The Lengthening Effect Revisited: A Reply to Prinzmetal and Wilson (1997) and Masin (1999)

Article excerpt

In the present study, the lengthening phenomenon (Tsal & Shalev, 1996), namely, the increase in perceived length of unattended lines, was reexamined in light of criticisms by Prinzmetal and Wilson (1997) and Masin (1999). Prinzmetal and Wilson suggested that the effect was not due to attentional factors but to the spatial interaction between the attended line and the cue used to direct attention. We have replicated the lengthening effect when both attended and unattended lines are preceded by cues at a nearby location, showing that the effect is not caused by spatial cues per se, but instead reflects an inherent property of the attentional system. Masin argued that the lengthening effect is not robust, because it occurs for some but not for all participants. In the present study, the lengthening effect was highly reliable, occurring for each participant for a variety of line lengths.

In a recent study (Tsal & Shalev, 1996), we demonstrated a directional effect of attention on length perception. Namely, we observed a systematic tendency to perceive a small vertical line as being longer when unattended than when attended. We explained this phenomenon by proposing that the visual field consists of a grid of small units termed attentional receptive fields (ARFs). When a stimulus appears within the boundaries of a unit, this unit signals its entire length to a central processor. That is, there is no spatial resolution within an ARF, and each unit operates on the basis of an all-or-none principle. In accordance with the resolution theory of visual attention (Tsal, Meiran, & Lamy, 1995) and with empirical findings by Cohen and Ivry (1991), we further proposed that the unattended visual field is composed of larger ARFs than is the attended field. When a line appears in the visual field, its ends cross the boundaries of adjacent units in most cases, so that these adjacent units are also activated. Because ARFs composing the unattended field are larger than those composing the attended field, an unattended line is systematically perceived as being longer than an attended one. The notion that attention improves spatial resolution is consistent with Yeshurun and Carrasco (1998, 1999,2000) and Carrasco, Williams, and Yeshurun (2002), who used a variety of perceptual tasks and showed that attention improves the quality of stimulus representation and that such enhancement results from fine spatial resolution.

The purpose of the present experiments was to reexamine this lengthening phenomenon in light of two recent studies questioning its validity (Prinzmetal & Wilson, 1997) and consistency (Masin, 1999). Prinzmetal and Wilson argued that the effect was due to the spatial interaction between the cues used to direct attention and the stimulus line, rather than to the operation of attention per se. Although Masin replicated the lengthening effect in one experiment, he argued that the effect is not reliable, because it occurred for some but not all participants in a second experiment. It is important to note that the three studies involved three different manipulations of attention: cuing (Tsal & Shalev, 1996), dual task (Prinzmetal & Wilson, 1997), and selective preparation (Masin, 1999). However, the arguments made by Prinzmetal and Wilson (1997) and by Masin (1999) are of sufficient importance to merit investigation within the framework of the cuing paradigm. Therefore, the purpose of the present experiment was not to replicate the studies of Prinzmetal and Wilson (1997) and Masin (1999), nor was it to assess the lengthening effect under different manipulations of attention. Instead, our objective was to reexamine the lengthening effect with a new version of a cuing paradigm that eliminates the potential problems pointed out by Prinzmetal and Wilson (1997) and by Masin (1999). Accordingly, we used a design that allowed assessing the effects of attention, while controlling for the possible confounding effects of the spatial cues, and performed a more detailed analysis to examine the lengthening effect for each participant. …