Green Stimuli Research Articles

Metabotropic glutamate receptor-mediated hetero-synaptic interaction of red- and green-cone inputs to LHC of carp retina

The role of presynaptic metabotropic glutamate receptor (mGluR) on the interaction of red- and green-cone signals was investigated in luminosity-type horizontal cell (LHC) of isolated carp retina. It was found that a dim red background could enhance LHC’s light response to green stimulus, and a dim green background was also able to increase the cell’s response to red flash. Such mutual color enhancement was eliminated by application of groups II and III mGluR antagonist ( S)-methyl-4-carboxyphenyl-glycine (MCPG). Furthermore, inhibition of glutamate uptake by using d-aspartate ( d-Asp) or dl-threo-β-hydroxy-aspartic acid (THA) completely blocked the mutual enhancement of color signals in LHC. However, the GABAergic feedback pathway in the outer retina was unnecessarily involved.

Screening effect diverts the swimming directions from diaphototactic to positive phototactic in a disk-shaped green flagellate Mesostigma viride.

We found diaphototactic behavior (i.e. the cells swim perpendicularly with respect to the incident light) in a strain with colorless eyespot of a unicellular disk-shaped green flagellate Mesostigma viride. Lacking pigments completely in the eyespot, the screening effect in this strain was due only to the central part of the chloroplast whose cross section was thin. The diaphototaxis was most obvious when unilateral green stimulus light (520-580 nm) was given, whereas positive phototaxis appeared when given blue light (430-490 nm). The choice between diaphototaxis and (ordinary) phototaxis depended entirely on the transmission (%T) of the cell body against each wavelength of the stimulus: the green light penetrated well (%T > 90%), whereas the blue light was considerably shaded by the chloroplast (50% < %T < 70%). The fraction of positive phototactically behaving cells against each wavelength was in proportion to the front-to-back contrast value obtained at each individual wavelength. The fraction of diaphototaxis was inversely proportional to it. In addition, bilateral stimulus irradiations to wild-type cell with colored eyespot provided useful information about the principle of the diaphototactic steering.

Ocular optics, electroretinography and primary open angle glaucoma.

Advancing the knowledge of ocular image quality is important in the development of new tests for the earlier detection of primary open angle glaucoma (POAG). Loss of blue sensitivity is an important feature of POAG; however, it is difficult to separate this loss from the effects of wavelength related intraocular light scatter and media absorption. A technique for recording the S-cone electroretinogram is described which may solve these problems. The effect of scattered light is avoided by a Ganzfeld stimulus and the absorption of short wavelength light is compensated by a photometric balancing of blue and green stimuli using the M-cone response. The modulation transfer function of the eye has enabled the true spatial tuning function of the retina to be determined by pattern electroretinography (PERG). This has demonstrated the role of retinal ganglion cells in the generation of the PERG. These techniques are shown to be highly sensitive tests in the early stages of POAG.

Color Responses of Flash Visual Evoked Potentials in Normal Adults, Normal Elderly Subjects and Dementia of the Alzheimer Type Using Light Emitting Diodes

Background: Since initially we could only employ the red light goggle‐shaped stimuli equipment with flash light emitting diodes (LEDs), we had little information concerning different color responses of visual evoked potentials (VEPs). On our request, Nihon Kohden (Tokyo, Japan) developed green light goggle‐shaped stimuli equipment with LEDs in 1999.The purpose of this study was to determine the effects of green and red flash VEPs in normal adults, normal elderly subjects and patients with dementia of the Alzheimer type (DAT). Methods: Subjects consisted of a normal adult group (n=30), normal elderly group (n=10), and DAT patients (n=7). Transient VEPs of 1 Hz were measured. The recording electrodes were placed at midline‐occipital, left‐occipital and right‐occipital sites in accordance with the ten‐twenty electrode system. Results: In the normal adult group, peak latencies of green flash VEPs increased significantly compared to those of red flash VEPs (N2, P3: p&lt;0.01, N3: p&lt;0.05). The N2 and P3 latencies of both red and green flash VEPs were delayed in the normal elderly and DAT groups compared with those in the normal adult group. The delayed latencies of red flash VEPs were greater than those of green flash VEPs. Conclusion: The difference in latency between green and red flash VEPs corresponds to the difference in the conducting time from three rods (each spectrum peak is 419–420 nm, 530–534 nm, and 558–564 nm) to the visual central system. Each channel from the three rods to the visual central system may have an individual speed reaction, individual ageing and pathological changes. The results indicate that the peak latency of red light reveal elderly and pathological change more easily than that of green light.

