Is discrimination enhanced at the boundaries of perceptual categories? A negative case

Abstract

The human visual system imposes discrete perceptual categories on the continuous input space that is represented by the ratios of excitations of the cones in the retina. Is discrimination enhanced at the boundaries between perceptual hues, in the way that discrimination may be enhanced at the boundaries between speech sounds in hearing? In the chromaticity diagram, the locus of unique green separates colours that appear yellowish from those that appear bluish. Using a two-alternative spatial forced choice and an adapting field equivalent to the Daylight Illuminant D65, we measured chromatic discrimination along lines orthogonal to the locus of unique green. In experimental runs interleaved with these performance measurements, we obtained estimates of the phenomenological boundary from the same observers. No enhancement of objectively measured discrimination was observed at the category boundary between yellowish and bluish hues. Instead, thresholds were minimal at chromaticities where the ratio of long-wave to middle-wave cone excitation was the same as that for the background adapting field.

Circuitry to explain how the relative number of L and M cones shapes color experience

“…In contrast, according to our hypothesis, paradigms that require subjects to describe the hue of stimuli only engage the small population of midget ganglion cells that receive S cone input representing the noncardinal “Hering primaries.” Whereas noncardinal representations of color are often believed to have a cortical origin, some groups, for example Bosten, Beer, and MacLeod (2015) and Danilova and Mollon (2012a, 2012b, 2014), have considered the possibility that retinal ganglion cells tuned to a noncardinal axis may underlie behavior under some stimulus conditions.…”

Superior discrimination for hue than for saturation and an explanation in terms of correlated neural noise

“…The chromatic separation of the discriminanda was increased or decreased symmetrically around the reference chromaticity according to the observer's accuracy. The staircase procedure tracked 79.4% correct [23]. The staircase terminated after 15 reversals, the last 10 being averaged to give the threshold.…”

“…In both laboratories, stimuli were presented on calibrated Mitsubishi CRT monitors (Diamond Pro 2070), controlled by Cambridge Research Systems (CRS) graphics systems (Visage in St Petersburg, VSG 2/3 in Cambridge). For calibration procedures and monitor settings, see [23].…”

Is discrimination enhanced at a category boundary? The case of unique red

“…However, if we consider the issue from a visual science perspective, rather than from the perspective of cognitive psychology, then it is not necessary that the same underlying channel should invariably determine both discrimination thresholds and the phenomenological equilibrium: Under some conditions the two measures may depend on different neural channels. In the case of unique green, we have previously found no indication of enhanced discrimination at the category boundary [35]. Rather thresholds were always minimal when the L/(L+M) coordinate corresponded to that of the white adapting field, suggesting that the discrimination was based on one of the classical 'cardinal' chromatic channels, a channel that extracted the ratio of excitation in long-and middle-wave cones [32].…”

“…The stimulus array was presented for 150 ms. This brief duration was chosen to minimize the disturbance to the observer's state of adaptation: Self-adaptation to a prolonged test stimulus -as would have been possible in some earlier studies -would blur any attempt to show enhanced discrimination at a specific locus in the chromaticity diagram, since the underlying chromatic channels would begin their adjustment towards a new equilibrium point (For discussion, see [35]).…”