Evaluación y caracterización de los 160 clones

Chapter Five: Lateral interference in arrays without middle letters 116 These effects of gap size on the exterior-letter advantage in filler trials of

Experiment 6 are not easily explained by the effects of overlaying mask contours. Even though overlaying mask contours may be more disrupting for targets presented

peripherally than for targets presented foveally, it is not clear how this could explain that the effect of position in the visual field appears to be different for interior letters and exterior letters. However, the effects of gap size observed in the filler trials of Experiment 6 on the exterior-letter advantage in nontarget letter-pairs could be quite easily explained if asymmetry of lateral interference between members of unilaterally presented letter pairs is added to the equation. Indeed, it could be that, when letter- pairs were moved away from the fixation point, the decrease in lateral interference for interior members from letter-pairs on the opposite side of the fixation point was (more than) compensated for by an increase in the asymmetry of lateral interference between interior and exterior members. That is, with small gaps, the asymmetry may have been small, but interference from letters across the gap would have been strong, while with large gaps interior members of non target letter-pairs would have suffered little

interference from letters across the gap, but the asymmetry between interior and

exterior members of nontarget letter-pairs may have been large, because of the increase in eccentricity of their position in the visual field.

However, because the target stimuli used in Experiment 6 always contained letter pairs on both side of the fixation point, the results of the filler trials of Experiment 6 tell us little about the role of the asymmetry of lateral interference

between interior letters and exterior letters of single, unilaterally presented, letter pairs. Therefore, Experiment 8 investigated the relative perceptibility of interior letters and exterior letters in single letter-pairs, presented unilaterally at four different distances from the fixation point. The locations occupied by letter pairs in Experiment 8 were the same as the locations occupied by nontarget letter-pairs in the test trials of

Experiment 6. If the asymmetry in the lateral interference between interior letters and exterior letters is larger for letter pairs presented further out into the periphery of the

visual field, it would be expected that the exterior-letter advantage increases as letters pairs are moved away from the fixation point. More specifically, although it is expected that both members are reported less accurately as letter pairs are presented in more eccentric locations due to a decrease in visual acuity further away from the fixation point (which was indeed the case for letter pairs in variable locations in

Experiment 6), if asymmetry in lateral interference between interior letters and exterior letters increases as letter pairs are moved away from the fixation point, the decrease in accuracy of report for interior letters should be steeper than for exterior letters. In particular, a differential effect of eccentricity for interior letters and exterior letters may be revealed if exterior letters are reported more accurately than interior letters even when the distance from the fixation point of these letters is equated. That is to say, when a letter pair is moved one letter-space away from the fixation point, interior letters would occupy the same screen position as previously occupied by exterior letters.

Finally, although backward pattern masks were used, mask contours overlaid only the positions occupied by letter pairs in the preceding target stimulus. Therefore, if exterior letters were reported slightly more accurately than interior letters in

Experiment 4, because mask contours were presented at both sides of the fixation point, no difference between interior letters and exterior letters of letter pairs, occupying the same positions as in Experiment 4, should be observed in Experiment 8. An example of target stimuli in each of the four distance conditions, and an example of mask stimuli is presented in Figure 5.9.

Method

Subjects. 16 subjects from the same population as previous experiments participated in a single 1-hr session in Experiment 8.

Stimuli. Stimuli were constructed by randomly selecting two letters from the letter set used in Experiment 1 (i.e., b, d, f, g, h, n, p, q, t, v, x z), the only

Figure 5.9a. An example of target displays with letter-pairs presented at four eccentricities on the left of the fixation point Experiment 8. Gap sizes refer to the corresponding conditions in the filler trials of Experiment 6.

EXPERIMENT 8

2-LETTER GAP 3-LETTER GAP

conditions in the filler trials of Experiment 6.

EXPERIMENT 8

2-LETTER GAP 3-LETTER GAP

Chapter Five: Lateral interference in arrays without middle letters 118 the letters occurred in interior and exterior positions an equal number of times. For each subject a different stimulus set was constructed. Stimulus sets contained 48 test stimuli.

Visual Conditions. For each trial a different mask was constructed. Only one mask configuration was used in this experiment; mask contours overlaid only the positions occupied by the letters in the preceding display (see Figure 5.9). The stimuli were presented on one side of the fixation point in adjacent positions, at the same locations as the letters in variable locations in Experiment 6. Thus, letters were presented at four different distances from the fixation point: The eccentricity from the fixation point to the edge of interior letters was approximately 0.16°, 0.45^, 0.74° and

1.04°, while the eccentricity from the fixation point to the edge of exterior letters was approximately 0.45°, 0.74°, 1,04° and 1.33° in the 2-letter, 3-letter, 4-letter and 5- letter gap conditions.

Design. Half the stimuli were presented at the left of the fixation point, and

half were presented at the right of the fixation point. Each stimulus was shown 4 ; times; once in each of the four distance conditions. Average exposure duration over all i subjects was 20.7 msec. All remaining aspects of Experiment 8 were identical to those i

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of Experiment 6. I

{ Results and Discussion

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Correct reports

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