Laminar microcircuitry of visual cortex producing attention-associated electric fields

Abstract

Cognitive operations are widely studied by measuring electric fields through EEG and ECoG. However, despite their widespread use, the neural circuitry giving rise to these signals remains unknown because the functional architecture of cortical columns producing attention-associated electric fields has not been explored. Here, we detail the laminar cortical circuitry underlying an attention-associated electric field measured over posterior regions of the brain in humans and monkeys. First, we identified visual cortical area V4 as one plausible contributor to this attention-associated electric field through inverse modeling of cranial EEG in macaque monkeys performing a visual attention task. Next, we performed laminar neurophysiological recordings on the prelunate gyrus and identified the electric-field-producing dipoles as synaptic activity in distinct cortical layers of area V4. Specifically, activation in the extragranular layers of cortex resulted in the generation of the attention-associated dipole. Feature selectivity of a given cortical column determined the overall contribution to this electric field. Columns selective for the attended feature contributed more to the electric field than columns selective for a different feature. Last, the laminar profile of synaptic activity generated by V4 was sufficient to produce an attention-associated signal measurable outside of the column. These findings suggest that the top-down recipient cortical layers produce an attention-associated electric field that can be measured extracortically with the relative contribution of each column depending upon the underlying functional architecture.

Keywords: CSD; ECoG; EEG; LFP; N2pc; V4; neuroscience; rhesus macaque.

Figure 1.. EEG traces and inverse source localization for the N2pc index of attention in monkeys.

(A) EEG was recorded from electrodes arranged according to the 10–20 system in monkeys performing visual search by shifting gaze to a colored oddball stimulus (monitor diagrams show two example arrays). Blue and red shading highlights mapping between visual hemifield and cerebral hemisphere. (B) Trial-averaged P5 and P6 EEG traces from monkey Z following presentation of search arrays with the target in either the right (blue) or left (red) visual hemifield as well as the difference (orange). The voltage difference between the target in the left versus right hemifields reveals the N2pc ~150 ms after array presentation. The N2pc was significant (dependent samples t test between polarizations averaged between 125 and 250 ms after array presentation (t(35) = 2.42, p = 0.02)). (C) Inverse solution of current distribution consistent with difference in voltage distribution during the N2pc (113–182 ms) when the target was in the left hemifield versus right hemifield using sLORETA. Current density is displayed over the three-dimensional (3D) boundary element model derived from a magnetic resonance scan of monkey Z. Data was clipped below the 85% maximum value for display purposes. Cyan disks indicate EEG electrode positions. Current density is concentrated beneath electrode P6 caudal to the lunate sulcus and in area V4 on the prelunate gyrus. Results are reproduced for a second monkey in Figure 1—figure supplement 1.

Figure 1—figure supplement 1.. N2pc distribution of monkey P (10–20 EEG recordings).

(A) EEG traces for right (blue) and left (red) visual hemifield target presentations. Organization of traces reflects electrode positions. Scale is consistent across traces and is indicated by OL. N2pc was found to be significant through an ANOVA measured as the interaction between posterior electrode sites, the target position in the array, and the set size sites (sites OR and OL, F(2,42) = 8.39, p < 0.001). (B) Inverse solution using sLORETA for N2pc (difference function mean 190–300 ms following array onset) during a right visual hemifield target presentation displayed over the three-dimensional (3D) render of MR scan for monkey P. Data clipped below 30% maximum value. Cyan cylinders indicate EEG electrode positions.

Figure 2.. Laminar profile of local field potential (LFP) and multiunit (MUA) attentional target selection during visual search task performance across monkeys Ca and He (n = 2).

