Abstract
Training can increase the performance in visual detection and discrimination tasks, a phenomenon of adult plasticity known as visual perceptual learning (VPL). Neural correlates of adult VPL include changes in the selectivity of neurons in visual areas. It is hypothesized that this plasticity is gated by reward signals, even in the absence of attention to the stimuli. Thus, mere pairing of reward with a stimulus can be sufficient to increase the representation of the paired stimulus. Indeed, previous fMRI studies in macaques in which electrical micro-stimulation of the ventral tegmental area (VTA-EM) was paired with a weak visual stimulus showed increased activations in posterior inferior temporal (PIT) cortex for the VTA-EM-paired stimulus (Arsenault & Vanduffel, Nature Communications, 2019). Here, we examined changes in neural responses and selectivity by recording spiking activity in PIT after pairing a grating orientation with VTA-EM. Two monkeys performed an orthogonal color discrimination task, while a low visibility grating was paired with VTA-EM. After 15-20 VTA-EM sessions, daily VTA-EM was followed by multi-unit activity (MUA) recordings in PIT during a fixation task presenting high visibility gratings of different orientations and a color discrimination task presenting low visibility gratings. The experiment was repeated with VTA-EM pairing of the orientation perpendicular to the paired orientation of the initial experiment. We observed that the relative proportion of units with a preference for the VTA-EM-paired orientation increased compared to the orthogonal orientation. A relative increase in mean spike rate was observed for the VTA-EM-paired orientation, as well. Significant effects of pairing were present for response variability (Fano factor) and noise correlations, but inconsistent over subjects, suggesting that the response changes did not result from attentional modulation. In conclusion, pairing VTA-EM with a low visibility grating stimulus while performing an orthogonal task induces changes in orientation preference of PIT neurons.