Exploring the roles of saturating and supersaturating contrast-response functions in conjunction detection and contrast coding

J. W. Peirce (2007, p. 1) has proposed that saturating contrast-response functions in V1 and V2 may form "a critical part of the selective detection of compound stimuli over their components" and that supersaturating (non-monotonic) functions allow even greater conjunction selectivity. Here, we argue that saturating and supersaturating contrast-response functions cannot be exploited by conjunction detectors in the way that Peirce proposes. First, the advantage of these functions only applies to conjunctions with components of lower contrast than the equivalent non-conjunction stimulus, e.g., plaids (conjunctions) vs. gratings (non-conjunctions); most types of conjunction do not have this property. Second, in many experiments, conjunction and non-conjunction components have identical contrast, sampling the contrast-response function at a single point, so the function's shape is irrelevant. Third, Peirce considered only maximum-contrast stimuli, whereas contrasts in natural scenes are low, corresponding to a contrast-response function's expansive region; we show that, for naturally occurring contrasts, Peirce's plaid detector would generally respond more weakly to plaids than to gratings. We also reassess Peirce's claim that supersaturating contrast-response functions are suboptimal for contrast coding; we argue that supersaturation improves contrast coding, and that the multiplicity of supersaturation levels reflects varying trade-offs between contrast coding and coding of other features.     

Effect of temporal sparseness and dichoptic presentation on multifocal visual evoked potentials

Multifocal VEP (mfVEP) responses were obtained from 13 normal human subjects for nine test conditions, covering three viewing conditions (dichoptic and left and right monocular), and three different temporal stimulation forms (rapid contrast reversal, rapid pattern pulse presentation, and slow pattern pulse presentation). The rapid contrast reversal stimulus had pseudorandomized reversals of checkerboards in each visual field region at a mean rate of 25 reversals/s, similar to most mfVEP studies to date. The rapid pattern pulse presentation had pseudorandomized presentations of a checkerboard for one frame, interspersed with uniform grey frames, with a mean rate of 25 presentations/s per region per eye. The slow pattern pulse stimulus had six presentations/s per region per eye. Recording time was 5.3 min/condition. For dichoptic presentation slow pattern pulse responses were 4.6 times larger in amplitude than the contrast reversal responses. Binocular suppression was greatest for the contrast reversal stimulus. Consideration of the signal-to-noise ratios indicated that to achieve a given level of reliability, slow pattern pulse stimuli would require half the recording time of contrast reversal stimuli for monocular viewing, and 0.4 times the recording time for dichoptically presented stimuli. About half the responses to the slow pattern pulse stimuli had peak value exceeding five times their estimated standard error. Responses were about 20% smaller in the upper visual field locations. Space-time decomposition showed that responses to slow pattern pulse were more consistent across visual field locations. We conclude that the pattern pulse stimuli, which we term temporally sparse, maintain the visual system in a high contrast gain state. This more than compensates for the smaller number of presentations in the run, and provides signal-to-noise advantages that may be valuable in clinical application.     

Effect of Contrast, Stimulus Density, and Viewing Distance on Multifocal Steady-State Visual Evoked Potentials (MSVs)

Purpose. We investigated the effects of image contrast, stimulus density, and viewing distance upon a multifocal steady-state visual evoked potential (MSV) METHOD: Methods. Fourteen adults with normal vision (mean age = 27.0 ± 6.6 years; 6 males) participated in the study. Each of the stimulus regions of the multifocal ensembles presented a contrast modulated grating, displaying spatial and temporal frequencies that evoke the spatial frequency doubling illusion. All subjects were tested at five contrasts: 0.06, 0.11, 0.22, 0.45, and 0.89; viewed at 16, 32, and 128 cm. A multivariate linear model estimated factors for each stimulus region, recording channel, number of stimuli (9 or 17 regions), and sex; and covariates for contrast, and octaves of viewing distance, and age. Results. The responses per unit area for the 17-region display were significantly larger than for the 9-region display (P < 10(-12)). The contrast-response function could be described by a power law with exponent 0.068 (P < 0.008). The effect of viewing distance was small but significant: response amplitude dropped by -0.17 ± 0.03 dB per octave of viewing distance (P < 10(-6)), or 10% over 8 octaves. Conclusions. The response per unit area indicated that cortical folding diminishes responses to larger stimuli. Viewing distance did not greatly affect response amplitude. This suggested that we can use similar, but scaled, stimuli to study central and peripheral disease. The rapidly saturating contrast responses imply that there would be nothing lost from testing at contrasts as low as 20% given that higher, saturating contrasts might mask visual field defects.     