Hippocampal lesions, contextual retrieval, and autoshaping in pigeons

Both pigeons and rats with damage to the hippocampus are slow to acquire an autoshaped response and emit fewer overall responses than control animals. Experiment 1 explored the possibility that the autoshaping deficit was due to an impairment in contextual retrieval. Pigeons were trained for 14 days on an autoshaping task in which a red stimulus was followed by reinforcement in context A, and a green stimulus was followed by reinforcement in context B. On day 15, the subjects were given a context test in which the red and green stimuli were presented simultaneously in context A and then later in context B. Both control and hippocampal animals showed context specificity, that is, they responded more to the red stimulus in context A and to the green stimulus in context B. In Experiment 2 we video-recorded the control and hippocampal animals performing the autoshaping task. Hippocampal animals tended to miss-peck the key more often than control animals. In addition, the number of missed pecks increased across days for hippocampal animals but not for control animals, suggesting that while the control animals increased their pecking accuracy, the hippocampal animals actually decreased their pecking accuracy. Our findings suggest that impairments in moving through space may underlie the hippocampal autoshaping deficit.

Perceived colour differences in displayed colours Part 2: spatial differences in soft copy imaging

This paper follows and extends previously reported investigations into the ability of observers to match surface colour samples by synthesis using red, green and blue stimuli provided by a cathode ray tube (CRT) monitor, controlled by a computer and adjusted by either an observer or the experimenter. Investigations are described into the ability of observers to perceive small colour perturbations applied to a colour patch, in the simultaneous presence of a reference and five other closely related patches, all the patches being displayed on a CRT computer display. Consistency is found between results obtained by different sets of observers at three locations, using local equipment for display. The results obtained are found to indicate sensitivities to perturbations that depend on the hue and chroma of the reference, and the hue direction perturbations. The surround to the colour patches is shown significantly to influence the sensitivity of observers to the perturbations, and this finding is related to predictions made using a colour appearance model. The data obtained are fitted by the observer response model hitherto successfully employed by the authors in previously reported investigations, and the goodness of fit is quantified statistically. Conclusions are drawn concerning the red, green and blue bit ranges and CIELAB colour differences of perturbations likely to be perceptible to observers in the presence of an unperturbed reference.

Fruit or aposematic insect? Context-dependent colour preferences in domestic chicks.

Colours are common stimuli in signalling systems. Requirements to function well as a signal sometimes conflict between different signallers, and the same colour stimulus is used to convey completely different messages to the same receiver. Fruits and aposematic insects both use red coloration as a signal, in the former case to signal profitability and in the latter case as a warning signal. In two experiments, we investigated whether the domestic chick, an omnivorous predator, differed in its unconditioned preference or avoidance of red and green stimuli depending on whether or not the stimulus was an insect. The experiments were designed as preference tests between red and green painted prey. The prey were live insects and artificial fruits (experiment 1), and, to investigate the effect of movement, live and dead insects (experiment 2). The chicks did not show any difference in pecking preference between red and green when fruit-like stimuli were used, but when the prey were insects, green prey were strongly preferred to red prey, and prey movement did not affect this bias. Thus, young chicks may recognize prey as insects and then discriminate between different prey colorations, or one type of food may elicit an unlearned colour preference-avoidance response that is absent with another type of food.

A comparative study of stimulus-specific pupil responses in the domestic fowl ( Gallus gallus domesticus) and the human

Pupil responses triggered by specific stimulus attributes such as spatial structure, colour and light flux changes were measured in eight domestic fowl. Comparative experiments were also carried out in human subjects. The results were unexpected in that large increments in light flux caused only small constrictions of the pupil. A red stimulus, on the other hand, caused a relatively large pupil response, but a green stimulus was less effective. This finding suggests that the size of the pupil, apart from being controlled by well-described pretectal pathways that mediate luminance responses, is also subject to other inputs. The pupil response in the domestic fowl may therefore make an effective quantitative indicator of things of significance to the animal. In some ways these observations are similar to other findings in primates in that the processing of stimulus attributes such as colour and structure that are not normally associated with the light reflex pathway can cause a pupil response. The fowl pupil does however respond very fast when large light flux changes or red stimuli are involved. Results obtained with sinusoidally modulated light flux changes reveal a short response latency of 105 ms (SD=8.3). In contrast, human responses measured for similar stimulus conditions reveal a latency of 434 ms (SD=36). The speed of pupil response in the fowl is significantly higher than in humans, but the response amplitude is usually small. Another interesting observation is the lack of sustained response to changes in ambient illumination. These findings suggest that the input to the pupilloconstrictor neurones in the fowl consists largely of transient neurones with little sustained component.

