Isolation and interaction of ON and OFF pathways in human vision: contrast discrimination at pattern offset

Contrast adaptation improves spatial integration

The effects of contrast adaptation and contrast area summation (spatial integration) were investigated using a contrast discrimination task. The task consisted of a target of variable size, and a pedestal with a fixed base contrast. Discrimination performance was examined for a condition in which the pedestal size was fixed, equal to the largest target size, and for a condition in which the pedestal size matched the target size and thus varied with it. Repeated performance of the task produced rapid within-session improvements for both conditions. For stimuli with a matching size of target and pedestal, the performance improved only for the larger targets, indicating the development of spatial integration, which was initially absent for these stimuli. However, the improvements were mostly temporary, and were not fully retained between subsequent daily sessions. The temporary nature of the sensitivity gains implies that they resulted, at least in part, from rapid adaptation to the stimulus contrast. We suggest that adaptation decorrelates and thus reduces the spatial noise generated by a high-contrast pedestal, leading to improved spatial integration (area summation) and better contrast sensitivity. A decorrelation model successfully predicted our experimental results.

Rapid Motion Adaptation Reveals the Temporal Dynamics of Spatiotemporal Correlation between ON and OFF Pathways

At the early stages of visual processing, information is processed by two major thalamic pathways encoding brightness increments (ON) and decrements (OFF). Accumulating evidence suggests that these pathways interact and merge as early as in primary visual cortex. Using regular and reverse-phi motion in a rapid adaptation paradigm, we investigated the temporal dynamics of within and across pathway mechanisms for motion processing. When the adaptation duration was short (188 ms), reverse-phi and regular motion led to similar adaptation effects, suggesting that the information from the two pathways are combined efficiently at early-stages of motion processing. However, as the adaption duration was increased to 752 ms, reverse-phi and regular motion showed distinct adaptation effects depending on the test pattern used, either engaging spatiotemporal correlation between the same or opposite contrast polarities. Overall, these findings indicate that spatiotemporal correlation within and across ON-OFF pathways for motion processing can be selectively adapted, and support those models that integrate within and across pathway mechanisms for motion processing.

Visual detection under uncertainty operates via an early static, not late dynamic, non-linearity

SIGNALS IN THE ENVIRONMENT ARE RARELY SPECIFIED EXACTLY: our visual system may know what to look for (e.g., a specific face), but not its exact configuration (e.g., where in the room, or in what orientation). Uncertainty, and the ability to deal with it, is a fundamental aspect of visual processing. The MAX model is the current gold standard for describing how human vision handles uncertainty: of all possible configurations for the signal, the observer chooses the one corresponding to the template associated with the largest response. We propose an alternative model in which the MAX operation, which is a dynamic non-linearity (depends on multiple inputs from several stimulus locations) and happens after the input stimulus has been matched to the possible templates, is replaced by an early static non-linearity (depends only on one input corresponding to one stimulus location) which is applied before template matching. By exploiting an integrated set of analytical and experimental tools, we show that this model is able to account for a number of empirical observations otherwise unaccounted for by the MAX model, and is more robust with respect to the realistic limitations imposed by the available neural hardware. We then discuss how these results, currently restricted to a simple visual detection task, may extend to a wider range of problems in sensory processing.

Sensitivity to stimulus onset and offset in the S-cone pathway

Previous work [Vassilev, capital A, Cyrillic., Mihaylova, M., Racheva, K., Zlatkova, M., & Anderson, R. S. (2003). Spatial summation of S-cone ON and OFF signals: Effects of retinal eccentricity. Vision Research, 43, 2875-2884; Vassilev, A., Zlatkova, M., Krumov, A., & Schaumberger, M. (2000). Spatial summation of blue-on yellow light increments and decrements in human vision. Vision Research, 40, 989-1000] has shown that spatial summation of brief S-cone selective stimuli depends on their polarity, increments or decrements, suggesting involvement of S-ON and OFF pathways, respectively. This assumption was tested in two experiments using a modified two-color threshold method of Stiles to selectively stimulate the S-cones. In the first experiment we measured detection threshold for small 100ms S-cone selective increments and decrements presented within three types of temporal window, rectangular, ramp onset/rapid offset and rapid onset/ramp offset. The ramp-onset threshold was higher than the ramp-offset threshold regardless of stimulus sign. In the second experiment we measured reaction time (RT) with near-threshold stimuli spatially coincident with the background to avoid spatial contrast. RT distribution for S-cone selective 500ms increments and decrements was unimodal and followed stimulus onset. An increase of stimulus duration to 1000 and 2000ms resulted in the appearance of responses following stimulus offset. The results suggest that, for brief S-cone selective increments or decrements, the human visual system is more sensitive to stimulus onset than to stimulus offset. Only for longer stimuli is the offset important, probably due to slow adaptation at a postreceptoral level.

