The decomposition of visual binding over time: Neuropsychological evidence from illusory conjunctions after posterior parietal damage

Patients with Balint's syndrome are known to make abnormal numbers of illusory conjunctions (ICs) when presented with multiple stimuli and asked to report the features of one. We used two converging procedures to assess the time course of these errors. In Experiments 1 and 2 the errors produced by a patient with Balint's syndrome, GK, were examined as a function of when he responded. We find that ICs were present even in GK's fastest responses, but that they also increased when GK responded slowly. In Experiment 3 we varied the exposure duration of the stimuli. With short stimulus exposures GK made ICs that he was certain were correct. With longer exposures there was an increase in the number of ICs where GK expressed uncertainty. In contrast to these "uncertain'' ICs, feature errors decreased as the exposure duration increased. We propose that the ICs present in GK's fastest responses, and that arise with short stimulus exposures, reflect impairments at a first stage of binding. In addition to this, "uncertain'' ICs arise on trials with slow responses, and with long exposures, due to performance then being affected by impairments to a second process dependent on bound features being consolidated into a more stable representation. The role of this consolidation process is limited when responses are made rapidly and exposure durations limited. This two-stage account is discussed in relation to other accounts of feature binding.