Responses of ectostriatal neurons during delayed matching-to-sample behavior in pigeons ( Columba livia)

We recorded from 71 neurons in the ectostriatum of awake behaving pigeons performing the delayed matching-to-sample task. The ectostriatum is considered to be equivalent to the primate extrastriate cortex. Many of the neurons ( n=56, 78.9%) were visually responsive to either or both the red and green stimuli. In addition, a large number of neurons ( n=51, 71.8%) exhibited delay activity, that is, a change in activity during the delay period when memory was required compared to the baseline intertrial interval period when memory was not required. We were more likely to encounter a delay neuron when the subjects performed well (>75% correct) compared to when the subjects performed poorly (<75% correct), suggesting that delay activity may carry some functional significance. Finally, the magnitude of the delay activity did not vary across the delay period but remained at a constant elevated or depressed level. Overall, our findings in pigeons are very similar to those reported for primates, and suggest that the pigeon may be an excellent model in which to study the neural mechanisms of visual information processing.

Receptor based models for spontaneous colour choices in flies and butterflies

AbstractColour is one of several stimuli used by herbivorous insects in host choice. Insects have between 2 and 5 different types of photoreceptors to catch quanta of different wavelengths of the spectrum. Many insects have been shown to possess opponent neural interactions between the receptors that enable them to see colour. I present simple models to describe colour choices as functions of the receptor quantum catches and linear interactions of the receptor types. Models are applied to data sets obtained from own experiments and from the literature, on Pieris brassicae and P. rapae (Lepidoptera, Pieridae), Papilio aegeus (Papilionidae), Dacus oleae (Diptera, Tephritidae) and Eristalis tenax (Syrphidae). In fruit flies, detection of green fruit is based on an inhibitory interaction between a green‐sensitive receptor type and a blue‐sensitive receptor type. This might explain the preference many herbivorous insects have for yellow over green stimuli. Pollen feeding in hoverflies might have evolved from yellow pollen being a super‐normal stimulus for herbivorous insects. In butterflies, an additional red‐receptor is involved in the colour choice for an oviposition substratum and leads to them choosing green and not yellow. The models introduced in this study open new perspective for a physiological understanding of the design of visual stimuli for monitoring and trapping pest insects.

Influence of stimulus color on the control of reaching-grasping movements.

This kinematic study aimed to determine whether color is a stimulus property involved in the control of reaching-grasping movements. Subjects reached and grasped a target-object, located either on the right or on the left of the subject's midline. A distractor, placed along the subject's midline, could be randomly presented. The colors, i.e., both chromaticity (red and green stimuli were presented) and lightness, of the target and distractor were varied in experiment 1. Only stimulus lightness and only stimulus chromaticity were varied in experiments 2 and 3, respectively. In experiment 4 subjects matched with their thumb and index finger the size of the target-stimuli presented in experiment 1. Chromaticity (experiments 1 and 3) of the target and distractor influenced grasp, but not reach. Maximal finger aperture was larger during grasping the red than the green target. Data collected in the matching task (experiment 4) confirmed a trend to overestimate the red target and to underestimate the green one. During grasp, hand shaping was influenced by distractor chromaticity when it was different from target chromaticity. Distractor lightness affected reach, but not grasp (experiments 1 and 2). Reach was slower when the distractor was lighter and arm trajectory veered away from it. The results of the present study suggest that color, that is the ensemble of chromaticity and lightness, is a stimulus property involved in the control of reaching-grasping. The different effects of target color on reach and grasp support the notion that intrinsic object properties, such as color, affect grasp more than reach. In addition, the different effects of distractor chromaticity and lightness on reach and grasp confirm that target-objects are visually extracted from surrounding cues by means of different processes, according to the required motor response.