Medidas psicofísicas e eletrofisiológicas da função visual do recém nascido: uma revisão

O sistema visual apresenta muitas funções ao nascimento. O processo de amadurecimento destas funções demanda um tempo variado. Neste trabalho, inicialmente descreveremos como a psicofísica e a eletrofisiologia visual tem colaborado para a medida e o estudo do desenvolvimento de três funções visuais: acuidade visual, sensibilidade ao contraste e visão de cores. Num segundo momento, discutimos sobre como a medida e o desenvolvimento destas funções podem estar prejudicados em patologias que afetam o sistema visual, como a prematuridade e a paralisia cerebral.

Metacontrast masking is specific to luminance polarity

A 1 degrees -spot was flashed up on a screen, followed by a snugly fitting annular mask. We measured the amount of masking as a function of stimulus luminance. The surround was always mid-gray, the masking ring was either black or white, and the luminance of the spot target ranged from 0% to 100% of white in 4% steps. Observers reported the apparent lightness of the masked spot by adjusting a matching spot.

RESULTS

A black annular mask made all spots that were darker than the gray surround appear to be transparent, that is, of the same luminance as the surround (complete masking). The black ring had virtually no masking effect on spots that were lighter than the surround. Conversely, a white ring made all spots that were lighter than the gray surround look apparently the same luminance as the surround (complete masking), but had virtually no masking effect on spots that were darker than the surround. In summary, a black ring masked spatial decrements but not increments, whilst a white ring masked spatial increments but not decrements. Thus masking occurred only when the spot and the ring had the same luminance polarity. This same-polarity masking still occurred when the target spot was larger than the 'donut hole' of the masking ring, so that the target and ring partly overlapped. This ruled out simple edge-cancellation theories. Instead, masking disrupts the filling-in process that normally propagates inward from the edges of a spot [Vision Res. 31 (7-8) (1991) 1221]. We conclude that metacontrast masking occurs within, but not between, separate visual ON and OFF pathways.

Attention during adaptation weakens negative afterimages

The effect of attention during adaptation on subsequent negative afterimages was examined. One of 2 overlapped outline figures was attended during a 7-10-s adaptation period. When the figures were readily perceptually segregated (on the basis of color or motion), the subsequent afterimages were initially weaker for the previously attended figure. This effect was confirmed by demonstrations that the onset of a single afterimage was delayed when an afterimage inducer was attended during adaptation compared with when a central digit stream or an overlapped (brightness-balanced) figure that did not generate an afterimage was attended. The attention effect was further confirmed using a criterion-independent (dot-integration) paradigm. The fact that selective attention during adaptation weakened or delayed afterimages suggests that attention primarily facilitates the adaptation of polarity-independent processes that modulate the visibility of afterimages rather than facilitating the adaptation of polarity-selective processes that mediate the formation of afterimages.

ON-pathway disturbance in two siblings

PURPOSE

To present two clinical cases diagnosed with predominant cone dystrophy and demonstrating early disturbance in the on-centre bipolar cells (ON-pathway).

METHODS

Electrophysiological findings are presented in two siblings with predominant cone dystrophy. The subjects showed no remarkable ophthalmoscopic or fluorescein angiographic retinal changes, but demonstrated progressive visual disturbance during their 20s.

RESULTS

The electroretinograms (ERGs) showed reduced dark-adapted responses but the positive component of the photopic ERG was absent. Response to 30 Hz flicker was severely reduced. Electroretinograms elicited by long-duration stimuli showed a loss of the b-wave, and the off-response was slightly reduced. In both patients, multifocal ERGs (m-ERGs) were more reduced within the central 10 degrees, where the ON-pathway is normally a major contributor.

CONCLUSION

We conclude that these patients may be affected by an abnormality of the synapses of the cone receptors and that their decrease in vision might, at least initially, be due to selective ON-pathway dysfunction.

Parkinson's disease changes the balance of onset and offset visual responses: an evoked potential study

OBJECTIVES

We investigated whether the transient pattern onset and offset visual evoked potential (VEP) can distinguish between patients with Parkinson's disease (PD) and normal subjects.

METHODS

Two horizontal sinusoidal gratings differing in spatial frequency, i.e. 1 and 4 cycles per degree, were presented to 17 patients with PD and 16 age-matched control subjects. We analyzed the responses in the time-domain and measured the latencies and amplitudes of N1 and P1 to the onset and the offset of the stimulus; we also derived the measures of offset N1 and P1 amplitude responses 'normalized' to onset N1 and P1 amplitude values, respectively (amplitude ratios).

