Function Of Retinal Eccentricity Research Articles

Acuity for fine-grain motion and for two-dot spacing as a function of retinal eccentricity: Differences in specialization of the central and peripheral retina

The brief presentation in the peripheral field of two closely spaced luminous point stimuli, in rapid sequence, induces the illusion of a single dot moving over an extended path. This fine-grain movement illusion (FGMI) is particularly compelling under conditions of dark adaptation. The strength of the motion percept, assessed by a rating-scale procedure, was found to correlate well, over different flash-flash onset delays, with an objective measure of the illusion requiring discrimination of the direction of the flash-flash sequence. A direction-discrimination measure was used to determine the minimum dot separation that would reliably elicit an FGMI at retinal eccentricities of 5–25 deg. For comparison, measures of static spatial acuity was made based on the minimum angle of resolution of two simultaneous dot flashes, and on the threshold for discriminating the separation of two simultaneous dot flashes with variable initial spacing. The spatial threshold for FGMI was lower than that for each of the static measures at all peripheral eccentricities, and it increased more slowly with eccentricity than the other spatial thresholds, suggesting the involvement of separate visual pathways for generating percepts of motion and percepts of shape or location. The finding that in the periphery the grain for motion detection was finer than that for spatial discrimination constrains a class of motion-perception models that form an initial spatial description of the stimulus and then compute a temporal derivative.

Is amblyopia spatial frequency or retinal locus specific?

The visual deficit associated with amblyopia is thought to be both spatial frequency and retinal locus dependent. However, most data that have been obtained can be accounted for by either of these factors or by a combination of both. We have tried to distinguish between these three possibilities by measuring contrast sensitivity at different retinal loci using discrete localized patches of grating. For five of nine amblyopes, we find the contrast sensitivity deficit to be constant across the retina for a given spatial frequency. In only two cases were there substantial changes as a function of retinal eccentricity. Therefore, most of our data suggest that the visual deficit in amblyopia is primarily spatial frequency and not retinal locus specific.

Contour interaction as a function of retinal eccentricity.

Targets (squares with a gap on one side) were presented 0°, 2°, or 5° from the center of the fovea. The targets were surrounded by bar masks, and the spacing between targets and masks was varied parametrically. The resulting functions were used to estimate the extent of contour interaction at different target eccentricities. Our estimates for the extent of contour interaction were approximately .1° for targets at the center of the fovea, .24° for targets located 2° from the center, and .8° for targets 5° into the periphery.

Cortical RF width, cortical magnification factor and retinal eccentricity in strobe-reared cats

Receptive field (RF) width was measured as a function of retinal eccentricity in areas 17 and 18 of normal and strobe-reared animals. In the normal animal RF width increases with eccentricity both in areas 17 and 18 but more steeply in area 18 than in area 17. The latter fact is in keeping with the smaller dimension of area 18 compared to area 17 (see Orban, 1984, for review). In normal animals there is an inverse relationship between receptive field (RF) width and cortical magnification (Hubel & Wiesel, 1974; Albus, 1975; Dow et al., 1981).

Exponent of the Broca-Sulzer flash duration as a function of retinal eccentricity.

The flash duration producing maximum brightness-enhancement changes as a power function of luminance, that is, the time locus of the Broca-Sulzer flash duration TB, decreases as luminance L increases: TB = k x L beta , where beta is the duration exponent. The exponent of TB was estimated for 1-deg white flash fovea, 10-, 20-, 30-, 40-, 50-, and 60-deg temporal eccentricities of the dark-adapted human right eye. The following method of brightness maximization was used: the observer adjusted the duration of constant luminance flashes to produce a maximally bright flash. Duration was adjusted by rotating a microprocessor-based potentiometer. The results showed that the duration of the brightest flash TB decreased as L increased with the negative power exponent. Furthermore, the size of the negative exponent decreased as a function of increasing retinal eccentricity. Time-dependent brightness processing in the fovea and periphery is discussed in terms of the psychophysical brightness power function.

Orientation selectivity of the human visual system as a function of retinal eccentricity and visual hemifield.

An adaptation method was used to determine the specificity of orientation-selective channels in the human visual system at different retinal eccentricities (up to 16 deg) in both hemifields of each eye. For a vertical test grating, the elevation in contrast threshold produced by adapting to a high-contrast grating of the same spatial frequency but variable orientation was equated with the contrast levels of a vertical adapting grating that produced equivalent effects (equivalent-contrast transformation). This enabled comparisons to be made between the orientation tuning of the aftereffect at different retinal loci. For the spatial frequency employed (3 cycles deg-1), no systematic change in orientation selectivity was found as a function of either retinal eccentricity or the hemifield (and hence the cerebral hemisphere) stimulated.

