Binary choice in patients with prefrontal or posterior brain damage. A relative judgement theory analysis
Introduction
Binary choice, the basic psychological process of choosing between two possible outputs, may be involved in many cognitive functions, probably at various stages of information processing. Considering the universal nature of the decision process, it is surprising that the ability to perform binary choice tasks has only been poorly addressed in patients with focal brain damage. Most studies concern patients with widespread cerebral damage such as severe closed head injury (e.g. [45]), dementia (e.g. [31]) and Parkinson's disease (e.g. [9]) and most of them suggest the presence of “cognitive slowing” in these diseases. Attempts to assess decision processes have mainly used reaction time (RT) paradigms, specifically the comparison of simple RT (SRT) and choice RT (CRT). Conflicting results have been obtained in patients with focal brain damage. Few studies have shown prolongation of CRT relative to SRT 6, 12 but others have found no difference in the extent of response slowing between SRT and CRT 1, 3, 4, 10, 11, 20. The interpretation of these results remains difficult because most studies did not take into account the amount of errors and therefore prevent any conclusions being drawn on the efficiency of decision processes. Second, a few studies have evaluated the effect of the lesion side but none of them has addressed the role of the lesion location within the hemisphere. The prefrontal cortex may be the main site of critical networks for decision-making and strategically organised cognitive processes such as planning [35], and lesions in this area result in cognitive impairment prominent on complex tasks 27, 37. Complex tasks may be viewed as successive processing stages which require a decision at each step [37], and it might be of interest to examine whether the impairment of basic decision processes contributes to the planning disorder. Consistently a few data suggest that the decision process involved in a go/no-go task is impaired in patients with frontal damage because their errors are, at least in part, related to the adoption of a liberal response criterion 20, 33.
Most studies have restricted the assessment of decision processes to the evaluation of one performance level measurement, namely RT or hits scores. One limitation to this approach is that it does not take into account the fact that humans are able to trade speed for accuracy [14]. For example, slowed CRT in brain damaged patients is frequently attributed to cognitive slowing. However, increased CRT may also reflect a strategy of more prolonged evidence gathering prior to decision which is only assessible by the reduction in error rate. For this reason, the evaluation of decision needs to take into account both speed and accuracy and to evaluate their relationship. This requires a theoretical model providing some specification of the key parameters regulating the process. Several mathematical models that treat the trade-off between speed and accuracy in terms of random walk process have been developed 23, 25, 43. They share in common two major assumptions: a differential accumulation of information with different signals and a decision rule based on that accumulation 25, 26. In addition, the relative judgement model of Link and Heath [25] assumes that the average drift rate towards the correct border is the same for both response alternatives, an assumption that appears roughly correct according to a previous study [21]. This model was chosen for the present investigation for two reasons: its predictions are free from the constraints imposed by the statistical distribution [24] and it provides response time predictions which have been matched to experimental data 21, 24. The relative judgement theory assumes a random walk beginning at any point (the starting criterion) between the two borders (Fig. 1). The presented stimulus is compared to an internal psychological referent and discrepancies between the stimulus and the referent are accumulated over time until one of the two response thresholds is exceeded. When either threshold is reached, the appropriate motor response is activated. Shift in starting criterion influences the probability of reaching one of the two borders. Equations developed by Link and Heath [25] which determine decision parameters from hits scores are provided in the appendix.
The use of such a model in brain-damaged patients may provide critical information on decision regulation which is not available from the examination of raw data. First, it allows the separation of decision from non-decision latencies and therefore the examination as to whether slowing of patients' CRT is related to increased duration of non-decision processes or longer decision time. Lengthening of decision time itself may be due to the adoption of a conservative speed accuracy trade-off or to slowing of decision process, and both possibilities are assessable by the distance traversed by the decision process and the time required for the average step towards the boundary. Second, it provides indications as to patients' ability to regulate the decision parameters according to various experimental conditions.
We have conducted several studies using the relative judgement model in order to examine the regulation of decision processes in patients with focal brain damage involving the frontal or the posterior cortex. This study used a procedure with varied response probabilities 21, 24 which has the advantage that avoids an increase in task complexity, a major complicating factor in patients with frontal lobe damage. The aims of this study were (a) to investigate the binary decision process using different response probabilities and (b) to examine methodological issues related to the use of relative judgement model in patients with focal brain damage.
Section snippets
Subjects
The study group consisted of 10 patients with prefrontal damage (frontal group), seven patients with vascular damage of the retro-rolandic cortex (posterior group) and nine control subjects free from central nervous system disorder. Exclusion criteria for all subjects were: previous neurological or psychiatric disorders, sensory or motor disorders, current psychoactive or antiepileptic medication, severe cardiac, respiratory or renal failure, diabetes mellitus, alcoholism, illiteracy. All
Simple and choice RT comparison
This analysis compared SRT and CRT measured in the equiprobable condition during the fourth session and was aimed to evaluate the effect of (a) the task and (b) the previous trial on the current one. Both the SRT and the equiprobable CRT tests used the same sequence of stimuli and they only differed in the nature of the task (detection/choice) and in the mode of response (forefinger/forefinger or middle finger). Response slowing on the CRT test is mainly due to the time required for “central
Acknowledgements
We thank Maryline Cabaret, Violaine Petit and Brigitte Debachy for their technical assistance in the neuropsychological assessment; Jean Pierre Pruvo for the neuroradiological evaluation; Didier Leys and Jean-Paul Lejeune who allowed us to investigate some of their patients, and Marc Jeannerod for his helpful comments. This study has been supported by grants from the Direction de la Recherche et des Etudes Doctorales (no. 000639), Faculté de Médecine de Lille and from Laboratoires Schering SA.
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