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.     

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.     

Two expressions of "surround suppression" in V1 that arise independent of cortical mechanisms of suppression

The preferred stimulus size of a V1 neuron decreases with increases in stimulus contrast. It has been supposed that stimulus contrast is the primary determinant of such spatial summation in V1 cells, though the extent to which it depends on other stimulus attributes such as orientation and spatial frequency remains untested. We investigated this by recording from single cells in V1 of anaesthetized cats and monkeys, measuring size-tuning curves for high-contrast drifting gratings of optimal spatial configuration, and comparing these curves with those obtained at lower contrast or at sub-optimal orientations or spatial frequencies. For drifting gratings of optimal spatial configuration, lower contrasts produced less surround suppression resulting in increases in preferred size. High contrast gratings of sub-optimal spatial configuration produced more surround suppression than optimal low-contrast gratings, and as much or more surround suppression than optimal high-contrast gratings. For sub-optimal spatial frequencies, preferred size was similar to that for the optimal high-contrast stimulus, whereas for sub-optimal orientations, preferred size was smaller than that for the optimal high-contrast stimulus. These results indicate that, while contrast is an important determinant of spatial summation in V1, it is not the only determinant. Simulation of these experiments on a cortical receptive field modeled as a Gabor revealed that the small preferred sizes observed for non-preferred stimuli could result simply from linear filtering by the classical receptive field. Further simulations show that surround suppression in retinal ganglion cells and LGN cells can be propagated to neurons in V1, though certain properties of the surround seen in cortex indicate that it is not solely inherited from earlier stages of processing.     

Temporal dynamics of contrast gain in single cells of the cat striate cortex

The response amplitude of cat striate cortical cells is usually reduced after exposure to high-contrast stimuli. The temporal characteristics and contrast sensitivity of this phenomenon were explored by stimulating cortical cells with drifting gratings in which contrast sequentially incremented and decremented in stepwise fashion over time. All responses showed a clear hysteresis, in which contrast gain dropped on average 0.36 log unit and then returned to baseline values within 60 s. Noticeable gain adjustments were seen in as little as 3 s and with peak contrasts as low as 3%. Contrast adaptation was absent in responses from LGN cells. Adaptation was found to depend on temporal frequency of stimulation, with greater and more rapid adaptation at higher temporal frequencies. Two different tests showed that the mechanism controlling response reduction was influenced primarily by stimulus contrast rather than response amplitude. These results support the existence of a rapid and sensitive cortically based system that normalizes the output of cortical cells as a function of local mean contrast. Control of the adaptation appears to arise at least in part across a population of cells, which is consistent with the idea that the gain control serves to limit the information converging from many cells onto subsequent processing areas.     

Effects of surround motion on receptive-field gain and structure in area 17 of the cat

Modulation of responses elicited by moving bars within the classical receptive fields (CRF) of cat area 17 neurons were studied as a function of the direction and velocity of drifting gratings in the surrounds. Several different types of modulation were observed; collectively, the responses of most cells, both simple and complex, were strongly modulated by surround motion. None of these cells appear to signal relative velocity between the CRF and its surround. The gain and spatiotemporal structure of the CRF mechanism were estimated using contrast-response functions and reverse correlation with spatiotemporal ternary white noise, respectively. These measurements were made in the presence of surround gratings shown to significantly modify responses elicited from the CRF. In all cases, the gain of the CRF mechanism was driven up or down relative to controls but the receptive-field structure did not change in any way. We conclude that neurons in cat area 17 act like scalable filters, meaning that their gains can be adjusted by stimuli in the surrounds without altering the properties of the CRF. This was verified by showing that velocity tuning curves were also unmodified by stimuli in the surround that did change the gain. Based in part on these data, we discuss the notion that primary visual cortex makes use of a double-opponent mechanism for the representation of local discontinuities in motion and orientation.     

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.     

