At the end of this page, you’ll find a full list of the papers I’ve co-authored during my academic career. If you’re looking for my work on a particular topic, you may want to look at one of the topic lists instead, which also include a summary of the work I’ve done.
Topics
I work on the psychology and neuroscience of learning and categorization. More specifically, I have published on the following topics within this area:
Publications
Definitions and Explanations
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Full text links lead to the full-text of the manuscript, normally as a PDF.
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Repository links lead to a location from which the raw data, analysis scripts, etc. can be publicly accessed. Where the full text of the paper is open access and contains such links within it, the repository link leads to the paper. If there is no repository link for a paper you’re interested in, please request that I add one.
Papers are ordered by date of online publication (date of last update for preprints). Journals differ in the time between online publication and allocation of an article to a journal issue, which means that sometimes papers are out of order in terms of the year of publication as expressed in their APA reference.
Complete list of publications
Longman, C.S., Milton, F. & Wills, A.J. (2023). Transfer of strategic task componewnts across unique tasks that share some common structure. Quarterly Journal of Experimental Psychology, accepted.
Schlegelmilch, R., Wills, A.J. & von Helversen, B. (2023). Towards re-unifying the theoretical perspectives on conditioning, category learning, probability learning and economic decisions from experience. preprint.
Schlegelmilch, R., Wills, A.J. & Lee, J.C. (2023). Is generalization general? A comparison of predictive and category learning and the influence of response format and framing. preprint.
Dome, L., & Wills, A. (2023). Errorless irrationality: removing error-driven components from the inverse base-rate effect paradigm. Proceedings of the 45th Annual Conference of the Cognitive Science Society, 237-243.
Dome, L. & Wills, A.J. (2023). Zebra reduction: Better generalization through distraction. preprint.
Dutli, J., Wills, A., & Oberauer, K. (2023). Introducing the ‘BenchmarksWM’ R data package. preprint.
Dome, L. & Wills, A.J. (2023). g-distance: On the comparison of model and human heterogeneity. preprint.
Hollins, T.J., Seabrooke, T., Inkster, A., Wills, A. & Mitchell, C.J. (2023). Pre-testing effects are target-specific and are not driven by a generalised state of curiosity. Memory, 31, 282-296.
Inkster, A.B., Mitchell, C. J., Schlegelmilch, R., & Wills, A. J. (2022). Effect of a context shift on the inverse base rate effect. Open Journal of Experimental Psychology and Neuroscience, 1, 22-29.
Inkster, A.B., Milton, F., Edmunds, C.E.R., Benattayallah, A., & Wills, A.J. (2022). Neural correlates of the inverse base-rate effect. Human Brain Mapping, 43, 1370-1380.
Spicer, S.G., Mitchell, C.J., Wills, A.J., Blake, K.L., and Jones, P.M. (2022). Theory protection: Do humans protect existing associative links? Journal of Experimental Psychology: Animal Learning & Cognition, 48, 1-16.
Schlegelmilch, R., Wills, A.J. & von Helversen, B. (2022). A Cognitive Category-Learning Model of Rule Abstraction, Attention Learning, and Contextual Modulation. Psychological Review, 129, 1211-1248.
Seabrooke, T., Mitchell, C. J., Wills, A. J., Inkster, A. B. & Hollins, T. J. (2022). The Benefits of Impossible Tests: Assessing the Role of Error-Correction in the Pretesting Effect. Memory & Cognition, 50, 296-311.
Dome, L., Edmunds, C. E. R. & Wills, A. J. (2021). SUSTAIN captures category learning, recognition, and hippocampal activation in a unidimensional vs. information-integration task. Proceedings of the Annual Meeting of the Cognitive Science Society, 43, 3013-3019.
Spicer, S.G., Wills, A.J., Jones, P.M., Mitchell, C.J. and Dome, L. (2021). Representing uncertainty in the Rescorla-Wagner model: Blocking, the redundancy effect, and outcome base rate. Open Journal of Experimental Psychology and Neuroscience, 1, 14-21.
Jones, P.M., Mitchell, C.J., Wills, A.J. & Spicer, S.G. (2021). Similarities and differences: Comment on Chan et al. Journal of Experimental Psychology: Animal Learning & Cognition, 47, 216-217.
Wills, A.J., Edmunds, C.E.R. & Milton, F. (2021). The rapid synthesis of integral stimuli. preprint.
