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The contrast sensitivity of the peripheral visual field to drifting sinusoidal gratings
Yuion Rus. Vol. 14. pp. 9OHOf.
Perpamon
Press
1974. Printed
in Great
LETTER THE CONTRAST
SENSITIVITY
Bntain
TO THE EDITORS
OF THE PERIPHERAL VISUAL FIELD TO DRIFTING SINUSOIDAL GRATINGS (Received 29 October 1973)
A recent study of spatial vision in the periphery of the visual field (Sharpe and Tolhurst, 1973a) showed that the selectivity of orientation channels determined by spatial adaptation is greater for moving than for stationary stimuli. Just the. reverse is true for central vision (Sharpe and Tolhurst, 1973b). Now, for central vision, well defined velocity channels have been shown to exist (Tolhurst, Sharpe and Hart, 1973); it was concluded on the basis of two types of evidence that the rate of movement of a drifting sine wave grating is probably analysed in terms of its temporal frequency (c/se4 rather than its actual velocity (deg/sec)..Firstly, when the dependence of contrast sensitivity upon temporal frequency of drift was determined it was found that the curves for different spatial frequencies (c/deg) all peaked at the same temporal frequency, about 6 c/set; therefore, the optimal velocity varied inversely as the spatial frequency. And secondly, movement threshold elevation curves produced by spatial adaptation to high contrast drifting gratings peaked at the adapting temporal frequency. In view of the known differences between peripheral and central vision with regard to the orientation specificity of channels responding to drifing gratings, one naturally wonders whether there may also be differences in the analysis of drift rate. Preliminary evidence already exists in the literature to suggest that this may indeed be the case. Sharpe (1972) found that the appearance of the entoptic shadows of the retinal blood vessels lying over the peripheral visual field depended on the temporal frequency of their controlled movement. At lower frequencies of movement (2-8 c/set) the highly detailed branching capillary networks were visible; at higher frequencies (10-20 c/set) the coarser arterioles became visible and the finer capillaries ceased to be visible. This is not what one would expect on the basis of the curves of Tolhurst et al. (1973, Fig. 1) relating contrast sensitivity for sinusoidal gratings to temporal frequency of drift for central vision; all spatial frequencies have their maximum contrast sensitivity at 6 c/set. To study this difference more quantitatively, the contrast sensitivity for drifting sinusoidal gratings viewed with the peripheral visual field was determined as a function of temporal frequency of drift. The subject fixated a small spot 10” to the left of the centre of the screen using his left eye alone. (For more details of
methods, see Sharpe and Tolhurst, 1973a.) The gratings were vertical and drifted away from the fovea on the . temporal hemtretina. Figure 1 shows the results. In contrast to the central part of the visual field, different spatial frequencies have different temporal frequencies for maximum contrast sensitivity; lower spatial frequencies have higher optimum temporal frequencies, e.g. the 15 c/deg curve peaks at 5 c/set. whereas the O-8c/deg curve peaks at 10 c/set. ..
b b
b
0 b
=a
b
b
b
b
b
b
I
0
”
_
.
m
......
.
.
.
._....
m
b _
lo
.
.
,._
loo
cRQaycT(Ha
Fig. 1. Contrast sensitivity as a function of temporal frequency for drifting gratings of different spatial frequencies viewed peripherally. 0.8 c/&g, (V); I.5 c/de& (0); 3.5 c/deg, (m); 5.5 c/deg, (A). Vertical arrows indicate the temporal frequency for maximum contrast sensitivity for each different spatial frequency. Gratings were vertical and drifted in a direction perpendicular to their orientation, away from the fovea. Viewed monocularly IO” into left temporal hemiretina. Screen subtended 3.8” of visual angle, and its space-averaged mean luminance was about 140 cd/m*. Subject was CRS, a well-corrected myope.