Systematic misestimation in a vernier task arising from contrast mismatch

Luminance signals mediated by the magnocellular (MC) pathway play an important role in vernier tasks. MC ganglion cells show a phase advance in their responses to sinusoidal stimuli with increasing contrast due to contrast gain control mechanisms. If the phase information in MC ganglion cell responses were utilized by central mechanisms in vernier tasks, one might expect systematic errors caused by the phase advance. This systematic error may contribute to the contrast paradox phenomenon, where vernier performance deteriorates, rather than improves, when only one of the target pair increases in contrast. Vernier psychometric functions for a pair of gratings of mismatched contrast were measured to seek such misestimation. In associated electrophysiological experiments, MC and parvocellular (PC) ganglion cells' responses to similar stimuli were measured to provide a physiological reference. The psychophysical experiments show that a high-contrast grating is perceived as phase advanced in the drift direction compared to a low-contrast grating, especially at a high drift rate (8 Hz). The size of the phase advance was comparable to that seen in MC cells under similar stimulus conditions. These results are consistent with the MC pathway supporting vernier performance with achromatic gratings. The shifts in vernier psychometric functions were negligible for pairs of chromatic gratings under the conditions tested here, consistent with the lack of phase advance both in responses of PC ganglion cells and in frequency-doubled chromatic responses of MC ganglion cells.     

Visual evoked potentials and magnocellular and parvocellular segregation

We have measured visual evoked potentials (VEPs) to luminance-modulated, square-wave alternating, 3-deg homogeneous disks for stimulus frequencies ranging from 1 Hz to 16.7 Hz. The aim of the study was to determine the range of frequencies at which we could reproduce the two-branched contrast-response (C-R) curves we had seen at 1 Hz (Valberg & Rudvin, 1997) and which we interpreted as magnocellular (MC) and parvocellular (PC) segregation. Low-contrast stimuli elicited relatively simple responses to luminance increments resulting in waveforms that may be the signatures of inputs from magnocellular channels to the visual cortex. At all frequencies, the C-R curves of the main waveforms were characterized by a steep slope at low contrasts and a leveling off at 10%-20% Michelson contrast. This was typically followed by an abrupt increase in slope at higher contrasts, giving a distinctive two-branched C-R curve. On the assumption that the low-contrast, high-gain branch reflects the responsivity of magnocellular-pathway inputs to the cortex, the high-contrast branch may be attributed to additional parvocellular activation. While a two-branched curve was maintained for frequencies up to 8 Hz, the high-contrast response was significantly compromised at 16.7 Hz, revealing a differential low-pass filtering. A model decomposing the measured VEP response into two separate C-R curves yielded a difference in sensitivity of the putative MC- and PC-mediated response that, when plotted as a function of frequency, followed a trend similar to that found for single cells. Due to temporal overlap of responses, the MC and PC contributions to the waveforms were hard to distinguish in the transient VEP. However, curves of time-to-peak (delay) as a function of contrast often went through a minimum before the high-contrast gain increase of the corresponding C-R curve, supporting the notion of a recruitment of new cell ensembles in the transition from low to high contrasts.     

The spatial characteristics of plaid-form-selective mechanisms

Rather little is known about the mechanisms that combine the outputs of orientation- and spatial frequency-selective channels. These can be studied by measuring the selective adaptation to compound stimuli over and above that expected from adaptation to the components alone (Peirce & Taylor, 2006). Here we investigated the contrast- and spatial phase-dependency of such mechanisms. A plaid was adapted in one visual hemi-field, while its constituent gratings were simultaneously adapted in the other hemi-field. Plaid-selective adaptation was most apparent with high-contrast probes, whereas adaptation to the component grating stimuli dominated at low contrasts. The mechanisms underlying this plaid-selective adaptation also appear to be insensitive to the spatial phase of the probes relative to the adaptor, whereas we find a clear phase-dependency for suprathreshold contrast adaptation to grating stimuli. These findings suggest that the visual system is equipped with mechanisms that conduct a global analysis of the plaid pattern, which are likely derived from the non-linear outputs of V1 complex cells.     

Effects on grating detection of vertically displaced peripheral gratings

Contrast threshold for a sinusoidal target in the simultaneous presence of the vertically displaced peripheral gratings was measured as a function of the peripheral contrast and the separation. When the signal and peripheral gratings were in phase, the low-contrast peripheral gratings produced the improvement in signal threshold, while the high-contrast gratings produced threshold elevation. The facilitation effect was extended to the separation equating to 10 grating cycles, whereas the inhibition effect was restricted to the region near the border. When the two gratings were out of phase, the low-contrast peripheral gratings produced threshold elevation, while the high-contrast gratings exerted little effect on signal threshold. The results were explained in terms of spatial summation and lateral inhibition.     