Responses were averaged across sessions (n = 30) at each of the depths (n = 15) relative to the L4/5 boundary (magenta to green). Difference between target (attended) and distractor (unattended) responses represented by the fill color corresponding to the recording channels’ laminar compartment. Top line of each trace combination is the attended condition, bottom trace is the unattended condition. Significant differences in magnitude of attention effect, averaged 150–190 ms after search array onset, across laminar compartment were detected through an ANOVA for both LFP (F(2, 442) = 22.43, p = 5.2e^–10) and MUA (F(2, 442) = 3.87, p = 0.022). Note the effect of attention in the MUA was largest in the middle layers (M_L2/3 = 2.68, M_L4 = 3.50, M_L5/6 = 2.38), consistent with previous reports (Nandy et al., 2017). Time of the N2pc as measured throughout the main text (150–190 ms following array onset) indicated with orange.

Figure 2—figure supplement 1.. Laminar alignment and receptive field mapping.

(A) Representative receptive fields (RFs) measured with gamma power across recording sites of a single array penetration. RFs across recording sites (z axis) are well aligned, indicating perpendicular penetration. Electrode positioned at left for reference. (B) Current source density (CSD) profile for the same session as (C). The initial sink following visual stimulation was used as a functional marker to determine the layer 4/5 boundary. Current sinks are indicated in red and sources in blue. The black horizontal line indicates the bottom of the granular input sink. Data are smoothed along depth and across time for visualization purposes. (C) Mean CSD profile following alignment of the 30 sessions (monkey Ca 21; He 9). Formatting identical to (B). (D) Columns’ RF locations across sessions and monkeys (cyan, monkey Ca; magenta, He). RF centers determined online, and diameters estimated from previous reports (see V4 RF mapping and electrode orthogonality for details). Concentric circles indicate eccentricities in degrees of visual angle (dva). Radial lines indicate angular positions relative to central fixation (black dot at top center). (E) Additional RF alignments using the magnitude of local field potential (LFP) response along depth, rather than gamma power as in (A), for electrodes found to be in cortex. Leftmost six are examples from monkey Ca (indicated with cyan bar at bottom) and rightmost two are from monkey He (indicated with magenta bar at bottom). Magnitude of LFP response was found for each recording site and normalized from smallest to largest response across visual space leading to the 0–1 normalization. Contours show the location of the largest LFP response along depth. Red vertical line through three-dimensional plot, and red point shown at the top of each plot, indicates the RF measured along depth with black vertical line indicating central fixation. Note the angle for monkey Ca indicates RF’s between 270 degrees (lower visual field vertical meridian) and 0 degrees (right-hand visual field horizontal meridian) and for monkey He indicates RF’s between 180 degrees (left-hand visual field horizontal meridian) and 270 degrees – both consistent with the contralateral positioning of recording chambers for each monkey (monkey Ca left V4, He right V4).

Figure 3.. Extracortical attention-associated signal and simultaneously recorded V4 synaptic currents during representative session.

(A) Extracortical event-related potential (ERP) voltages after search array presentation, averaged over all trials when the target was presented contra- (solid) or ipsilateral (dashed) to the electrode. Inset magnifies the N2pc interval defined as the difference in potentials 150–190 ms after the array appeared (orange highlight). (B) Simultaneous current source density (CSD) when the target appeared in the population receptive field of the column. Dashed lines indicate boundaries between supragranular (L2/3), granular (L4), and infragranular (L5/6) layers. CSD values were interpolated and smoothed along depth for display only. Current sinks have hotter hues, and current sources, cooler. The earliest sink arises in putative L4, likely from rapid feedforward transmission, followed by intense, prolonged sinks in L2/3 accompanied by weaker source in L5/6. (C) CSD evoked by distractor in the receptive field has similar pattern. (D) Subtraction of CSD responses to target versus distractor in receptive field. The only statistically significant differences (determined through a t test across time-depth with p < 0.05, outlined by magenta line) were due to a current sink in L2/3 that arose gradually ~100 ms after array presentation. This relative sink was associated with a weak relative source in L5/6. (E) Simultaneous mutual information between CSD and the extracortical signal for L2/3 (blue), L4 (purple), and L5/6 (green). Times with significant mutual information were computed through Monte Carlo shuffle simulations (MCS). N2pc interval is highlighted. Intervals with significant mutual information persisting for at least 10 ms are indicated by horizontal bars. No mutual information with EEG was observed in L4. (F) Information transmission about target position from V4 CSD to the extracortical signal. Conventions as in E.

