.

Temporal and chromatic properties of motion mechanisms

We measured threshold contours in color space for detecting drifting sinusoidal gratings over a range of temporal frequencies, and for identifying their direction of motion. Observers were able to correctly identify the direction of motion in all directions of color space, given a sufficiently high contrast. At low temporal frequencies we found differences between luminance and isoluminance conditions; for isoluminance there was a marked threshold elevation for identification when compared to detection. The threshold elevation for identification is dependent on eccentricity as well as on temporal frequency. At high temporal frequencies there were no differences between detection and identification thresholds, or between thresholds for luminance and isoluminance. A quantitative analysis of the threshold contours allowed us to identify two mechanisms contributing to motion: a color-opponent mechanism with a high sensitivity at low temporal frequencies and a luminance mechanism whose relative sensitivity increases with temporal frequency. An analysis of the cone contributions to motion detection and identification showed that L-cones dominated threshold behavior for both detection and identification at high temporal frequencies. There was a weak S-cone input to motion detection and identification at high temporal frequencies.

Contributions of human long-wave and middle-wave cones to motion detection

1. It has been suggested that motion may be best detected by the luminance mechanism. If this is the most sensitive mechanism, motion thresholds may be used to isolate the luminance mechanism and study its properties. 2. A moving (1 cycle deg-1), vertical, heterochromatic (red-plus-green), foveal grating was presented on a bright yellow (577 nm wavelength) field. Detection and motion (direction identification: left versus right) thresholds were measured for different amplitude ratios of the red and green components spatially summed in phase or in antiphase. Threshold contours plotted in cone-contrast co-ordinates (L',M') for the long-wave (L) and middle-wave (M) cones, revealed two motion mechanisms: a luminance mechanism that responds to a weighted sum of L and M contrasts, and a spectrally opponent mechanism that responds to a weighted difference. 3. Detection and motion thresholds, measured at 1-4 Hz, were identical for luminance gratings, having equal cone contrasts, L' and M', of the same sign. For chromatic gratings, with L' and M' of opposite sign, motion thresholds were higher than detection thresholds. A red-green hue mechanism may mediate chromatic detection, and a separate spectrally opponent motion mechanism may mediate motion. 4. The red-green hue mechanism was assessed from 1 to 15 Hz with an explicit hue criterion. The detection contour had a constant slope of one, implying equal L' and M' contributions of opposite sign. For motion identification, L' and M' contributed equally at 1 Hz, but the M' contribution was attenuated at higher velocities. 5. The cone-contrast metric provides a physiologically relevant comparison of sensitivities of the two motion mechanisms. At 1 Hz, the spectrally opponent motion mechanism is approximately 4 times more sensitive than the luminance mechanism. As temporal frequency is increased, the relative sensitivities change so that the luminance mechanism is more sensitive above 9 Hz. 6. The less sensitive motion mechanism was isolated with a quadrature phase protocol, using a pair of heterochromatic red-plus-green gratings, counterphase flickering in spatial and temporal quadrature phase with respect to each other. One grating was set slightly suprathreshold and oriented in cone contrast (L',M') so as to potentiate a single motion mechanism, the sensitivity of which was probed with the second grating, which was varied in (L',M'). This allowed us to measure the motion detection contour of the less sensitive luminance mechanism at low velocities.(ABSTRACT TRUNCATED AT 400 WORDS)

Spectral efficiency measured by heterochromatic flicker photometry is similar in human infants and adults

Spectral efficiency functions based on heterochromatic flicker photometry (HFP) were measured for three adults and 42 infants using a rapid visually-evoked potential (VEP) method. A 5 degrees-diameter, broadband standard (0.6 cd/m2) was presented in square-wave counterphase (15 Hz) with one of 13 monochromatic lights (420-660 nm; 20 nm steps). The intensity of the monochromatic light was continuously varied while extracting the phase-locked VEP amplitude of the fundamental component. HFP functions measured psychophysically by the method of adjustment were also obtained for the adults. Adult HFP functions from the two methods were found to be essentially the same. Both of these functions were compared to Vos'-modified 2 degrees V(lambda) function and the 10 degrees CIEV(lambda) function. The mean adult data were slightly better fit to the 2 degrees V(lambda) function than to the 10 degrees CIEV(lambda) function, although there was an elevation in sensitivity at 420 and 440 nm. Infant HFP functions were similar to Vos' modified V(lambda) except for an elevation in efficiency at short wavelengths. The mean infant HFP function agreed better with the 10 degrees CIEV(lambda) function than Vos'-modified V(lambda) function, but infant sensitivity was elevated by 0.4 log units at 420 nm compared to the 10 degrees CIE observer. The elevation found at short wavelengths for both adults and infants is attributed to individual and age-related variation in the density of the ocular media, and to reduced macular pigment screening resulting from use of a 5 degrees field size.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection and discrimination of moving stimuli: the effects of color, luminance, and eccentricity

Psychophysical detection and direction discrimination thresholds for 1c/o, 1-Hz Gabors are plotted in a Weberian long-middle-wavelength-sensitive cone contrast plane. The shape of these threshold contours suggests linear cone contributions to additive (delta L/Lb + delta M/Mb) and opponent (delta L/Lb - delta M/Mb) postreceptoral mechanisms. The opponent mechanism dominates thresholds at the fovea, but sensitivity decreases rapidly with eccentricity in comparison with the additive mechanism. Cone contributions to the mechanisms vary in a small and nonsystematic manner across the retina. The experiments show that the additive mechanism is directionally sensitive at detection threshold. At all eccentricities studied (0-24 degrees), 0.3-log-unit suprathreshold contrasts are necessary for the opponent mechanism to signal direction of motion.

Estimation of linear detection mechanisms for stimuli of medium spatial frequency

Detection thresholds were obtained for a circularly-symmetric Gabor profile and Craik-Cornsweet profiles presented on a large white adapting field. These stimuli possessed peak spatial power between 1 and 6 c/deg. Their contrast was represented in an L, M and S cone contrast space. Detection thresholds were obtained for many vectors close to specific but theoretically important planes within this space. These data were fitted with a model comprising independent mechanisms, each a weighted sum of cone contrasts. The fit revealed a chromatic mechanism driven by delta L/L-delta M/M with no S cone input. Within cone contrast space, this mechanism was more sensitive than both a luminance mechanism with little S cone input but considerable variation in relative L to M cone input, and a blue-yellow chromatic mechanism.