HZPPOCAMPUS, VOL. 2, NO. 1, PAGES 90-91, JANUARY 1992 aid of configural cues. A point overlooked by Sutherland and Rudy (1989) was that many reports (e.g., Holland, 1984; Rescorla, 1985; Davidson and Rescorla, 1986; Davidson and Jarrard, 1989) argued against the configural cue interpretation of serial-compound discrimination and related problems. Moreover, Sutherland and Rudy (1989) cited no data indiSupport for Configural Association Theory: cating that rats solve the serial-compound problem in the manner required by their view. It seems that the lack of eviNow You See It, Now You Don’t . ~dence for configural learning is of little, if any, concern when the effects of hippocampal damage support CAS theory-but To the Editors: crucial when contradictory data emerge. ( 2 ) By predicting Ross et al. (1984) reported that aspiration lesions of hip- that hippocampal damage should not produce impairments in pocampus impaired performance on a Pavlovian serial-com- serial-compound discriminations, CAS theory now seems to pound discrimination task in which a light (B) signaled when be surprised by our finding (and that of Ross et al.) that seriala tone (A) would be followed by food (US). We replicated compound discrimination performance was disrupted by astheir study with an added group that received highly selective piration lesions. If the data from our unimpaired ibotenateibotenate lesions of hippocampus (Jarrard and Davidson, lesioned rats supports CAS theory, the data from our (and 1991). We found, as did Ross et al., that rats with aspiration those of Ross et al.) impaired aspiration lesioned rats refute lesions of hippocampus were unable to solve the serial-com- it. It is difficult to see how their view can account for these pound problem. But we found no impairments for ibotenate- opposing results since both were obtained under the same lesioned rats. We concluded that “the hippocampus is not conditions (in our study) within the same type of discriminecessary for learning and performance of either conditional nation problem. Rudy and Sutherland (above) were quick to remind us or nonconditional Pavlovian discriminations” (p. I 14) and that “the impaired conditional responding found in aspiration about a previously stated qualification of CAS theory: if Ross hippocampals must be due to extrahippocampal damage” (p. et al.’s aspiration-lesioned rats had used a simple light-food association to solve the serial-compound discrimination, CAS 107). As a secondary point, we commented briefly that our find- theory would expect no impairment. But we also remember ings failed to support several views of hippocampal function, the following: “An important feature of the Ross et al. (1984) including Sutherland and Rudy’s (1989) configural associa- study was that their animals did not use the simple association tion (CAS) theory. We shall elaborate that comment in re- strategy to solve the problem” (Sutherland and Rudy, 1989, sponse to Rudy and Sutherland’s contention that our data p. 136). In addition to testing for configural associations, our study tested the simple association hypothesis by assessing actually support their account. Sutherland and Rudy (1989, p. 129; Rudy and Sutherland, whether or not discrimination performance was based on re1989) claimed that CAS theory could explain the findings re- sponding elicited directly by light. We found that the light ported by Ross et al. (1984). Their account requires that rats alone elicited little responding relative to the light-tone serial solve the serial-compound problem by “forming a configural compound. Although one might get a different impression representation of the light-tone serial compound.” Accord- based on their letter, we concluded that conditional discrimingly, the aspiration lesioned rats of Ross et al. were impaired ination performance was not based on a simple light-food asbecause the construction of configural representations de- sociation (see Jarrard and Davidson, 1991, p. 113). Curiously, Rudy and Sutherland portray Rescorla (1985) as pends on the integrity of the hippocampal formation. In response to our finding of no impairment following ibo- adopting a simple association account of serial-compound tenate lesions of hippocampus, Rudy and Sutherland now discrimination learning. But Rescorla (1985) stated explicitly state that “It is the opposite [their italics] result that would that facilitators (a designation given to the light or B stimulus) go against configural association theory, i.e., if animals with are not simple Pavlovian excitors (i.e., are not direct exciselective hippocampal damage had failed to solve their serial tatory associates of the US). Rescorla (1985, p. 317) sumcompound discrimination.” Hence, where CAS theory once marized his facilitation research as follows: “The picture that attempted to draw support from data showing that hippo- emerges from these experiments is that a facilitator cannot campectomy disrupted serial-compound discrimination per- be easily thought of as either a simple excitor or as a generator formance, it now claims support from the opposite outcome. of a configural cue.” Rather, Rescorla (e.g., 1985; 1991; DavThis change in position appears to be based on our data idson and Rescorla, 1986) and others (e.g., Davidson and Jarindicating that our rats (and presumably those of Ross et al.) rard, 1989) have favored the idea that facilitators modulate used other than configural cues to solve the serial-compound the excitatory strength of other stimuli. That is, rats solve discrimination problem. Rudy and Sutherland argue: “given serial-compound discriminations because stimulus B comes that the transfer test indicated that a nonconfigural solution to modulate the capacity of stimulus A to elicit a response. was used; configural association theory predicts that a se- Hence, although it appears to be the basis for solving a task lective hippocampus lesion would not impair performance on purportedly explained by Sutherland and Rudy’s (1989) thetheir [i.e., Jarrard and Davidson, 19911 task.” We have two ory, conditioned modulation does not seem to be encomcomments. ( 1 ) We deserve little credit for the discovery that passed by either their configural or their simple association rats can solve serial-compound discriminations without the view. ~~
LETTERS TO THE EDITOR Terry L . Davidson Department of Psychological Sciences Purdue University West Lafayette, Indiana Leonard E . Jarrard Department of Psychology Washington & Lee University Lexington, Virginia
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