Demanda Judiciales

The method of limits begins by presenting a stimulus intensity well above or well below

threshold. When starting with a low stimulus, an ascending series ensues—the stimulus is increased every trial until the subject detects the stimulus. The series then terminates and a new series begins. When starting with a high stimulus, a descending series ensues—the stimulus is decreased until the subject can no longer detect the stimulus. Averaging multiple termination points give the 50% threshold. Figure 2.5 shows an ascending and descending

Figure 2.5: Method of Limits [adapted from Swan et al. (2007)].

series with the 50% threshold estimate. Proportions of detections at each stimulus level can also be analyzed to determine the entire psychometric function (Swan et al., 2007).

2.3.3.4 Adaptive Staircases

Anadaptive staircaseis a psychophysics procedure that results in stimulus levels concentrating

near a point of interest. A starting stimulus level is chosen and the following stimulus levels are determined by preceding stimulus levels and subject responses (Levitt, 1970). For a one- up-one-down staircase, the stimulus level decreases after the subject successfully detects the stimulus and increases after the subject fails to detect the stimulus. Figure 2.6 shows a one- up-one-down adaptive staircase, it converges to the 50% detection threshold. Other adaptive staircases converge to different threshold levels, e.g., a one-up-two-down staircase converges to the 70.7% detection threshold. Values near the 70.7% detection threshold are helpful for exploring the region between the 50% PSE and the 75% PSE+JND.

The step size (the change in stimulus level) may start high and then become lower each time a staircase reversal occurs. This allows the threshold to converge faster than when using a constant step size.

Figure 2.6: A one-up-one-down adaptive staircase. The stimulus intensities converge to a 50% detection threshold. [adapted from Razzaque (2005)].

2.3.4

Judgment Tasks

Detection rates, psychometric functions, and thresholds vary depending upon the experimen- tal task. For example, experiment designs differ in how many presentations the user sees before making a judgment as to whether the stimulus is present or not.

2.3.4.1 Yes/No Tasks

In ayes/no task, the experimenter provides a presentation and the subject states if she believes

the presentation contained the stimulus or not. Yes/no tasks can be performed quickly, since they require only a single presentation to be judged.

Yes/no tasks assume the subject knows what the “signal present” looks like. By providing a reference presentation with no stimulus before the test presentation, the subject is better able to judge what the “signal present” looks like.

The disadvantage of a yes/no task is that there is a large amount of bias because the resulting threshold largely depends upon the subject’s belief of “how much” perceived signal constitutes a “yes” response. A liberal responser may almost always say “yes” resulting in a low threshold whereas a conservative responder may almost always say “no” resulting in very different thresholds. Yes/no tasks are useful for determining single subjects’ thresholds (assuming their criterion remains the same throughout the experiment) but yes/no tasks are not very useful for computing statistics across subjects due to the large variance between subjects.

2.3.4.2 Same/Different Tasks

In asame/different task, the subject states if she believes two presentations are are the same

specific cues; the subject is free to discriminate on any basis she chooses. This is useful when a change in stimulus level causes several effects that are not well defined, or when “greater” and “less” are not well defined. The subject only states if the presentations different, without stating in what way they are different or if one is “greater” or “less” then the other.

The disadvantage of a same/different task is, like a yes/no task, there is a large amount of bias; the amount of difference required for the subject to state “different” is a subjective judgment. A subject may almost always choose “different” due to a criterion where even the slightest possibility of differences between the presentations results in a “different” judgment. Or a subject may almost always choose “same” unless the presentations are completely different.

2.3.4.3 Identification Tasks

In an n-alternative forced-choice (nAFC) identification task multiple presentations are provided, with one presentation containing the stimulus. The subject selects which presentation she believes contains the stimulus. When the stimulus is too weak to be detected, the PSE will take on a range of values with a detection rate of 1/n where n is the number of alternatives. In this case the threshold is often defined to be the stimulus level with a detection rate halfway between guessing and perfect judgment: 1/n+ ((n−1)/n)/2. (e.g., the 75% threshold for a 2AFC task).

Bias is smaller for an nAFC identification task than for a yes/no task and a same/different task because for an nAFC identification task, the subject is forced to choose between alternative presentations. Assuming the presentations are randomized, subjects are forced to choose the “best” answer and their choice is largely independent of their tendency to be a liberal or a conservative responder.

2AFC identification tasks assume that the subject knows what “signal present” looks like. If the subject perceives two presentations to be different, but she does not understand what a stimulus looks like then she may consistently choose the wrong presentation (the presentation with no stimulus instead of the presentation with the stimulus).

Athree-interval two-alternative forced-choice (3I-2AFC) identification task is similar to a

2AFC identification task, except that the first presentation contains a reference stimulus and the subject chooses from the following two presentations. This has the advantages of both the same/different task (since the subject chooses which of the two is different from the reference) and the 2AFC identification task (since bias is minimized). The 3I-2AFC identification task also has an advantage over a 3AFC identification task because the subject knows the first presentation is a reference presentation with no stimulus. The disadvantage of the 3I-2AFC identification task (and the 3AFC identificaiton task) compared to all previously described tasks is that trials take a longer period of time when presentations are presented sequentially

in time.

2.4

Latency

I definesystem delay in an HMD system to be the true time from the start of head movement to the time a pixel resulting from that movement responds to the viewpoint change without any compensation (e.g., prediction). I consider pixel response to be a change of intensity of 50% from starting intensity to intended intensity, unless otherwise noted.

I define latency to be the effective time from the start of head movement to the time a pixel resulting from that movement responds to the viewpoint change. If no pixel motion occurs due to system delay, then I consider latency to be zero. Compensation techniques can reduce latency but cannot reduce system delay. Section 2.4.4 provides an overview of some common compensation techniques to reduce system delay and latency.

2.4.1

Human Factors

Latency in an HMD-based system causes visual cues to lag behind other perceptual cues, creating sensory conflict. With latency and some head motion, the visual scene presented to a subject in an HMD moves incorrectly. This scene motion due to latency is known as “swimming” and has serious usability consequences. In this section, information is derived from Allison et al. (2001) unless otherwise referenced.