80 INTRUSION ERRORS

%

20

ECCENTRICITY

position for location errors and intrusion errors indicates that the differential effect of eccentricity on accuracy of report for interior letters and exterior letters was not reflected in just one type of error.

The results of Experiment 8 show that, even in single letter-pairs, the visual acuity gradient is not the only factor determining the relative perceptibility of interior letters and exterior letters. Most importantly, however, this factor becomes stronger, and the offset of the visual acuity gradient becomes larger, as the eccentricity of letter pairs increases. This clearly indicates that, rather than the visual acuity gradient, the asymmetry of lateral interference between interior letters and exterior letters was the important factor determining the relative perceptibility of interior letters and exterior letters.

Obviously, the possibility that pattern masks affected the relative perceptibility of interior letters and exterior letters cannot be excluded, since mask contours overlaid letter positions at every eccentricity. For example, it could be that mask contours overlaying exterior letters caused more lateral interference for interior letters than the interference for exterior letters caused by mask contours overlaying interior letters. Flowever, since this would have to assume an asymmetry in lateral interference, it only strengthens the case for the role of asymmetry of lateral interference between interior letters and exterior letters. Furthermore, although eccentricity may have weakened the effects of overlaying mask contours, such that masking was progressively weaker for exterior letters than for interior letters, a difference in the effectiveness of overlaying mask contours at different eccentricities could not explain the difference in accuracy of report between interior and exterior letters presented at the same eccentricity. Finally, the slight advantage for exterior letters over interior letters, when letter pairs occupied the same location in the visual field as letter pairs presented in Experiment 4, indicates that a similar difference between interior letters and exterior letters in Experiment 4, was not caused by mask contours presented at the opposite side of the fixation point.

Chapter Five: Lateral interference in arrays without middle letters 122 Conclusions

The first indication to emerge from the results of the experiments reported in this chapter is that the exterior-letter advantage in gapped arrays reflects a difference in the amount of lateral interference suffered by interior letters and exterior letters. Two possible sources of interference contributing to this difference have been identified. First, the perceptibility of interior letters relative to exterior letters may be limited by lateral interference from letters across the gap. Second, the asymmetry in lateral interference between interior letters and exterior letters may contribute to the exterior- letter advantage when the asymmetry is large enough to offset the visual acuity

gradient. However, the role of asymmetry of lateral interference in the exterior-letter advantage is probably more important when interior letters and exterior letters occupy more eccentric locations in the visual field than in letter pairs of arrays with 3-letter gaps centred across the fixation point. Therefore, the first source of interference contributing to the exterior-letter advantage will be discussed in more detail.

The results of Experiment 6 indicated that interior letters suffered more lateral interference from letters across the gap than exterior letters. At first, this was assumed to occur because the distance between interior letters and nontarget letter-pairs is smaller than the distance between exterior letters and nontarget letter-pairs. However, the effect of gap size also indicated that the absolute distance between the target and distractors across the gap was not the dominant factor in the amount of lateral interference suffered by interior letters relative to exterior letters. Indeed, interior letters still suffered more lateral interference than exterior letters when the distance to nontarget letter-pairs was equated for letters in either position. Rather, the position of letters relative to each other and to letters across the gap appeared to be the dominant factor in determining the amount of lateral interference suffered by letters in gapped arrays. Furthermore, the relative position of letters in gapped arrays appeared to be so important that performance for exterior letters was no different in arrays with 5-letter gaps than in arrays with 2-letter gaps. Although it is not clear how this difference

between interior letters and exterior letters occurs, it may be that spatial anchors provided by exterior boundaries were available for exterior letters, while no, or less useful, spatial anchors were provided by interior boundaries for interior letters (see Chapter Four).

However, even though interior letters may have suffered lateral interference from letters on the opposite side of the fixation point, the nature of this interference is not clear. The distance between letters on either side of the fixation point appears to exclude the possibility that feature interactions across the gap decreased the

perceptibility of interior letters relative to exterior letters. Furthermore, interior letters may have suffered lateral interference from nontarget letter-pairs and nontarget digit- pairs but not from pairs of ampersands, suggesting that the physical similarity between letters and digits, the fact that letters and digit both frequently occur in linear arrays, or the familiarity of letters and digits played a crucial role in the exterior-letter advantage in gapped arrays.

In Chapter Three and in the discussion of Experiment 6, it was suggested that the availability of spatial anchors for exterior letters may have been part of the explanation for the absence of an effect of middle letters on performance for exterior letters. That is, interior letters in complete arrays may have suffered lateral

interference from middle letters, which can explain the increase in accuracy of report for interior letters when middle letters were removed. However, exterior letters did not appear to suffer lateral interference from middle letters, as removing middle letters did not affect performance for exterior letters. Therefore, it was suggested that exterior letters may have been 'shielded' from lateral interference produced by middle letters. In particular, the effects of mask configuration observed in Experiments 3-5, indicated that this may, indeed, have been the case.

However, before any conclusions are drawn about the role of spatial anchors in the exterior-letter advantage, the nature of lateral interference between letters on either side of the fixation point in gapped arrays, and between middle letters and interior

Chapter Five: Lateral interference in arrays without middle letters 124 letters in complete arrays, was examined in Experiments 9-11, reported in Chapter Six.