For each recording session, we verified the laminar position of the electrode contacts by computing the evoked potential (ERP) profiles for brief visual stimulation during a passive fixation task (full-field black screen that flashed white for 100 ms,
and then returned to black). LFP responses were processed to obtain ERP traces for each contact (over 100 trials). We computed the current source density (CSD) by using the second spatial derivative of the LFP time-series across equally spaced laminar contacts using the iCSD toolbox for MATLAB (Pettersen et al., 2006). We analyzed the laminar CSD profile to verify the presence of a primary sink in the granular layer in each of the 34 recording sessions (the contact with the sink centroid served as granular layer reference at 0 μm). We then analyzed all the contacts above see more and below the reference and grouped them into one of three possible layers: supragranular, granular, and infragranular (see Supplemental Experimental Procedures). We measured spike count correlations (rSC) between Lumacaftor pairs of neurons in different layers. The calculation of rSC for a pair of neurons responding to particular stimulus orientation (θ) is as follows: equation(1) rsc(θ)=∑k=1N(rik−ri)(rjk−rj)Nσiσj=∑k=1Nrikrjk−rirjNσiσj,where
N is the number of trials, rik is the firing rate of neuron i in trial k, ri is the mean firing rate, and σi is the SD of the responses for neuron i ( Bair et al., 2001). We transformed the firing rates of neurons into Z scores, rik → zik = (rik − ri)/σi to eliminate the effect of stimulus orientation on the computation of noise correlations. To compute noise correlations for all stimulus orientations θ1, θ2,…, θn, we calculated for each neuronal pair the correlations rsc(θ1), rsc(θ2),…rsc(θn) and then averaged them in order to obtain the noise correlation coefficient for that pair:
equation(2) rSC=E[rsc(θ1),rsc(θ2),…,rsc(θn)]. To remove potential artifacts in the calculation of correlation coefficient, such as slow-wave fluctuations in responses across trials, all the neurons underwent detrending in which the spike counts for each trial were high-pass filtered using a linear-phase finite impulse response filter (Bair et al., 2001; Kohn and Smith, 2005). We thank D. Gutnisky and K. Josić for comments medroxyprogesterone on the manuscript and S. Pojoga for assistance during monkey training. This work was supported by grants from NEI, NIH EUREKA Program, Pew Scholars Program, and James S. McDonnell Foundation (V.D.), and an NIH Vision Training Grant (B.J.H). “
“To survive in an ever-changing environment, creatures must be able to predict what is going to occur next in order to plan their reactions appropriately. The natural world is not random: natural stimuli are highly redundant due to the physical properties of the world. For example, Ruderman and Bialek (1994) showed that there are strong statistical dependencies between luminance values in different pixels of natural scenes, and Nelken et al.