Neuropsychologia 40 (2002) 1488–1493 Efficiency of temporal order discrimination as an indicator of bradyphrenia in Parkinson’s disease: the inspection time loop task Beverly A. Shipley a , Ian J. Deary a,∗ , Jennifer Tan a , Gayle Christie a , John M. Starr b a Department of Psychology, University of Edinburgh, 7 George Square, Edinburgh EH8 9JZ, Scotland, UK b Royal Victoria Hospital, 13 Craigleith Road, Edinburgh EH4 2DN, Scotland, UK Received 28 March 2001; received in revised form 17 September 2001; accepted 17 September 2001 Abstract To investigate the bradyphrenia hypothesis of Parkinson’s disease (PD), 32 patients undertook an information-processing task which measured their efficiency of temporal order discrimination. Their performance was compared with 31 non-PD controls matched on age, sex, years of full-time education and pre-morbid IQ. The task was novel and designed to be sensitive to the clinical phenomenon of bradyphrenia (slowing of mental abilities), in the context of temporal order discrimination without confounding from motor ability deficits. The test (the inspection time loop task; ITloop) required judgements as to the temporal sequence of four single letters. The stimulus duration of the letters in each sequence ranged from 100 to 700 ms. The PD group had a significantly lower mean score on the ITloop task than did controls (P = 0.02). PD patients perform more poorly on temporal order discrimination judgements even when the task makes no motor demands. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Parkinson’s disease; Dopamine; Inspection time; Information processing; Mental-speed; Backward masking 1. Introduction The hypothesis that bradyphrenia significantly contributes to the cognitive deficits seen in Parkinson’s disease (PD) originates in the early 1900s when, in 1922, the French scientist Naville introduced the term to describe the slowing of cognitive processes after studies looking into PD noted retardation in ‘complex’ frontal lobe tasks. Some studies of bradyphrenia in PD have focused on par- allels between cognitive decline in healthy elderly partici- pants and that in PD (e.g. [11]). Repeatedly, studies have shown distinct cognitive slowing in healthy elderly partici- pants compared to healthy younger controls and that this de- cline is associated with the slowing of the processing of in- formation [14]. Studies investigating cognitive decline in PD have also noted the presence of slowed information process- ing or bradyphrenia [11]. Therefore, if PD is an accelerated form of some of the cognitive deficits of normal ageing then participants should show an accentuated deficit in task per- formance when compared to age matched healthy controls. In the examination of cognitive slowing in healthy elderly subjects, the favoured assessment methods of speed of in- ∗ Corresponding author. Tel.: +44-131-650-3452; fax: +44-131-651-1771. E-mail address: i.deary@ed.ac.uk (I.J. Deary). formation processing involve tests such as the Digit Sym- bol subtest from the Wechsler battery and sundry reaction time procedures. All of these require motor responses under time pressure. However, since motor slowing (bradykinesia) is symptomatic of PD and affects performance on all timed tests, it is clear that timed motor tests are unsuited to PD stud- ies, as results may not provide unequivocal evidence of men- tal slowing. This concern about motor responses prompted cognitive scientists investigating PD to move to psychophys- ical procedures, such as inspection time [23], to estimate the efficiency of mental processing [14,17]. The main strength of tasks such as inspection time is the absence of the need for a speeded motor response. Temporal discrimination is another motor-free measure of mental processing and has been described as “a measure of the minimum time interval required between two successive auditory, visual or somaes- thetic stimuli for them to be perceived as separate” [1]. Tests of this ilk, used to measure the hypothesised general slow- ing of cognitive abilities in PD patients, suggest that there is slowed speed of processing in those suffering from PD [17]. To date, though, there have been few studies specifi- cally aimed at examining bradyphrenia in PD in the absence of motor demands. In the present study, we employed the inspection time loop task (ITloop). The ITloop task was constructed and de- veloped by Deary [4] as a visual form of Warren’s auditory 0028-3932/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII:S0028-3932(01)00195-6