Colour vision in fallow deer: a behavioural study

To examine whether fallow deer, Dama dama, have colour vision, we trained four adult females on a two-choice discrimination task, where a positive chromatic stimulus (green) and a negative achromatic stimulus (grey) had similar brightness. The criterion for learning was set at 80% correct responses. To exclude the possibility that the hinds used small differences in brightness to distinguish between the green and the grey stimulus, we conducted a test that differed from the training situation. A light green positive stimulus combined with a dark grey negative stimulus was alternated with a dark green positive stimulus combined with a light grey negative stimulus on every second trial. The positive green stimuli had different reflectance spectra. After training, each of the four hinds showed over 80% correct responses in the test. These results suggest that fallow deer can use colour vision in a discrimination situation by generalizing over slightly different colours, at least in the range of the green spectrum.

The unseen color aftereffect of an unseen stimulus: insight from blindsight into mechanisms of color afterimages.

We show here that, in the absence of a direct geniculostriate input in human subjects, causing loss of sight in the visual half-field contralateral to the damage, the pupil responds selectively to chromatic modulation toward the long-wavelength (red) region of the spectrum locus even when the stimulus is isoluminant for both rods and cones and entirely restricted to the subjects' "blind" hemifields. We also show that other colors are less or wholly ineffective. Nevertheless, red afterimages, generated by chromatic modulation toward the green region of the spectrum locus, also cause constrictions of the pupil even when green stimuli are themselves completely ineffective in the blind hemifield. Moreover, human subjects with damage to or loss of V1 are typically completely unaware of the stimulus that generates the aftereffect or of the aftereffect itself, both of which can be seen clearly in normal vision. The results show that pupillary responses can reveal the processing of color afterimages in the absence of primary visual cortex and in the absence of acknowledged awareness. This phenomenon is therefore a striking example of "blindsight" and makes possible the formulation of a model that predicts well the observed properties of color afterimages.

Scanning laser densitometry and color perimetry demonstrate reduced photopigment density and sensitivity in two patients with retinal degeneration

Purpose. To test the feasibility of scanning laser densitometry with a modified Rodenstock scanning laser ophthalmoscope (SLO) to measure the rod and cone photopigment distribution in patients with retinal diseases. Methods. Scanning laser densitometry was performed using a modified Rodenstock scanning laser ophthalmoscope. The distribution of the photopigments was calculated from dark adapted and bleached images taken with the 514 nm laser of the SLO. This wavelength is absorbed by rod and cone photopigments. Discrimination is possible due to their different spatial distribution. Additionally, to measure retinal sensitivity profiles, dark adapted two color static perimetry with a Tübinger manual perimeter was performed along the horizontal meridian with 1° spacing. Results. A patient with retinitis pigmentosa had slightly reduced photopigment density within the central ±5° but no detectable photopigment for eccentricities beyond 5°. A patient with cone dystrophy had nearly normal pigment density beyond ±5°, but considerably reduced photopigment density within the central ±5°. Within the central ±5°, the patient with retinitis pigmentosa had normal sensitivity for the red stimulus and reduced sensitivity for the green stimulus. There was no measurable function beyond 7°. The patient with cone dystrophy had normal sensitivity for the green stimulus outside the foveal center and reduced sensitivity for the red stimulus at the foveal center. The results of color perimetry for this patient with a central scotoma were probably influenced by eccentric fixation. Conclusion. Scanning laser densitometry with a modified Rodenstock SLO is a useful method to assess the human photopigment distribution. Densitometry results were confirmed by dark adapted two color static perimetry. Photopigment distribution and retinal sensitivity profiles can be measured with high spatial resolution. This may help to measure exactly the temporal development of retinal diseases and to test the success of different therapeutic treatments. Both methods have limitations at the present state of development. However, some of these limitations can be overcome by further improving the instruments.

Input of long- and middle-wavelength-sensitive cones to orientation discrimination

Orientation discrimination thresholds for Gabor stimuli as a function of spatial frequency [0.6, 1, 2, or 4 cycles per degree (cpd)] and contrast were determined for several directions in the color plane spanned by the long-wavelength-sensitive (L) and medium-wavelength-sensitive (M) cones. For Gabor stimuli with carrier frequencies of 2 or 4 cpd we do not find a systematic and robust advantage of a particular color direction in the LM-cone plane when the stimuli are equated in terms of the sum of the absolute L- and M-cone contrasts. Luminance (L+M) and equiluminant (L–2M) stimuli of identical overall L- and M-cone contrast yield identical orientation discrimination thresholds for the 2-cpd stimuli for the entire available contrast range (1–11%). For the very-low-spatial-frequency stimuli, orientation discrimination thresholds are lower for equiluminant stimuli than for luminance stimuli of the same cone contrast; for 4 cpd, orientation discrimination thresholds are slightly higher for equiluminant red–green stimuli than for luminance-defined stimuli.

