PRUEBA C: INTERPRETACIÓN DE UNA OBRA, ESTUDIO O FRAGMENTO

Duncan et al. (1994) defined the term attentional dwell time as the time period during which “an object that must be identified continues to occupy attentional capacity”(p. 313). They used a dual task paradigm as illustrated in Figure 2.2. Two alphanumeric characters were presented on each trial, each at one of the four locations (left, right, top, bottom) of the display in a random order and separated in time. One of the two characters was a green digit (“2” or “5”) that only appeared at one of the horizontal locations. The other was a red letter (“L” or “T”) that only appeared at one of the vertical locations. Either stimulus was presented for 40 to 65 ms, with a random temporal separation between 0 – 900 ms (from onset to onset, referred to as “stimulus onset asynchrony”, SOA). The presentation of either stimulus was immediately followed by a masking pattern which lasted for 250 ms to interrupt further processing of the stimulus6. There were three conditions in the experiment: Participants were asked to report the identity of both characters, of only the digit, or of only the letter. The interference of the first target (T1) on the second (T2) was measure by the accuracy of the T2 identification. The major finding was that when both characters must be identified, the T2 identification was impaired the most severe at the SOA of 100 to 300 ms. The interference increased with increasing SOA and peaked at the SOA of about 200 ms. It than declined gradually and recovered at SOA of about 450 ms. When only one of the characters must be identified, the identification was independent of the SOA and similar to the performance for T1 in the dual task. To enable a better comparability with visual search, Duncan et al. (1994) required the participants to indicate whether a predefined target is present or absent in a second experiment. They found similar patterns in the performance as the identification task. To differentiate the deployment of attention from shifts in space, the authors also conducted a variant of the design where both 6_{The technique of presenting a masking stimulus immediately after a brief (i.e., only presented}

for a very short time interval) visual stimulus (usually a target stimulus to be identified) is referred to as “backward masking.”

Figure 2.2: Sequence of frames on a single trial of the attentional time paradigm by Duncan et al. (1994). Reprinted from “Direct measurement of attentional dwell time in human vision” by J. Duncan, R. Ward, and K. Shapiro, 1994, Nature,

369(6478), p. 313. Copyright 1994 by the Nature Publishing Group. Reprinted with permission.

stimuli were presented successively at fixation and obtained similar estimates of attentional dwell time. The authors drew the conclusion that identifying a single item occupies attentional capacity for several hundred milliseconds.

Moore et al. (1996) argued that the estimate of attentional dwell time using the dual task paradigm of Duncan et al. (1994) may not be applicable to explaining the processing time of stimuli in visual search paradigm. They questioned the use of masks following each target and pointed out that the masking makes the discrimination more difficult because stimuli in standard visual are typically not masked. In two experiments, Moore et al. (1996) employed the same dual task paradigm as Duncan et al. (1994) and manipulated the masking of T1. They compared the interference on T2 identification under the conditions a) with immediate T1 masking, b)with delayed and c) without T1 masking. The accuracy of T2 identification (measured by two alternative forced choice) was impaired at short SOAs in all conditions but the interference was most profound with T1

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masking. Consistent with the findings of Duncan et al. (1994), the impairment first increased and peaked at the SOA of 200 ms in all conditions. However, the T2 identification under the condition without (or with delayed) T1 masking began recovering sooner (at SOA of 200 ms) than with immediate T1 masking (at SOA of 400 ms). At SOAs between 350 to 500 ms, the T2 accuracy was significantly higher without T1 masking than with T1 masking. The authors concluded that the estimate of attentional dwell time based on the data of Duncan et al. (1994) overestimated the time required to process a visual stimulus and suggested instead a value of approximately 200 ms based on their data.

Another finding of Moore et al. (1996) concerns the accuracy of T1 identi- fication. Although no systematic dependence of the T1 accuracy on SOA was observed, it was significantly higher without backward masking (≥97% and ≥94%) than with backward masking (84 – 90% and 75 – 85%). This indicates that backward masking interfered with the identification of T1 backwards. This finding is not surprising insofar that brief masks following a brief target stimulus have been demonstrating the ability to reduce or eliminate the identification of the target stimulus in various applications (e.g., Breitmeyer et al., 2006).

