Perceptual decisions that are used to select particular actions can appear to be formed in an intentional framework, in which sensory evidence is converted directly into a plan to act. However, because the relationship between perceptual decision-making and action selection has been tested primarily under conditions in which the two could not be dissociated, it is not known whether this intentional framework plays a general role in forming perceptual decisions or only reflects certain task conditions. To dissociate decision and motor processing in the brain, we recorded from individual neurons in the lateral intraparietal area (LIP) of monkeys performing a task that included a flexible association between a decision about the direction of random-dot motion and the direction of the appropriate eye-movement response. We targeted neurons that responded selectively in anticipation of a particular eye-movement response. We found that these neurons encoded the perceptual decision in a manner that was distinct from how they encoded the associated response. These decision-related signals were evident regardless of whether the appropriate decision-response association was indicated before, during, or after decision formation. The results suggest that perceptual decision-making and action selection are different brain processes that only appear to be inseparable under particular behavioral contexts.

Gold JI, Law C-T, Connolly P, Bennur S. The relative influences of priors and sensory evidence on an oculomotor decision variable during perceptual learning. J Neurophysiol 100: 2653-2668. First published August 27, 2008 doi:10.1152/jn.90629.2008. Choice behavior on simple sensory-motor tasks can exhibit trial-to-trial dependencies. For perceptual tasks, these dependencies reflect the influence of prior trials on choices that are also guided by sensory evidence, which is often independent across trials. Here we show that the relative influences of prior trials and sensory evidence on choice behavior can be shaped by training, such that prior influences are strongest when perceptual sensitivity to the relevant sensory evidence is weakest and then decline steadily as sensitivity improves. We trained monkeys to decide the direction of random-dot motion and indicate their decision with an eye movement. We characterized sequential dependencies by relating current choices to weighted averages of prior choices. We then modeled behavior as a drift-diffusion process, in which the weighted average of prior choices provided an additive offset to a decision variable that integrated incoming motion evidence to govern choice. The average magnitude of offset within individual training sessions declined steadily as the quality of the integrated motion evidence increased over many months of training. The trial-by-trial magnitude of offset was correlated with signals related to developing commands that generate the oculomotor response but not with neural activity in either the middle temporal area, which represents information about the motion stimulus, or the lateral intraparietal area, which represents the sensory-motor conversion. The results suggest that training can shape the relative contributions of expectations based on prior trends and incoming sensory evidence to select and prepare visually guided actions. I N T R O D U C T I O NPerformance on trial-based sensory-motor tasks can exhibit numerous forms of sequential dependence. For example, both response times and choice are sensitive to effects of the previous trial, including repeated actions, task switching, and errors (Botvinick et al. 2001;Cho et al. 2002;Laming 1979Laming 1968Luce 1986). For tasks in which reward probability depends on sequential patterns of choices, these dependencies can reflect rational strategies for choosing future outcomes based on prior history Behrens et al. 2007;Corrado et al. 2005;Davidson and McCarthy 1988;Herrnstein 1961;Kennerley et al. 2006;Lau and Glimcher 2005;Sugrue et al. 2004;Williams 1988). In contrast, for perceptual tasks in which the present stimulus is the exclusive factor determining the rewarded choice, these dependencies can only hinder performance. The goal of this study was to better understand how these dependencies evolve as perceptual sensitivity to sensory cues that instruct the rewarded choice improves with training.We trained monkeys on a one-interval, two-alternative direction-discrimination task in which they d...

Prolonged and severe stress leads to cognitive deficits, but facilitates emotional behaviour. Little is known about the synaptic basis for this contrast. Here, we report that in rats subjected to chronic immobilization stress, long-term potentiation (LTP) and NMDA receptor (NMDAR)-mediated synaptic responses are enhanced in principal neurons of the lateral amygdala, a brain area involved in fear memory formation. This is accompanied by electrophysiological and morphological changes consistent with the formation of ‘silent synapses’, containing only NMDARs. In parallel, chronic stress also reduces synaptic inhibition. Together, these synaptic changes would enable amygdalar neurons to undergo further experience-dependent modifications, leading to stronger fear memories. Consistent with this prediction, stressed animals exhibit enhanced conditioned fear. Hence, stress may leave its mark in the amygdala by generating new synapses with greater capacity for plasticity, thereby creating an ideal neuronal substrate for affective disorders. These findings also highlight the unique features of stress-induced plasticity in the amygdala that are strikingly different from the stress-induced impairment of structure and function in the hippocampus.

Summary Neurons in cortical sensory areas respond selectively to sensory stimuli, and the preferred stimulus typically varies among neurons so as to continuously span the sensory space. However, some neurons reflect sensory features that are learned or task-dependent. For example, neurons in the lateral intraparietal area (LIP) reflect learned associations between visual stimuli. One might expect roughly even numbers of LIP neurons would prefer each set of associated stimuli. However, in two associative learning experiments and a perceptual decision experiment, we found striking asymmetries: nearly all neurons recorded from an animal had a similar order of preference among associated stimuli. Behavioral factors could not account for these neuronal biases. A recent computational study proposed that population-firing patterns in parietal cortex have “one-dimensional” dynamics on long time scales, a possible consequence of recurrent connections that could drive persistent activity. One-dimensional dynamics would predict the biases in selectivity that we observed.

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