OMNETICS
3.2 Data selection
A prior data analysis was required to ensure that the recorded data met the essential require-ments of the working hypothesis formulated in Section 1.4. As the hypothesis implied investigat-ing the routinvestigat-ing of neuronal signals correspondinvestigat-ing to attended and non-attended visual objects, I had to verify that the recorded neuronal signals were indeed caused by the visual objects presented on the screen.
I calculated the γ-PSD of the recorded signals to use it as a measure of the stimulus-driven neuronal activation and thus as a basis for data selection. Only signals from neuronal populations sufficiently activated by the visual stimuli were suitable for testing the hypothesis, so I set a criterion to exclude the data with little stimulus-driven neuronal activation from the upcoming analysis. The criterion was based on evaluation of the characteristicγ-PSD value (see Equation 2.23): this normalized γ-PSD value was required to lie above a threshold value of 1. Applying the criterion rendered a portion of the recorded data in which the stimulus-driven activity was at least double as high as the spontaneous activity. I investigated the effect of applying this criterion on the recorded data.
I chose a set of example recording sites for showing neuronal activity of the corresponding neuronal populations. In case of the epidural recording I chose site A corresponding to a neuronal population in area V4, and sites B and C corresponding to two neuronal populations in area V1 of the monkey’s left brain hemisphere. In case of the intracortical recording I chose site E in area V4 and site D in area V1 of the monkey’s right brain hemisphere. These sites were expected to fulfill the requirements of the working hypothesis, as was confirmed later on. The location of the recording sites can be found in Figures 3.1 and 3.2.
Figures 3.3 and 3.4 show the time course of the normalized PSD averaged across all available data of example V4 and a V1 recording sites prior to applying the criterion described above.
Figure 3.3 corresponds to the epidural recording, and Figure 3.4 - to the intracortical recording.
The black rectangular marks the time-frequency range (45 to 81 Hz and the two-cycles-span from 3.35 to 6.15 sec) in which the PSD in each trial was further averaged to become a measure for estimation of the intertrial PSD variation. These characteristic γ-PSD values (one per trial) are plotted in Figures 3.5 and 3.6 to demonstrate their variety among single trials.
As expected, PSD varied much stronger in case of the intracortical recording with regularly moved electrodes. In fact, recording conditions using the epidural array were different as com-pared to those using the intracortical array. In the first case the electrodes were chronically implanted, whereas recording with the intracortical array required moving the electrodes before and after each recording session. So being relatively sure that chronically implanted electrodes record from the same neuronal population in each session, one could not claim the same in case of electrodes that had been moved daily, despite the effort of always hitting the same spot in the brain tissue. This supported the idea of setting a PSD-based criterion to extract data most likely produced by the same neuronal populations which got similarly (strong) activated by the presented visual stimuli.
I applied the criterion defined by Equation 2.23 to the acquired data. This criterion required that the stimulus-driven activity should be at least double as high as the spontaneous activity
V4 V1
Figure 3.3: Time-frequency plots of the normalized PSD recorded at example V4 and V1 sites with the epidural array. The plot on the left hand side corresponds to the V4 recording site A, and the plot on the right hand side corresponds to the V1 recording site B (see Figure 3.11 for the site’s location). Recording took place while the monkey performed the shape-tracking task schematically shown at the bottom (see Figure 2.3 for a comparison). Condition 1 A/e (see Figure 2.4), averaging over 2676 trials in each plot.
(see Section 2.6.4 for details). By this means, only sufficiently activated recording sites (as stated by Equation 2.24) were subjected to further analysis. Figures 3.7 and 3.8 show the effect of applying this criterion to the data recorded with the epidural and with the intracortical array, respectively.
In case of the epidural recording, 11,4% of the data from V4 site A did not meet the selection criterion and were discarded (blue dashed line in the left plot of Figure 3.7), so that the average PSD values of the remaining data slightly increased in respect to the average across all data (see the solid red line being slightly above the solid blue line in the same plot). All data from V1 site B satisfied the selection criterion, so in this case no data was discarded (see the right plot of Figure 3.7).
In case of the intracortical recording, 17,2% of the data from V4 site E did not meet the selection criterion and were discarded (dashed blue line in the left plot of Figure 3.8), so that the average PSD values of the remaining data increased considerably in respect to the average across the original data set before selection (see the solid red line being well above the solid blue line in the same plot). The data from V1 site D contained a 2,8% fraction not fulfilling the selection criterion (see the dashed blue line in the left plot of Figure 3.8). As expected, the average PSD values of the remaining data slightly increased with respect to the average across all data (see the solid red line being slightly above the solid blue line in the same plot).