RESULTS

Absolute and normalized offset P1 amplitude is a distinguishing feature of PD patients from controls. Offset P1 amplitude was significantly larger in PD patients than in controls, particularly to the lower spatial frequency stimulus (P<0.01 for absolute and P<0.001 for normalized values, respectively).

CONCLUSIONS

We conclude that the pattern onset/offset VEP amplitude provides a simple measure to evaluate visual processing deficits in PD and could contribute to an understanding of the pathophysiology of these changes.

On- and off-responses of the photopic electroretinograms in X-linked juvenile retinoschisis

PURPOSE

To examine the physiologic condition of the middle retinal layer of patients with X-linked juvenile retinoschisis (xlRS) by studying the on- and off-responses of the photopic electroretinograms (ERGs).

METHODS

Eleven unrelated Japanese men (mean age; 24.9 +/- 7.6 years) who were clinically diagnosed with xlRS and molecularly confirmed as having XLRS1 mutations were investigated. For the photopic ERGs, the a-, b- and d-wave amplitudes elicited by long duration stimuli were recorded, and the responses from the xlRS patients were compared to those recorded from normal subjects (n = 14, mean age, 27.5 +/- 4.5 years). We also examined the relationship between the photopic ERG responses and the genotype.

RESULTS

No significant difference was found between the a- and d-wave amplitudes in the xlRS patients (34.2 +/- 8.7 microV, 52.5 +/- 10.4 microV, respectively), and those in normal subjects (40.4 +/- 10.3 microV, 44.7 +/- 6.3 microV, respectively). The mean b-wave amplitude in the xlRS patients was significantly smaller (10.5 +/- 7.7 microV) than the mean of normal subjects (46.4 +/- 10.2 microV) (P < 0.0001). No significant correlation was found between the ERG responses and the locus of the mutation.

CONCLUSION

The photopic ERG demonstrated considerable impairment of the on-pathway arising from an abnormality of the on-bipolar cells or possibly secondary to Müller cell abnormality in xlRS.

Increment and decrement detection on temporally modulated fields

Increment and decrement probe thresholds were measured during the presentation of two types of temporal masking stimuli. In Experiment 1, we measured thresholds for increment or decrement rectangular probes presented during the presentation of an increment or decrement Gaussian masking stimulus. We find that thresholds are higher when the probe and the Gaussian mask are of the same sign (e. g. both increments). However, both types of Gaussian mask raised increment and decrement probe thresholds above steady state conditions. In Experiment 2, we presented increment or decrement probes at one of eight possible phases of a 1 Hz luminance-modulated sine wave. For both increment and decrement probes, threshold variation with phase is non-sinusoidal in shape, but increment and decrement probe thresholds vary as a function of the sinusoid phase. These experiments show that increment and decrement thresholds vary as a function of the adaptation state of the visual system, and as a function of the direction of change in the adaptation state. Data from both experiments are discussed in terms of a recent neurophysiological model [Hood & Graham (1998). Threshold fluctuations on temporally modulated backgrounds: a possible physiological explanation based upon a recent computational model. Visual Neuroscience, 15 (5), 957-967]. We find that the predicted ON- and OFF-pathway responses do not correlate in a straightforward manner with the psychophysical thresholds, suggesting that detection of increment and decrement probes may not be performed exclusively by one pathway. Our data have implications for modeling visual performance under conditions where visual adaptation is dynamic, such as when scanning complex images or natural scenes.

Flicker adaptation can be explained by probability summation between ON- and OFF-mechanisms

This study considered whether the dynamics of flicker adaptation can be explained in terms of probability summation over time between independent ON- and OFF-processors. Foveal thresholds were measured for flickering probes, and for static ON- and OFF-probes that have durations consistent with the flicker duty cycle. Thresholds were obtained using a probe-flash stimulus onset asynchrony paradigm. The outcome of the experiment suggests that flicker thresholds are lower than the component ON- and OFF-thresholds, and that the flicker response can be predicted by assuming probability summation between the ON- and OFF-mechanisms.

