A task in which response is guided by an internal representation of a stimulus in the absence of that stimulus

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Delay tasks were introduced into experimental psychology by Hunter (1913), who found that a variety of species can learn to respond to a light stimulus after a delay, which in his hands ranged from a few seconds in rats to about half an hour in children. The procedures were later refined by Yerkes and Yerkes (1928) in their investigation on mnemonic capabilities in the chimpanzee. Since then, a variety of delay tasks have been developed and proven highly useful in the measurement of short-term and "working memory, "recognition, and "recall.

Delay tasks comprise an heterogeneous family whose members are united solely by the fact that a delay is introduced between the stimulus and the opportunity to respond to it (definition 1).1 Other than that, these tasks could be used to tax different facets of learning and memory, "attention and motivation, innate response predispositions ("a priori), response strategies, and "planning. The delay period may be blank, or, alternatively, the delay is filled with other stimuli, which could serve as distracters. A common assumption is that in all the delay tasks, correct "performance taps into the ability to hold in memory information about the stimulus in the absence of that stimulus (definition 2). In some delay tasks, the validity of this assumption must not be taken for granted. For example, consider the spatial delayed alternation task. The subject is first rewarded for selecting one of two positions in a response chamber or a "maze. This is followed by a delay, after which the correct response is choosing the alternate position. Thus the subject must remember over the delay that it had responded to position A, and master the rule that the next response is B, and so on ("learning set). The memory used is a combination of recall (of the last response) and recognition (of the situation after the delay). The problem with this type of spatially guided delay paradigm is that the subject could adopt a strategy of orienting toward the correct position while still in the sample phase, hence eliminating the need to rely on the internal representation of the stimulus over the delay (Hunter 1913; Steckler etal. 1998a).

Indeed some authors distinguish 'delayed response' from 'delayed comparison' tasks (Steckler et al. 1998a). In delayed response tasks, all the information necessary for the internal representation of the correct response is available before the delay, and the behaviour could represent a "habit response that only awaits the "cue in the test. In delayed comparison, additional information must be supplied during the test, involving the comparison of past and present situations. To emit a correct response, the subject must recombine the information carried over the delay with the on-line test information. Examples for delayed comparison tasks are provided by the delayed matching to sample (DMTS) and the delayed nonmatching to sample (DNMTS) tasks. Here the sample stimulus, usually a visual one, is presented and then withdrawn. After a delay, the sample stimulus is presented again along with one or more additional stimuli. The subject has to respond by either choosing the previous stimulus (DMTS) or a different stimulus (DNMTS). As it is not a spatially guided response to a recurrent situation (compare with delayed spatial alternation above), the subject cannot 'cheat' by orienting during the training phase and merely release the planned motor response in the test.

A particularly useful DNMTS task is the trial unique DNMTS (Gaffan 1974; Mishkin and Delacour 1975).

Delayed Response Task Monkey

Fig. 24 Delayed response tasks are used to measure object Recognition, visuospatial memory, and working memory. (a) A visuospatial task. The monkey is placed in a Wisconsin General Testing Apparatus (Harlow and Bromer 1938).A food reward is placed in one of two wells in front of the cage. Both wells are then covered. After a delay (usually a few seconds or minutes), the monkey has to uncover the baited well. This task measures the ability to remember visuospatial information over the delay. (b) Delayed matching and nonmatching to sample tasks. A visually conspicuous object is placed over a central reward-baited well. The monkey replaces the object to obtain the food reward. After a delay, the monkey is confronted with the same object paired with a new one. In the delayed matching-to-sample task (left), displacement of the familiar object is rewarded. This test measures visual recognition and the association of an object with reward. In the delayed nonmatching-to-sample task (right), displacement of the nonfamiliar object is rewarded. In the trial-unique version of this task, each object is seen in only one trial during the whole series of tests. The trial-unique nonmatching-to-sample is appropriate for quantifying recognition memory, because reward is only an incentive to perform the task, and the ability to associate a specific object with reward does not contribute to success on the test.The monkey masters the nonmatching tasks faster than the matching tasks, because it has an *a priori tendency to explore novelty. (Modified from Mishkin and Appenzeller 1987; Dudai 1989.)

