Electrophysiological correlates of stimulus equivalence processes

Abstract

Research reported here concerns neural processes relating to stimulus equivalence class formation. In Experiment 1, two types of word pairs were presented successively to normally capable adults. In one type, the words had related usage in English (e.g., uncle, aunt). In the other, the two words were not typically related in their usage (e.g., wrist, corn). For pairs of both types, event-related cortical potentials were recorded during and immediately after the presentation of the second word. The obtained waveforms differentiated these two types of pairs. For the unrelated pairs, the waveforms were significantly more negative about 400 ms after the second word was presented, thus replicating the "N400" phenomenon of the cognitive neuroscience literature. In addition, there was a strong positive-tending wave form difference post-stimulus presentation (peaked at about 500 ms) that also differentiated the unrelated from related stimulus pairs. In Experiment 2, the procedures were extended to study arbitrary stimulus-stimulus relations established via matching-to-sample training. Participants were experimentally naïve adults. Sample stimuli (Set A) were trigrams, and comparison stimuli (Sets B, C, D, E, and F) were nonrepresentative forms. Behavioral tests evaluated potentially emergent equivalence relations (i.e., BD, DF, CE, etc.). All participants exhibited classes consistent with the arbitrary matching training. They were also exposed also to an event-related potential procedure like that used in Experiment 1. Some received the ERP procedure before equivalence tests and some after. Only those participants who received ERP procedures after equivalence tests exhibited robust N400 differentiation initially. The positivity observed in Experiment 1 was absent for all participants. These results support speculations that equivalence tests may provide contextual support for the formation of equivalence classes including those that emerge gradually during testing.

Keywords: N400; arbitrary matching; equivalence; mouse click; normally capable adults.

Fig 1

Waveforms obtained for related (broken lines) and unrelated (solid lines) word pairs presented during Experiment 1. The notations (F3, Fz, F4, C3, Cz, C4) refer to locations for electrode placements according to the International 10–20 system. A “z” (zero) refers to an electrode placed on the midline. F and C refer to “frontal” and “central,” respectively. Even numbers refer to electrode positions on the right hemisphere; odd numbers refer to those on the left hemisphere.

Fig 2

Difference waves by arithmetic subtraction of waveforms shown in Figure 1. The notations (F3, Fz, F4, C3, Cz, C4) refer to locations for electrode placements according to the International 10–20 system. A “z” (zero) refers to an electrode placed on the midline. F and C refer to “frontal” and “central,” respectively. Even numbers refer to electrode positions on the right hemisphere; odd numbers refer to those on the left hemisphere.

Fig 3

Stimuli presented during matching-to-sample procedures and electrophysiological testing in Experiment 2. The trigrams constituted Set A and the forms ordered left-to-right constituted Sets B, C, D, E, and F.

Fig 4

Difference waves obtained from participants who were exposed to behavioral equivalence tests prior to electrophysiological testing. The notations (F3, Fz, F4, C3, Cz, C4) refer to locations for electrode placements according to the International 10–20 system. A “z” (zero) refers to an electrode placed on the midline. F and C refer to “frontal” and “central,” respectively. Even numbers refer to electrode positions on the right hemisphere; odd numbers refer to those on the left hemisphere.

Fig 5

Difference waves obtained from the first 60 trials of electrophysiological testing obtained with participants who were exposed to behavioral equivalence tests after that testing. The notations (F3, Fz, F4, C3, Cz, C4) refer to locations for electrode placements according to the International 10–20 system. A “z” (zero) refers to an electrode placed on the midline. F and C refer to “frontal” and “central,” respectively. Even numbers refer to electrode positions on the right hemisphere; odd numbers refer to those on the left hemisphere.

Fig 6

Difference waves obtained from the second 60 trials of electrophysiological testing obtained with participants who were exposed to behavioral equivalence tests after that testing. The notations (F3, Fz, F4, C3, Cz, C4) refer to locations for electrode placements according to the International 10–20 system. A “z” (zero) refers to an electrode placed on the midline. F and C refer to “frontal” and “central,” respectively. Even numbers refer to electrode positions on the right hemisphere; odd numbers refer to those on the left hemisphere.