Binocular interactions and steady-state VEPs. A study in normal and defective binocular vision (Part II)

EEG measurement for the effect of perceptual eye position and eye position training on comitant strabismus

One of the clinical features of comitant strabismus is that the deviation angles in the first and second eye positions are equal. However, there has been no report of consistency in the electroencephalography (EEG) signals between the 2 positions. In order to address this issue, we developed a new paradigm based on perceptual eye position. We collected steady-state visual evoked potentials (SSVEPs) signals and resting-state EEG data before and after the eye position training. We found that SSVEP signals could characterize the suppression effect and eye position effect of comitant strabismus, that is, the SSVEP response of the dominant eye was stronger than that of the strabismus eye in the first eye position but not in the second eye position. Perceptual eye position training could modulate the frequency band activities in the occipital and surrounding areas. The changes in the visual function of comitant strabismus after training could also be characterized by SSVEP. There was a correlation between intermodulation frequency, power of parietal electrodes, and perceptual eye position, indicating that EEG might be a potential indicator for evaluating strabismus visual function.

Short-term monocular patching boosts the patched eye's response in visual cortex

Purpose: Several recent studies have demonstrated that following short-term monocular deprivation in normal adults, the patched eye, rather than the unpatched eye, becomes stronger in subsequent binocular viewing. However, little is known about the site and nature of the underlying processes. In this study, we examine the underlying mechanisms by measuring steady-state visual evoked potentials (SSVEPs) as an index of the neural contrast response in early visual areas.

Methods: The experiment consisted of three consecutive stages: a pre-patching EEG recording (14 minutes), a monocular patching stage (2.5 hours) and a post-patching EEG recording (14 minutes; started immediately after the removal of the patch). During the patching stage, a diffuser (transmits light but not pattern) was placed in front of one randomly selected eye. During the EEG recording stage, contrast response functions for each eye were measured.

Results: The neural responses from the patched eye increased after the removal of the patch, whilst the responses from the unpatched eye remained the same. Such phenomena occurred under both monocular and dichoptic viewing conditions.

Conclusions: We interpret this eye dominance plasticity in adult human visual cortex as homeostatic intrinsic plasticity regulated by an increase of contrast-gain in the patched eye.

Steady-state contrast response functions provide a sensitive and objective index of amblyopic deficits

PURPOSE

Visual deficits in amblyopia are neural in origin, yet are difficult to characterize with functional magnetic resonance imagery (fMRI). Our aim was to develop an objective electroencephalography (EEG) paradigm that can be used to provide a clinically useful index of amblyopic deficits.

METHODS

We used steady-state visual evoked potentials (SSVEPs) to measure full contrast response functions in both amblyopic (n = 10, strabismic or mixed amblyopia, mean age: 44 years) and control (n = 5, mean age: 31 years) observers, both with and without a dichoptic mask.

RESULTS

At the highest target contrast, the ratio of amplitudes across the weaker and stronger eyes was highly correlated (r = 0.76) with the acuity ratio between the eyes. We also found that the contrast response function in the amblyopic eye had both a greatly reduced amplitude and a shallower slope, but that surprisingly dichoptic masking was weaker than in controls. The results were compared with the predictions of a computational model of amblyopia and suggest a modification to the model whereby excitatory (but not suppressive) signals are attenuated in the amblyopic eye.

CONCLUSIONS

We suggest that SSVEPs offer a sensitive and objective measure of the ocular imbalance in amblyopia and could be used to assess the efficacy of amblyopia therapies currently under development.

Factors contributing to the outcome of sensory testing in patients with anomalous binocular correspondence

Traditional teaching on anomalous retinal correspondence (ARC) identifies the dissociative quality of a sensory test as the primary factor influencing the outcome of correspondence testing. However, these tests differ also in function and format. This study compared one mildly dissociating test and one highly dissociating test to evaluate the subjective visual direction of both the deviation point and the fovea of the deviating eye in 74 patients with ARC. Subjects were more likely to demonstrate an ARC response, and to do so after a significantly shorter period of time following change in alignment, on the minimally dissociating tests. Eighty-five percent demonstrated the presence of a pseudo-fovea at the deviation point, while only 39% had evidence of rewiring of the deviated fovea. The mean angle of deviation of the latter group was significantly larger than that of the former group (P < 0.001). Exotropic patients were more likely to rewire the fovea than esotropic patients (P < 0.005). The mean time needed to rewire a pre-existing ARC following a change in deviation was 7.7 ± 1 months. Results of this study indicate that variables influencing sensory test results include: a) retinal element evaluated, b) magnitude of the deviation, c) direction of the deviation, and d) age of onset of the strabismus. Results of sensory testing in patients with ARC may be misinterpreted if these factors are not taken into consideration.

Fourier analysis of steady-state visual evoked potentials in subjects with normal and defective stereo vision

The purpose of this work was to study the second harmonic in the steady state pattern visual evoked potential (ssVEP) to various stimulus frequencies in subjects with normal and defective binocularity. ssVEPs were elicited by 4 c/deg sinusoidal gratings, with temporal frequencies ranging from 5 to 20 Hz (exp. 1) and 15 to 27.5 Hz (exp. 2). Responses were Fourier analysed and power and phase of the second harmonic to stimulus frequency were measured. For power, binocular enhancement in a bimodal fashion was found both in normals and in subjects with defective binocularity. The power with binocular stimulation was significantly higher in the normal group in the high frequency domain. Latency, estimated from the phase-frequency function, was longer in the group with defective binocularity, but this was statistically significant only for the high frequency domain. The results suggest that a visual system with normal binocular function can follow a stimulus with high temporal frequency more accurately than a system with disturbed binocularity.