Stimulus alternation and the Purkinje shift

Rapid Adaptation of Night Vision

Apart from the well-known loss of color vision and of foveal acuity that characterizes human rod-mediated vision, it has also been thought that night vision is very slow (taking up to 40 min) to adapt to changes in light levels. Even cone-mediated, daylight, vision has been thought to take 2 min to recover from light adaptation. Here, we show that most, though not all adaptation is rapid, taking less than 0.6 s. Thus, monochrome (black-white-gray) images can be presented at mesopic light levels and be visible within a few 10th of a second, even if the overall light level, or level of glare (as with passing headlamps while driving), changes abruptly.

Opponent-processing effects on the field spectral sensitivity of pattern-elicited electroretinograms

Field spectral sensitivities of the pattern-elicited electroretinogram (PERG) were obtained from two subjects using a modified version of Stiles' two-color increment threshold procedure. A 540 nm checkerboard test pattern of 38 degrees dia was alternated at 8 Hz on a uniform adapting field of the same size. Test intensity, field wavelength and field intensity were varied parametrically. The derived field spectral sensitivity does not resemble that of any individual class of cones; it roughly approximates the photopic luminosity function V lambda, but with sensitivity dips at 540 and 600 nm, which may be associated with an adaptation of the red/green (R/G) opponent site. Thus, it is proposed that the PERG reflect activities in both the luminance and the R/G opponent channels.

Isolation of scotopic human electroretinograms using color adaptation and pattern reversal stimuli

The human electroretinogram was recorded in response to alternations of a 500 nm checkerboard stimulus pattern presented against adaptation backgrounds of different wavelengths. A modification of Stiles' color adaptation paradigm was adapted to electroretinography. The results provided electrophysiological t.v.i. curves (threshold versus intensity curves) and permitted a determination of spectral sensitivity. The t.v.i. curves matched those seen in psychophysics. The spectral curve showed a close agreement with the scotopic CIE function.

Retinal receptive fields under different adaptation levels studied with pattern-evoked ERG

The pattern-evoked electroretinogram (PERG) was studied in three normal subjects under different levels of adaptation using pattern onset-offset stimulation. The occurrence of a response maximum (spatial selectivity) in the onset response at a certain spatial frequency can be interpreted as reflecting the activity of neurons having antagonistic center-surround receptive fields and allows an estimation of receptive-field center sizes. The present data suggest that in the light-adapted state, the majority of receptive fields, dominating the response can be estimated to have center diameters of 6-13 min of arc. With increasing dark adaptation, the spatial selectivity shifts from high to low spatial frequencies over 1.3-2.4 octaves, indicating that large receptive fields are active in the dark and small fields in the light. The physiological mechanisms of these changes are discussed on the basis of findings obtained from single-unit studies.

Changes in spatial selectivity of pattern-ERG components with stimulus contrast

Electrical mass responses of the visual system to stripe patterns of varying fineness (spatial frequency) can show either an amplitude maximum at a medium spatial frequency, a behavior termed "spatial selectivity," or a monotonic decrease in amplitude with increasing spatial frequency. The former behavior is probably mediated by neurons having a center-surround receptive field structure and the latter by neurons lacking this antagonism. The pattern-evoked human electroretinogram was studied in this report using different spatial frequencies and pattern contrasts. The positive component of the response showed a spatial selectivity only at low contrast but was not spatially selective at the highest contrast. The negative component showed a spatial selectivity at all contrast levels. The data indicate that if pattern-related responses activated by antagonistic receptive fields are to be studied, low contrast values should be employed and attention should be paid to the negative component of the response.

Effects of stimulus location and pattern upon the visually evoked cortical potential and the electroretinogram

The pattern properties of the visually evoked potential and the electroretinogram have been investigated for phase alternated patterns of checks presented to the near periphery of the retina. When the eye was light-adapted, coarse patterns became relatively more effective for eliciting the evoked potential as the stimulus was moved away from the fovea. Fine patterns were most effective in the center of the field. The electroretinogram responded best to coarse spatial frequencies at all retinal locations. At a lower level of light adaptation the sensitivity of the evoked potential shifted to coarse spatial frequencies in the center of the field, but did not change appreciably in the periphery. The results may reflect some of the dynamic properties of visual fields.

Stimulus alternation and fast retinal potentials: photopic anc scotopic contributions

Human retinal oscillatory potentials in response to an alternating checker-board pattern stimulus were studied in two subjects over an intensity range of 7.5 log units. Under scotopic conditions, two wavelets (S1 and S2) could be recorded. At an intensity of 1.9 photopic log Td four high frequency oscillations (O1--O4) were noticed and a discontinuity was observed in the corresponding luminance curve of the b-wave together with a sudden decrease in the magnitude of the standard deviation of the amplitude measures. The oscillations were noticed only on the ascending slope of the b-wave. With increasing stimulus intensity, their latency decreased at a slower rate than that of the b-wave and their number decreased. Each wavelet had an amplitude maximum at a certain stimulus intensity level. It was suggested that O1--O4 were generated by the activation of the photopic system and that S1 and S2 were of scotopic origin.

Transient visually evoked potential

A light delivered to the human eye will instigate changes in electrical potentials recorded over the visual cortex that last for some finite time, at least several hundred milliseconds. If the rate of stimulation is sufficiently low, the response is completed before the next visual stimulus arrives; the cortical potential is then called a transient evoked response. This review of the transient evoked potential describes the techniques by which the response is obtained, the problems inherent in its use, and procedures by which these difficulties can be overcome or minimized. A summary of the research results from the field enumerates those findings which have been well established and replicated by many investigators, those that are more controversial or as yet inconclusive, and the many practical applications of the technique for science and medicine. Finally, the needs for an underlying physiological theory and for future research are discussed.

Visually evoked potentials: theory, techniques and clinical applications

The visually evoked potential (VEP), the recording of which has recently been made possible by the development of computer averaging techniques, is a gross electrical signal generated by the occipital region of the cortex in response to visual stimulation. It is more specific than the electroencephalogram (EEG) and more sensitive to changes in the visual stimulus; thus, it can provide ophthalmologists and vision researchers with information about the human visual system that is unavailable by other methods. Clinically, the VEP is of special value in the areas of refraction, infant acuity, diseases of the optic nerve, color blindness, amblyopia and field defects. Theory, techniques and instrumentation are described, and applications of the VEP to clinical situations and to vision research are discussed.