Seabrooke, T., Mitchell, C. J., Wills, A. J., & Hollins, T. J. (2021). Pretesting boosts recognition, but not cued recall, of targets from unrelated word pairs. Psychonomic Bulletin & Review, 28, 268-273.
Edmunds, C.E.R., Inkster, A.B., Jones, P.M., Milton, F., & Wills, A.J. (2020). Absence of cross-modality analogical transfer in perceptual categorization. Open Journal of Experimental Psychology and Neuroscience, 1, 3-13.
Wills, A.J. (2020). Editorial. Open Journal of Experimental Psychology and Neuroscience, 1, 1-2.
Spicer, S.G., Mitchell, C.J., Wills, A.J., & Jones, P.M. (2020). Theory protection in associative learning: humans maintain certain beliefs in a manner that violates prediction error. Journal of Experimental Psychology: Animal Learning and Cognition, 46, 151-161.
Wills, A.J., Ellett, L., Milton, F., Croft, G., & Beesley, T. (2020). A dimensional summation account of polymorphous category learning. Learning and Behavior, 48, 66-83.
Milton, F., McLaren, I.P.L., Copestake, E., Satherley, D., & Wills, A.J. (2020). The effect of pre-exposure on overall similarity categorization. Journal of Experimental Psychology: Animal Learning and Cognition, 46, 65-82.
Wills, A.J., Edmunds, C.E.R., Le Pelley, M.E., Milton, F., Newell, B.R., Dwyer, D.M., & Shanks, D.R. (2019). Dissociable learning processes, associative theory, and testimonial reviews: A comment on Smith and Church (2018). Psychonomic Bulletin & Review, 26, 1988-1993.
Seabrooke, T., Mitchell, C.J., Wills, A.J., Waters, J.L., & Hollins, T.J. (2019). Selective effects of errorful generation on recognition memory: The role of motivation and surprise. Memory, 27, 1250-1262.
Wills, A.J. (2019). Open science, open source and R. Linux Journal, 295, 166-176.
Seabrooke, T., Hollins, T.J., Kent, C., Wills, A.J., & Mitchell, C.J. (2019). Learning from failure: Errorful generation improves memory for items, not associations. Journal of Memory and Language, 104, 70-82.
Seabrooke, T., Wills, A.J., Hogarth, L., & Mitchell, C.J. (2019). Automaticity and cognitive control: Effects of cognitive load on cue-controlled reward choice. Quarterly Journal of Experimental Psychology, 72, 1507-1521.
Schlegelmilch, R., Wills, A.J., & von Helversen, B. (2018). CALM - A Process Model of Category Generalization, Abstraction and Structuring. In T. Rogers, M. Rau, X. Zhu, & C.W. Kalish (Eds.). Proceedings of the 40th Annual Conference of the Cognitive Science Society (pp. 2436-2441). Austin, TX: Cognitive Science Society.
Sambrook, T.D., Wills, A.J., Hardwick, B., & Goslin, J. (2018). Model-free and model-based reward prediction errors in EEG. NeuroImage, 178, 162-171.
Lea, S.E.G., Pothos, E.M., Wills, A.J., Leaver, L.A., Ryan, C.M.E., & Meier, C. (2018). Multiple Feature Use in Pigeons’ Category Discrimination: The Influence of Stimulus Set Structure and the Salience of Stimulus Differences. Journal of Experimental Psychology: Animal Learning and Cognition, 44, 114-127.
Edmunds, C.E.R., Milton, F., & Wills, A.J. (2018). Due process in dual process: Model-recovery simulations of decision-bound strategy analysis in category learning. Cognitive Science, 42, 833-860.
Edmunds, C.E.R., Wills, A.J., & Milton, F. (2019). Initial training with difficult items does not facilitate category learning. Quarterly Journal of Experimental Psychology, 72, 151-167.
Edmunds, C.E.R., Milton, F., & Wills, A.J. (2017). Due process in dual process: A model-recovery analysis of Smith et al. (2014). In A. Gunzelmann, A. Howes, T. Tenbrink, & E.J. Davelaar (Eds.). Proceedings of the 39th Annual Conference of the Cognitive Science Society (pp. 1979-1984). Austin, TX: Cognitive Science Society.