Current source density analysis of ON/OFF channels in the frog optic tectum

In the vertebrate retina, it is well known that an ON/OFF dichotomy is present. In other words, ON-center and OFF-center cells participate in segregated pathways morphologically and physiologically. However, there is no doubt that integration of both channels is necessary to generate the complicated response properties of visual neurons in higher optic centers. So far, functional organization of the ON and OFF channels in the optic centers has not been demonstrated at the level of neuronal populations. In this review article, we summarize our experimental approaches to demonstrate functional organization of the ON and OFF channels using current source density (CSD) analysis in the frog optic tectum. First, we show that one-dimensional CSD analysis, assuming constant conductivity, is applicable in the tectal laminated structure. The CSD depth profile of a response to electrical stimulation of the optic tract is composed of three current sinks (A, B, and D) in the retinorecipient layers and two current sinks (C and E) below those layers. This result is in agreement with previous morphological and physiological findings, and shows that CSD analysis is very useful to demonstrate the flow of visual information processing. Second, CSD analysis of tectal responses evoked by diffuse light ON and OFF stimuli reveals obviously different distributions of synaptic activity in the laminar structure. Two or three current sinks (I, II and III) are generated in response to ON stimulation only in the retinorecipient layers, while up to six current sinks (IV, V, VI, VII, VIII and IX) to OFF stimulation throughout the tectal layers. Based on well known properties of retinal ganglion cells of the frog, possible neuronal mechanisms underlying each current sinks and their functional roles in visually guided behavior are considered.

Pattern detection in the presence of maskers that differ in spatial phase and temporal offset: threshold measurements and a model

Four experiments are described in which brief Gabor patterns are detected in the presence of full-field gratings or Gabor patterns that are superimposed in space, but vary in spatial phase and temporal offset (SOA). E1: Threshold versus masker contrast (TvC) functions were determined for relative phases of 0, 90, 180 and 270 degrees at SOA = 0. For 0 degree relative phase, TvC functions decrease (facilitation) and then increase (masking) as contrast increases. For 90 degrees, there is little or no facilitation and thresholds increase with masker contrast. For 180 degrees, the form of the TvC function varies with observer and conditions. E2: Like E1, except that maskers are Gabor patterns. TvC functions are similar in form to those for full-field maskers, but there is less masking. E3: Forward masking. TvC functions were determined for relative phases of 0, 90, and 180 degrees at SOA = -33 ms. The forms of the TvC functions for 0 and 180 degrees are reversed relative to those at SOA = 0. E4: TvP (threshold versus phase) functions were determined for SOA's of -100, -67, -33, 0 and 33 ms at a constant masker contrast of 0.063. Masking occurs at all relative phases. For simultaneous and backward masking, the threshold is minimum for a relative phase of 0 and maximum at 180 degrees. For forward masking, the form of the function is inverted. A model of pattern masking and facilitation (Foley, J. M. (1994a) Journal of the Optical Society of America A, 11, 1710-1719) is extended to account for masker phase and SOA effects. The model assumes four mechanisms tuned to phases 90 degrees apart, and divisively inhibited by stimuli of all phases. Performance depends on the detection strategy of the observer.

Nonlinearities of near-threshold contrast transduction

The existence of analytic threshold nonlinearities was probed with 2AFC incremental threshold functions for both local and extended test patterns on stationary matched pedestals of the same and opposite sign. In contrast to the facilitation effect with same-sign pedestals, sensitivity with opposite-sign pedestals first deteriorated up to the mask detection level, abruptly improved and then deteriorated again. Analytic solutions for the transducer function with additive noise were derived to account for the incremental data in all conditions. The results for positive difference-of-Gaussian (DoG) stimuli (whose increment made the central spot lighter) and for 10 c deg-1 Gabor stimuli were consistent with accurate hard-threshold behavior with best-fitting d' powers from 17 to 358. The 10 c deg-1 data further implied that contrast gain control was operating throughout the subthreshold range. The results for negative DoGs (whose increment corresponds to the darkening of the central spot) and 2 c deg-1 Gabor profiles were consistent with mild nonlinearities having d' powers of 1.6-3. Significant differences between the nonlinearities for positive and negative DoGs indicate that only a small portion, if any, of the near-threshold nonlinearity could be attributed to uncertainty. Our analysis suggests that, with low spatial frequency gratings, detection was based on those bars that become darker; with high-frequency gratings, on the bars that become brighter.

S-cone signals to temporal OFF-channels: asymmetrical connections to postreceptoral chromatic mechanisms

