Processing of scotopic increments and decrements.

Rod and cone photoreceptors send their signals to ON- and OFF-retinal ganglion cells through different pathways in the primate retina. We hypothesized that increments and decrements of light may be processed differently by the rod-bipolar pathway because of the funneling of the rod signal through the rod bipolar cell. We tested this hypothesis using a psychophysical adaptation paradigm, which has provided evidence that photopic increments and decrements of light are processed by ON- and OFF-pathways in the human visual system. We had observers adapt to either a rapid-on or rapid-off sawtooth waveform, under both photopic and scotopic conditions. We then measured detection thresholds for one cycle of a rapid-on or rapid-off sawtooth stimulus. For photopic stimuli, sawtooth adaptation asymmetrically raised thresholds for test stimuli in a manner that depended on the polarity of the adaptation stimulus. For scotopic stimuli, thresholds were raised, but no significant selective adaptation effect was found. By repeating the photopic condition with sawtooth stimuli which had been filtered using an impulse response function derived for the rod system, we demonstrated that the lack of selective adaptation was not a consequence of the sluggish temporal response of the rod-bipolar pathway. We conclude instead that the reduced effectiveness of sawtooth adaptation is due to channeling of rod photoreceptor signals through the rod bipolar cell before reaching ON- and OFF-ganglion cells.

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.

Cone contributions to the photopic spectral sensitivity of the zebrafish ERG.

Microspectrophotometry studies show that zebrafish (Danio rerio) possess four cone photopigments. The purpose of this study was to determine the cone contributions to the zebrafish photopic increment threshold spectral-sensitivity function. Electroretinogram (ERG) b-wave responses to monochromatic lights presented on a broadband or chromatic background were obtained. It was found that under the broadband background condition, the zebrafish spectral-sensitivity function showed several peaks that were narrower in sensitivity compared to the cone spectra. The spectral-sensitivity function was modeled with L - M and M - S opponent interactions and nonopponent S- and U-cone mechanisms. Using chromatic adaptation designed to suppress the contribution of the S-cones, a strong U-cone contribution to the spectral-sensitivity function was revealed, and the contributions of the S-cones to the M - S mechanism were reduced. These results show that the b-wave component of the ERG receives input from all four cone types and appears to reflect color opponent mechanisms. Thus, zebrafish may possess the fundamental properties necessary for color vision.

The peripheral flicker effect: desensitization of the luminance pathway by static and modulated light.

Previous studies have shown that luminance flicker, presented peripheral to a foveal test target, increases thresholds for target detection: the peripheral flicker (PF) effect. These studies have also shown that thresholds are elevated more for luminance targets, relative to chromatic targets. In the present study we examined the specificity of the PF effect on the luminance mechanism and assessed the contribution of modulated stray-light to the test field, as well as longer range spatial interactions. We found that the presence of a foveal luminance pedestal, as well as PF, caused a notch to appear in the spectral sensitivity function around 570 nm. This result confirms the hypothesis that the PF effect decreases the sensitivity of the luminance pathway. To assess the contribution of stray-light to the PF effect, we modulated a luminance pedestal without the presence of PF in order to simulate the stray-light effect in isolation. A decrease in sensitivity for wavelengths around 570 nm occurred with modulated stray-light, suggesting that modulated stray-light contributes substantially to this effect. We then minimized the modulated stray-light by phase-reversing a checkerboard pattern in the periphery. A significant, though smaller, threshold elevation to mid-spectrum stimuli was obtained, suggesting that long range spatial effects are also active in the PF effect. We conclude that the PF effect causes a desensitization of foveal luminance pathways via local and more long range spatial interactions. Our results are consistent with previous data which suggest that the PF effect is due to selective adaptation of cells in the magnocellular pathway (M-cells). Our data imply that local network adaptation may be a property of the magnocellular pathway.

An electrophysiological metric of activity within the ON- and OFF-pathways in humans.

Several animal studies have shown an anatomical and functional separation between the ON- and OFF-pathways in the retina and in the lateral geniculate nucleus. Psychophysical studies in humans have also documented separate pathways that process increments and decrements of light. However, at the level of the visual cortex, there is electrophysiological evidence of interactions between the ON- and OFF-pathways. In addition, psychophysical studies have shown that these pathways can exhibit differential sensitivity and be differentially adapted. These findings motivated an electrophysiological study to gather further evidence of processing within the ON- and OFF-pathways in the human visual system. Using sawtooth stimulus modulation, we measured the visual evoked potential (VEP) before and after adaptation to both rapid-on and rapid-off sawtooth stimuli. The effect of adaptation was determined by comparing the VEP response in three test conditions: without adaptation, after adaptation to the same sawtooth polarity, and after adaptation to the opposite sawtooth polarity. The results reveal a selective adaptation effect, which provides physiological evidence for separate processing of increments and decrements in the human visual system. We conclude that with appropriate stimulus parameters, the VEP can serve as an objective measure of processing within the ON- and OFF-pathways in humans.