Visual evoked potentials for reversals of red-green gratings with different chromatic contrasts: asymmetries with respect to isoluminance

Human visual evoked potentials (VEPs) were recorded for abrupt 6.25-Hz reversals of 2 c/deg square-wave gratings combining red-green contrast with different levels of luminance contrast. Response characteristics-- amplitudes and peak latencies as a function of luminance contrast--were compared for four different pairs of red-green colors and an isochromatic yellow grating. For each of the red-green color pairs, the plots of VEP amplitudes and latencies were nonsymmetrical with respect to isoluminance. The amplitude dropped to a minimum within a region of rapid phase change, at a different contrast for each color pair but always at a luminance contrast for which the greener color had the higher luminance. When the contrast-response curve for each of the four red-green pairs was modeled by a simple |CL - CM| opponency of L- and M-cone contrast using a fixed CL/CM weighting ratio of about two, there was a close correspondence between the contrast giving a null in the modeled response and that giving a minimum in the VEP amplitude. So for the stimulus parameters applied here, the reversal VEP appeared to be dominated by L/M-opponent response contributions for which the signed CL/CM-cone weighting ratio was close to a value of minus two rather than to a value of minus one, which is characteristic of the psychophysical red-green detection mechanism and representative of CL/CM weighting ratios for precortical cells in the parvocellular pathway.     

Effects of monocular deprivation on the development of visual inhibitory interactions in kittens.

A visual-evoked-potential (VEP) masking technique was used to assess the effects of short- and long-term monocular deprivation on the development of visual inhibitory interactions in kittens. VEP contrast-response curves were recorded in response to contrast-reversed sinusoidal gratings, both with and without superimposed high-contrast masks. The contrast-response curves measured from the nondeprived eye were similar to those of normal cats: with no mask VEP amplitudes increase with contrast up to saturation at about 10% contrast; parallel masks shift the curves to the right, decreasing thresholds; and orthogonal masks decrease the slope of the contrast-response curves without affecting thresholds. After monocular deprivation (either brief or extensive), the contrast-response curves without mask did not show the typical response saturation, and neither parallel nor orthogonal mask had any effect on the contrast-response curves. The masking effects did not return after 100 days of normal vision, although contrast sensitivity and acuity recovered to about half of the normal levels during that period. The results indicate that the inhibitory intracortical circuitry that mediates the orientation-dependent masking effects are highly vulnerable to visual experience.     

The pattern-pulse multifocal visual evoked potential

PURPOSE: To define the pattern-pulse multifocal visual evoked potential (PPMVEP) and determine its characteristics in a sample of normal subjects in terms of amplitude of response attainable, the variation in waveform across visual field, and distribution of potential over the scalp and to compare pattern-pulse with contrast-reversal multifocal stimuli. METHODS: VEPs were obtained by concurrently stimulating 60 regions of a cortically scaled dartboard with pulses of pattern contrast. Responses were recorded from normal subjects, by using a 32-channel electroencephalogram recording system, and elementary responses to each region were estimated by multiple regression of each of the response channel signals on stimulus signals. Left-eye, right-eye, and binocular viewing conditions were concurrently tested by dichoptic stimulation. A direct comparison was then made with contrast-reversal stimulation. RESULTS: Response waveform sets for 12 subjects varied in maximum amplitude from 1.8 to 6.8 micro V. A stereotypical distribution of waveforms held in most subjects, depending primarily on the polar angle location of the stimulus within the visual field. In a direct comparison with a contrast-reversal multifocal analysis, the pattern-pulse responses had similar waveforms and scalp topography, but were 15 times larger in amplitude. Root mean square (RMS) signal-to-noise ratio (SNR) was 1.9 times higher with pattern-pulse stimulation, corresponding to a reduction of 73% in recording time to achieve the same SNR. CONCLUSIONS: The PPMVEP can simultaneously characterize 60 regions of the visual field for both eyes in less than 7 minutes. A general methodology is illustrated that allows multifocal analysis with flexible choice of stimulus conditions.     