Shared target selection for combined version-vergence eye movements.

Primates frequently make rapid binocular eye movements to reorient gaze in both direction and depth. To explain the unequal movements made by the two eyes, it often is assumed that they result from the combined action of a conjugate saccadic system and a vergence contribution. Clearly such a scheme can only yield coordinated binocular movements if both systems are guided by a shared or coupled target selection mechanism. To investigate the degree of cooperation at this level, we studied binocular refixations to target-nontarget double-stimuli in three-dimensional (3-D) space. Binocular eye movements were recorded in seven subjects using the scleral coil technique. In the experiments, 20% of trials were composed of a green target and a red nontarget, presented at the same time, but at different locations in 3-D space. These were alternated randomly with single-target trials (80%) in which the green stimulus was presented randomly at one of eight possible positions in 3-D space. Instructions to the subject emphasized either the speed or accuracy of response. Our findings show that typical features of the saccadic response to double-stimuli (bistability, averaging, and a speed-accuracy trade-off), as found in earlier two-dimensional studies, are also prevalent for initial binocular refixations to double stimuli in 3-D space. When the first saccadic response is directed to one of the two stimuli, the vergence system almost invariably makes the same choice. Likewise, when the saccadic system makes a short-latency averaging response, the vergence system shows a similar compromise. Statistical analysis shows a high correlation between saccadic and vergence target selection, strongly suggesting that the amplitude computation process of both subsystems is due to a common target selection stage that has access to information about stimulus location in 3-D space.

Temporal analysis of the topographic ERG: chromatic versus achromatic stimulation.

The topographic electroretinogram evoked by multi-focal exchange of black and white or red and green stimuli was analysed into linear and non-linear Wiener kernels. The first-order (temporally linear) response showed a biphasic waveform which inverted as the luminance ratio of the exchanged colours passed through unity (established both psychophysically and photometrically). A short latency non-linearity which was dependant on luminance contrast was observed in both chromatic and achromatic ERG. However, in the chromatic second-order response, a long-latency non-linearity, foveally prominent, with a distinct skew in power towards the nasal retina, appeared around the isoluminant point, between the points of silent substitution for the L and M-cone types. Modelling of the second-order responses showed that over a wide range of luminance ratios, the chromatic ERG is well described by a linear combination of the achromatic (contrast-dependent) component and the response at isoluminance. The difference in second-order response between coloured and black and white stimulation, at the same luminance contrast, showed that the long-latency non-linearity is recorded when the red and green cone types are operating out of phase and peaks in amplitude at a green/red luminance ratio of 0.8. This interpretation was confirmed by the lack of the long-latency non-linearity in colour-anomalous subjects (whether deficient in the L or the M-cone type). A marked similarity exists between the properties of the long-latency non-linearity and the frequency-doubled response generated in the ganglion cells of the magnocellular pathway.

Variation in Sensitivity, Absorption and Density of the Central Rod Distribution with Eccentricity

Purpose: To assess the human rod photopigment distribution and sensitivity with high spatial resolution within the central ±15° and to compare the results of pigment absorption, sensitivity and rod density distribution (number of rods per square degree). Method: Rod photopigment density distribution was measured with imaging densitometry using a modified Rodenstock scanning laser ophthalmoscope. Dark-adapted sensitivity profiles were measured with green stimuli (17′ arc diameter, 1° spacing) using a Tübingen manual perimeter. Sensitivity profiles were plotted on a linear scale and rod photopigment optical density distribution profiles were converted to absorption profiles of the rod photopigment layer. Results: Both the absorption profile of the rod photopigment and the linear sensitivity profile for green stimuli show a minimum at the foveal center and increase steeply with eccentricity. The variation with eccentricity corresponds to the rod density distribution. Conclusion: Rod photopigment absorption profiles, retinal sensitivity profiles, and the rod density distribution are linearly related within the central ±15°. This is in agreement with theoretical considerations. Both methods, imaging retinal densitometry using a scanning laser ophthalmoscope and dark-adapted perimetry with small green stimuli, are useful for assessing the central rod distribution and sensitivity. However, at present, both methods have limitations. Suggestions for improving the reliability of both methods are given.

