The Non-inversion Of Similarity Judgment And Difference Judgment

Similarity serves as a central construct in theoretical accounts of many cognitive processes, such as perception, classification, learning, memory, problem-solving and reasoning. Previous research has suggested that similarity judgment and difference judgment are inverses of one another. The philosopher James Mill said:"distinguishing differences and similarities is the same thing; a similarity being nothing but a slight difference". That is, similarity and difference judgments are logical opposites; as similarity increases, difference will decrease to the same extent. For instance, if wolves and dogs have a similarity rating of 4 on a 1-to-5 scale, they ought to have a difference rating of 2. Furthermore, in the words of William James, "To abstract the ground of either difference or likeness (where it is not ultimate) demands an analysis of the given objects into their parts". In other words, the underlying basis for similarity is shared attributes or constituents. For example, a sparrow and a crow are perceived to be similar because of their many common features (e.g. beak, wings).However, studies have found that similarity and difference judgments are not necessarily logical opposites. At least in certain cases, the perception of similarity is not inversely related to the perception of difference. At first, in addition to shared features or attributes, the relations among features also influence on similarity judgments, and features and relations may have different roles in similarity judgments and difference judgments, which induce no-inversion especially when both relational similarity and attributional similarity are considered. Secondly, like structural relations and attributes, thematic relations also have effect on similarity, and the thematic relations are more salient for similarity than for difference. In this case, the similarity judgment and difference judgment are also not logical opposites. Although previous researchers have explored this question in several prior experiments, however, there has been considerable debate about the cognitive mechanism. What's more, few studies have investigated the neural basis of such influence, and especially, the relation between similarity judgment and difference judgment are left unknown.The event-related potential (ERP) was used to investigate the time course of similarity judgment and difference judgment, and provide neural evidences of the non-inversion of similarity and difference judgments in certain cases. Specifically, two ERP experiments were included in this study:The first experiment mainly explored the processing of spatial relation and features and their influence on perceptual similarity judgment and difference judgment. Event-related potentials were recorded from 22 healthy adults using an S1-S2 paradigm, in which a remembered geometric shape (S1) presented in the center of a screen had to be compared with a second geometric shape (S2) on the basis of either similarity or difference. In both tasks, participants were required to compare the features and relative spatial relation of S1 with S2, and make a "large" or "small" judgment. Our findings suggest that the P2 amplitude elicited by S2 under conditions of a feature difference was larger than that without a feature difference, possibly related to differences in perceptual processing. The P3 amplitude was larger in difference judgment than in similarity judgment, and appeared to be most sensitive to the extent of cognitive processing needed for categorization. It is proposed that the mental processes of similarity judgment can be divided into three stages:selective attention (N1), perceptual processing (P2), and comparison and judgment (P3).The second experiment mainly explored the processing of thematic relation and category relation and their influence on conceptual similarity judgment and difference judgment. Unlike the first experiment, the material in this experiment is not geometric pictures but noun words. Similarly,20 healthy adults were required to compare the semantic relation of S1 with S2, and make a "large" or "small" judgment in similarity and difference judgment. Our findings suggest that the N400 amplitude elicited by S2 under conditions of semantic unrelated words was larger than that semantic difference, while larger P600 amplitude was evoked by category related words than thematic related words. Additionally, the LPC (750-950ms) wave evoked by similarity judgment was larger than difference judgment in taxonomic related condition. The mental processes of conceptual similarity judgment can be divided into three stages:selective attention (N1), semantic processing and integration (N400 or P600), and comparison and judgment (LPC). Generally speaking, the non-inversion of similarity judgment and difference judgment were evident in the behavioral data of both experiments. In our study, however, some results which are inconsistent with previous were found, partly because the tasks and requirements were different from those used in past research. The ERP data indicated that the processing of spatial relation and features, thematic and category relations has certain differences, and their influence on similarity judgment and difference judgment are also inconsistent.(1) Differences in the processing of spatial relation and features, thematic relation and category relation were evident in ERP data. Specifically, the difference between structural relation and feature/attribution for picture information mainly reflected on P2 waves. In addition, for linguistic information, the processing of semantic unrelated words and semantic related words mainly reflected on N400 waves, while the difference between category-related words and thematic-related words were mainly found on the mean amplitude of P600 waves.(2) The similarity could be divided to perceptual similarity and conceptual similarity based on the dual-coding model of the picture-word information. It is noted that the representation of perceptual and conceptual similarity is not same, which may be the reason why the results between two experiment are inconsistency while comparing similarity to difference judgments. That is, differences in the processing of perceptual similarity judgment and difference judgment were mainly found on the mean amplitude of P3 waves. However, differences in the processing of conceptual similarity judgment and difference judgment mainly reflected on LPC amplitude. These results may be considered complementary as they were viewed in the earlier literature, rather than mutually exclusive, which reflect the essence of this cognitive process. In conclusion, these, together with previous results, suggested that two complementary processes, namely, perceptual similarity and conceptual similarity, should be differentiated while exploring similarity in future studies.(3) Based on previous studies and our results, the time course of similarity should be consist of three stages at least, that is, selective attention (N1), perceptual processing (P2) or semantic processing and integration (N400 or P600), and comparison and judgment (P3). Comparison and judgment is the core process of similarity judgment, while the former two processes are the basis of comparison and judgment.