Visual evoked potentials: advances in clinical and basic sciences.

Neuromagnetic techniques were used in conjunction with magnetic resonance imaging (MRI) techniques to: (1) localize and characterize cortical sources evoked by visual stimuli presented at different locations in the lower right visual field; (2) examine the superposition of cortical responses by comparing the summation of responses to the presentation of single stimuli with responses to paired stimuli; and (3) examine the spatial resolution of magnetoencephalographic (MEG) techniques by comparing the identified source locations evoked by the presentation of single vs. paired stimuli. Using multi-dipole, non-linear minimization analyses, three sources were localized for each stimulus condition during the initial 80-170 ms poststimulus interval for all subjects. In addition to an occipital source, two extrastriate sources were identified: occipital-parietal and occipital-temporal. Each source evidenced a systematic shift in location associated with changes in stimulus placement parallel to the vertical meridian. To our knowledge, this is the first demonstration of retinotopic organization of extrastriate areas, using non-invasive neuromagnetic techniques. The paired presentation of stimuli reflected superposition of the responses evoked by single stimuli but only for early activity up to 150 ms poststimulus. Undersummation was evident after 150 ms. All sources identified for single stimuli were also identified in the paired-stimulus responses; but at the expense of larger errors for some of the estimated parameters.

Evoked Potentials, Visual↗

PubMed1999

Application of double voxel functional spectroscopy to event-related cognitive experiments.

The hemodynamic response to functional activation can be regarded as the convolution of the neuronal response with an unknown kernel. As such, it introduces an intrinsic blurring that limits the attainable temporal resolution of functional magnetic resonance (fMR) techniques. This study demonstrates that by measurement of displacements in activation onsets between different types of trial, it is nevertheless possible to obtain a subsecond temporal accuracy in fMR. A single trial stimulation paradigm was adopted: a simple search task embedded in a longer period of visual flicker stimulation that produced reliable activations in the primary visual cortex and supplementary motor area. Data were acquired from both of these regions using double voxel functional spectroscopy.

Evoked Potentials, Visual↗

PubMed1999

Visual completion of partly occluded grating in infants under 1 month of age.

Four groups of eight infants (3 weeks of age on average) were each habituated to one of four displays consisting of a grating of either low (0.4 cpd) or high (1.2 cpd) spatial frequency, whose central portion was covered up with a horizontal occluder which was either narrow (1.33 degrees) or broad (4.17 degrees). These habituation displays are referred to as LN (low spatial frequency grating and narrow occluder), LB (low and broad), HN (high and narrow), and HB (high and broad) displays. Posthabituation-test displays consisted of a complete grating (CG) of the same frequency as the habituated grating along with a separate grating (SG) whose central portion was replaced with a black gap of the same height as the occluder in the habituation displays. Infants habituated to the LN display looked significantly longer at the SG than the CG display during posthabituation-test trials. Infants habituated to the LB and HN displays looked at the CG and SG displays, almost equally. In contrast, infants habituated to the HB display looked longer at the CG than the SG display. These results show that infants under 1 month of age can perceive the continuation of the grating behind the occluder, and that their visual completion on habituation displays can be evoked according to the interaction between the spatial frequency of the grating and the occluder height.

Evoked Potentials, Visual↗