Longman, C.S., Milton, F., Wills, A.J., & Verbruggen, F. (2018). Transfer of learned category-response associations is modulated by instruction. Acta Psychologica, 184, 144-167.
Wills, A.J., & Hollins, T.J. (2017). In defence of effect-centric research. Journal of Applied Research in Memory and Cognition, 6, 43-46.
Wills, A.J., O’Connell, G., Edmunds, C.E.R., & Inkster, A.B. (2017). Progress in modeling through distributed collaboration: Concepts, tools, and category-learning examples. Psychology of Learning and Motivation, 66, 79-115.
Milton, F., Bealing, P., Carpenter, K.L., Bennattayallah, A., & Wills, A.J. (2017). The neural correlates of similarity- and rule-based generalization. Journal of Cognitive Neuroscience, 29, 150-166.
Lawson, R., Chang, F., & Wills, A. J. (2017). Free classification of large sets of everday objects is more thematic than taxonomic. Acta Psychologica, 172, 26-40.
O’Connell, G., Myers, C.E., Hopkins, R.O., McLaren, R.P., Gluck, M.A., & Wills, A.J. (2016). Amnesic patients show superior generalization in category learning. Neuropsychology, 30, 915–919.
Le Pelley, M.E., Mitchell, C.J., Beesley, T., George, D.N., & Wills, A.J. (2016). Attention and associative learning in humans: An integrative review. Psychological Bulletin, 142, 1111-1140.
Edmunds, C.E.R., & Wills, A.J. (2016). Modeling category learning using a dual-system approach: A simulation of Shepard, Hovland and Jenkins (1961) by COVIS. In A. Papfragou, D. Grodner, D. Mirman, & J.C. Trueswell (Eds.). Proceedings of the 38th Annual Conference of the Cognitive Science Society (pp. 69-74). Austin, TX: Cognitive Science Society.
Edmunds, C.E.R., Wills, A.J., & Milton, F.N. (2016). Memory for exemplars in category learning. In A. Papfragou, D. Grodner, D. Mirman, & J.C. Trueswell (Eds.). Proceedings of the 38th Annual Conference of the Cognitive Science Society (pp. 2243-2248). Austin, TX: Cognitive Science Society.
Carpenter, K., Wills, A.J., Bennattayallah, A., & Milton, F. (2016). A comparison of the neural correlates that underlie rule-based and information-integration category learning. Human Brain Mapping, 37, 3557–3574.
Roberts, H., Watkins, E., & Wills, A.J. (2017). Does rumination cause “inhibitory” deficits? Psychopathology Review, 4, 341-376.
Maes, E., De Filippo, G., Inkster, A., Lea, S.E.G., De Houwer, J., D’Hooge, R., Beckers, T., & Wills, A.J. (2015). Feature- versus rule-based generalization in rats, pigeons and humans. Animal Cognition, 18, 1267-1284.
Wills, A.J., Inkster, A.B., & Milton, F. (2015). Combination or Differentiation? Two theories of processing order in classification. Cognitive Psychology, 80, 1-33.
Hogarth, L., Zhimin, H., Chase, H.W., Wills, A.J., Troisi II, J., Leventhal, M., Mathew, A.R., & Hitsman, B. (2015). Negative mood reverses devaluation of goal-directed drug-seeking favouring an incentive learning account of drug dependence. Psychopharmacology, 232, 3235-3247.
Edmunds, C.E.R., Milton, F., & Wills, A.J. (2015). Feedback can be superior to observational training for both rule-based and information-integration category structures. Quarterly Journal of Experimental Psychology, 68, 1203-1222.
Wills, A.J., Lavric, A., Hemmings, Y., & Surrey, E. (2014). Attention, predictive learning, and the inverse base-rate effect: Evidence from event-related potentials. NeuroImage, 87, 61-71.
Yeates, F., Wills, A.J., Jones, F.W., & McLaren, I.P.L. (2015). State trace analysis: Dissociable processes in a connectionist network? Cognitive Science, 39, 1047-1061.
Milton, F., Copestake, E., Satherley, D., Stevens, T., & Wills, A. J. (2014). The effect of pre-exposure on family resemblance categorization for stimuli of varying levels of perceptual difficulty. In P. Bello, M. Guarini, M. McShane, & B. Scassellati (Eds.). Proceedings of the 36th Annual Conference of the Cognitive Science Society (pp. 1018-1023). Austin, TX: Cognitive Science Society. Published: August 2014.