What is the advantage to the visual system of such an organization? In central vision, where the optimal temporal frequency is the same for all spatial frequencies, the optimal velocity will be inversely proportional to the spatial frequency. In the periphery, however, the lower the spatial frequency, the higher the preferred
905
906
Letter to the Ldltors
temporal frequency; this relationship increases the preferred velocity for low spatial frequencies to more than what an inverse proportionality would produce. This organization would tend to promote the capability of the peripheral visual field for detecting rapidly moving coarse detail. That the periphery is specialized for the detection of moving stimuli is suggested by the greater orientation specificity for moving stimuh; its preference for coarser detail is shown by the fact that the contrast sensitivity curve peaks at a lower spatial frequency (about 2c/deg) than in the fovea (about 4 c/deg) (Sharpe and Tolhurst, 1973a). For the higher spatial frequencies. this type of organization will decrease the preferred velocity in the periphery relative to the fovea. Now, the distance between individual photoreceptors increases with increasing eccentricity, and the number of retinal ganglion cells per square millimeter decreases (Stone. 1965).
This has the effect of increasing the time between stimulation of adjacent detectors in the periphery relative to the fovea, when the slow drifts of fixational eye movements cause images to slide over the retinal mosaic; in a sense. then. the periphery “sees” images moving at a “slower” velocity. uo see low contrast fine detail in the periphery. images must move successively from one ’ Recipient of a Studentship from the MRC of Canada. Present address: Aviation Medical Research Unit. Department of Physiology, McGill University. Montreal. Quebec. Canada.
photoreceptor to the next and temporal summatton 15 needed (Sharpe. 1972).] Decreasing the preferred tcmporal frequency in the periphery will decrease the preferred velocity to counteract this effect; as well. this will bring the preferred velocity for higher harmonics closer to the velocity of the slow drifts of fixational eye movements. Ph~sioloyical Laborator\,. University of Cantbridge. Cambridge. England
C. R. SHAKPE'
REFERENCES
Sharpe C. R. (1972) The visibility and fading of thin lines visualized by their controlled movement across the retina. J. Physiol., Land. 222, 113 134. Sharpe C. R. and Tolhurst D. J. (1973a) Orientation and spatial frequency channels in peripheral vision. Vision Rex 13,2103-2112. Sharpe C. R. and Tolhurst D. J. (1973b) Temporal modulation and the orientation-specificity of human channels. Perceptioti 2. 2329.
Stone J. (1965) A quantitative analysis of the distribution of ganglion cells in the eat’s retina. J. camp. New-of. 124, 337-352. Tolhurst D. J., Sharpe C. R. and Hart G. (1973) The analysis of the drift rate of moving sinusoidal gratings. Vision Rex 13,254s2555.
Perpamon
Press
1974. Printed
in Great
LETTER THE CONTRAST
SENSITIVITY
Bntain
TO THE EDITORS
OF THE PERIPHERAL VISUAL FIELD TO DRIFTING SINUSOIDAL GRATINGS (Received 29 October 1973)
A recent study of spatial vision in the periphery of the visual field (Sharpe and Tolhurst, 1973a) showed that the selectivity of orientation channels determined by spatial adaptation is greater for moving than for stationary stimuli. Just the. reverse is true for central vision (Sharpe and Tolhurst, 1973b). Now, for central vision, well defined velocity channels have been shown to exist (Tolhurst, Sharpe and Hart, 1973); it was concluded on the basis of two types of evidence that the rate of movement of a drifting sine wave grating is probably analysed in terms of its temporal frequency (c/se4 rather than its actual velocity (deg/sec)..Firstly, when the dependence of contrast sensitivity upon temporal frequency of drift was determined it was found that the curves for different spatial frequencies (c/deg) all peaked at the same temporal frequency, about 6 c/set; therefore, the optimal velocity varied inversely as the spatial frequency. And secondly, movement threshold elevation curves produced by spatial adaptation to high contrast drifting gratings peaked at the adapting temporal frequency. In view of the known differences between peripheral and central vision with regard to the orientation specificity of channels responding to drifing gratings, one naturally wonders whether there may also be differences in the analysis of drift rate. Preliminary evidence already exists in the literature to suggest that this may indeed be the case. Sharpe (1972) found that the appearance of the entoptic shadows of the retinal blood vessels lying over the peripheral visual field depended on the temporal frequency of their controlled movement. At lower frequencies of movement (2-8 c/set) the highly detailed branching capillary networks were visible; at higher frequencies (10-20 c/set) the coarser arterioles became visible and the finer capillaries ceased to be visible. This is not what one would expect on the basis of the curves of Tolhurst et al. (1973, Fig. 1) relating contrast sensitivity for sinusoidal gratings to temporal frequency of drift for central vision; all spatial frequencies have their maximum contrast sensitivity at 6 c/set. To study this difference more quantitatively, the contrast sensitivity for drifting sinusoidal gratings viewed with the peripheral visual field was determined as a function of temporal frequency of drift. The subject fixated a small spot 10” to the left of the centre of the screen using his left eye alone. (For more details of
methods, see Sharpe and Tolhurst, 1973a.) The gratings were vertical and drifted away from the fovea on the . temporal hemtretina. Figure 1 shows the results. In contrast to the central part of the visual field, different spatial frequencies have different temporal frequencies for maximum contrast sensitivity; lower spatial frequencies have higher optimum temporal frequencies, e.g. the 15 c/deg curve peaks at 5 c/set. whereas the O-8c/deg curve peaks at 10 c/set. ..
