Biomedical subjects

D B Kirk

Publications and source records attributed to D B Kirk.

4 recordsLinked to original sources

PubMed1986

The pi mechanisms of W S Stiles: an historical review.

The origins, development, and status of the pi mechanism theory are reviewed. The paper is divided into four sections. In the first section Stiles's general ideas about 'color mechanisms' are examined, and it is concluded that foremost amongst these is a mathematical theory that specifies certain formal rules or laws that should govern a certain class of observations. In the case of pi mechanisms, the class of observations is that of two-color thresholds, and the defining laws are the two well-known displacement laws. Five other laws that two-color increment-threshold observations should obey, if the latter are governed by ideal pi mechanisms, are abstracted from Stiles's writings. In the second section literature pertinent to the testing of the seven Stilesian laws is reviewed, and it is asked whether or not the seven pi mechanisms of Stiles do in fact obey the laws. In the third section the relation of the pi mechanism concept to physiological concepts is examined, and its relation to the 'cone fundamental' is discussed; the evidence pertinent to the question: "Are any of the pi mechanisms of the single-fundamental type?" is then reviewed. The last section is devoted to the evolution of Stiles's ideas in the period after 1959 when Stiles's own investigations and those of others propelled him to reject the initial (1953) pi mechanism theory as an adequate characterization of the data of the two-color threshold.

Adaptation, Ocular↗

PubMed1985

Electroretinograms evoked by sinusoidal excitation of human cones.

The amplitude and phase of the fundamental Fourier component of the human electroretinogram (e.r.g.) were recorded with a synchronous detection method under conditions in which each of the three species of cones can be assumed most sensitive in turn. Weber-Fechner behaviour is well established at, or more distal in the retina than, the source of these voltages. Results over the frequency range 7-50 Hz exclude a diffusion model of human flicker perception. The e.r.g. phase vs. frequency plot found with a 'red' test differs from that obtained with a 'green'. The shapes of the e.r.g. field sensitivity action spectra agree with those of the subject's IIj(mu) (j = 3, 4 and 5) mechanisms of Stiles and with in situ measurements of the absorbance spectra of human cone pigments. Threshold phase with each test was independent of background wave-length but, consistent with the results in 3, the phase of the response to the 'red' test (25 Hz) differed significantly from that to the 'green'. If these differences resulted from the absorption of test photons of different colours at different points along the outer segment (independent of cone spectral sensitivity), they would be as clear on dichromats as on trichromats. Results on a protanope are inconsistent with this prediction. We infer that differences in phase are due to the different kinetics of different cone mechanisms and that the e.r.g.s studied here are evoked by exciting only the most sensitive cone mechanism, even though dark-adaptation studies prove that at e.r.g. threshold the test is well above psychophysical threshold for all three cone species. If the inference in 5 is correct, studies of sensitivity across the retina suggest that the spatial distribution of long-, middle- and short-wave-sensitive cones in the human retina differ remarkably.

Dark Adaptation↗

PubMed1984

The color-matching functions for lambda greater than or equal to 540 nm: incompatibility with the blue fundamental.

The question of the blue fundamental contribution to foveal color matches for lambda greater than or equal to 540 nm is examined. It is shown that, with two almost universally accepted assumptions, such a blue fundamental contribution leads to a contradiction with the observed color-matching functions. It is concluded that the blue fundamental does not contribute to foveal color matches for lambda greater than or equal to 540 nm.

Color Perception↗

PubMed1982

Color discrimination at threshold: the approach through increment threshold sensitivity.

A close correlation between Stiles's increment threshold (pi-mechanism) approach and color discrimination at threshold was found: a sharp transition from detection of two lights via two pi-mechanisms (pi 1 and pi 4) to detection via only one pi-mechanism (pi 1) was paralleled by a sharp decline from excellent discrimination between those two lights (even with the rods bleached) to no discrimination. This parallel between detection and discrimination suggests that Stiles's approach can isolate single visual pathways with distinct perceptual correlates.

Adult↗