The effects of luminance and chromatic background flicker on the human visual evoked potential.

Previous studies report that background luminance flicker, which is asynchronous with signal averaging, reduces the amplitude and increases the latency of the pattern-onset visual evoked potential (VEP). This effect has been attributed to saturation of the magnocellular (m-) pathway by the flicker stimulus. In the current study, we evaluate this hypothesis and further characterize this effect. We found that flicker had similar effects on the pattern-onset and pattern-reversal VEP, suggesting that the reversal and onset responses have similar generators. Chromatic flicker decreased latency of the chromatic VEP whereas luminance flicker increased peak latency to luminance targets. This result indicates that luminance flicker saturates a rapidly conducting m-pathway whereas chromatic flicker saturates a more slowly conducting parvocellular (p-) pathway. Finally, evoked potentials to chromatic and luminance stimuli were recorded from 34 electrodes over the scalp in the presence of static and asynchronously modulated backgrounds. An equivalent dipole model was used to assess occipital, parietal, and temporal lobe components of the surface response topography. Results showed that chromatic flicker reduced activity to a greater extent in the ventral visual pathway whereas luminance flicker reduced activity to a greater extent in the dorsal visual pathway to parietal lobe. We conclude that the VEP to isoluminant color and luminance stimuli contains both m- and p-pathway components. Asynchronous flicker can be used to selectively reduce the contribution of these pathways to the surface recorded VEP. Our results provide evidence of parallel pathways in the human visual system, with a dorsal luminance channel projecting predominantly to the posterior parietal lobe and a ventral color channel projecting predominantly to inferior temporal lobe.

The contributions of ON- and OFF-pathways to contrast sensitivity and spatial resolution in goldfish.

DL-2-amino-4-phosphonobutyric acid (APB) reduces the sensitivity of ON- and OFF-responses in goldfish retina, although the ON-responses are reduced significantly more than the OFF-responses. This paper describes the effects of APB on behavioral sensitivity of goldfish to spatial sinusoidal gratings. Fish were classically conditioned to suppress respiration upon presentation of gratings drifting at 1 Hz; contrast thresholds were measured by an observer-based two-alternative forced-choice procedure. Thresholds were repeated following intraocular injections of APB or physiological saline. Saline had no effect, but APB dramatically reduced contrast sensitivity and shifted contrast sensitivity functions to lower spatial frequencies. The results suggest that both ON- and OFF-pathways are necessary for normal spatial vision and that the effects of APB are consistent with the disruption of both ON- and OFF-pathways.

Short-term changes in the response characteristics of the human visual evoked potential.

The present study examined how the response characteristics of the visual evoked potential (VEP) varied during the course of trials using a sinusoidal grating stimulus that reversed contrast in a square-wave manner. To accomplish this, amplitude and phase values were derived in short segments during the course of continuous stimulation for three subjects. When stimulus spatial frequencies of 0.77 or 1.55 c/deg were used, VEP amplitude remained at a stable value throughout the trial. At 3.1 c/deg, 6-12 sec were required for VEP amplitude to increase to a stable value, which was on average 204% greater than the value noted during the first few seconds of the trial. At 6.2 and 12.4 c/deg, VEP amplitude changes were more complex, first increasing and then decreasing substantially, to levels that were on average 63.8% and 38% of the peak reached earlier in the trial. In all cases, VEP phase decreased during the trial. The magnitude of this decrease ranged up to 50 deg, corresponding to an approx. 10.5 msec delay for the 6.65 Hz stimulation rate used. Prior exposure to an adapting grating diminished the changes in VEP amplitude and advanced the phase changes. Therefore, these changes appear to represent a form of contrast adaptation that is restricted to responses to high spatial frequencies. In addition, the present results provide evidence against a fundamental assumption of signal averaging--that an invariant stimulus will evoke an invariant response.

Anatomy and physiology of the visual system.

This article reviews the anatomy and physiology of the visual system. The physical stimulus is coded by several separate and parallel pathways at multiple sites in the nervous system. We outline the major parallel pathways of the system, from their beginning in the retina, to the multiple cortical areas that receive visual input. Emphasis has been placed on the functional properties of the neurons of these pathways and the various cortical processing areas as they are currently known. Clinical syndromes as a result of damage to specific visual cortical areas are also described.

Effect of sawtooth polarity on chromatic and luminance detection.