Figure 3—figure supplement 1.. Mutual information measures for the extracortical signal, V4 current source density (CSD), and target position.

(A) Mutual information between target position (binarily coded contra- or ipsi-presentation) and the extracortical signal along time (top) aligned on array onset with 95% confidence interval (CI) cloud estimated from subsampling 75% of the data 100 times and recomputing. Significance established through Monte Carlo simulations is indicated below. Only intervals where significance >10 ms were included. Orange region indicates N2pc. (B) Mutual information between target position (binarily coded inside or opposite column receptive field [RF]) and each laminar compartment (L2/3 [blue], L4 [purple], and L5/6 [green]). Panel organization identical to (A). (C) Mutual information between the extracortical signal and each laminar compartment. Panel organization identical to (A). (D–F) Population averages (n = 30) mutual information measures. Same organization as representative session, (A–C) respectively. Clouds around averages denote 95% CI across sessions. Statistical measures for population averages reflect interval’s where 75% sessions were found to be significant through Monte Carlo simulations for >10 ms.

Figure 4.. Grand average demonstrating the link between V4 current source density (CSD) and the extracortical attention-associated electric field.

Conventions as in Figure 3. (A) Average event-related potential (ERP) across all sessions and animals with the target contra- (solid) or ipsilateral (dashed). The N2pc interval is indicated by orange shading. (B) Average V4 CSD with the target in (top) or out of the receptive field (RF) (center) with the difference between the two at the bottom. (C) Grand average information transmission about target position from V4 layers to the extracortical signal as a function of time (left). Average +2 SEM of information transmission during the N2pc window (right). Panel below shows that information transmission from L2/3 and in L5/6 was significantly greater than that from L4 (t test p < 0.05). Timepoints with significant information transmission were assessed through Monte Carlo simulations during >75% of sessions. Intervals with significance persisting for at least 10 ms are indicated by horizontal bars, color coded for each laminar compartment (bottom).

Figure 4—figure supplement 1.. Individual monkey physiology and information transmission.

Results for monkey Ca (n = 21) in left column and He (n = 9) in right column. (A) Extracortical signal traces for target contralateral (solid line) and ipsilateral (dashed line) to recording site. Orange highlight represents the average time of N2pc used throughout the rest of the manuscript (150–190 ms). (B) Current source density (CSD) profile for target in receptive field (RF) (top), outside RF (center), and the difference between the two (bottom). Horizontal boundaries indicate laminar compartments. (C) Information transmission computed at each timepoint regarding target position from laminar CSD to the extracortical signal (top). Blue represents L2/3, purple represents L4, and green represents L5/6. Timepoints where 66% of recorded sessions showed significant information transmission for more than 10 consecutive milliseconds through Monte Carlo simulations for each laminar compartment are shown at the bottom.

Figure 4—figure supplement 2.. Grand average results with expanded interval.

Conventions as in Figure 3. (A) Average event-related potential (ERP) across all sessions and animals with the target contra- (solid) or ipsilateral (dashed). The N2pc interval is indicated by orange shading. (B) Average V4 current source density (CSD) with the target in (top) or out of the receptive field (RF) (center) with the difference between the two at the bottom. (C) Grand average information transmission about target position from V4 layers to the extracortical signal as a function of time (left). Average +2 SEM of information transmission during the N2pc window (right). Panel below shows that information transmission from L2/3 and in L5/6 was significantly greater than that from L4 (t test p < 0.05). Timepoints with significant information transmission were assessed through Monte Carlo simulations during >75% of sessions. Intervals with significance persisting for at least 10 ms are indicated by horizontal bars, color coded for each laminar compartment (bottom). Note that elevated information transmission persists into the 200–250 ms interval. Coupled with the sustained sink/source pattern observed along V4 layers and the inversion of the ERP polarization, this might indicate the coexistence of the N2pc and Pd during this interval with the polarization of the Pd masking the persistent N2pc in the ERP.