Inkster, A. B., Milton, F. N., & Wills, A. J. (2014). Does incidental training increase the prevalence of overall similarity classification? A re-examination of Kemler Nelson (1984). In P. Bello, M. Guarini, M. McShane, & B. Scassellati (Eds.). Proceedings of the 36th Annual Conference of the Cognitive Science Society (pp. 649-653). Austin, TX: Cognitive Science Society. Published: August 2014.
Carmantini, G. S., Cangelosi, A., & Wills, A. (2014). Machine learning of visual object categorization: An application of the SUSTAIN model. In P Bello, M Guarini, M McShane, & B Scassellati (Eds.). Proceedings of the 36th Annual Conference of the Cognitive Science Society (pp. 290-295). Austin, TX: Cognitive Science Society. Published: August 2014.
Roberts, H., Watkins, E.R., & Wills, A.J. (2013). Cueing an unresolved personal goal causes persistent ruminative self-focus: an experimental evaluation of control theories of rumination. Journal of Behavior Therapy and Experimental Psychiatry, 44, 449-455. Published: December 2013.
Wills, A.J., Milton, F., Longmore, C.A., Hester, S., & Robinson, J. (2013). Is overall similarity classification less effortful than single-dimension classification? Quarterly Journal of Experimental Psychology, 66, 299-318.
Yeates, F., Jones, F.W., Wills, A.J., Aitken, M.R.F., & McLaren, I.P.L. (2013). Implicit learning: A demonstration and a revision to a novel SRT paradigm. In M. Knauff, M. Pauen, N. Sebanz, & I. Wachsmuth (Eds.). Proceedings of the 35th Annual Conference of the Cognitive Science Society (pp. 3829-3834). Austin, TX: Cognitive Science Society.
Wills, A.J., Longmore, C.A., & Milton, F. (2013). Impulsivity and overall similarity classification. In M. Knauff, M. Pauen, N. Sebanz, & I. Wachsmuth (Eds.). Proceedings of the 35th Annual Conference of the Cognitive Science Society (pp. 3783-3788). Austin, TX: Cognitive Science Society.
Wills, A.J. (2013). Models of categorization. In D. Reisberg (Ed.). Oxford Handbook of Cognitive Psychology (pp. 346-357). Oxford: Oxford University Press.
Yeates, F., Wills, A.J., McLaren, R.P., & McLaren, I.P.L. (2013). Modeling human sequence learning under incidental conditions. Journal of Experimental Psychology: Animal Behavior Processes, 39, 166-173.
Newell, B.R., Moore, C.P., Wills, A.J., & Milton, F. (2013). Reinstating the frontal lobes? Having more time to think improves ‘implicit’ perceptual categorization. A comment on Filoteo, Lauritzen and Maddox (2010). Psychological Science, 24, 386-389.
Wills, A.J., & Pothos, E.M. (2012). On the adequacy of Bayesian evaluations of categorization models: Reply to Vanpaemel & Lee (2012). Psychological Bulletin, 138, 1253-1258.
Yeates, F., Jones, F.W., Wills, A.J., Aitken, M.R.F., & McLaren, I.P.L. (2012). Implicit Learning: A Demonstration and a Novel SRT Paradigm. In N Miyake, D Peebles, & RP Cooper (Eds.). Proceedings of the 34th Annual Conference of the Cognitive Science Society (pp. 1185-1190). Austin, TX: Cognitive Science Society.
Wills, A.J., & Pothos, E.M. (2012). On the adequacy of current empirical evaluations of formal models of categorization. Psychological Bulletin, 138, 102-125.
Wills, A.J. (2011). Learning from errors: Perspectives from basic research. In E. Wuttke, & J. Seifried (Eds.). Learning from Errors at School and at Work (pp. 143-153). Verlag Barbara Budrich.
Kovacs, T., & Wills, A.J. (2012). Generalization versus discrimination in learning. In N.M. Seele (Ed.). Encyclopedia of the Sciences of Learning. Springer.
Wills, A.J., Barrasin, T.J., & McLaren, I.P.L. (2011). Working memory capacity and generalization in predictive learning. In L. Carlson, C. Holscher, & T. Shipley (Eds.). Proceedings of the 33rd Annual Conference of the Cognitive Science Society (pp. 3205-3210). Austin, TX: Cognitive Science Society.