Brightness exponent as a function of retinal eccentricity in the peripheral visual field: effects of dark and light adaptation.

Using a method of direct magnitude estimation, the exponent of the brightness power function was determined under dark and light adaptation at luminance levels well above threshold. The exponent was estimated for functions describing the brightness of stimuli presented at the fovea and the following peripheral retinal loci: 10, 20, 30, 40, and 50 deg nasally eccentric to the fovea along the horizontal meridian of the right eye. The exponent for a 1-sec flash was found to be approximately .33 at the fovea and increased slightly with increasing retinal eccentricity.The effect of adaptation on the brightness exponent was not so large when the target luminance was set well above threshold.

Response characteristics of visual and extravisual neurons in the pulvinar and lateral posterior nuclei of the cat

Extracellular single cell recordings were carried out from the pulvinar and lateralis posterior nuclei of cats maintained on a mixture of oxygen and nitrous oxide. Of the 320 cells tested for responses to visual and extravisual stimuli, 42.5% were found to be visually sensitive, and less than 10% yielded auditory and/or somatosensory responses. For most of the exclusively visually responsive neurons (37.8%) specific response properties could not be delimited (and these were classified as diffuse), whereas the next largest category was movement-sensitive (36.9%), and some were also directionally selective (18%) or showed orientation specificity (7.3%). The size of the visual receptive fields varied considerably, and there was a clear tendency for receptive field areas to increase as a function of retinal eccentricity. Of the 70 cells tested for binocularity, 54.3% responded to stimulation of either eye, 38.6% responded only to stimulation of the contralateral eye, and less than 7.1% were only ipsilaterally responsive. Polymodal and extravisually responsive cells were quite rare and these did not appear to be localized to any particular subarea of the pulvinar-lateralis posterior complex. Only three cells (in ventral lateralis posterior) were encountered which showed a clear spatial correspondence of visual and extravisual receptive fields.

Naso-temporal differences in human reaction time in the peripheral visual field

Visual reaction times (RTs) were measured using two trained, right-handed subjects with normal vision. A target was presented to 11 retinal loci: fovea; and 10°, 20°, 40° and 50° on both the nasal and the temporal sides of the fovea along the horizontal meridian of the right eye. Stimulus luminance was fixed at 8.50 cd/m 2. RT measured on the basis of a right-hand response increased as a function of retinal eccentricity from the fovea. Moreover, RT to stimulation on the nasal side of the retina was significantly faster than RT to stimulation on the temporal side at any corresponding retinal locations. Furthermore, the data suggested that the mean naso-temporal difference in RT increased as a function of the distance from the fovea. It was hypothesized that inter-hemispheric transmission time may be a major determinant of naso-temporal differences in RT.

Responses of visual, somatosensory, and auditory neurones in the golden hamster's superior colliculus.

1. The response characteristics of visual, somatosensory, and auditory neurones in the golden hamster's superior colliculus were investigated.2. As has been noted for other mammalian species, a distinct difference between the functional organizations of the superficial and deeper layers of the superior colliculus was observed.3. Neurones in the superficial layers were exclusively visual, with small receptive-fields, and generally did not show response decrements with repeated stimulation. The sizes of the receptive-fields did not vary appreciably as a function of retinal eccentricity.4. In the deeper layers, visual receptive-fields were large, or could not be accurately delimited, and response habituation was often evident. In addition, many cells in the deeper layers of the colliculus responded only to somatosensory stimuli. Far fewer cells, which appeared to be confined to the caudal portions of the colliculus, responded to auditory stimuli. Polymodal cells were also encountered.5. Selectivity to opposing directions of movement was tested for ninety-four visual cells. Using a ;null' criterion, 27.7% of these cells were judged to be directionally selective. A distribution of the preferred directions of these cells showed a significant preference for movement with an upper-nasal component. With a statistical criterion, 60.6% of these cells were considered to show a significant asymmetry in responding to movement in opposing directions.6. Directional selectivity was also tested for ninety-two cells following acute, unilateral, lesions of the visual cortex. For the eighty cells recorded, homolateral to the ablated cortex, 27.5% were judged as directionally selective using the statistical criterion, while 12.5% were selective with the ;null' criterion. Of the twelve cells isolated in the colliculus, contralateral to the lesions, seven were judged as directionally selective with the statistical, and three with the ;null' criterion.7. The effects of visual cortical lesions upon directional selectivity appeared to be confined to cells in the superficial layers of the colliculus. It was suggested that directional selectivity of many cells in the superficial layers of the tectum of the hamster is organized cortically.8. A clear spatial correspondence was observed for the receptive-fields of visual, somatosensory, and auditory neurones.9. As has been suggested for other species, the hamster's superior colliculus appears to play an important role in orienting the animal toward visual, somatosensory, and auditory stimuli.

