Neuropsychologia 42 (2004) 577–583 Visual inspection time in Parkinson’s disease: deficits in early stages of cognitive processing Andrew M. Johnson a,* , Quincy J. Almeida b,e , Con Stough c , James C. Thompson c , Rene Singarayer d , Mandar S. Jog e a Faculty of Health Sciences, University of Western Ontario, London, Ont., Canada N6A 3K7 b Department of Kinesiology, University of Waterloo, Waterloo, Ont., Canada c Swinburne Centre for Neuropsychology, Swinburne University, Hawthorn, Vic., Australia d Department of Biochemistry, McGill University, Montreal, Que., Canada e Department of Clinical Neurological Sciences, London Health Sciences Centre, London, Ont., Canada Received 6 February 2003; received in revised form 11 June 2003; accepted 30 October 2003 Abstract Inspection time (IT) is a simple information processing paradigm dependent on a participant’s ability to identify physical properties of a stimulus presented for a specified time interval. In contrast with reaction time (RT) studies, the dependent variable of interest in IT is not related to the motoric speed with which the individual is able to respond, but rather the minimum presentation time necessary for participants to reliably identify physical properties of the stimulus. It is well documented that individuals with Parkinson’s disease (PD) experience significant impairment on tests of simple RT, but it is unclear whether such deficits can be interpreted as ‘pure’ slowness of information processing, or a delay in the selection and output of a motor response. In the first experiment described here, a sample of ‘optimally medicated’ PD patients was compared with an age-matched control group, on an IT task. Results of this experiment suggested that individuals with PD required significantly longer stimulus presentation times than healthy participants. The second experiment compared a sample of PD patients (tested both “ON” and “OFF” their typical dopaminergic medications), with an age-matched control group, on the same test of IT. Results again indicated a significant IT deficit in participants with PD, and suggested that these deficits do not significantly resolve with levodopa treatment. Overall, the results of these two experiments suggest that information processing deficits associated with PD are distinct from motor impairment. These findings are further discussed in terms of existing neurochemical models of information processing ability. © 2003 Elsevier Ltd. All rights reserved. Keywords: Parkinson’s disease; Inspection time; Information processing; Perceptual speed 1. Introduction Inspection time (IT) is a simple information processing paradigm dependent on a participant’s ability to identify physical properties of a stimulus presented within a lim- ited time interval (Deary & Stough, 1996; Nettelbeck, 1982; Vernon, 1986). In contrast with reaction time (RT) studies, the dependent variable of interest in an IT task is not re- lated to the time required to select a motor response, nor the time required to transmit the selected motor response to the appropriate body segment. Measures of IT can thus be considered an important indicator of information processing speed, particularly with regards to the early, or input, stages (Petrill, Luo, Thompson, & Detterman, 2001). * Corresponding author. Tel.: +1-519-661-2111x82215; fax: +1-519-850-2432. E-mail address: ajohnson@uwo.ca (A.M. Johnson). The dissociation between information processing and speed of motor output is particularly interesting within pop- ulations demonstrating clinically impaired movement, such as Parkinson’s disease (PD). It is well documented that individuals with PD experience significant impairment on tests of simple RT (Gauntlett-Gilbert & Brown, 1998), and yet it is unclear whether such deficits can be interpreted as ‘pure’ slowness at the input stage, or a delay in the output of a selected motor response to the limb (Adams, Victor, & Ropper, 1997). To the extent, therefore, that IT is a valid predictor of information processing speed, it may be useful in distinguishing between bradykinesia (slowness of move- ment) and bradyphrenia (slowness of cognition) in PD. Given its reliance on the speed at which participants can accurately apprehend the properties of a visually presented stimulus, IT is often described as a measure of perceptual speed. Although few previous studies have explicitly exam- ined IT in PD, individuals with PD have been shown to 0028-3932/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropsychologia.2003.10.011