Milton, F., Viika, L., Henderson, H., & Wills, A.J. (2011). The effect of time pressure and the spatial integration of the stimulus dimensions on overall similarity categorization. In L. Carlson, C. Holscher, & T. Shipley (Eds.). Proceedings of the 33rd Annual Conference of the Cognitive Science Society (pp. 795-800). Austin, TX: Cognitive Science Society.
Wills, A.J., Graham, S., Koh, Z., McLaren, I.P.L., & Rolland, M.D. (2011). Effects of concurrent load on feature- and rule-based generalization in human contingency learning. Journal of Experimental Psychology: Animal Behavior Processes, 37, 308-316.
Welham, A.K., & Wills, A.J. (2011). Unitization, similarity and overt attention in categorization and exposure. Memory and Cognition, 39, 1518-1533.
McLaren, I.P.L., Wills, A.J., & Graham, S. (2011). Representation development, perceptual learning and concept formation. Behavioral and Brain Sciences, 34, 141-142.
Pothos, E.M., & Wills, A.J. (2011). Formal approaches in categorization. Cambridge University Press.
Goto, K., Lea, S.E.G., Wills, A.J., & Milton, F. (2011). Interpreting the effects of image manipulation on picture perception in pigeons (Columba livia) and humans (Homo sapiens). Journal of Comparative Psychology, 125, 48-60.
McLaren, I.P.L., Wills, A.J., & Graham, S. (2010). Attention and Perceptual Learning. In C. Mitchell, & M. Le Pelley (Eds.). Attention and associative learning: From brain to behaviour. Oxford: Oxford University Press.
Beesley, T., Wills, A.J., & Le Pelley, M.E. (2010). Syntactic transfer in artificial grammar learning. Psychonomic Bulletin and Review, 17, 122-128.
Wills, A.J., Lea, S.E.G., Leaver, L.A., Osthaus, B., Ryan, C.M.E., Suret, M.B., Bryant, C.M.L., Chapman, S.J., & Millar, L. (2009). A comparative analysis of the categorization of multidimensional stimuli: I. Unidimensional classification does not necessarily imply analytic processing; evidence from pigeons (Columba livia), squirrels (Sciurus carolinensis), and humans (Homo sapiens). Journal of Comparative Psychology, 123, 391-405.
Lea, S.E.G., Wills, A.J., Leaver, L.A., and Ryan, C.M.E., Bryant, C.M.L., & Millar, L. (2009). A comparative analysis of the categorization of multidimensional stimuli: II. Strategic information search in humans (Homo sapiens) but not in pigeons (Columba livia). Journal of Comparative Psychology, 123, 406-420.
Milton, F., & Wills, A.J. (2009). Eye movements in overall similarity and single-dimension sorting. In N.A. Tattgen, & H. van Rijn (Eds.). Proceedings of the 31st Annual Conference of the Cognitive Science Society (pp. 1512-1517). Austin, TX: Cognitive Science Society.
Hopewell, L., Leaver, L.A., Lea, S.E.G., & Wills, A.J. (2010). Grey squirrels (Sciurus carolinensis) show a feature-negative effect specific to social learning. Animal Cognition, 13, 219-227.
Wills, A.J. (2009). Prediction errors and attention in the presence and absence of feedback. Current Directions in Psychological Science, 18, 95-100.
Milton, F., Wills, A.J., & Hodgson, T.L. (2009). The neural basis of overall similarity and single-dimension sorting. NeuroImage, 46, 319-326.
Lea, S.E.G., & Wills, A.J. (2008). Use of multiple dimensions in learned discriminations. Comparative Cognition and Behavior Reviews, 3, 115-133.
Milton, F., & Wills, A.J. (2008). The influence of perceptual difficulty on family resemblance sorting. In B.C. Love, K. McRae, & M. Sloutsky (Eds.). Proceedings of the 30th Annual Conference of the Cognitive Science Society (pp. 2273-2278). Austin, TX: Cognitive Science Society.
Milton, F., Longmore, C.A., & Wills, A.J. (2008). Processes of overall similarity sorting in free classification. Journal of Experimental Psychology: Human Perception and Performance, 30, 407-415.
Haslam, C., Wills, A.J., Haslam, S.A., Kay, J., Baron, R., & McNab, F. (2007). Does maintenance of colour categories rely on language? Evidence to the contrary from a case of semantic dementia. Brain and Language, 103, 251-263.