The relation of apparent brightness to contrast threshold on a photopic background :Dependence on retinal position and target size

The luminance necessary for constant apparent brightness was measured as a function of retinal eccentricity at several photopic brightness levels and compared with the corresponding increment threshold functions. Thresholds rose substantially from the fovea to the periphery, but the contrast required for constant brightness showed little change as a function of eccentricity. A change of target size had relatively little effect on the shapes of the equal brightness curves. Two contributing factors to the difference between the threshold and equal brightness curves are systematic changes in (1) the apparent brightness at threshold (2) the exponent of the brightness power function. A neural mechanism is proposed which accounts for the results. A comparison with known retinal ganglion cell properties leads to the conclusion that this mechanism must be located proximally to the retina.

Photochromatic intervals as a function of retinal eccentricity for stimuli of different size.

Absolute light and color thresholds for broad-band stimuli were determined between 0° and 15° using test fields from 7′ to 104′ in diameter. Photochromatic intervals increase with retinal eccentricity. In the fovea, they are greatest for short wavelengths, followed by medium, and then by long wavelengths; in the periphery, the order for short and medium wavelengths is reversed. Toward the peripheral retina, interval size for short and medium wavelengths increases at a negative rate and for long wavelengths at a positive rate. Photochromatic intervals vary with stimulus size. The two variables change in the same direction for short wavelengths and in opposite directions for medium wavelengths. No correlation is found for long wavelengths.

Evoked cortical responses to checkerboard patterns: Effect of check-size as a function of retinal eccentricity

Visually evoked cortical responses were obtained from 3 Ss as they viewed checkerboard patterned light flashes (checks subtended from 0–180′ of arc) presented to different areas of the retina in respect to the fovea (0–27.5° eccentricity). An inverted “U” —shaped function was obtained between response amplitude and check-size. The check-size which elicited the greatest amplitude responses depended on eccentricity of retinal stimulation: when the foveal area was stimulated, relatively small checks (subtending 15–30′ of arc) evoked the greatest amplitude responses; when progressively more peripheral areas of the retina were stimulated (up to 7.5° eccentricity), progressively larger check-sizes evoked the greatest amplitude responses (subtending up to 60′ of arc). Check-size had little effect on evoked responses when the retina was stimulated 12.5–27.5° from the fovea. The results were discussed in relationship to the assumed size of retinal receptive field centers at different eccentricities in animals and humans.

Variation of the magnitude of the horizontal-vertical illusion with retinal eccentricity

The horizontal-vertical illusion was studied as a function of retinal eccentricity. It was found that the relation of illusion magnitude to vertical eccentricity is described by a U-shaped function with large amounts of reversed illusion for the more eccentric positions. Substantial effects due to horizontal eccentricity were also obtained, but these were not consistent across subjects. It is suggested that the flattening of the peripheral zones of the refracting surfaces of the eye may be involved in the variation of the illusion with retinal position, and that the astigmatic properties of the central portions of these surfaces may be a prime factor in the usual horizontal-vertical illusion.

Difference limen as a function of retinal eccentricity and background brightness.

The goal of this experiment was to obtain difference limens at several combinations of background brightness and retinal eccentricity. Background brightnesses were 0.0001, 0.001, 0.01, 0.1, 1, and 10 ft-L; retinal locations were the fovea and 2°, 6°, and 10° in the nasal portion. Monocular difference limens were determined using a 1° test stimulus superimposed at the center of a 27° background. The technique of threshold determination was the ascending series of the Method of Limits. The findings suggest that the curve describing log ΔB vs log background brightness becomes steeper as retinal eccentricity increases. At the three higher background brightnesses log ΔB increased as a function of eccentricity; at the three lower brightnesses log ΔB decreased with increasing eccentricity. In both cases the curves were negatively accelerated. It is suggested that the rising functions represent cone and the falling ones, rod thresholds. It appeared that the slopes of the curves relating ΔB to eccentricity were not as great at intermediate background brightnesses as they were at extreme values. This finding suggests a gradual rather than a sudden transition from rod to cone activity as brightness increases.