Visual Sensory Units and the Minimal Angle of Resolution*

Vernier acuity, crowding and cortical magnification

“…It is of special interest to attempt to relate the decline in visual performance with eccentricity to changes in the anatomy and physiology of the visual system (Weymouth, 1958;Cowey and Rolls, 1974;Rovamo et al, 1978;Koenderink et al 1978;Drasdo, 1977;Virsu and Rovamo, 1979;Schwartz 1980Schwartz , 1983. Our results, in agreement with Westheimer (1982) show that vernier acuity falls off faster with eccentricity than does grating acuity.…”

“…Weymouth, 1958;Cowey and Rolls, 1974;Drasdo, 1977;Rovamo et al, 1978). For a variety of resolution tasks, the retinal periphery performs as well as central vision when the stimulus size is "scaled" in proportion to the inverse of the linear cone density at each eccentricity (Weymouth, 1958;Koenderink et al, 1978;Rovamo et al, 1978;Virsu and Rovamo, 1979;Virsu, 1983;Wilson, 1970).…”

“…The inverse of the cortical magnification factor, M -1 (the number of degrees of visual space/mm of cortex) rises approximately linearly with eccentricity (Daniel and Whitteridge, 1961;Hubel and Wiesel, 1974;Dow et al, 1981) as do many spatial acuities (Weymouth, 1958;Center et al, 1981). Figure 10 replots the physiological data from Dow et al's two monkeys.…”

Peripheral positional acuity: Retinal and cortical constraints on 2-dot separation discrimination under photopic and scotopic conditions

“…For tasks which scale with eccentricity, there is often a striking resemblance between the rates of change of the behavioural thresholds and the morphology of either the retina or primary visual cortex. The variation of human resolution and contrast sensitivity within the central 10 deg parallels that of human and monkey cone spacing (Rolls & Cowey, 1970;Thibos, Cheney & Walsh, 1987;and Williams & Coletta, 1987), ganglion cell separationz (Weymouth, 1958;Rolls & Cowey, 1970;Drasdo, 1977;Rovamo, Virsu & Nasanen, 1978;Levi, Klein & Aitsebaomo, 1985;and Schein, 1987) receptive field size (Dow, Snyder & Bauer, 1981). The precision of bisecting the space between two features also scales to a single factor for a wide range of feature-separations , although its scaling factor is comparable to current estimates of the monkey Vl cortical magnification factor (Dow et al, 1981;Tootell, Silverman, Switkes & De Valois, 1982;and Van Essen, Newsome & Maunsell, 1984) which falls off three to four times faster than contrast sensitivity and resolution .…”

Topography of the retina and striate cortex and its relationship to visual acuity in rhesus monkeys and squirrel monkeys

“…There may be an explanation for this relationship in terms of the anatomy of the visual system, for with increasing eccentricity from the fovea Osterberg (1935) has noted a fall in cone density, Weymouth (1958) and van Buren (1963) have found a monotonic decrease in ganglion cell density, and Daniel and Whitteridge (1961) have found that the magnification factor at the visual striate cortex in monkeys falls monotonically. The magnification factor (M) is the number of mm of cortex measured linearly along the cortex representing each linear degree of visual field.…”