Congruency effects in the letter search task: Semantic activation in the absence of priming

Attentional Focus Manipulations Affect Naming Latencies of Neutral But Not of Incongruent Stroop Trials

People are slower and more error-prone when indicating the color of incongruent color words compared to that of neutral stimuli. This Stroop effect results from the concurrent semantic analysis of the word stimulus. It has long been considered a prime example of the automaticity of semantic activation. However, coloring as well as cuing only a single letter both reduce the Stroop effect to the point of being absent. Proposed underlying mechanisms include the blocking of semantic activation, an improved selectivity between the interfering stimulus dimensions, and slowed color processing. In order to test the validity of these differing accounts of the single-letter Stroop effect, we compared vocal responses to standard and single-letter Stroop stimuli in two experiments. Irrespective of whether participants maintained a wide (Experiment 1) or a narrow (Experiment 2) attentional focus, both single-letter coloring and single-letter cuing increased reaction times to neutral Stroop stimuli but left those to incongruent stimuli unaffected. Both curtailed semantic activation and improved selection for action should, however, speed up reactions to incongruent stimuli. Our data thus support an explanation of the single-letter Stroop phenomenon in terms of impeded color processing, possibly as a result of color-color interference.

Semantic activation and letter search: Blocking or suppression?

Two experiments investigated associative priming with a letter-search prime task where either the prime and letter probe were presented simultaneously, or the letter probe appeared 200 ms (Experiment 1) or 300 ms (Experiment 2) after the prime. Weak associative priming was observed in both experiments, but unlike Stolz and Besner (1996) we found no evidence that priming was increased when the probe was delayed. However, strong associative priming was observed when a semantic decision had to be made on the prime (Experiment 3). Our results are consistent with an account where semantic activation of the prime occurs but its action on the target is suppressed by the prime task. The persistence of weak priming effects with the letter search task is explained in terms of the low-frequency items used.

Electrophysiology reveals semantic priming at a short SOA irrespective of depth of prime processing

The otherwise robust behavioral semantic priming effect is reduced to the point of being absent when a letter search has to be performed on the prime word. As a result the automaticity of semantic activation has been called into question. It is unclear, however, in how far automatic processes are even measurable in the letter search priming paradigm as the prime task necessitates a long prime-probe stimulus-onset asynchrony (SOA). In a modified procedure, a short SOA can be realized by delaying the prime task response until after participants have made a lexical decision on the probe. While the absence of lexical decision priming has already been demonstrated in this design it seems premature to draw any definite conclusions from this purely behavioral result since event related potential (ERP) measures have been shown to be a more sensitive index of semantic activation. Using the modified paradigm we thus recorded ERP in addition to lexical decision times. Stimuli were presented at two different SOAs (240 ms vs. 840 ms) and participants performed either a grammatical discrimination (Experiment 1) or a letter search (Experiment 2) on the prime. Irrespective of prime task, the modulation of the N400, the ERP correlate of semantic activation, provided clear-cut evidence of semantic processing at the short SOA. Implications for theories of semantic activation as well as the constraints of the delayed prime task procedure are discussed.

Letter search does not affect semantic priming in a probe naming task

The normally robust semantic priming effect observed in lexical decision is usually reduced to the point of being absent, when a letter search has to be performed on the prime. It has been argued that semantic activation is thus not an automatic process but rather cognitively controlled and therefore adaptable to task demands. We examined the effects of letter search priming on pronunciation times and found a reliable semantic priming effect, following letter search that was not affected at all relative to a standard condition, where participants silently read the prime. Thus the nature of the prime task did not seem to affect the processing mode employed, semantic access occurred even though attention was focused on surface properties of the prime.