Perirhinal cortex and feature-ambiguous discriminations

Abstract

Perirhinal cortex is often regarded as part of a medial temporal lobe (MTL) memory system, mediating memory selectively, and having little role in other functions such as perception (e.g., Squire and Zola-Morgan 1991; Buffalo et al. 1998, 1999; Squire et al. 2004). We and others have argued against this idea, suggesting instead that perirhinal cortex may have a role in perception (e.g., Buckley and Gaffan 1997; Murray and Bussey 1999; Buckley et al. 2001; Bussey and Saksida 2002). Evidence to support this view has come from recent work indicating that lesions of perirhinal cortex can impair the discrimination of visual stimuli. Perirhinal cortex is not, however, important for all types of visual discrimination (Buckley and Gaffan 1997; Hampton and Murray 2002), only for those requiring representations of complex conjunctions of features (Saksida and Bussey 1998; Murray and Bussey 1999; Buckley et al. 2001; Eacott et al. 2001; Bussey and Saksida 2002; Bussey et al. 2002, 2003). These discriminations possess a high degree of “feature ambiguity,” a property of visual discriminations that can emerge when features are a part of both rewarded and unrewarded stimuli (Bussey and Saksida 2002). In one such study (Bussey et al. 2003), we used complex featureambiguous “morphed” photographic stimuli to test this idea (Fig. 1). Monkeys were first trained on a pairwise discrimination of unmorphed photographs (e.g., images 1 and 40 in Fig.1). This was followed by the critical performance test session, in which the monkeys were presented with three pairs of stimuli: the trained pair, and two pairs in which the photographs had been morphed together to different degrees, thus creating three levels of perceptual difficulty. It was found that monkeys with perirhinal cortex lesions were impaired on the perceptually difficult, but not the perceptually easy, pairs (Fig. 2). This impairment was specific to complex, high feature-ambiguity discriminations: The lesions did not affect difficult color or size discriminations, indicating that the impairment was not due simply to the difficulty of the discriminations. Levy et al. (2005) investigated visual discrimination ability in amnesic patients using similar stimulus materials to those used in our monkey study. In their report, these authors challenge our previously published results, and call for a reanalysis of our previously published data, suggesting that an impairment in new learning may explain the poor performance in our monkeys with perirhinal cortex damage. Specifically they ask: “In the case of the study with blended images, it was suggested that learning was required during initial discrimination learning but not during the 32 test trials with blended images. Accordingly, it was suggested that impaired performance during the trials with blended images reflected impaired visual perception, not an impairment in learning and memory. This interpretation was based on an analysis of variance that indicated that no further learning occurred across the 32 trials (the 32 trials were partitioned into eight four-trial blocks). There was also no group x trial-block interaction. Yet, one wonders how monkeys performed on the first one or two blocks of the test trials with blended images. Did generalization fail altogether? One also wonders whether the impaired performance during these test trials could reflect to some degree the fact that control monkeys improved more during the test trials than the monkeys with perirhinal lesions. Did no learning occur at all? Accordingly, it