, 2009 and Britten, 2008). Correlated noise among pairs of neurons was examined in two groups of animals: one group (“naive”) was only trained to fixate; the other group (“trained”) also learned to perform a fine heading discrimination task. Noise correlations were significantly weaker in trained than naive animals, whereas tuning curves, response variability, and discrimination thresholds of individual neurons were similar. Importantly,
training reduced noise correlations uniformly, regardless of tuning similarity between pairs of neurons. As a result, if all neurons contribute equally to perception, this change in correlated noise is unlikely to account for improvements in perceptual sensitivity with training. MG-132 price Monkeys were presented with two types of heading stimuli while maintaining fixation on a head-fixed target: inertial motion delivered by a motion platform in the absence of optic flow (vestibular
condition) and optic flow stimuli presented while the animal was stationary (visual condition, see Experimental Procedures for details). Consistent with previous findings (Gu et al., 2006 and Takahashi et al., 2007), many MSTd neurons were tuned to heading direction, and their responses mainly followed the Gaussian velocity profile of the stimulus (Figure 1A). We analyzed responses obtained during the middle 1 s of the stimulus period, during which neuronal activity was robust. Tuning curves of two simultaneously recorded cells are shown in Figures learn more 1B and 1C. The similarity of heading tuning between pairs of neurons was quantified as the
Pearson correlation coefficient of mean responses across all stimulus directions (“signal correlation”, rsignal). For this example pair Terminal deoxynucleotidyl transferase of neurons, rsignal = 0.83 and 0.79 for the visual and vestibular conditions, respectively. As in other cortical areas, the spike counts of MSTd neurons in response to an identical stimulus vary from trial to trial, as illustrated in Figure 1D (visual condition) and Figure 1E (vestibular condition). Each datum in these plots represents the spike counts of the two neurons from a single trial. Because heading direction varied across trials, spike counts from individual trials have been z-scored to remove the stimulus effect and allow pooling of data across directions (see Experimental Procedures). “Noise correlation” is then computed as the Pearson correlation coefficient of the normalized trial-by-trial spike counts, and reflects the degree of correlated variability across trials. For this example pair of cells, there was a weak positive correlation, such that when one neuron fired more spikes, the other neuron did as well (visual condition: rnoise = 0.29, p = 0.04, Figure 1D; vestibular condition: R = 0.14, p = 0.3, Figure 1E). We first examined whether correlated noise in MSTd depends on stimulus modality (Figure 1F).