Visual perceptual learning (VPL) is influenced by the interaction of attention andreinforcement. Interestingly, VPL can occur in absence of stimulus-directedattention (Seitz et al., 2009). Such task-irrelevant perceptual learning (TIPL) hasbeen hypothesized to result from the temporal coincidence of a stimulus andneuromodulatory signals triggered by reinforcement. Consequently, TIPL offersan opportunity to isolate the role of neuromodulatory signals in the regulation ofVPL and plasticity while excluding the role of attention. Although severalneuromodulatory centers may be involved in VPL, the ventral tegmental area(VTA) is a good candidate because its neurons are activated by reinforcement,have widespread connections and release dopamine that facilitates plasticity.Therefore, we microstimulated monkey VTA (VTA-EM) to determine its causal rolein VPL and cortical plasticity. In Exp 1 we looked for plasticity. To do this we performed fMRI mapping (preassociation)to determine the response to 4 different grating stimuli [(L/R visualfield) x (45°/135°)]. During all phases, the animals performed a difficult,orthogonal color discrimination task (80% correct performance) during gratingpresentation to avoid grating-directed attention. Next came a cue-VTA-EMassociation phase in which all gratings were shown but one was temporallyassociated with VTA-EM (20 sessions). Again, the color task was used to controlattention. Finally, post-association fMRI mapping was identical to phase 1. Theanalysis demonstrated representation-specific enhancements (post vs. pre) infMRI activity for the stimulated grating orientation within the ‘trained’ visual field,especially pronounced in PIT. These changes were not found for gratingsdisplayed in the ‘untrained’ visual field.In Exp 2 we looked for VPL effects. Exp 2 also consisted of pre- and postassociationphases bookending a cue-VTA-EM association phase (6 - 20 sessions).Within the pre- and post-association phases the animals discriminated betweenupward and downward random dot motion stimuli presented in the left or rightvisual field at 6 motion coherences (0 - 50%). The cue-VTA-EM association phaseconsisted of bilateral presentations of 0 and 2% motion stimuli with only one 2%coherence stimulus being associated with VTA-EM. The color task was again usedto avoid directed attention to the motion stimuli. Enhanced performance forlower motion coherences (2 - 12%) were consistently observed in the test phasefor the ‘trained’ visual field. These results provide causal evidence for the role ofprimate VTA in VPL and stimulus-specific plasticity in the absence of stimulusorientedattention.
展开▼
