Contrast Reversal Stimulus Research Articles

Assessing Lateral Interaction in the Synesthetic Visual Brain.

In grapheme-color synesthesia, letters and numbers evoke abnormal colored perceptions. Although the underlying mechanisms are not known, it is largely thought that the synesthetic brain is characterized by atypical connectivity throughout various brain regions, including the visual areas. To study the putative impact of synesthesia on the visual brain, we assessed lateral interactions (i.e., local functional connectivity between neighboring neurons in the visual cortex) by recording steady-state visual evoked potentials (ssVEPs) over the occipital region in color-grapheme synesthetes (n = 6) and controls (n = 21) using the windmill/dartboard paradigm. Discrete Fourier Transform analysis was conducted to extract the fundamental frequency and the second harmonics of ssVEP responses from contrast-reversing stimuli presented at 4.27 Hz. Lateral interactions were assessed using two amplitude-based indices: Short-range and long-range lateral interactions. Results indicated that synesthetes had a statistically weaker signal coherence of the fundamental frequency component compared to the controls, but no group differences were observed on lateral interaction indices. However, a significant correlation was found between long-range lateral interactions and the type of synesthesia experience (projector versus associator). We conclude that the occipital activity related to lateral interactions in synesthetes does not substantially differ from that observed in controls. Further investigation is needed to understand the impact of synesthesia on visual processing, specifically in relation to subjective experiences of synesthete individuals.

Retinal pathology in multiple sclerosis: insight into the mechanisms of neuronal pathology

Sir, We congratulate Calabresi et al . (2010) for the scientific commentary regarding retinal pathology in multiple sclerosis. The authors have excellently summarized the pathological mechanism in multiple sclerosis in the brain and retina. We would like to make a contribution to the retinal aspects of their comments. Full-field flash electroretinogram is the mass electrical activity of the retina to a flash of light. Pattern electroretinogram reflects the retinal response to a structured, contrast-reversal stimulus, mostly a checkerboard pattern. There have been many studies investigating the functional and structural retinal changes in the retinas of patients with multiple sclerosis. Mostly, optical coherence tomography has been used to explore the structural changes that show retinal nerve fibre layer thinning. This change was …

Frequency-doubling in the early visual system underlies sensitivity to second-order stimuli

There are two primary visual cortical areas in the cat, each receiving a direct LGN projection. Area 17 neurons are described reasonably well by linear filters selective for the orientation and spatiotemporal frequencies of stimuli. Area 18 neurons are thought to be analogous except that they are selective for lower spatial and higher temporal frequencies. However, about half of Area 18 neurons possess a nonlinearity that makes them sensitive to beat frequencies and other second-order stimuli. Because second-order responsive neurons are tuned for the orientation of the high spatial frequency carrier of a beat stimulus, it has been argued that this selectivity arises from a nonlinear combination of Area 17 inputs (Mareschal & Baker, 1998). Alternatively, this selectivity may originate from their LGN inputs, Y-cells. Consistent with this, Y-cells are responsive to low spatial frequencies but also show frequency-doubled responses to contrast-reversing stimuli at the high spatial frequencies used to construct second-order stimuli. Furthermore, the activity of retinal ganglion Y-cells modulates at the temporal frequency of second-order stimuli (Demb et al., 2001). To date however, it has been assumed, but not tested, that Y-cell tuning properties cannot account for the orientation selectivity of second-order responses in Area 18. We asked if LGN responses to second-order stimuli can account for those in Area 18. Our results indicate that while X-cells are unresponsive to these stimuli, Y-cells behave similarly to Area 18 neurons. First, Y-cells show tuning for the orientation of the carrier of a beat stimulus. Second, the preferred temporal frequency and cut-off is greater for drifting sinusoidal gratings than for the envelope of a beat. Third, the temporal frequency tuning of the carrier resembles that of Area 18 neurons. Together, these findings suggest that the frequency-doubling nonlinearity of Y-cells is sufficient to account for second-order responses in cat Area 18.

Adaptive changes of inner retina function in response to sustained pattern stimulation

We have characterized adaptive changes of inner retina function in response to sustained pattern stimulation in 32 normal subjects with an age range 23–77 years by measuring changes of the pattern electroretinogram (PERG) as a function of time. Contrast-reversal stimuli had square-wave profile in space and time, with peak spatial and temporal frequency and high contrast to maximize response amplitude. The PERG signal was sampled over 5min with a resolution of 15s. PERG signals were non-stationary, resulting in either progressive amplitude decline or even enhancement to a plateau, with a time course that could be well described by an exponential function with a time constant of 1–2min. Higher initial amplitudes were generally associated with amplitude decline, and lower initial amplitudes with enhancement. The delta amplitude (plateau minus initial) was a linear function of the initial amplitude. The magnitude of delta decreased with decreasing initial amplitude and inverted its sign for initial amplitudes about 1/3 lower than the maximum initial amplitude measured, but still about 3–4 times larger than the noise. Amplitude decline was generally associated with phase lag, whereas amplitude enhancement was associated with phase advance. Altogether, PERG generators appear to slowly adjust their gain in order to keep their sustained activity at an intermediate level that is rather independent of the level of activity at stimulus onset. This behavior is reminiscent of a buffering mechanism, where glial cells may play a primary role. An energy-budget model of neural–vascular–glial interaction is provided together with an equivalent electrical circuit that accounts for the results.