Psychophysical tests of S-cone contributions to temporal ON- and OFF-channels were conducted. Detection thresholds for S-cone modulation were measured with two kinds of test stimuli presented on a CRT: a rapid-on sawtooth test and a rapid-off sawtooth test, assumed to be detected differentially by temporal ON- and OFF-channels, respectively. S-cone related ON- and OFF-temporal responses were separated by adapting for 5 min to 1 Hz monochromatic (420, 440, 450, 540, or 650 nm in separate sessions) sawtooth flicker presented in Maxwellian view. Circular test stimuli, with a sawtooth temporal profile and a Gaussian spatial taper, were presented for 1 s in one of four quadrants 1.0 degree from a central fixation point. A four-alternative forced-choice method combined with a double-staircase procedure was used to determine ON- and OFF-thresholds in the same session. Following adaptation, the threshold elevation was greater if the polarity of the test stimulus was the same as the polarity of the sawtooth adaptation flicker, consistent with separate ON- and OFF-responses from S-cones. This asymmetrical pattern was obtained, however, only when the adaptation stimuli appeared blue with a little redness. When the adaptation flicker had a clear reddish hue component, the threshold elevation did not depend on the polarity of the sawtooth test stimuli. These results are consistent with a model in which OFF-signals originating from S cones are maintained by a postreceptoral mechanism signaling redness, but not by a postreceptoral chromatic mechanism signaling blueness.

Spatiotemporal impulse response and cortical magnification

According to a model of the spatiotemporal weighting function (Manahilov, V. Spatiotemporal visual response at suprathreshold stimuli. Vision Research, 1995, 35, 227-237; and Triphasic temporal impulse responses and Mach bands in time. Vision Research, 38, 447-458) the waveform of the temporal-impulse response and the cortical spread of the spatial-impulse response should not depend on the retinal site of stimulation. To verify these model predictions, the spatiotemporal responses to brief near-threshold lines presented in the fovea and the near retinal periphery were studied. The effect of an inducing stimulus on the threshold for pattern detection of a test stimulus was measured, assuming that the pattern-detection threshold was determined by the test peak response. The spatial spread of the line response expressed in visual-field units was increased with eccentricity. The temporal-impulse responses to foveal and peripheral stimuli were similar. The model of the weighting function was used to evaluate the relative magnification factor for the retinal location tested. The calculated cortical spatial-impulse responses did not depend on the stimulation site. The data obtained are in line with the cortical magnification theory of peripheral vision.

Dynamic contrast perception assessed by pattern masking

The perceived contrast of a pulsed grating varies markedly with the exposure duration and spatial frequency of the grating. We studied dynamic changes in perceived grating contrast with a pattern-masking paradigm. We measured masking of a brief, localized test pattern (a D6 stimulus, 30 ms in duration) by fixed-contrast cosine grating patterns of varying duration (50-500 ms). The cosine mask pattern had spatial frequency of either 1 or 6 cycles per degree (cpd) at a contrast of 0.3. The D6 test pattern was centered on a light bar of the mask and was either positive peak contrast (same-polarity test and mask) or negative peak contrast (opposite-polarity test and mask). In Experiment 1, the test and mask had simultaneous onset. With a 6-cpd mask, the same-polarity test-threshold elevation versus mask-duration function increases monotonically. For a 1-cpd mask, the same-polarity threshold-mask-duration function is nonmonotonic, with peak masking effect produced by a grating pulse of 80-100 ms. These masking effects are closely congruent with known dynamic contrast effects. With negative tests, masking-duration functions are elevated from same-polarity functions and are essentially similar in shape for 1- and 6-cpd masks. The elevated thresholds suggest inhibitory interaction between ON and OFF pathways, with a similar time course across spatial frequency. In Experiment 2, the D6 test was delayed from mask onset by 33 ms. Positive contrasts only were employed. For 1-cpd stimuli, the delay of test greatly reduced masking at all mask durations and eliminated the nonmonotonic function. This suggests that for low-spatial-frequency patterns, perceived contrast is determined by an early peak component of the neural response. But for 6-cpd stimuli, masking of the delayed test was somewhat greater at all mask durations, consistent with a gradually increasing underlying neural response to the grating. Finally, in Experiment 3, same-polarity masking effects at both spatial frequencies were replicated with negative-contrast test and mask (OFF pathway mediation). This indicates that the ON and OFF pathways have similar response dynamics.

Dynamics of adaptation at high luminances: adaptation is faster after luminance decrements than after luminance increments

As is well known, dark adaptation in the human visual system is much slower than is recovery from darkness. We show that at high photopic luminances the situation is exactly opposite. First, we study detection thresholds for a small light flash, at various delays from decrement and increment steps in background luminance. Light adaptation is nearly complete within 100 ms after luminance decrements but takes much longer after luminance increments. Second, we compare sensitivity after equally visible pulses or steps in the adaptation luminance and find that detectability is initially the same but recovers much faster for pulses than for increment steps. This suggests that, whereas any residual threshold elevation after a step shows the incomplete luminance adaptation, the initial threshold elevation is caused by the temporal contrast of the background steps and pulses. This hypothesis is further substantiated in a third experiment, whereby we show that manipulating the contrast of a transition between luminances affects only the initial part of the threshold curve, and not later stages.