Hanna Benoni

Diluting the burden of load: perceptual load effects are simply dilution effects.

The substantial distractor interference obtained for small displays when the target appears alone is reduced in large displays when the target is embedded among neutral letters. This finding has been interpreted as reflecting low-load and high-load processing, respectively, thereby supporting the theory of perceptual load (Lavie & Tsal, 1994). However, a possible alternative interpretation of this effect is that the distractor is similarly processed in both displays, yet its interference in the large ones is diluted by the presence of the neutral letters. We separated the effects of load and dilution by introducing dilution displays. They contained as many letters as the high-load displays but were clearly distinguished from the target, thus allowing for a low-load processing mode. Distractor interference obtained under both the low-load and high-load conditions disappeared under the dilution condition. Hence, the display size effect traditionally misattributed to perceptual load is fully accounted for by dilution. Furthermore, when dilution is controlled for, it is high load not low load producing greater interference.

Where have we gone wrong? Perceptual load does not affect selective attention

The theory of perceptual load (Lavie & Tsal, 1994) proposes that with low load in relevant processing left over resources spill over to process irrelevant distractors. Interference could only be prevented under High-Load Conditions where relevant processing exhausts attentional resources. The theory is based primarily on the finding that distractor interference obtained in low load displays, when the target appears alone, is eliminated in high load displays when it is embedded among neutral letters. However, a possible alternative interpretation of this effect is that the distractor is similarly processed in both displays, yet its interference in the large displays is diluted by the presence of the neutral letters. We separated the possible effects of load and dilution by adding dilution displays that were high in dilution and low in perceptual load. In the first experiment these displays contained as many letters as the high load displays, but their neutral letters were clearly distinguished from the target, thereby allowing for a low load processing mode. In the second experiment we presented identical multicolor displays in the Dilution and High-Load Conditions. However, in the former the target color was known in advance (thereby preserving a low load processing mode) whereas in the latter it was not. In both experiments distractor interference was completely eliminated under the Dilution Condition. Thus, it is dilution not perceptual load affecting distractor processing.

Conceptual and methodological concerns in the theory of perceptual load

The present paper provides a short critical review of the theory of perceptual load. It closely examines the basic tenets and assumptions of the theory and identifies major conceptual and methodological problems that have been largely ignored in the literature. The discussion focuses on problems in the definition of the concept of perceptual load, on the circularity in the characterization and manipulation of perceptual load and the confusion between the concept of perceptual load and its operationalization. The paper also selectively reviews evidence supporting the theory as well as inconsistent evidence which proposed alternative dominant factors influencing the efficacy of attentional selection.

Controlling for dilution while manipulating load: Perceptual and sensory limitations are just two aspects of task difficulty

Lavie and de Fockert (2003) proposed that perceptual load and sensory limitations reflect two distinct mechanisms producing opposite effects: Perceptual load eliminates distractor interference, whereas sensory limitations enhance it. Tsal and Benoni (2010a) suggested that these results may have been due to the confounding effect of dilution—that is, to the presence (perceptual load) or absence (sensory load) of neutral items capable of diluting distractor interference. In the present study, we jointly manipulated dilution with perceptual load and with sensory degradation. The results show that, with both perceptual and sensory manipulations, the presence of diluting items eliminates distractor interference, whereas controlling for dilution increases distractor interference. We therefore conclude that perceptual and sensory limitations are just two aspects of task difficulty.

Attentional sets influence perceptual load effects, but not dilution effects

Perceptual load theory [Lavie, N. (1995). Perceptual load as a necessary condition for selective attention. Journal of Experimental Psychology: Human Perception and Performance, 21, 451–468.; Lavie, N., & Tsal, Y. (1994) Perceptual load as a major determinant of the locus of selection in visual attention. Perception & Psychophysics, 56, 183–197.] proposes that interference from distractors can only be avoided in situations of high perceptual load. This theory has been supported by blocked design manipulations separating low load (when the target appears alone) and high load (when the target is embedded among neutral letters). Tsal and Benoni [(2010a). Diluting the burden of load: Perceptual load effects are simply dilution effects. Journal of Experimental Psychology: Human Perception and Performance, 36, 1645–1656.; Benoni, H., & Tsal, Y. (2010). Where have we gone wrong? Perceptual load does not affect selective attention. Vision Research, 50, 1292–1298.] have recently shown that these manipulations confound perceptual load with “dilution” (the mere presence of additional heterogeneous items in high-load situations). Theeuwes, Kramer, and Belopolsky [(2004). Attentional set interacts with perceptual load in visual search. Psychonomic Bulletin & Review, 11, 697–702.] independently questioned load theory by suggesting that attentional sets might also affect distractor interference. When high load and low load were intermixed, and participants could not prepare for the presentation that followed, both the low-load and high-load trials showed distractor interference. This result may also challenge the dilution account, which proposes a stimulus-driven mechanism. In the current study, we presented subjects with both fixed and mixed blocks, including a mix of dilution trials with low-load trials and with high-load trials. We thus separated the effect of dilution from load and tested the influence of attentional sets on each component. The results revealed that whereas perceptual load effects are influenced by attentional sets, the dilution component is not. This strengthens the notion that dilution is a stimulus-driven mechanism, which enables effective selectivity.

Bottom-up capture is a top-down phenomenon.

pAttentional capture produced by a task-irrelevant color singleton is assumed to reflect stimulus-driven processing, completely governed by bottom-up factors. Results from two converging paradigms - search reaction time and free letter report - support the conclusion that singleton capture is fully determined by (top-down) expectation. The first series of experiments resulted in strong expectation-based capture when the color singleton is expected in a given location (or color) but is not actually presented. For example, when participants are presented with a sequence of trials with a red singleton (among green items) randomly switching among four positions, attention ends up being captured by red stimuli within a non-singleton (four color) display (red, green, blue and yellow) presented unexpectedly. In the second series of experiments we eliminated singleton expectation by embedding singleton displays with visually similar but non-singleton displays. For example, participants were presented with nine letter circular displays with all possible combinations of red and white letters, of which a fourth of the trials were singleton displays (one red letter among eight white letters, or one white letter among eight red letters). In this condition singleton capture was completely eliminated (i.e. did not exceed chance - calculated separately for each participant). We conclude that singlet on capture is a top-down phenomenon originated from pervasive mental programs that direct the attentional system to seek out unique items in the visual field irrespective of task demands. We speculate that the rationale for such programs may derive from the fact that whereas processing several identical non-singleton items is redundant, processing the singleton, which is statistically most likely to be missed (one vs. many), provides important unique information. Meeting abstract presented at VSS 2015.