Evoked potentials to dynamic random dot stimuli with varying dot density ratios of disparity to background

A systematic study was made of visual evoked responses to dynamic random dot stimuli containing controllable, monocularly visible contrast cues. Ratios of dot densities for the centrally presented, disparate figure and the background were varied in steps of 1/8 maximum density between 0/8 and 8/8. The figure was either a square or an equivalent area of nebulous shape. A 30-arc min disparity was compared with binocular nondisparate and monocular conditions. Evoked responses (scalp sites 02, 01, T6, T5) were averaged for each of 24 disparity-contrast-shape conditions. At all contrast levels, response amplitudes and latency over the left hemisphere was significantly greater than over the right hemisphere. For 30 arc min disparity, amplitudes in the 8/8 condition were significantly smaller than in conditions where stimulus/background contrast could afford monocular depth cues. Hemisphere amplitude differences diminished as contrast decreased. Factor analyses isolated two overlapping components in the response to disparate stimuli. The earlier, at 236 ms latency, may index the stereoscopic stimulus features. The later, at 295 ms, peaking at maximum contrast and present in all suprathreshold nondisparity conditions, may index contrast features of the stimulus. The results indicate the importance of controlling dot density ratios in electrophysiological studies of the stereoscopic response to random dot stimuli.     

Contrast sensitivity of form and motion discrimination during binocular rivalry

Binocular rivalry, which is induced by presenting the two eyes with incompatible stimuli, results in periods where one eye's stimulus is seen and the other stimulus is suppressed. We measured the depth of suppression in two ways, with very different results. First, two similar forms were briefly presented to one eye: the difference in shapes required to discriminate the forms was substantially greater during suppression than during dominance. Second, the two forms were made sufficiently different in shape to be easily distinguishable at high contrast, and contrast was lowered to find the threshold for discrimination of the forms. Contrast sensitivity did not differ between the suppression and dominance states. These results were replicated with a motion discrimination task: suppression markedly worsened the ability to distinguish increases from decreases in speed but did not elevate the minimum contrast required for the same task. We interpret the results in terms of steep contrast-response functions in visual cortex beyond the primary area.     

Luminance-contrast mechanisms in humans: visual evoked potentials and a nonlinear model

Isolated-checks were luminance-modulated temporally to elicit VEPs. Bright or dark checks were used to drive ON or OFF pathways, and low or high-contrast conditions were used to emphasize activity from magnocellular or parvocellular pathways. Manipulation of stimulus parameters and frequency analysis of the VEP were performed to obtain spatial and contrast-response functions. A biophysical explanation is offered for why the opposite polarity stimuli drive selectively ON and OFF pathways in primary visual cortex, and a lumped biophysical model is proposed to quantify the data and characterize changes in the dynamics of the system with contrast given a limited number of parameters. Response functions were found to match the characteristics of the targeted pathways.     