b b
b
0 b
=a
b
b
b
b
b
b
I
0
”
_
.
m
......
.
.
.
._....
m
b _
lo
.
.
,._
loo
cRQaycT(Ha
Fig. 1. Contrast sensitivity as a function of temporal frequency for drifting gratings of different spatial frequencies viewed peripherally. 0.8 c/&g, (V); I.5 c/de& (0); 3.5 c/deg, (m); 5.5 c/deg, (A). Vertical arrows indicate the temporal frequency for maximum contrast sensitivity for each different spatial frequency. Gratings were vertical and drifted in a direction perpendicular to their orientation, away from the fovea. Viewed monocularly IO” into left temporal hemiretina. Screen subtended 3.8” of visual angle, and its space-averaged mean luminance was about 140 cd/m*. Subject was CRS, a well-corrected myope.
What is the advantage to the visual system of such an organization? In central vision, where the optimal temporal frequency is the same for all spatial frequencies, the optimal velocity will be inversely proportional to the spatial frequency. In the periphery, however, the lower the spatial frequency, the higher the preferred
905
906
Letter to the Ldltors
temporal frequency; this relationship increases the preferred velocity for low spatial frequencies to more than what an inverse proportionality would produce. This organization would tend to promote the capability of the peripheral visual field for detecting rapidly moving coarse detail. That the periphery is specialized for the detection of moving stimuli is suggested by the greater orientation specificity for moving stimuh; its preference for coarser detail is shown by the fact that the contrast sensitivity curve peaks at a lower spatial frequency (about 2c/deg) than in the fovea (about 4 c/deg) (Sharpe and Tolhurst, 1973a). For the higher spatial frequencies. this type of organization will decrease the preferred velocity in the periphery relative to the fovea. Now, the distance between individual photoreceptors increases with increasing eccentricity, and the number of retinal ganglion cells per square millimeter decreases (Stone. 1965).
This has the effect of increasing the time between stimulation of adjacent detectors in the periphery relative to the fovea, when the slow drifts of fixational eye movements cause images to slide over the retinal mosaic; in a sense. then. the periphery “sees” images moving at a “slower” velocity. uo see low contrast fine detail in the periphery. images must move successively from one ’ Recipient of a Studentship from the MRC of Canada. Present address: Aviation Medical Research Unit. Department of Physiology, McGill University. Montreal. Quebec. Canada.
photoreceptor to the next and temporal summatton 15 needed (Sharpe. 1972).] Decreasing the preferred tcmporal frequency in the periphery will decrease the preferred velocity to counteract this effect; as well. this will bring the preferred velocity for higher harmonics closer to the velocity of the slow drifts of fixational eye movements. Ph~sioloyical Laborator\,. University of Cantbridge. Cambridge. England
C. R. SHAKPE'
REFERENCES
Sharpe C. R. (1972) The visibility and fading of thin lines visualized by their controlled movement across the retina. J. Physiol., Land. 222, 113 134. Sharpe C. R. and Tolhurst D. J. (1973a) Orientation and spatial frequency channels in peripheral vision. Vision Rex 13,2103-2112. Sharpe C. R. and Tolhurst D. J. (1973b) Temporal modulation and the orientation-specificity of human channels. Perceptioti 2. 2329.
Stone J. (1965) A quantitative analysis of the distribution of ganglion cells in the eat’s retina. J. camp. New-of. 124, 337-352. Tolhurst D. J., Sharpe C. R. and Hart G. (1973) The analysis of the drift rate of moving sinusoidal gratings. Vision Rex 13,254s2555.