It is well established that visual information travels from the eye by means of parallel pathways, with two principal pathways being the magnocellular (M) and parvocellular (P) pathways.
1 These two pathways subserve different visual functions, with the M pathway being achromatic and showing high temporal frequency tuning, and the P pathway showing chromatic sensitivity as well as high spatial resolution.
2,3 The retinal ganglion cells within each pathway,
4 along with corresponding cells in the lateral geniculate nucleus,
5 display anatomical and physiological differences,
4 prompting researchers to investigate whether different pathways may be differentially affected in disease.
6 Although direct measurement of the functional responses of each pathway is possible electrophysiologically in animal models, such invasive electrophysiological measures cannot be performed in human observers. Conventional electrophysiological measures in humans, such as the electroretinogram or visual evoked potential, do not distinguish between M and P pathway function. Therefore, finding behavioral methods that can distinguish between M and P pathways is important for human research.
There is a substantial overlap in the spatiotemporal tuning of the M and P pathways,
3 making it difficult to isolate the response of one pathway using simple contrast detection tasks. Pokorny and Smith
7 described a method that exploits contrast gain differences in the M and P pathways
5 to better isolate each pathway behaviorally. Briefly, their method involves a contrast discrimination task in which an observer is presented with a small group of luminous increments (pedestals), with one differing from the others in luminance (pedestal + ΔL). The observer must discriminate which target is more intense among a group. If the pedestals are briefly flashed (the pulsed-pedestal paradigm), the M pathway response saturates, leaving the nonsaturated P pathway to perform the discrimination task. If, however, the pedestals are steadily presented for a period before the presentation of the ΔL to be discriminated (the steady-pedestal paradigm), local adaptation mechanisms that are highly effective for the M, but not the P, pathway allow the M pathway to regain sensitivity and preferentially perform the discrimination task. An advantage of the method of Pokorny and Smith
7 is that M and P isolating stimuli are identical at the time of the contrast discrimination judgment, differing only in terms of the eye's adaptation state immediately preceding this judgment. Because of this, the influence of preretinal factors, such as refractive error and media opacities, would be expected to be similar for both stimuli, unlike if M and P functions were attempted to be separated through the use of grating targets of differing spatial frequencies or color. Their method has been used widely to measure how M and P functioning is altered in ocular, systemic, and cortical conditions (see review by Pokorny
6).
The method of Pokorny and Smith
7 method was used originally in comprehensive laboratory investigations on young, healthy observers, and involved comparatively long periods of adaptation. For example, participants preadapted to a uniform display for 2 minutes, and then to the steadily presented pedestal for a further 1 minute, before any measurements commenced.
7 Translations of their method for use in more clinical populations similarly have tended to use long adaptation times, although the selection of these times is somewhat ad hoc: for example, 2-minute preadaptation and 1-minute pedestal adaptation in a study of Leber's optic neuropathy,
8 2 minutes preadaptation plus 30 seconds before each stimulus condition for a study of amblyopia,
9 and 1 minute pedestal adaptation only (no preadaptation) in studies of glaucoma.
10,11 Given that test time often is limited with clinical observers, it would be useful to know if preadaptation and pedestal adaptation times might be reduced without significantly altering results. Investigation of the time course of adaptation processes reveals mechanisms operating on timescales of tens of milliseconds to a few seconds for contrast-based tasks,
12 suggesting that there may be substantial scope for reducing adaptation times currently used in current clinical investigations that use the Pokorny and Smith
7 method. Protracted adaptation times in the order of minutes do exist for tasks such as brightness perception, however.
13
Most threshold determining procedures inherently assume that threshold remains constant over time, and so are not suited to measuring adaptation effects that might change rapidly. We used a modified method, the “method of a thousand ZESTs,”
14,15 that can estimate thresholds with a temporal resolution of a couple of seconds. Using this technique, we explored the role of the preadaptation period in the Pokorny and Smith
7 protocol, as well as adaptation to, and recovery from, a steady pedestal. By investigating this, we aimed to determine what preadaptation and stimulus-adaptation times might be used in clinical studies to allow data collection in as short a time as possible.
The efficiency of yes/no and two alternative forced choice (2-AFC) ZEST procedures have been explored,
16,17 with yes/no being markedly more efficient due to having a much smaller false alarm rate (typically a few percent, in contrast to the 50% false alarm rate in 2-AFC experiments). The four alternative forced choice (4-AFC) procedure used in the method of Pokorny and Smith
7 has a false alarm rate of 25% that is intermediate to yes/no and 2-AFC procedures, suggesting a suitable number of presentations for a 4-AFC would be intermediate between the 8 presentations recommended for yes/no
16 and the 30 recommended for 2-AFC.
18 In addition to exploring adaptation times, we also assessed another important question in terms of test efficiency: how many stimulus presentations are required to return reliable threshold estimates in the Pokorny and Smith
7 method?