Standing Waves and Traveling Waves Distinguish Two Circuits in Visual Cortex

The visual cortex represents stimuli through the activity of neuronal populations. We measured the evolution of this activity in space and time by imaging voltage-sensitive dyes in cat area V1. Contrast-reversing stimuli elicit responses that oscillate at twice the stimulus frequency, indicating that signals originate mostly in complex cells. These responses stand clear of the noise, whose amplitude decreases as 1/frequency, and yield high-resolution maps of orientation preference and retinotopy. We first show how these maps are combined to yield the responses to focal, oriented stimuli. We then study the evolution of the oscillating activity in space and time. In the orientation domain, it is a standing wave. In the spatial domain, it is a traveling wave propagating at 0.2-0.5 m/s. These different dynamics indicate a fundamental distinction in the circuits underlying selectivity for position and orientation, two key stimulus attributes.

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.

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.

Development of receptive field properties of retinal ganglion cells in kittens raised with a convergent squint.

The effects on retinal ganglion cell receptive fields of rearing kittens with convergent squint, surgically induced on the 12th post-natal day, were investigated by utilizing the extracellular single unit recording technique. The data revealed that responses of cross-eyed cat ganglion cells to contrast reversal stimuli were severely depressed and the retinal region exhibiting the best responses varied according to the degree of convergent misalignment of the eyes displayed by each animal. Receptive field sizes of X-type (but not Y-type) units located within 10 degrees of the area centralis of cross-eyed cats were significantly larger than those in normally reared cats. Finally, the encounter rate for units exhibiting non-linearity of spatial summation (Y-type) were much lower in cross-eyed cats. The results suggest developmental alterations in the retinal neurophysiology of common cats reared with a large convergent squint.

Classification of cat retinal ganglion cells into X- and Y-cells with a contrast reversal stimulus

A contrast reversal (alternating phase) stimulus was used to study the responses of 150 retinal ganglion cells from 15 adult cats. Because the majority of the cells did not show perfect linear spatial summation, a ratio of the firing rates at two time periods was used to express the degree of nonlinearity. Y-cells showed a high degree of nonlinearity, and their mean null ratio was significantly lower than that of X-cells. With the stimulus at the null position, X-cells had an unmodulated discharge rate which was significantly higher than maintained activity, while the firing rate of Y-cells was lower than maintained activity. With the stimulus placed at an eccentric position in the receptive field, X-cells responded in a sustained manner, while Y-cells respond transiently. Because of these observations, we conclude that X-cells correspond to the sustained cells, while Y-cells correspond to the transient cells.

Development of X- and Y-cells in kittens

A contrast reversal stimulus was used to classify the retinal ganglion cells of kittens 3--12 weeks of age. At 3 and 4 weeks of age, the majority of the units were classified as Y-cells. The percentage of X-cells was similar to that of the adult at 5--6 weeks of age although other response properties were not adult-like. The angular size of the receptive field center was significantly larger than that of the adult through 5--6 weeks of age. An estimate of the position of the vertical meridian was obtained from the positions of the receptive fields in the visual field which permitted the calculation of the posterior nodal distance in kittens of various ages. From the posterior nodal distance, the linear extent of the receptive field center was determined. This showed that there is only a very slight increase in the linear size of the receptive field center from 3 weeks to adulthood.

Properties of X- and Y-cells in the rabbit retina.

Bevelled glass microelectrodes were used to record spike potentials extracellularly from the ganglion cells of the rabbit retina. Good responses were obtained from the isolated retina or eye-cup preparation for at least 12 hr. Using a contrast reversal stimulus, 63.8% (67/105) of the units showed linear spatial summation (X-cells), and 21.0% (22/105) showed nonlinear spatial summation (Y-cells). The X- and Y-cells in the rabbit retina had physiological properties which were similar to those in cat retina. Directionally selective cells (15.2%) were found to respond poorly, if at all, to the contrast reversal stimulus.

Properties of X- and Y-type retinal ganglion cells in Siamese cats

There is ample evidence that the visual system of the Siamese cat is different from common cats. These abnormalities suggest possible retinal origins, although no documentation exists. In the present study, single unit recordings were made from 91 misrouted and 209 normally-routed optic tract fibers in Siamese cats. Electrophysiological responses of the misrouted fibers did not differ from those found in the normally-routed fibers of the Siamese cat with the exception of depressed responses to contrast reversal stimuli. X/Y classification of units and experiments on receptive field center sizes, intensity-response functions, and responses to flicker failed to demonstrate significant differences between the misrouted and normally-routed fibers in Siamese cats. These results were not affected by different degrees of interocular misalignment exhibited by the Siamese cat studied. Response properties of retinal ganglion cells in Siamese cats, however, were found to be quite abnormal when compared with common cats. Only 14% (42/300) of all units studied were Y-cells in Siamese cats in comparison to35% (60/170) in common cats. The percentage of Y-units also was correlated with the severity of interocular misalignment in Siamese cats, i.e. the greater the misalignment of the eyes, the lower the percentage of Y-cells. Experiments on response to contrast reversal stimuli, intensity-response functions and responses to flicker revealed that the ganglion cells in Siamese cats are not as responsive as those in common cats.

Siamese cats: abnormal responses of retinal ganglion cells.

Comparison of optic tract recordings in Siamese and ordinary cats reveals that Siamese cats have a significantly lower percentage of Y-cells than of X-cells. In addition, Siamese cats show depressed responses to a contrast-reversal stimulus, a result that supports the lower spatial contrast sensitivity demonstrated behaviorally by these animals. Both physiological findings suggest neurophysiological anomalies in the Siamese retina.