Psychophysical studies have documented that many observers show lower thresholds for rapid-off than for rapid-on sawtooth luminance modulation. This finding, together with physiological findings from chromatically opponent ganglion cells of the macaque monkey, prompted a search for a similar bias in psychophysical detection of chromatic increments and decrements of light. Using a luminance pedestal in conjunction with a luminance background to favor detection by chromatic mechanisms, we measured spectral sensitivity for rapid-on and rapid-off sawtooth stimuli presented spatially coextensive with the pedestal. There were two different pedestal chromaticities: one broadband, and the second composed only of long-wavelength light to enhance short-wavelength-sensitive, cone-mediated detection. Spectral-sensitivity measurements for different wavelength stimuli revealed no systematic differences across the visible spectrum as a function of sawtooth waveform polarity or pedestal chromaticity. Similarly, temporal contrast-sensitivity functions for hetero-chromatically modulated red-green sawtooth stimuli did not reveal an asymmetry in sensitivity for rapid-red and rapid-green chromatic change. Some of the observers showed a higher sensitivity for luminance modulated rapid-off sawtooth stimuli, as also noted in previous studies. This asymmetry was not found when a white luminance pedestal and background was used. These results suggest that the cone inputs to chromatically opponent ON- and OFF-center cells are sufficiently balanced to provide equivalent psychophysical thresholds for chromatic increments and decrements of light.

APB alters photopic spectral sensitivity of the goldfish retina.

Because the glutamate analog 2-amino-4-phosphonobutyric acid (APB) alters synaptic transmission at the outer plexiform layer in goldfish we asked whether intraocular injection of ABP would alter the spectral sensitivity of the retina. The spectral sensitivity of the ON and OFF components of the optic nerve response (ONR) in goldfish was measured in the presence and absence of APB, under four chromatic adaptation condition. APB decreased absolute sensitivity and altered spectral sensitivity for both ON and OFF responses under each adaptation condition. The spectral sensitivity of the OFF response was altered most at short wavelengths, in a manner consistent with a change in the balance of additive cone inputs. For the ON response, the effects of APB were consistent with a change in spectral antagonism, particularly between M- and L-cones. These results suggest that the activity in the retinal cone pathways in goldfish can be influenced by a mechanism incorporating an APB-sensitive receptor, and that this receptor may be intimately involved with setting the balance of cone inputs to spectrally-opponent neurons.

DL-2-amino-4-phosphonobutyric acid does not eliminate "ON" responses in the visual system of goldfish.

DL-2-Amino-4-phosphonobutyric acid (APB) suppresses activity in retinal ON pathways. It is generally assumed that loss of the ON pathway would result in loss of ON responses in the visual system. We tested this assumption by recording activity from the optic nerves of intact goldfish (Carassius auratus) before and after intraocular injection of APB. Whole-nerve responses to increments and decrements of light were compared to electroretinogram responses and to tectal evoked potentials. APB severely reduced the amplitude of the electroretinogram b-wave but left ON and OFF responses from the optic nerve and tectum intact, although decreased in sensitivity. We conclude that APB does not completely eliminate ON responses in the visual system, at least in goldfish. The selectivity and effectiveness of APB must be evaluated in other species before this agent can be relied upon as a useful tool in understanding the roles of ON and OFF pathways in visual function.

Spectral sensitivity of ON and OFF responses from the optic nerve of goldfish.

The vertebrate retina processes visual information in parallel neural pathways known as the ON and OFF pathways. These pathways encode increments and decrements of light independently as excitatory responses. We examined the photopic spectral response of ON and OFF mechanisms in goldfish by measuring the sensitivity of optic nerve responses to the onset and termination of stimuli of various wavelengths. Using various adapting backgrounds, we found that the ON and OFF responses have different spectral sensitivities. The weighting of the cone inputs to the responses was estimated by an algebraic summation model. This model suggests that for the ON response, input from S-cones is stronger and more independent than for the OFF response, and M- and L-cones show stronger antagonism in the ON response than in the OFF response. The OFF response probably receives input from all cone types, but spectral antagonism is weak and its dominant input is from L-cones.

Sensitivity of ERG components from dark-adapted goldfish retinas treated with APB.

The electroretinogram (ERG) of the dark-adapted goldfish was examined before and after intravitreal injection of DL-2-amino-4-phosphonobutyric acid (APB). APB abolished the b-wave and decreased absolute sensitivity of the remaining waveform, which was composed of a vitreal-negative component followed by a vitreal-positive component. The sensitivity, time course and amplitude of these components differed from ERGs obtained from animals treated with sodium aspartate. Spectral sensitivity of both post-APB components closely resembled that of the normal dark-adapted b-wave. The results suggest that APB does not act selectively on any particular class of photoreceptors or photoreceptor pathways in the dark-adapted goldfish retina.

APB selectively reduces visual responses in goldfish to high spatiotemporal frequencies.

Visual responses of goldfish to rotating square-wave gratings were recorded before and after intraocular injection of 2-amino-4-phosphonobutyric acid (APB). High doses of APB reduced the rate of optokinetic nystagmus (OKN) to a relatively high spatial frequency grating moving at a high temporal frequency. Responses to a low spatial frequency grating were not altered, nor were responses to the higher spatial frequency when it rotated slowly. The effects of APB were transient and lasted no longer than 3 d. We conclude that APB reduces OKN to high spatiotemporal frequencies in goldfish.