Wills, A.J., Lavric, A., Croft, G., & Hodgson, T.L. (2007). Predictive learning, prediction errors and attention: Evidence from event-related potentials and eye tracking. Journal of Cognitive Neuroscience, 19, 843-854.
Wills, A.J., Noury, M., Moberly, M.J., & Newport, M. (2006). Formation of category representations. Memory and Cognition, 34, 17-27.
Lea, S.E.G., Wills, A.J., & Ryan, C.M.E. (2006). Why are artificial polymorphous concepts so hard for birds to learn? Quarterly Journal of Experimental Psychology, 59, 251-267.
Zwickel, J., & Wills, A.J. (2005). Integrating associative models of supervised and unsupervised categorization. In A.J. Wills (Ed.). New Directions in Human Associatve Learning (pp. 101-123). Lawrence Erlbaum Associates.
Le Pelley, M.E., Oakeshott, S.M., Wills, A.J., & McLaren, I.P.L. (2005). The outcome-specificity of learned predictiveness effects: Parallels between human causal learning and animal conditioning. Journal of Experimental Psychology: Animal Behavior Processes, 31, 2226-236.
Wills, A.J. (2005, Ed.). New Directions in Human Associative Learning. Hove: Lawrence Erlbaum Associates.
Bryant, C.M., Jones, G.J.F., & Wills, A.J. (2004). Integration of psychological models in the design of artificial creatures. Proceedings of the AISB 2004 symposium on Emotion, Cognition and Affective Computing (pp. 9-20).
Milton, F., & Wills, A.J. (2004). The influence of stimulus properties on category construction. Journal of Experimental Psychology: Learning, Memory and Cognition, 30, 407-415.
Wills, A.J., Suret, M.B., & McLaren, I.P.L. (2004). The role of category structure in determining the effects of stimulus preexposure on categorization accuracy. Quarterly Journal of Experimental Psychology, 57B, 79-88.
Goto, K., Wills, A.J., & Lea, S.E.G. (2004). Global-feature classification can be acquired more rapidly than local-feature classification in both humans and pigeons. Animal Cognition, 7, 109-113.
Lochmann, T., & Wills, A.J. (2003). Predictive history in an allergy prediction task. In F. Schmalhofer, R.M. Young, & G. Katz (Eds.). Proceedings of EuroCogSci 03: The European Cognitive Science Conference (pp. 217-222). Mahwah, NJ: Lawrence Erlbaum Associates.
Welham, A.K., Schnadt, M.J., & Wills, A.J. (2003). Speeded categorization: the effects of perceptual processing and decision-making time. In R. Alterman, & D. Kirsh (Eds.). Proceedings of the 25th Annual Conference of the Cognitive Science Society (pp. 1218-1223). Mahwah, NJ: Lawrence Erlbaum Associates.
Zwickel, J., & Wills, A.J. (2002). Is competitive learning an adequate account of free classification? In W.D. Gray, & C.D. Schunn (Eds.). Proceedings of the 24th Annual Conference of the Cognitive Science Society (pp. 982-987). Mahwah, NJ: Lawrence Erlbaum Associates.
Wills, A.J. (2002). Adapting to a response deadline in categorization. In W.D. Gray, & C.D. Schunn (Eds.). Proceedings of the 24th Annual Conference of the Cognitive Science Society (pp. 938-943). Mahwah, NJ: Lawrence Erlbaum Associates. Published: August 2002.
Wills, A.J., Reimers, S., Stewart, N., & McLaren, I.P.L. (2000). Tests of the ratio rule in categorization. Quarterly Journal of Experimental Psychology, 53A, 983-1011.
Wills, A.J., & McLaren, I.P.L. (1998). Perceptual learning and free classification. Quarterly Journal of Experimental Psychology, 51B, 235-270.
Jones, F.W., Wills, A.J., & McLaren, I.P.L. (1998). Perceptual categorization: connectionist modelling and decision rules. Quarterly Journal of Experimental Psychology, 51B, 33-58.
Wills, A.J., & McLaren, I.P.L. (1997). Generalization in human category learning: A connectionist explanation of differences in gradient after discriminative and non-discriminative training. Quarterly Journal of Experimental Psychology, 50A, 607-630.