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Biomedical subjects

M R Harter

Publications and source records attributed to M R Harter.

7 recordsLinked to original sources
PubMed1978

Size-specific information channels and selective attention: visual evoked potential and behavioral measures.

Subjects were shown a stimulus (diffuse light or a checkerboard pattern with checks subtending 12' or 35' of arc) and were instructed to remember and attend (give a reaction time response to or count) that stimulus during the course of an approximately 6-min trial. A trial consisted of the random presentation of 8 stimulus flashes (diffuse light and checkerboards with 9, 12, 18, 24, 35, 45, 94 and 95 min checks) at a rate of 1/555--930 msec. Visual evoked potentials and reaction times were averaged to each of the 8 stimuli. The purpose of the experiment was to assess how specifically the relevant stimulus could be attended, as indicated by VEPs and behavioral responses to the relevant and 7 irrelevant stimuli. Attention to one check size resulted in greater amplitude VEP components to that size than to other sizes: the greater the discrepancy between the flashed and attended check size, the smaller the VEP amplitude. Such size tuning was first evident in the VEP 160 msec after stimulation and most evident at 260 msec. The bandwidth of the VEP amplitude size-tuning functions became progressively narrower from 160 up to 260 msec, that point in time when the RT response was initiated. The width at 260 msec was similar to that indicated by the behavioral RT data (modal latency of 322 msec). The functional components of the VEP appeared to reflect the effects of attention on the activity of cortical size channels or detectors.

Attention↗
PubMed1977

An objective indicant of binocular vision in humans: size-specific interocular suppression of visual evoked potentials.

Evoked cortical potentials (VEPs) to grid patterns flashed to one eye were suppressed in amplitude when grid patterns were continuously presented to the other eye. The degree of interocular suppression of VEPs was influenced by the stereoacuity of the subjects. VEPs were obtained to each of two grid sizes flashed to one eye (individual squares subtending 15 and 60 min of arc) and changes in amplitude of these VEPs were considered as a function of four stimuli continuously presented to the other eye (diffuse light, 15, 30, and 60 min of arc squares in grids). Interocular suppression of VEPs was greater (a) when the continuously presented grid was of high (38.00 mL) as compared to low (00.38 mL) intensity, (b) when the continuous and flashed grids were of the same as compared to different sizes, and (c) in six subjects who had good as compared to six subjects who had poor binocularity. Eleven of the twleve subjects could be classified correctly as having good or poor binocularity on the basis of statistically significant interocular suppression of VEPs. The results were interpreted in terms of centrally located binocular neurons responsive to specific grid sizes or spatial frequencies and the decreased functioning of such neurons in subjects with poor binocularity.

Evoked Potentials↗
PubMed1977

Maturation of evoked potentials and visual preference in 6-45-day-old infants: effects of check size, visual acuity, and refractive error.

Visual evoked potentials (VEPs) and the percentage time fixated (PTF) were investigated in response to checkerboard light flashes in 10 human infaed as a function of the size of check in the evoking stimulus (diffuse light, 11, 22, 45, 90 and 180 min of arc), the refractive lens strength the checkerboards were viewed through (-6 to +6 diopters), and the age of the infants (6-26 or 27-45 days). Check size significantly influenced VEP amplitude in infants as young as 6 days. The 11' checks evoked greater responses that diffuse light suggesting a visual acuity of better than 20/220. Only the 27--45-day-olds behaviorally discriminated the checks, PTF indicating an acuity of 20/120. Evoked potential refraction with spherical lention between VEP amplitude and check size measured from different VEP components at different ages indicated the function contained two modes or components. The first mode was inverted "U-shaped" and was obtained in response to check sizes less than 45'. It was primarily due to changes in amplitude of the early VEP components (less than 210 msec after the evoking stimulus) and was poorly correlated with the behavioral PTF measure. It was proposed that this mode reflected subcortical activity. The second mode was a linear increase in amplitude as check size was increased from 45' to 180'. It was primarily due to changes in the amplitude of late VEP components (240--400 msec after the evoking stimulus) and was highly correlated with the percentage time the infants fixated the various check sizes. It was proposed that this mode reflected cortical activity. Age selectively influenced the late VEP components and the PTF behavioral measure, these measures being influenced by check size only in the 27--45-day-old infants. This change in responsivity of late VEP components and the transition from passive to more active and discriminating visual preference, suggest the onset of increased cortical function between 28 and 45 days of age.

Age Factors↗
PubMed1977

Objective determination of human visual acuity: pattern evoked potentials.

The natural visual acuities of 15 adult persons were predicted on the basis of changes in visual evoked potentials (VEP's) to flashed patterns of various sized dots. An objective method was used to quantify the VEP's, based on the minimum-sized stimulus that would elicit a pattern VEP- the VEP pattern threshold. This measure was highly correlated with recognition and resolution measures of perceptual visual acuity (r's as high as 0.89). The regression equation between the VEP measures and predicted perceptual measures of acuity enabled the objective estimation of perceptual acuity to within +/- 0.29 decimal units.

Adult↗