5 EUROPEAN JOURNAL OF BEHAVIOR ANALYSIS NUMBER 1 (SUMMER 2008) 2008, 9, 5 - 11 5 Matching-to-sample performance is usu- ally explored using a conditional discrimina- tion 2 procedure in which a sample stimulus determines the probability of reinforcement for matching responses to a particular com- parison stimulus. One type of conditional discrimination procedure is the successive one (Wasserman, 1976). In this procedure one of two sample stimuli (S) is followed by one of the comparison stimuli (C) and reinforcement (S R ) (a positive trial), or the other comparison stimulus and no reinforcement (a negative trial). Positive and negative trials take place in a successive order and the sequence can be programmed before the experimental session begins. Given this procedure, each positive trial can be seen as a particular structural sequence of the conditional probabilities between S, C and SR. One type of positive trial could be S1-C1- SR, in which following the response to S1, C1 will be presented and a response to it will be followed by SR. Another type of positive trial could be S2-C2-SR, which represents a difer- ent structural sequence. In the case of both this type of conditional discrimination task and the traditional two-choice or simultaneous type (Cumming & Berryman, 1961), the diference between structural sequences can be seen as an important factor in the development of con- trol by the sample stimulus (see, e.g., Cohen, Looney, Brady, & Aucella, 1976; Delong & Wasserman, 1981; Eckerman, 1970; Goeters, Blakely, & Poling, 1992). 1 Corresponding author: Isaac Camacho, Av. de los Barrios No. 1 Los Reyes Iztacala Tlalnepantla Edo. De México, México, Phone: 56231211, E- mail: isaac_camacho@hotmail.com We thank Kennon A. Lattal for his time, detailed comments, and valuable suggestions on this paper. Also, we thank Sergio Moreno and Jorge Nava for their valuable support. 2 In the present paper statements referring to conditional discrimination are treated in the same way as in other reports in which matching-to-sample is the type of procedure used. Diferent Structural Sequences: A positive Efect of Reinforcement Delay in a Conditional Discrimination Task Isaac Camacho 1 CICS/ UST-IPN, México and Francisco Aguilar and Claudio Carpio FES Iztacala-UNAM, México Nine pigeons were exposed to a matching-to-sample task in which the reinforcer delay was varied. Speciic sample stimulus, comparison stimulus, and reinforcement conditional probabilities were used to establish diferent structural sequences. Pigeons were assigned to three diferent groups. Reinforcer delays for the three groups were 3 s and 1 s for group 1, 2 s for group 2, and 0 s for group 3. Pigeons in group 1 performed similarly to those in group 3, while those in group 2 performed most poorly. hese data indicate that matching-to-sample performance may be enhanced by diferent structural sequences even when the element that makes the diference—reinforcer delay—has been previously reported as having a negative efect on matching accuracy. Key words: conditional discrimination, structural sequence, reinforcement delay, pigeons