Dynamics of distributed 1D and 2D motion representations for short-latency ocular following

Integrating information is essential to measure the physical 2D motion of a surface from both ambiguous local 1D motion of its elongated edges and non-ambiguous 2D motion of its features such as corners or texture elements. The dynamics of this motion integration shows a complex time course as read from tracking eye movements: first, local 1D motion signals are extracted and pooled to initiate ocular responses, then 2D motion signals are integrated to adjust the tracking direction until it matches the surface motion direction. The nature of these 1D and 2D motion computations are still unclear. One hypothesis is that their different dynamics may be explained from different contrast sensitivities. To test this, we measured contrast-response functions of early, 1D-driven and late, 2D-driven components of ocular following responses to different motion stimuli: gratings, plaids and barberpoles. We found that contrast dynamics of 1D-driven responses are nearly identical across the different stimuli. On the contrary, late 2D-driven components with either plaids or barberpoles have similar latencies but different contrast dynamics. Temporal dynamics of both 1D- and 2D-driven responses demonstrates that the different contrast gains are set very early during the response time course. Running a Bayesian model of motion integration, we show that a large family of contrast-response functions can be predicted from the probability distributions of 1D and 2D motion signals for each stimulus and by the shape of the prior distribution. However, the pure delay (i.e. largely independent upon contrast) observed between 1D- and 2D-motion supports the fact that 1D and 2D probability distributions are computed independently. This two-pathway Bayesian model supports the idea that 1D and 2D mechanisms represent edges and features motion in parallel.     

Full-field electroretinogram response to increment and decrement stimuli

Purpose

The d-wave is typically elicited after the termination of an increment flash, but a decrement flash provides an alternative, and perhaps more appropriate, stimulus to elicit the d-wave. Here, we investigated the affects of stimulus polarity on the electroretinogram (ERG) response.

Methods

ERG responses elicited to increment and decrement flashes of varying intensity and duration from different background levels were measured from human participants to assess the b-wave and d-wave responses as a function of adaptation level and flash polarity. Response amplitudes were measured using standard metrics for waveform analysis.

Results

The amplitude of the b-wave is larger than the d-wave regardless of flash polarity when using different background levels which maximized the dynamic range of the two waveforms. However, when response amplitudes are measured from a common background, the d-wave elicited with decrement flash was larger than the b-wave elicited by an increment flash. This trend was evident across a range of background levels. The b-wave and d-wave become separate entities when flash duration reaches approximately 50 ms. Rapid-on and rapid-off sawtooth stimuli were also tested against increment and decrement step stimuli that were matched in mean luminance. These two stimulus types produced different amplitude b-wave and d-wave responses, suggesting asymmetric effects of the two stimulus types on the retinal response.

Conclusions

We conclude that the response properties of the b-wave and d-wave are influenced by the duration, polarity and waveform of the stimulus, as well as the background from which the stimuli arise.     

Pattern reversal VER as a tool for evaluating unbalanced visual inputs between the two eyes

We studied the effects of unequal or unbalanced visual inputs on the binocular system of normal subjects by measuring the amplitudes of the binocularly and the monocularly recorded VER and determining the value of binocular summation. The stimuli used were patterns that were either fused or not fused, of unequal luminosity and of unequal image size. When identical patterns were delivered to each eye and the patterns were fused, the binocular VER demonstrated a larger amplitude than the monocular VER, resulting in a binocular summation that was prominent in the low-contrast stimulus pattern. With stimulus patterns of higher contrast, the amplitudes of the binocular and the monocular VER did not differ greatly, and the value of binocular summation was significantly decreased. When fusion was disturbed with a base-in prism, the binocular VER displayed smaller amplitudes than the monocular VER, indicating binocular interaction or inhibition. With a small interocular luminosity difference, the binocular VER exhibited a larger amplitude than the monocular VER (binocular summation), but as the difference exceeded a certain level (0.6 to 0.8 log units), the binocular VER amplitudes were smaller than those of the monocular VER (binocular inhibition). With very large luminosity differences, the binocular VER amplitudes were almost similar to those of the monocular VER (suppression). When the perceived image size of the retina was altered with an aniseikonic lens, the binocular VER displayed larger amplitudes than those of the monocular VER (binocular summation) in aniseikonia equal to or less than 3.0%. At 5.0% aniseikonia, the binocular VER amplitudes were almost equal to the monocular VER (zero summation). At aniseikonia equal to or larger than 8.0%, the binocular VER amplitudes were significantly smaller than those of the monocular VER (binocular inhibition or interaction).     