Similarities and dissimilarities between pattern VEPs and motion VEPs

The contrast response functions (CRF) of pattern-appearance and motion-onset VEPs for periodic stimuli (gratings) were compared. The CRF for pattern-appearance is accelerative for the P100 component and compressive for the N200 component. Contrary to these results, the CRF for motion-onset shows an almost negligible slope for both components within the contrast range tested (0.5–64%). To better isolate the neural contributions to these different VEP components, we studied the effects of prior adaptation to stationary and moving gratings. Adaptation to stationary gratings has no effect on both VEP components for motion-onset and the P100 component for pattern-appearance, but did reduce the amplitude of the N200 for pattern-appearance. Adaptation to slow (1 deg/s) and fast (4 deg/s) gratings left the P100 amplitudes unaltered, while it significantly reduced the N200 amplitudes for both pattern-appearance and motion-onset. These results suggest that the N200 component of the motion-onset VEP is generated by motion-dependent neurons, whereas the same component for pattern-appearance arises from contrast-dependent neurons. The observed differences between P100 and N200 components appear to reflect the activity of both transient and sustained neural mechanisms. This revised version was published online in July 2006 with corrections to the Cover Date.     

Grating detection and identification: comparison of postadaptation reaction times

The effect of pattern adaptation on grating detection and grating identification was investigated by the method of reaction time. The test and adapting stimuli were sinusoidal vertical gratings. Adaptation caused a spatial frequency-selective increase of the mean detection reaction times. However, the mean identification reaction times were delayed regardless of the adapting spatial frequency. The similarity of the present results with some data about visually evoked potentials is discussed.     

Contrast response of temporally sparse dichoptic multifocal visual evoked potentials

Temporally sparse stimuli have been found to produce larger multifocal visual evoked potentials than rapid contrast-reversal stimuli. We compared the contrast-response functions of conventional contrast-reversing (CR) stimuli and three grades of temporally sparse stimuli, examining both the changes in response amplitude and signal-to-noise ratio (SNR). All stimuli were presented dichoptically to normal adult human subjects. One stimulus variant, the slowest pattern pulse, had interleaved monocular and binocular stimuli. Response amplitudes and SNRs were similar for all stimuli at contrast 0.4 but grew faster with increasing contrast for the sparser stimuli. The best sparse stimulus provided an SNR improvement that corresponded to a recording time improvement of 2.6 times relative to that required for contrast reversing stimuli. Multiple regression of log-transformed response metrics characterized the contrast-response functions by fitting power-law relationships. The exponents for the two sparsest stimuli were significantly larger (P < 0.001) than for the CR stimuli, as were the mean response amplitudes and signal-to-noise ratios for these stimuli. The contrast-dependent response enhancement is discussed with respect to the possible influences of rapid retinal contrast gain control, or intracortical and cortico-geniculate feedback.     

The range of spatial frequency contingent color aftereffects

Checkerboard stimuli contain two-dimensional Fourier components oriented 45 degrees from edges of the individual checks. Adaptation to such stimuli and testing with rectilinear gratings showed that contingent color aftereffects are associated with the Fourier components rather than the edges of such patterned stimuli at high spatial frequencies. With low spatial frequency (0.8 c/d) contingent color aftereffects were aligned with the edges rather than the Fourier components. These data, obtained with a color cancellation procedure, are in agreement with previous reports of spatial frequency-contingent color aftereffects and indicate the range over which such effects can be obtained.     

Development of visual inhibitory interactions in kittens.

This study was designed to monitor the development of inhibitory interactions elicited in the cat visual system by oriented visual stimuli. Steady-state visual-evoked potentials (VEPs) were recorded from the scalp of 11 behaving and alert kittens while they viewed contrast-reversed sinusoidal gratings. In adult cats, the form of VEP contrast-response curves (the amplitude of second harmonic modulation as a function of stimulus contrast) was modified by superimposing a mask grating on the test. Parallel masks displaced the curves to a higher contrast region (probably via contrast gain-control mechanisms), increasing contrast threshold without affecting the slope of the curve. Orthogonal gratings, on the other hand, decrease the slope of the curve without affecting threshold (so called cross-orientation inhibition: Morrone et al., 1981). These effects are similar to those previously reported in human VEPs (Morrone & Burr, 1986; Burr & Morrone, 1987) and single cortical cat cells (Morrone et al., 1982). For young kittens of 20 days, the orthogonal mask had no effect whatsoever on the response curves, and the effect of the parallel mask was much less than for adult cats. At about 40 days, the orthogonal mask began to attenuate responses multiplicatively, and by 50 days the amount of multiplicative attenuation had reached adult levels. The effect of the parallel mask (as indicated by the increase in threshold elevation) increased gradually from 20-50 days. The results are consistent with the existence of at least two types of inhibition in cat visual neurones that develop at different rates.     

