The authors trained 2 homing pigeons (Columba livia) on a directed forgetting task with 3 cues: a remember cue that was followed by a memory test and the opportunity to obtain a reward, a forget cue that was not followed by a memory test or a reward, and a free-reward cue that was not followed by a memory test but was followed by a free reward. The authors examined the activity of single neurons in the avian nidopallium caudolaterale, an area equivalent to the primate prefrontal cortex. Following the remember cue there was sustained neural activity during the delay period, whereas following the forget cue the neural activity in the delay period was significantly reduced. The activity following the free-reward cue mirrored that following the remember cue. The authors discuss the extent to which the findings are in line with the view that the sustained activity reflects memory for the sample stimulus or memory for reward.
We review our studies examining neural correlates of directed forgetting and executive control in the avian prefrontal cortex. One of the fundamental forms of executive control is the ability to selectively filter information, retaining that which is critical for the current purposes and discarding that which is not. In our first experiment, we trained birds on a directed-forgetting version of a delayed matching-to-sample task. Following a sample stimulus, a bird heard either a remember tone indicating that a memory test would follow, or a forget tone indicating that no memory test would be given. We found that neural activity in the avian prefrontal cortex increased when the bird was told to remember, and decreased when the bird was told to forget. Behavioral probe tests confirmed that the animals were forgetting on forget trials.Although the sustained activation observed on remember trials and the absence of such activation on forget trials could be a code of remembering and forgetting the sample stimulus, it could also be a code of the possibility of obtaining a reward. To address this issue we conducted a second study in which we used three cues: remember, forget, and forget-reward. The forget-reward cue instructed the subject to forget the sample yet at the same time provided a free reward. Neural activity on forget-reward trials matched that on remember trials tentatively indicating that the sustained activation on remember trials might be a reward code rather than a sample stimulus code. Behavioral probe tests, however, failed to indicate that the animals were forgetting on forget-reward trials, and hence it still is possible that the sustained activation could be a code for memory of the sample stimulus. © 2008 Elsevier Inc. All rights reserved.Keywords: Sustained activation; Neural correlates of memory; Delay cells; Executive control; Prefrontal cortex; Nidopallium caudolaterale That we remember is self evident, yet how remembering is expressed in the activity of neurons is almost completely unknown. One possible mechanism that has been proposed is that of sustained activation [4,9,20]. Sustained activation is best understood with reference to the delayed matching-to-sample (DMS) task, a classic test of short-term memory in animals [23]. At the end of an intertrial interval (ITI), a sample stimulus (e.g., a circle or a line) is presented on the center of three projectors (see Fig. 1). After the subject makes a response to the sample stimulus, the sample stimulus is turned off for a period of time (delay) during which memory for the sample stimulus is activated. At the end of the delay period, both stimuli (circle and line) appear on the side projectors (comparison). A correct response requires * Corresponding author.E-mail address: colombo@psy.otago.ac.nz (M. Colombo). the subject to select the comparison stimulus that was the same as the sample stimulus. In the case of trial n in Fig. 1, the correct response is to press the circle stimulus, which results in a reward. Selecting the incorrect sti...
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