Contrast invariance of functional maps in cat primary visual cortex

Neurons in cat primary visual cortex (V1) are clustered according to their preference for stimulus position, orientation, spatial frequency, and eye of presentation, thereby giving rise to functional maps. It is not known, however, whether a similar arrangement is present for stimulus contrast. Neurons in cat V1 vary considerably in their contrast responses, and might be clustered in a systematic fashion in this respect. Additionally, stimulus contrast might affect other functional maps. For example, there has been debate over whether the contrast threshold of neurons in cytochrome oxidase blobs is lower than elsewhere. Here we have imaged intrinsic signals to measure orientation maps in cat V1 at a range of contrast levels. We determined, on a pixel-by-pixel basis, contrast-response functions and orientation tuning curves. The fit parameters describing contrast responses were more or less uniform: We found no regions where neurons have lower contrast threshold than elsewhere. If such regions do exist, their functional maps must be substantially weaker than maps of orientation preference. Moreover, we found that contrast has no impact on maps of orientation preference: The orientation selectivity of each pixel is invariant with stimulus contrast. The contrast invariance that we demonstrate at the level of maps is well known at the level of single neurons. It suggests that neurons contributing to a pixel response generally have similar orientation preferences or similar contrast responses. The latter explanation is likely to hold in pinwheel centers, where preferred orientation of nearby neurons can differ markedly. In summary, our data suggest that contrast is represented uniformly over the surface of cat V1, and changes in contrast do not affect maps of orientation preference.     

Objective visual field determination in forensic ophthalmology with an optimized 4-channel multifocal VEP perimetry system: a case report of a patient with retinitis pigmentosa

We present the case of a 59-year-old male patient with progressive vision impairment and consecutive visual field narrowing (“tunnel view”) for 7 years and a known retinitis pigmentosa for 5 years. The remaining Goldmann perimetric visual field at time reported was less than 5°. A request for blindness-related social benefits was rejected because an ophthalmologic expert assessment suggested malingering. This prompted us to assess an objective determination of the visual field using multifocal VEPs. Objective visual field recordings were performed with a four-channel multifocal VEP-perimeter using 58 stimulus fields (pattern reversal dartboard stimulus configuration). The correlated signal data were processed using an off-line method. At each field, the recording from the channel with the maximal signal-to-noise ratio (SNR) was retained, thus resulting in an SNR optimized virtual recording. Analysis of VEP signals was performed for each single field and concentric rings and compared to an average response measured in five healthy subjects. Substantial VEP responses could be identified in three fields within the innermost ring (eccentricity, 1.7°) for both eyes, although SNR was generally low. More eccentric stimuli did not elicit reliable VEP responses. The mfVEP recording was correlated with perimetric visual field data. The current SNR optimization by using the channel with the largest SNR provides a good method to extract useful data from recordings and may be appropriate for the use in forensic ophthalmology.     

The direction-selective contrast response of area 18 neurons is different for first- and second-order motion