Figure 4—figure supplement 3.. Information theoretic relationship between V4 current source density (CSD) and extracortical signal persists when accounting for stimulus identity.

Grand average information transmission about target position from V4 layers to the extracortical signal as a function of time (left). Information transmission was calculated twice for each recording session – once only taking into account only trials when the item in the receptive field (RF) was red (e.g., red target in RF and red distractor in RF trials) and once only taking into account trials when the item in the RF was green. The average of those two computations was taken for each session and the traces shown here are the average across sessions (n = 30). Average +2 SEM of information transmission during the N2pc window (right). Panel below shows that information transmission from L2/3 and in L5/6 was significantly greater than that from L4 (t test p < 0.05). Timepoints with significant information transmission were assessed through Monte Carlo simulations during >75% of sessions. Intervals with significance persisting for at least 10 ms are indicated by horizontal bars, color coded for each laminar compartment (bottom).

Figure 4—figure supplement 4.. Microsaccades do not explain information theoretic relationship between V4 current source density (CSD) and extracortical signal during the N2pc.

(A) Example saccade main sequence from one session in monkey Ca. Microsaccades detected during task performance highlighted in lower left. (B) Grand average information transmission about target position from V4 layers to the extracortical signal as a function of time (left). Information transmission was calculated across all sessions (n = 30) from both monkeys. Only trials where no microsaccades were detected between array onset and saccade for behavioral choice were included in the information theoretic computation. Average +2 SEM of information transmission during the N2pc window (right). Panel below shows that information transmission from L2/3 and in L5/6 was significantly greater than that from L4 (t test p < 0.05). Timepoints with significant information transmission were assessed through Monte Carlo simulations during >75% of sessions. Intervals with significance persisting for at least 10 ms are indicated by horizontal bars, color coded for each laminar compartment (bottom).

Figure 5.. Contribution of columnar feature selectivity to the N2pc.

Conventions as in Figure 3. (A) Visual search array configurations used for color selectivity analyses. (B) Laminar profiles of red/green color selectivity across all sessions. The hue of each point across cortical depth signifies the value of a color selectivity index (CSI), derived from local gamma power. CSI values < 0 (>0) indicate preference for green (red). CSI is smoothed across adjacent channels for display. Sessions are sorted from left to right based on a column color selectivity index (CCSI) that estimates each column’s combined selectivity. A bar plot of session-wise CCSI is plotted below. Asterisks indicate columns with significant color-selectivity (Wilcoxon signed rank, p < 0.05). Asterisk color indicates monkey (Ca cyan; He magenta). (C) Average event-related potentials (ERPs) for trials when a red (top) or green (bottom) target or distractor appeared in the receptive field (RF) of the 17 color selective columns. Conventions as in Figure 3. (D) Difference in current source density (CSD) when the target relative to distractor appeared in the columnar population RF when a red (top) or green (bottom) stimulus appeared in the RF (n = 17). (E) Average ERP for trials when the preferred color (top) or non-preferred color (bottom) target or distractor appeared in the RF (n = 17). Conventions as in Figure 3. (F) Difference in CSD when the target relative to distractor appeared in the RF with the preferred (top) or non-preferred (bottom) color. (G) Average difference in information transmission between laminar CSD and N2pc when preferred relative to non-preferred stimulus color appeared in RF. Conventions as before. More information was transmitted when a stimulus of the preferred color appeared in the RF. (H) Correlation between difference in information transmission across color columns and CCSI for each session for L2/3 (blue, top), L4 (purple, center), and L5/6 (green, bottom). Spearman correlation reported in lower right of each plot with data from all 30 sessions. Color-specific information transmission scaled with magnitude of color selectivity. (I) Information transmission for columns with (solid, n = 17) and without (dashed line, n = 13) feature selectivity for L2/3 (top), L4 (middle), and L5/6 (bottom). Intervals with significant differences are plotted below at two alpha levels for a two-sample t test (filled: 0.05; unfilled: 0.1). Bars plot average with upper 95% confidence interval of information transmission during the N2pc for columns with (left) and without (right) feature selectivity. Significant differences are indicated with a bracket and p value from a two-sample t test.