Moore et al. (1996) also pointed out two reasons why the estimate of attention dwell time using the dual task paradigm of Duncan et al. (1994) does not necessarily correspond to the minimum required processing time of a stimulus in visual search. Both arguments concern the sequential presentation of stimuli in the dual task paradigm. First, shifts of attention used in the dual task paradigm may differ from those used in visual search. Shifts may be delayed when stimuli are presented asynchronously because it is not possible to plan a shift until the stimulus to attend to is displayed. In standard visual search, stimuli are presented simultaneously and remain visible and unchanged until a response is made, which enables the planning of a sequence of shifts on the onset of stimulus display. Second, a simultaneous presentation also enables exploiting the parallel processing capacity of the early processing stage, such that the average time attention dwells on a stimulus is shorter.

Note that under the notion of two stage processing, the attentional dwell time estimated using the dual task paradigm does not only include the time during which attention is deployed on an object, but also necessarily the time of

early parallel processing which is not supposed to involve attention. The same problem applies to the estimate based on the RT × set size slope. Even if visual search were strictly parallel and exhaustive, the slope would reflect in an ideal situation the marginal time cost for processing an item added to the search set. Yet the early parallel processing stage has to be completed for the identification of an object. Thus the corresponding time is necessarily included in the marginal time cost. However, under the assumption of parallel processing of the first stage, this time should be reflected only partially in the marginal time cost.

Theeuwes et al. (2004) also addressed the problem of applicability of the attentional dwell time paradigm to explaining findings from visual search. They argued that the attentional dwell time paradigm was a variant of rapid serial visual presentation (RSVP) from the attentional blink research (see next section), where the operation of attention in working memory and not of attention in perception plays a key role. The authors suggested using an experimental paradigm that was more similar to the situation of standard visual search to infer on the attentional dwell time. They adopted an essentially different experimental paradigm, as illustrated in Figure 2.3. The inference on attentional dwell time relies neither on measuring the increment time of processing additional items, nor the impairment of identification of a second target (accuracy under speed constraint). Rather, it is based on the assumption that the RT to a stimulus should be shorter when attention is at the same (or a nearer) location at its appearance than when it is deployed at another (or a farther) location at its appearance. More specifically, the task includes the identification of a letter stimulus and the detection of a probe stimulus and this dual task forces observer to switch attention serially from one location to the next. After the initialization of a trial, an arrow appeared in the center of the display, pointing to one of the four corners of the display. The arrow indicated the location to which attention had to be shifted and remained visible and unchanged until the end of the trial. After 400 ms of the appearance of the arrow in the center, the target display was presented. It contained four stimuli, each at one of the four corners. The target arrow was presented at the location the center arrow was pointing to and it pointed to the target letter itself. The target letter was either the letter “E” or the mirror-reversed “E”. The other two stimuli were a distractor arrow and a

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distractor letter. Observer was required to identify the target letter. In this way, the identification of the target arrow was necessary for the identification of the target letter. That is, the processing of the target arrow must be completed before the start of the processing of the target letter to determine which item was the target letter. After a variable amount of time (50 to 650 ms), the target display was removed. On half of the trials, a small square probe was flashed for 33 ms at one of the four corners 50 ms after the removal of the target display, resulting in an SOA of 100 to 700 ms. Observer was required to press the space key as soon as possible once they detected the probe. At the end of the trial, observer indicated the identity of the target letter by pressing a key. The authors compared the mean RT to the probe presented at the target arrow location, at the target letter location and the distractor location depending on the SOAs. At short SOA (100 – 200 ms), probe RT at the first attended location was descriptively shorter than at the second attended location (not significant). At longer SOAs (300 – 500 ms), probe RT at the second attended location was significantly shorter. The authors inferred that attention was at the first location within the first 200 ms and at the second location after 300 ms. They concluded that the attentional dwell time should be around 250 ms.

Figure 2.3: Sequence of frames on a probe-present trial in the experiment of Theeuwes et al. (2004). Reprinted from “A new estimation of the duration of attentional dwell time” by J. Theeuwes, R. Godijn and J. Pratt, 2004, Psychonomic

Bulletin and Review, 11(1), p. 61. Copyright 2004 by the Psychonomic Society. Reprinted with permission.