Cortical neurons selective for the direction of motion often exhibit some limited response to motion in their nonpreferred directions. Here we examine the dependence of neuronal direction selectivity on stimulus contrast, both for first-order (luminance-modulated, sine-wave grating) and second-order (contrast-modulated envelope) stimuli. We measured responses from single neurons in area 18 of cat visual cortex to both kinds of moving stimuli over a wide range of contrasts (1.25-80%). Direction-selective contrast response functions (CRFs) were calculated as the preferred-minus-null difference in average firing frequency as a function of contrast. We also applied receiver operating characteristic analysis to our CRF data to obtain neurometric functions characterizing the potential ability of each neuron to discriminate motion direction at each contrast level tested. CRFs for sine-wave gratings were usually monotonic; however, a substantial minority of neurons (35%) exhibited nonmonotonic CRFs (such that the degree of direction selectivity decreased with increasing contrast). The underlying preferred and nonpreferred direction CRFs were diverse, often having different shapes in a given neuron. Neurometric functions for direction discrimination showed a similar degree of heterogeneity, including instances of nonmonotonicity. For contrast-modulated stimuli, however, CRFs for either carrier or envelope contrast were always monotonic. In a given neuron, neurometric thresholds were typically much higher for second- than for first-order stimuli. These results demonstrate that the degree of a cell's direction selectivity depends on the contrast at which it is measured, and therefore is not a characteristic parameter of a neuron. In general, contrast response functions for first-order stimuli were very heterogeneous in shape and sensitivity, while those for second-order stimuli showed less sensitivity and were quite stereotyped in shape.     

Accuracy of isolated-check visual evoked potential technique for diagnosing primary open-angle glaucoma

Purpose

The aim of this study was to determine the diagnostic accuracy, sensitivity and specificity of isolated-check visual evoked potentials (icVEP) in primary open-angle glaucoma (POAG).

Methods

Ninety POAG patients and sixty-six healthy controls were recruited consecutively. All subjects underwent icVEP and visual field testing. Swept icVEP response functions were obtained by increasing contrast in six stimulus steps, recording the electroencephalogram synchronized to the stimulus display’s frame rate and calculating the corresponding signal-to-noise ratio (SNR) of the response at the fundamental frequency to evaluate visual function. Depth of modulation of the check luminance was increased as follows: 2, 4, 8, 14, 22 and 32%, about an equal level of standing contrast, so that the pattern appeared and disappeared at a frequency of 10.0 Hz. SNR above 0.85 was deemed to be significant at the 0.1 level and SNR above 1 significant at the 0.05 level.

Results

The results show that SNR is contrast dependent. It significantly rose as contrast increased. The areas under receiver-operating-characteristic curves (AUCs) indicating classification accuracy for all POAG cases in comparison with normal subjects were 0.790 (sensitivity 91.1%, specificity 69.7%) with the cutoff SNR of 0.85, and 0.706 (sensitivity 95.6%, specificity 51.5%) with the cutoff SNR of 1. The AUC of early glaucoma cases (EG) in comparison with normal subjects was 0.801 (sensitivity 93.3%, specificity 69.7%) with the cutoff SNR of 0.85, and 0.717 (sensitivity 97.8%, specificity 51.5%) with the cutoff SNR of 1.

Conclusion

icVEP has good diagnostic accuracy (high sensitivity and moderate specificity) in distinguishing early POAG patients from healthy subjects. It might be a promising device to use in conjunction with complementary functional and structural measures for early POAG detection.

     

Contrast adaptation in striate cortex of macaque

We have characterized the contrast-response relationships for simple and complex cells in striate cortex of macaque monkey, before and during adaptation to high-contrast sinusoidal gratings of the optimal spatial-frequency and orientation. Adaptation brings about systematic changes in the steepness of contrast-response curves and in the effective contrast of stimuli. Adaptation reduces the detectability of low-contrast gratings by almost a factor of three, but by extending the operating range of most cells it appears to improve the discriminability of high-contrast stimuli that previously gave rise to responses of saturating amplitude.     