Figure 5—figure supplement 1.. Single session example (monkey Ca) of the observed difference in information transmission depending on columnar color preference.

(A) Extracortical event-related potential averaged across correctly performed trials (n = 2992) for a single session with the target stimulus presented contralateral to the recording electrode (solid line; n_red = 742, n_green = 766) or ipsilateral to the recording electrode (dashed line; n_red = 752, n_green = 732) for trials where the target is red (top) or green (bottom). (B) Average difference in current source density (CSD) profile for correctly performed trials between target present in receptive field (RF) and distractor present in RF for red item in RF trials (top) and green item in RF trials (bottom). (C) Difference in information transmission between the preferred color and the non-preferred color for the same single session as in panels A and B.

Figure 5—figure supplement 2.. Attentional modulation is present in cortical columns not selective for an attentional target feature present in the pop-out search task.

(A) Cortical columns found to be non-selective for red or green were identified across both monkeys (n = 2) and selected for further analysis (n = 13). (B) The N2pc was observable in the sessions where no significant feature selectivity was present. Contra- versus ipsilateral target presentations (relative to the recording electrode) were plotted in time relative to the search array onset. Intervals of N2pc as measured in the main text (150–190 ms following array onset) is highlighted in orange. (C) Current source density (CSD) profiles for the target in receptive field (RF) (top), outside RF (middle), and the difference between the two (bottom) for the feature non-selective columns (n = 13). (D) Multiunit spiking activity averaged across non-selective columns (n = 13). Recording channels shown for upper (blue), middle (magenta), and lower (green) laminar compartments. Top line of each trace is the response in the attended condition with the bottom line being the unattended condition. Fill reflects the difference between attention conditions.

Figure 6.. Comparing an estimated field potential generated from the current source density (CSD) across the cortical columns to the actually observed extracortical event-related potential (ERP).

Black lines indicate the empirically measured event-related potential (ERP_obs, top), averaged across sessions. The pink line indicates the estimated ERP calculated from the synaptic currents across V4 columns, averaged across sessions (ERP_cal, center). Synaptic currents at each electrode are measured and divided by the Euclidean distance of the electrode from the extracortical surface (see Materials and methods; Nicholson and Llinas, 1971; Kajikawa and Schroeder, 2011). ERP for target present in the receptive field (RF) versus target opposite the RF is shown as solid and dashed lines, respectively (example array for each condition shown at top right). Clouds around ERP_cal lines indicate 95% confidence intervals across sessions for each condition. Note that despite differences in overall waveshape (which are likely due to the fact that V4 is not the only contributor to the attention-independent, visually evoked ERP), the timing of differences within signal types can be compared. The congruence in polarization of the difference in potentials is of similar note.

Figure 7.. Feature mosaic hypothesis.

(A) A map of preferred color in area V4 derived from optical imaging (Tanigawa et al., 2010) with corresponding color columns in area V4. (B) Relative contributions of color selective cortical columns to the N2pc when a red (left) or green (right) target appears in the receptive field (RF). Intensity of pyramidal neuron activity is indicated by saturation in the diagram. The mesoscopic columns produce electric fields (dashed lines) that sum to produce the equivalent event-related potential (ERP).

Author response image 1.