Thu, 21 Aug 2008 09:45:42 BST CiteULike: klouie's Nakahara http://www.citeulike.org/user/klouie/author/Nakahara CiteULike.org en-gb Copyright © 2004-2008 citeulike.org http://www.citeulike.org/user/klouie/article/890133 <i>Neuron, Vol. 41, No. 2. (22 January 2004), pp. 269-280.</i><br /><br />Midbrain dopamine (DA) neurons are thought to encode reward prediction error. Reward prediction can be improved if any relevant context is taken into account. We found that monkey DA neurons can encode a context-dependent prediction error. In the first noncontextual task, a light stimulus was randomly followed by reward, with a fixed equal probability. The response of DA neurons was positively correlated with the number of preceding unrewarded trials and could be simulated by a conventional temporal difference (TD) model. In the second contextual task, a reward-indicating light stimulus was presented with the probability that, while fixed overall, was incremented as a function of the number of preceding unrewarded trials. The DA neuronal response then was negatively correlated with this number. This history effect corresponded to the prediction error based on the conditional probability of reward and could be simulated only by implementing the relevant context into the TD model. Dopamine neurons can represent context-dependent prediction error. H Nakahara H Itoh R Kawagoe Y Takikawa O Hikosaka Neuron, Vol. 41, No. 2. (22 January 2004), pp. 269-280. 2006-10-09T14:04:22-00:00 2004 Neuron 0896-6273 41 2 269 280 dopamine neurophysiology reward http://www.citeulike.org/user/klouie/article/705089 <i>Nature, Vol. 401, No. 6754. (14 October 1999), pp. 699-703.</i><br /><br />Knowledge or experience is voluntarily recalled from memory by reactivation of the neural representations in the cerebral association cortex. In inferior temporal cortex, which serves as the storehouse of visual long-term memory, activation of mnemonic engrams through electric stimulation results in imagery recall in humans, and neurons can be dynamically activated by the necessity for memory recall in monkeys. Neuropsychological studies and previous split-brain experiments predicted that prefrontal cortex exerts executive control upon inferior temporal cortex in memory retrieval; however, no neuronal correlate of this process has ever been detected. Here we show evidence of the top-down signal from prefrontal cortex. In the absence of bottom-up visual inputs, single inferior temporal neurons were activated by the top-down signal, which conveyed information on semantic categorization imposed by visual stimulus-stimulus association. Behavioural performance was severely impaired with loss of the top-down signal. Control experiments confirmed that the signal was transmitted not through a subcortical but through a fronto-temporal cortical pathway. Thus, feedback projections from prefrontal cortex to the posterior association cortex appear to serve the executive control of voluntary recall. Top-down signal from prefrontal cortex in executive control of memory retrieval. H Tomita M Ohbayashi K Nakahara I Hasegawa Y Miyashita doi:10.1038/44372 Nature, Vol. 401, No. 6754. (14 October 1999), pp. 699-703. 2006-06-21T02:44:16-00:00 1999 Nature 0028-0836 401 6754 699 703 callosotomy it memory memory_retrieval monkey neurophysiology prefrontal split_brain http://www.citeulike.org/user/klouie/article/482007 <i>J Neurophysiol, Vol. 95, No. 2. (February 2006), pp. 567-584.</i><br /><br />Expectation of reward motivates our behaviors and influences our decisions. Indeed, neuronal activity in many brain areas is modulated by expected reward. However, it is still unclear where and how the reward-dependent modulation of neuronal activity occurs and how the reward-modulated signal is transformed into motor outputs. Recent studies suggest an important role of the basal ganglia. Sensorimotor/cognitive activities of neurons in the basal ganglia are strongly modulated by expected reward. Through their abundant outputs to the brain stem motor areas and the thalamocortical circuits, the basal ganglia appear capable of producing body movements based on expected reward. A good behavioral measure to test this hypothesis is saccadic eye movement because its brain stem mechanism has been extensively studied. Studies from our laboratory suggest that the basal ganglia play a key role in guiding the gaze to the location where reward is available. Neurons in the caudate nucleus and the substantia nigra pars reticulata are extremely sensitive to the positional difference in expected reward, which leads to a bias in excitability between the superior colliculi such that the saccade to the to-be-rewarded position occurs more quickly. It is suggested that the reward modulation occurs in the caudate where cortical inputs carrying spatial signals and dopaminergic inputs carrying reward-related signals are integrated. These data support a specific form of reinforcement learning theories, but also suggest further refinement of the theory. Basal Ganglia orient eyes to reward. O Hikosaka K Nakamura H Nakahara doi:10.1152/jn.00458.2005 J Neurophysiol, Vol. 95, No. 2. (February 2006), pp. 567-584. 2006-01-26T18:34:25-00:00 2006 J Neurophysiol 0022-3077 95 2 567 584 basal_ganglia review reward