Latency and variability of the visual evoked magnetic response: a comparison of two single channel magnetometers

The latency variation of the P100M from minute to minute, between morning and afternoon and from day to day was investigated in an unshielded environment using two single channel magnetometers. Latency variation was greatest from minute to minute with relatively little longer term variation. The two magnetometers differed both in mean latency and in the degree of variation. This may be attributed to variation in the performance of the filters which were set a narrow bandwidth for recording in an unshielded environment.

Estimating ECAP threshold from the variability of the response

Electrical compound action potentials (ECAPs) of the cochlear nerve are used clinically for quick and efficient cochlear implant parameter setting. The ECAP is the aggregate response of nerve fibres at various distances from the recording electrode, and the magnitude of the ECAP is therefore related to the number of fibres excited by a particular stimulus. Current methods, such as the masker-probe or alternating polarity methods, use the ECAP magnitude at various stimulus levels to estimate the neural threshold, from which the parameters are calculated. However, the correlation between ECAP threshold and perceptual threshold is not always good, with ECAP threshold typically being much higher than perceptual threshold. The lower correlation is partly due to the very different pulse rates used for ECAPs (below 100 Hz) and clinical programs (hundreds of Hz up to several kHz). Here we introduce a new method of estimating ECAP threshold for cochlear implants based upon the variability of the response. At neural threshold, where some but not all fibers respond, there is a different response each trial. This inter-trial variability can be detected overlaying the constant variability of the system noise. The large stimulus artefact, which requires additional trials for artefact rejection in the standard ECAP magnitude methods, is not consequential, as it has little variability. The variability method therefore consists of simply presenting a pulse and recording the ECAP, and as such is quicker than other methods. It also has the potential to be run at high rates like clinical programs, potentially improving the correlation with behavioural threshold. Preliminary data is presented that shows a detectable variability increase shortly after probe offset, at probe levels much lower than those producing a detectable ECAP magnitude. Care must be taken, however, to avoid saturation of the recording amplifier saturation; in our experiments we found a gain of 300 to be optimal.

Variability of the permetric response in normals and in glaucoma

This study investigated the variability of response associated with various perimetric techniques, with the aim of improving the clinical interpretation of automated static threshold perirnetry. Evaluation of a third generation of perimetric threshold algorithms (SITA) demonstrated a reduction in test duration by approximately 50% both in normal subjects and in glaucoma patients. SITA produced a slightly higher, but clinically insignificant, Mean Sensitivity than with the previous generations of algorithms. This was associated with a decreased between-subject variability in sensitivity and hence, lower confidence intervals for normality. In glaucoma, the SITA algorithms gave rise to more statistically significant visual field defects and a similar between-visit repeatability to the Full Threshold and FASTPAC algorithms. The higher estimated sensitivity observed with SITA compared to Full Threshold and FASTPAC were not attributed to a reduction in the fatigue effect. The investigation of a novel method of maintaining patient fixation, a roving fixation target which paused immediately prior lo the stimulus presentation, revealed a greater degree of fixational instability with the roving fixation target compared to the conventional static fixation target. Previous experience with traditional white-white perimetry did not eradicate the learning effect in short-wavelength automated perimetry (SWAP) in a group of ocular hypertensive patients. The learning effect was smaller in an experienced group of patients compared to a naive group of patients, but was still at a significant level to require that patients should undertake a series of at least three familiarisation tests with SWAP.