Position-based motion perception for color and texture stimuli: effects of contrast and speed

Motion can be perceived either through low-level, motion-energy detection or through tracking the change in position of features. Previously we have shown that, while luminance-based motion likely is detected with velocity-sensitive motion-energy units, patterns defined by texture or binocular disparity ('second-order' stimuli) were tracked by a position-sensitive mechanism (Seiffert & Cavanagh (1998) Vision Research, 38, 3569-3582). Here, we use the same technique, measuring motion amplitude thresholds of oscillating gratings over a range of temporal frequencies and find that the motion of low-contrast equiluminant red/green gratings is also detected with position tracking. In addition, we find that as contrast or speed increases these results change: high-contrast or high-speed equiluminant color or texture-based motion is detected by velocity-sensitive mechanisms. These results help resolve the dispute over the processes which detect the motion of non-luminance based stimuli. Both systems are available, but their relative efficiency changes as a function of contrast and speed. A position-tracking process is more sensitive at low contrasts and low speeds whereas a motion-energy system is more sensitive at high contrasts and high speeds.     

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.     

Temporal asymmetry in motion masking: a shortening of the temporal impulse response function

Morgan and Chubb observed a striking temporal asymmetry in motion masking (Vis. Res. 39 (1999) 4217). Motion was produced with a two-frame sequence of gratings presented in spatial quadrature phase; the second grating (100 ms) was presented immediately after the first grating (100 ms), with no temporal overlap. The contrast threshold for detecting the direction of motion of the stimulus pair was facilitated when the first grating was of low-contrast and the second grating was of high-contrast, but strong masking occurred when the order was reversed, so the high-contrast grating came first. We replicated this result, but showed that the masking mostly disappeared when the two gratings temporally overlapped only slightly. The high sensitivity to the precise temporal pattern of the stimulus can be explained by a small temporal 'shortening' of the temporal impulse response function (IRF) as stimulus contrast is increased. The IRF is biphasic with a negative inhibitory lobe. When the first grating has high-contrast, its flash response (owing to the shortening of the IRF) may be in a fairly strong negative phase by the time that the positive response to the second, lower-contrast grating has reached appreciable strength--this reduces the magnitude of the motion signal generated by the two flashes and can account for the masking. A shortening of the IRF with increased contrast (a nonlinearity) is supported by psychophysical studies in humans and by recordings of magnocellular retinal ganglion cells in macaque, and the present results bolster this concept.     

Two-dimensional optokinetic nystagmus induced by moving plaids and texture boundaries. Evidence for multiple visual pathways.

Horizontal and vertical components of optokinetic nystagmus (OKN) were measured using the magnetic search coil technique in normal human adults during presentation of simple and complex moving patterns. Simple patterns were gratings moving horizontally and obliquely. Complex moving patterns consisted of plaids formed by superimposed oblique motion of two sets of gratings or of illusory contours formed by offset discontinuities in gratings. Slow-phase OKN gains (eye velocity divided by stimulus velocity) induced by high-contrast type I and type II plaids were comparable with those generated by one-dimensional moving gratings. The axis of OKN for high-contrast plaids was along the resultant direction determined by the intersection-of-constraints rule and not along any component. With low-contrast presentations, OKN induced by type I patterns remained in the resultant direction, but the OKN direction induced by type II patterns was biased toward the components' directions. The OKN generated by texture boundaries embedded in real pattern motion was measured for motion of illusory contours having systematically varying directions. The gain of OKN induced by real motion was independent of the direction of illusory contour motion, but the gain to illusory contour motion decreased with increasing contour angles. All these results suggest that input signals for driving the optokinetic system come from visual areas extracting higher order two-dimensional motion information.     

Long-lasting afterimages caused by neural adaptation

If the retinal location of a sinusoidal grating alternates back and forth in half-period steps, the temporal average stimulation is equivalent to that produced by a homogeneous field of light. Adaptation to this stimulus causes strong patterned afterimages, however, and they last longer than 2 min in the dark. The afterimages appear as gratings but at twice the spatial frequency of the adaptation gratings even though only the fundamental frequency is seen during adaptation. The appearance of this illusory grating afterimage indicates that a nonlinear intensity scaling precedes the site of some afterimages, the origin of which is neural and not photochemical. The neural origin was confirmed by the results of various experiments in which the eye was pressure blinded during adaptation. Blinding eliminated all negative afterimages otherwise caused by weakly bleaching primary stimuli but left positive afterimages intact. Two classes of long-lasting afterimages, the “sensitivity images” and “bleaching images”, are required to explain our results.