Stimulus-response compatibility in representational space
Introduction
When simple light stimuli are presented to either the left or the right visual hemifield, a human observer’s manual reaction times (RTs) are consistently faster for the hand ipsilateral to the stimulus than for the contralateral hand (3, 4for overviews). Historically this RT difference has been interpreted as an interhemispheric transmission time since the afforded visuomotor integration requires a callosal transfer in the case of responding with the contralateral but not the ipsilateral hand [23]. While this explanation is still considered valid for simple unimanual go—no go decision tasks (in which, for any given block of trials, only one hand is responding), interhemispheric transmission is not the source of RT differences between the hands in bimanual RT tasks in which the choice of whether or not to respond has been replaced by the choice of whether the response should be initiated by the left or the right hand 1, 4. The former type of decision is typically required in the ‘‘Poffenberger paradigm’’ [23], the latter in experiments studying spatial stimulus-response (S-R) compatibility 1, 3. In the Poffenberger paradigm, crossing the hands, so that the right hand manipulates the key in the left hemispace and the left hand the key in the right hemispace, does not alter the RT advantage of the right hand for the right stimulus and of the left hand for the left stimulus 1, 3. However, hand crossing leads to a reversal of the hand-hemispace interaction in a S-R compatibility paradigm 1, 7, 26, 27. Hence, intermanual RT differences in choice RT tasks to unilateral targets arise from an incongruency between stimulus and response key rather than between stimulus and responding hand. As a consequence, these differences are typically accounted for in terms of an interference between two conflicting spatial codes, one for the location of the stimulus and one for the location of response initiation [27].
The two experiments reported here show that spatial S-R compatibility effects can also be observed when target stimuli are presented in the center of the visual field, provided subjects associate some stimuli with the left half of a mental image and others with the right. The rationale for these experiments is rooted in clinical observations of patients with neglect syndrome which suggest that the attributes ‘‘left’’ and ‘‘right’’ may be applied as much to the mental representation of an object as to the object itself. Bisiach and Luzzatti [5]requested two patients with left-sided visual neglect after right hemisphere lesions to imagine a familiar place and to describe details of the imagined scene. Both patients largely failed to report items located to the left of their imagined vantage point. Since Bisiach and Luzzatti’s classic paper, a considerable number of additional cases of ‘‘representational neglect’’ have been reported [e.g. 15, 16, 20, 24]. We are not aware, however, of any study, with normal subjects, that quantitatively investigated differential accessibility to the left and right halves of a mental image.
In one of our experiments, subjects were instructed to imagine a 12-unit ruler and give speeded unimanual responses to single numbers (1 to 11, without number 6) presented in the center of the visual field. The critical decision was whether the number represented a distance shorter or longer than 6 units. The second experiment required subjects to imagine a clock face and to decide whether the centrally presented number indicated a time earlier or later than 6 o’clock. While in the first experiment (RULER), the numbers 1–5 are mentally represented to the left of the numbers 7–11, there is a left-right reversal in the second experiment (CLOCK), where the numbers 7–11 are mentally represented to the left of the numbers 1–5. For both experiments we predicted an S-R compatibility effect, reflected by a left hand RT advantage for numbers smaller than six in Experiment 1 and a similar advantage for numbers larger than six in Experiment 2.
Section snippets
Subjects
Thirty-two male college or university students, right-handed according to a 13-item scale [9], volunteered for the experiments. Their mean age was 23.8 years (range: 16–38 years) and their mother tongue Swiss German. None of them had a history of neurological or psychiatric illness. Half of the subjects were randomly assigned to Experiment 1, the other half to Experiment 2.
Stimuli
In both experiments, the stimuli were the numbers 1–11 (without the number 6) presented in the center of a computer screen
Results
The mean error rate in experiment 1 (RULER) was 2.19% (range: 0–6%), in experiment 2 (CLOCK) 2.56% (range: 0–12%).
Fig. 2 displays the mean RTs of correct decisions for both experiments and for the left and right hands separately. A three-way ANOVA with ‘‘experiment’’ (RULER vs CLOCK) as the between-subjects factor and ‘‘hand’’ (left vs right) and ‘‘number size’’ (<6 vs >6) as the two repeated measures revealed a significant main effect for experiment (F(1,31) = 7.9; P < 0.01) and a triple
Discussion
In two experiments we asked subjects to give speeded unimanual responses to numbers ranging from 1–11 (without number 6) randomly presented in the center of the visual field. In Experiment 1 numbers were to be conceived of as distances on a ruler, in Experiment 2 as hours on a clock-face. Subjects had to decide whether a particular number represented a distance shorter or longer than 6 cm (Exp. 1) or a time of day earlier or later than 6 o’clock (Exp. 2). In the RULER task, ‘‘small’’ numbers
Acknowledgements
We thank Dr Dorothea Weniger (Zurich) for having drawn our attention to the SNARC effect and Dr Klaus Willmes (Aachen) for his helpful comments on an early version of this report. Kirsten Taylor’s editorial help is much appreciated as well. This study is part of the first author’s doctoral dissertation. In preliminary form it has been presented at the 158th Annual Meeting of the Swiss Neurological Society, Geneva, 14–16 November, 1996, and at the 15th European Workshop on Cognitive
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