Fig. 24 Delayed response tasks are used to measure object Recognition, visuospatial memory, and working memory. (a) A visuospatial task. The monkey is placed in a Wisconsin General Testing Apparatus (Harlow and Bromer 1938).A food reward is placed in one of two wells in front of the cage. Both wells are then covered. After a delay (usually a few seconds or minutes), the monkey has to uncover the baited well. This task measures the ability to remember visuospatial information over the delay. (b) Delayed matching and nonmatching to sample tasks. A visually conspicuous object is placed over a central reward-baited well. The monkey replaces the object to obtain the food reward. After a delay, the monkey is confronted with the same object paired with a new one. In the delayed matching-to-sample task (left), displacement of the familiar object is rewarded. This test measures visual recognition and the association of an object with reward. In the delayed nonmatching-to-sample task (right), displacement of the nonfamiliar object is rewarded. In the trial-unique version of this task, each object is seen in only one trial during the whole series of tests. The trial-unique nonmatching-to-sample is appropriate for quantifying recognition memory, because reward is only an incentive to perform the task, and the ability to associate a specific object with reward does not contribute to success on the test.The monkey masters the nonmatching tasks faster than the matching tasks, because it has an *a priori tendency to explore novelty. (Modified from Mishkin and Appenzeller 1987; Dudai 1989.)

This paradigm has been extensively used in the analysis of visual recognition in the "monkey. In brief, a monkey is placed in a test enclosure, such as the Wisconsin General Testing Apparatus (WGTA, Figure 24) (Harlow and Bromer 1938), and presented with a visually conspicuous 'junk' object over a central baited food well. The monkey learns to uncover the well and retrieve the food reward. After a delay, in which a screen is lowered to hide the manipulation of stimulus tray from the monkey, the same object is paired with another, novel 'junk' object, each presented over a lateral well. The monkey must now avoid the familiar object and displace the new one. The procedure is repeated, with new 'junk' objects in each trial. As the objects are unique to each trial, no object-reward associations are formed (the reward here is merely an incentive to perform the test). The results could therefore be construed as representing 'pure' visual recognition combined with the innately dispositioned rule of'go for the new one'. Furthermore, the information from one trial is irrelevant to the next, and therefore this procedure is also useful in tapping "working memory.

Selected delay tasks are key components of test batteries used in the analysis of monkey "models of the human "amnesic syndrome (Zola-Morgan and Squire 1985). This is because performance on such tasks, e.g. DNMTS, maps on to multiple features of the amnesic syndrome, including the dependence on the length of the delay, the sensitivity to interference, and the independence of sensory modality. Attempts have been made to develop reliable DNMTS procedures for rodents as well, to facilitate the molecular and "neurogenetic analysis of amnesia models, but the interpretation of the behavioural data is still unsettled (e.g. Mumby 1995); it is not unlikely that monkeys will run happily in huge "mazes in memory labs around the world before rats will be able to solve the monkey's favourite delay tasks.

Performance on delay tasks is subserved by multiple brain regions, including "cortical and subcortical (Eichenbaum et al. 1994; Mishkin and Murray 1994; Brown and Xiang 1998; Parker and Gaffan 1998; Steckler et al. 1998b; "hippocampus, "limbic system). The involvement of some cortical regions is modality specific, e.g. inferotemporal cortex in vision (Mishkin and Murray 1994) or somatosensory cortex in haptic tasks (Zhou and Fuster 1996), whereas the involvement of other cortical regions, i.e. perirhinal and parahip-pocampal cortex, is modality independent (Suzuki et al. 1993). Visuospatial delayed response tasks rely heavily on frontal function (Jacobsen and Niseen 1937; Mishkin 1957), and the performance on these tasks is correlated with prefrontal neuronal activity (Fuster 1973; Friedman and Goldman-Rakic 1994; "retrieval, "working memory). Delay tasks are also useful in assessment and research of human neurological and affective disorders. For example, poor performance of chronic schizophrenics on visuospatial delay tasks is cited to support the notion that frontal pathology is involved (Pantelis et al. 1997).

Selected associations: Attention, Instrumental conditioning, Learning set, Monkey, Working memory

''Delay task' should not be confused with 'delay conditioning', which is a protocol of 'classical conditioning. In delay conditioning, the onset of the unconditioned stimulus occurs after the onset, but prior to the offset, of the conditioned stimulus; hence the response in delay conditioning does not have to be guided by off-line information.

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