Processing of color- and noncolor-coded signals in the gourami retina. III. Ganglion cells.

The dynamics of intracellular responses from ganglion cells, as well as that of spike discharges, were studied with the stimulus regimens and analytic procedures identical to those used to study the dynamics of the responses from horizontal and amacrine cells (,). The stimuli used were large fields of red and green light given as a pulsatile input or modulation about a mean luminance by a white-noise signal. Spike discharges evoked by a white-noise stimulus were analyzed in exactly the same manner as that used for analysis of analog responses. The canonical nature of kernels allowed us to correlate the first- and second-order components in a spike train with those of the intracellular responses from horizontal, amacrine, and ganglion cells. Both red and green stimuli given alone in darkness produced noncolor-coded responses from all ganglion cells. In the case of some cells, steady red illumination changed the polarity or waveform of the response to green light. Color-coded ganglions responded only to simultaneous color contrast. Nonlinearities recovered from intracellular responses, and spike discharges were similar to those found in responses from amacrine cells and were of two types, one characteristic of the C amacrine cells and the other characteristic of the N amacrine cells. The first-order kernels of most ganglion cells could be divided into two basic types, biphasic and triphasic. The combination of kernels of these two basic types with different polarities can produce a wide range of responses. Addition of two types of second-order nonlinearity could render color coding in this relatively simple retina as an extremely complex process. Color information appeared to be represented by the polarity, as well as the waveform, of the first-order kernel. The response dynamics is a means of transmission of color-coded information. Second-order components carry information about changes around a mean luminance regardless of the color of an input. Some spike discharges produced a well-defined cross-kernel between red and green inputs to show that a particular time sequence of red and green stimuli was detected by the retinal neuron network. The similarity between signatures of second-order kernels for both amacrine and ganglion cells indicates that signals undergo a minimal transformation in the temporal domain when they are transmitted from amacrine to ganglion cells and then transformed into a spike train. Under our experimental conditions, a single spike train carried simultaneously information about red and green inputs, as well as about linear and nonlinear components. In addition, the spike train also carries a cross-talk component. A spike train is a carrier of multiple signals. Conversely, many types of information in a stimulus are independently encoded into a spike train.

Processing of color- and noncolor-coded signals in the gourami retina. II. Amacrine cells.

The same set of stimuli and analytic methods that was used to study the dynamics of horizontal cells () was applied to a study of the response dynamics and signal processing in amacrine cells in the retina of the kissing gourami, Helostoma rudolfi. The retina contains two major classes of amacrine cells that could be identified from their morphology: C and N amacrine cells. C amacrine cells had a two-layered dendritic field, whereas N cells had a monolayered dendritic field. Both types of amacrine cell were tracer-coupled but coupling was more extensive in the N amacrine cells. Responses from C amacrine cells lacked a DC component and had a small linear component that was <10% in terms of mean square error (MSE); the second-order component often accounted for >50% of the modulation response. The C amacrine cells did not show any characteristic color coding under any stimulus condition. Most responses of N cells to a pulsatile stimulus consisted of a series of depolarizing transient potentials and steady illumination did not generate any DC potential in these cells. The response to a white-noise modulated input was composed of well-defined first- and second-order components and, possibly, higher-order components. The response evoked by a red or green white-noise-modulated stimulus given alone was not color coded. Modulated red illumination in the presence of a green illumination elicited a color-coded response from >70% of N amacrine cells. Color information was carried not only by the polarity but also by the dynamics of the first-order component. No convincing evidence was obtained to indicate that the second-order component might be involved in color processing. Some N amacrine cells produced a well-defined (second-order) interaction kernel to show that the temporal sequence of red and green stimuli was a parameter to be considered. In a complex cell such as an amacrine cell, responses evoked by a pulsatile stimulus given in darkness and by modulation of a mean luminance could be very different in terms of their characteristics. It was not always possible to predict the response evoked by one stimulus from observing the cell's response to another stimulus. This is because, in N cells, a flash-evoked (nonsteady state) response is composed largely of nonlinear components whereas a modulation (steady state) response is composed of linear as well as nonlinear components.