Neuropsychologia 40 (2002) 1849–1857 Visual search in Alzheimer’s disease: a deficiency in processing conjunctions of features A. Tales a,1 , S.R. Butler a , J. Fossey b , I.D. Gilchrist c , R.W. Jones b , T. Troscianko d,∗ a Burden Neurological Institute, Frenchay Hospital, Bristol BS16 1 JB, UK b The Research Institute For The Care Of The Elderly, St. Martin’s Hospital, Bath BA2 5RP, UK c Department of Experimental Psychology, University of Bristol, 8 Woodland Road, Bristol BS8 1TN, UK d School of Cognitive and Computing Sciences, University of Sussex, Brighton BN1 9QH, UK Received 19 February 2001; received in revised form 20 May 2002; accepted 20 May 2002 Abstract Human vision often needs to encode multiple characteristics of many elements of the visual field, for example their lightness and orien- tation. The paradigm of visual search allows a quantitative assessment of the function of the underlying mechanisms. It measures the ability to detect a target element among a set of distractor elements. We asked whether Alzheimer’s disease (AD) patients are particularly affected in one type of search, where the target is defined by a conjunction of features (orientation and lightness) and where performance depends on some shifting of attention. Two non-conjunction control conditions were employed. The first was a pre-attentive, single-feature, “pop-out” task, detecting a vertical target among horizontal distractors. The second was a single-feature, partly attentive task in which the target element was slightly larger than the distractors—a “size” task. This was chosen to have a similar level of attentional load as the conjunction task (for the control group), but lacked the conjunction of two features. In an experiment, 15 AD patients were compared to age-matched controls. The results suggested that AD patients have a particular impairment in the conjunction task but not in the single-feature size or pre-attentive tasks. This may imply that AD particularly affects those mechanisms which compare across more than one feature type, and spares the other systems and is not therefore simply an ‘attention-related’ impairment. Additionally, these findings show a double dissociation with previous data on visual search in Parkinson’s disease (PD), suggesting a different effect of these diseases on the visual pathway. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Attentional mechanisms; Dementia; Vision 1. Introduction For appropriate environmental interaction, our visual sys- tem must be able to detect and recognise visual objects. The analysis of a complex visual scene is traditionally seen as being made up of several distinct but inter-related functional stages and anatomical regions and to be variously reliant on the need for attention. The first of these stages, feature extraction, is generally accepted to be pre-attentive or au- tomatic in nature. At this level, local patches of the scene are encoded as a set of basic features [37] which include size, orientation, colour, distance, brightness, position, and motion. This encoding is performed in parallel. In a subse- quent feature integration stage, the sequential action of at- ∗ Corresponding author. Tel.: +44-774-7000-764; fax: +44-1273-671320. E-mail address: tomt@cogs.susx.ac.uk (T. Troscianko). 1 Present address: The BRACE Centre, Department of Care of the Elderly, Bristol University, Blackberry Hill Hospital, Manor Road, Bristol BS16 2EW, UK. tention on different locations in the visual scene results in the binding together or conjoining of features to allow more complex processes, such as object recognition to occur. The sequential nature of this attentional process means that it cannot operate simultaneously (in parallel) on the whole vi- sual scene; rather, the serial deployment of focused attention throughout the scene is required to conjoin the features by which the target can be defined, according to Treisman and Gelade [39]. The efficiency by which a particular target can be found in a natural visual scene has been emulated in the labora- tory by using the technique of visual search. In a typical experiment, a participant views a display in which there is a varying number (N) of distractor elements, and there may or may not be a pre-specified target element present. The participant is asked to indicate whether or not the specified target is present, and the reaction time (RT) of the response is measured. Search time and accuracy systematically vary with the type of target and the number of distractor elements (set size). 0028-3932/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII:S0028-3932(02)00073-8