Cone signal contributions to electroretinograms [correction of electrograms] in dichromats and trichromats

PURPOSE: To find out how the different cone types contribute to the electroretinogram (ERG) by quantifying the contribution of the signal pathways originating in the long (L-) and the middle (M-) wavelength-sensitive cones to the total ERG response amplitude and phase. METHODS: ERG response amplitudes and phases were measured to cone-isolating stimuli and to different combinations of L- and M-cone modulation. Conditions were chosen to exclude any contribution of the short wavelength-sensitive (S-) cones. The sensitivity of the ERG to the L and the M cones was defined as the cone contrast gain. RESULTS: In the present paper, a model is provided that describes the ERG contrast gains and ERG thresholds in dichromats and color normal trichromats. For the X-chromosome-linked dichromats, the contrast gains of only one cone type (either the L or the M cones) sufficed to describe the ERG thresholds for all stimulus conditions. Data suggest that the M-cone contrast gains of protanopes are larger than the L-cone contrast gains of deuteranopes. The response thresholds of the trichromats are modeled by assuming a vector summation of signals originating in the L and the M cones. Their L- and M-cone contrast gains are close to a linear interpolation of the data obtained from the dichromats. Nearly all trichromats had larger L- than M-cone contrast gains. Data from a large population of trichromats were examined to study the individual variations in cone weightings and in the phases of the cone pathway responses. CONCLUSIONS: The data strongly suggest that the missing cone type in dichromats is replaced by the remaining cone type. The mean L-cone to M-cone weighting ratio in trichromats was found to be approximately 4:1. But there is a substantial interindividual variability between trichromats. The response phases of the L- and the M-cone pathways can be reliably quantified using the response phases to the cone-isolating stimuli or using a vector addition of L- and M-cone signals.     

Signal and noise in P300 recordings to visual stimuli

The P300 of the event-related potential is typically obtained for infrequent target stimuli that are embedded in a sequence of frequent irrelevant stimuli. The P300 has been suggested as a marker of high-level cognitive processing and might be useful in ophthalmology to confirm the diagnosis of a functional disorder. However, typical P300 measurements require relatively lengthy recording sessions. It would therefore be desirable to minimize the required time and to maximize the signal-to-noise ratio by finding the optimal balance between parameters such as stimulus probability and the number of target trials or the recording time. This is different from previous studies, which assessed the amplitude only. We recorded event-related potentials to visual stimuli using standard oddball paradigms with various target frequencies ranging from 2:1 (target majority) to 1:16 (massive non-target majority). We compared the signal-to-noise ratios for a fixed number of target trials as well as for a fixed total recording time and assessed effects of the immediate stimulus history. As expected, P300 amplitudes depend strongly on target infrequency. This did not reach saturation within the range tested. For a given number of target trials, the signal-to-noise ratio also increases with target infrequency. For a given recording duration, the signal-to-noise ratio is optimal around 1:8. In the 1:4 condition, the signal-to-noise ratio can be improved by excluding trials that were preceded by a target trial.     

Differential contributions of magnocellular and parvocellular pathways to the contrast response of neurons in bush baby primary visual cortex (V1)

How neurons in the primary visual cortex (V1) of primates process parallel inputs from the magnocellular (M) and parvocellular (P) layers of the lateral geniculate nucleus (LGN) is not completely understood. To investigate whether signals from the two pathways are integrated in the cortex, we recorded contrast-response functions (CRFs) from 20 bush baby V1 neurons before, during, and after pharmacologically inactivating neural activity in either the contralateral LGN M or P layers. Inactivating the M layer reduced the responses of V1 neurons (n = 10) to all stimulus contrasts and significantly elevated (t = 8.15, P < 0.01) their average contrast threshold from 8.04 (+/- 4.1)% contrast to 22.46 (+/- 6.28)% contrast. M layer inactivation also significantly reduced (t = 4.06, P < 0.01) the average peak response amplitude. Inactivating the P layer did not elevate the average contrast threshold of V1 neurons (n = 10), but significantly reduced (t = 4.34, P < 0.01) their average peak response amplitude. These data demonstrate that input from the M pathway can account for the responses of V1 neurons to low stimulus contrasts and also contributes to responses to high stimulus contrasts. The P pathway appears to influence mainly the responses of V1 neurons to high stimulus contrasts. None of the cells in our sample, which included cells in all output layers of V1, appeared to receive input from only one pathway. These findings support the view that many V1 neurons integrate information about stimulus contrast carried by the LGN M and P pathways.