PSEUDORANDOM SEQUENCES IN THE STUDY OF EVOKED POTENTIALS

A first comparison of the human multifocal visual evoked magnetic field and visual evoked potential

Our objectives were to determine the feasibility of recording reliable multifocal visual evoked magnetic fields (mfVEFs), to investigate the maximum stimulus eccentricity for which the mfVEF responses can be obtained, and to study how this changes with checksize (spatial frequency tuning). Using a checksize of 30', we recorded 8-channel pattern-onset mfVEFs three times to obtain responses from 19 channels located around the inion. Multifocal visual evoked potentials (mfVEPs) were recorded under the same conditions. Eccentricity changes with spatial frequency were studied using checksizes from 7.5' to 60'. We obtained, for the first time, reliable mfVEFs, and found they could be elicited from more peripheral stimulus elements than could mfVEPs. The larger the checksize, the greater the eccentricity reached.

The topography of visual evoked response properties across the visual field

Visual evoked potentials (VEPs) to luminance and pattern reversal stimulation were derived for a large number of small areas throughout the central visual field. In one study, the field was tested with a stimulus array consisting of 64 equal-area patches. Local response components were extracted by independent m-sequence modulation of the patches. Field topographies were compared between and within subjects using different electrode placements. The subject-dependent local variability observed in response characteristics is attributed to contributions from two or more cortical representations of the visual field and to inter-subject variations in gross cortical anatomy. The second study used luminance modulation of 56 patches across a 15 degrees field, scaled to activate approximately equal cortical areas in area V1. This produced many robust signals at all eccentricities. Bipolar and double differential ("1-dimensional Laplacian") signals were compared. The double differencing reduced contributions from distant or distributed sources, enhancing nearby current source activity, and greatly improved S/N for many stimulus locations. The high-resolution visual field maps demonstrated that clinical field testing using the VEP is not feasible because of effects of cortical convolutions on responses. However, the vast improvement in data quality and quantity make it a useful tool for VEP source localization and identification.

An analysis of the VEP to luminance modulation and of its nonlinearity

Identification of the VEP luminance modulation system was carried out using a relatively long pseudorandom binary m-sequence stimulus. The complete first order cross-correlation function provides a "fingerprint" of the nonlinearity in the time domain. Volterra kernel slices up to third order are recognized in the complete first order cross-correlation function. They all lie close to the major diagonal. Higher order kernel slices are not measurable. A cascade block model (sandwich model) analysis shows the nonlinearity to be an asymmetric rectification. Luminance increase causes a stronger response than does luminance decrease. The pre-filter gain function is low-pass. The post-filter gain function is band-pass tuned near 10 Hz. Coherency spectra are used to examine the channel structure of the system. Two channels operating near 8 and 15 Hz are easily identified. Another broad channel (or possibly multiple channels) is present above 30 Hz. In many subjects, the pseudorandom stimuli allow a complete system identification to be carried out in less than 82 sec.

Application of computer averaging techniques to the study of learned volitive heartbeat control

Research on the acquisition of conscious cardiovascular control (Yellin 1984, 1986) utilized computer averaging of electrocardiographic (ECG) cycles to demonstrate and document precise, reliable and sustainable time-locking of heartbeat to external stimuli on demand. ECG cycles were treated as evoked responses, and successive epochs were summated, beginning with Zeitgeber-stimulus (ZS) onset, and with the duration of each epoch equaling the ZS's interstimulus interval. Cardiac synchrony was exhibited by event-modulated (time-locked) cardiac responses (EMCRs) and in average ECGs.