ABSTRACT
The present paper focuses on a classic hyperacuity, Vernier acuity-the ability to discriminate breaks in the collinearity of lines or edges on the order of only arcseconds of visual angle. We measured steady-state sweep visual evoked potentials (sVEPs) in response to 6 Hz periodic breaks in collinearity (Vernier offsets) in horizontal squarewave gratings. Vernier thresholds, estimated by extrapolating the amplitude of the first harmonic (1F) to 0 µV, were measured for gratings with 4%, 8%, 16%, 32%, 64%, and 80% contrast, with gaps of 0, 2, or 5 arcmin introduced between neighboring bar elements that formed the Vernier offsets. Thresholds for the 2F response component provided an estimate of motion thresholds. The data confirmed and extended evidence that the odd- and even-harmonic components reflect cortical activity of different neurons (i.e., neurons that respond asymmetrically to the periodic breaks in alignment and neurons that respond symmetrically to the local relative motion cue of the stimulus). Suprathreshold data (peak amplitude, response slope, and response phase at the peak amplitude) provided additional independent evidence of this notion. Vernier thresholds decreased linearly as contrast increased, with a slope of approximately -0.5 on log-log axes, similar to prior psychophysical results. The form of contrast dependence showed more similarity to measures of magnocellular ganglion cell spatial precision than measures from parvocellular ganglion cells. Our data thus support the hypothesis that magnocellular ganglion cell output from the retina has the requisite properties to support cortical calculation of Vernier offsets at a hyperacuity level...
Subject(s)
Humans , Male , Female , Adult , Evoked Potentials, Visual , Visual AcuityABSTRACT
To investigate the effects of early visual stimulation on the development of tonus and posture control and of grating visual acuity of preterm infants, eight infants received visual stimulation that induced head motion during the first 6 months of life in weekly sessions. Their performance was compared with that of unstimulated preterm infants of same age (n = 6). Both groups were evaluated in the developmental scale of Gesell and had their grating visual acuity measured by swept visually evoked potentials (sweep-VEP). Visuomotor performance and adoption and maintenance of postures were significantly better for the stimulated group (p < .05) compared to the unstimulated group. There was no statistical difference between the two groups for the development of visual acuity. Visuo-motor and postural development improved with the early visuo-motor stimulation program. The lack of influence of this program in the development of visual acuity may be attributed to differences in the pathways involved in the processing of visual acuity of patterns and in the responses elicited by visuo-motor stimulation.