I applied criterion 2.23 to all recorded data for each recording site and discarded data files in which the criterion was not fulfilled. Figure 3.9 summarizes the results of data selection for all stimuli conditions, i.e. for the shape-tracking task with and without distracter. If the criterion
V4 V1
Figure 3.4: Time-frequency plots of the normalized PSD recorded with the intra-cortical array at example V4 and V1 sites. The plot on the left hand side corre-sponds to the V4 recording site E, and the plot on the right hand site correcorre-sponds to the V1 recording site D (see Figure 3.12 for the site’s location). Recording took place while the monkey performed the shape-tracking task schematically shown at the bottom (see Figure 2.3 for a comparison). Condition 1 A/i (see Figure 2.4), averaging over 1346 trials in each plot.
was fulfilled for at least 100 trials, a given recording site was considered sufficiently activated by the stimuli and thus selected for further analysis (see the highlighted arrays’ parts in Figure 3.9).
Figure 3.9 demonstrates once again, that the two visual stimuli, i.e. shape sequences, were represented in two distinctly different V1 neuronal populations. Indeed, in case of the shape-tracking task without distracter - meaning that only one shape sequence was shown on the screen at a time - for each selected V1 recording site the visual stimulus caused neuronal activation only when presented at one of the two possible locations (see the highlighted V1-part of the two arrays in Figure 3.9: one of the two upper values was always zero). At V4 sites, however, neuronal activation was observed mostly for both locations of the shape sequence, which was reflected by existence of trials satisfying criterion 2.23 for both stimulation conditions (see the two upper values in the highlighted V4-part of both arrays in Figure 3.9). This suggested, that the receptive field of the neuronal population at such a V4 site comprised both of the two possible locations, at which the shape sequence was presented.
In case of the shape-tracking task with distracter - meaning that two shape sequences were shown simultaneously on the screen - sufficient neuronal activation at all selected recording sites was observed in both conditions (see the two bottom values in the highlighted parts of both arrays in Figure 3.9).
The approximate location of the selected recorded sites on both arrays placed on the monkey’s brain is schematically shown in Figure 3.10.
Figure 3.5: Normalized average single trialγ-PSD recorded in V4 and in V1 with the epidural array. PSD was averaged from 45 to 81 Hz and in the two-cycles-span from 3.35 to 6.15 sec for one of the V4 (top plot, corresponding to site A shown in Figure 3.11) and one of the V1 (bottom plot, corresponding to site B shown in Figure 3.11) sites recorded with the epidural array at Condition 1 A/e (see Figure 2.4). Normalization was done according to Equation 2.21. Each of the 2676 values corresponds to a single trial, and the vertical lines separate different data files. Within each file an averaged PSD-value is shown as a short black horizontal line. The common average PSD is shown as a long dashed red line stretching throughout all the trials. Time course of the PSD averaged through all trials is shown in Figure 3.3.
Figure 3.6: Normalized average single trialγ-PSD recorded in V4 and in V1 with the intracortical array. PSD was averaged from 45 to 81 Hz and in the two-cycles-span from 3.35 to 6.15 sec for one of the V4 (top plot, corresponding to site E in Figure 3.12) and one of the V1 (bottom plot, corresponding to site D in Figure 3.12) sites recorded with the intracortical array at Condition 1 A/i. Normalization was done according to Equation 2.21. Each of the 1346 values corresponds to a single trial, and the vertical lines separate different data files. Within each file an averaged PSD-value is shown as a short black horizontal line. The common average PSD is shown as a long dashed red line stretching throughout all the trials. Time course of the PSD averaged through all trials is shown in Figure 3.4.
V4 V1
Figure 3.7: Applying the data selection criterion to the data from the epidural recording. Normalized PSD averaged in the gamma band from 45 to 81 Hz and in the two-cycles-span from 3.35 to 6.15 sec at recording sites A (left plot) and B (right plot) under Condition 1 A/e. See Figure 3.11 for the site’s location and Figure 2.4 for the details on stimulation condition. The shape-tracking task is schematically shown at the bottom (see Figure 2.3 for a comparison). Solid blue line corresponds to all data (2676 trials), solid red line corresponds to data satisfying the criterion in Equation 2.23 (2371 trials for V4, left plot), and dashed blue line corresponds to data not satisfying the criterion (305 trials for V4, left plot). In case of the shown V1 site all data satisfied the criterion (see the right plot).
V4 V1
Figure 3.8: Applying the data selection criterion to the data from the intracortical recording. Normalized PSD averaged in the gamma band from 45 to 81 Hz and in the two-cycles-span from 3.35 to 6.15 sec at recording sites E (left plot) and D (right plot) under Condition 1 A/i. See Figure 3.12 for the site’s location and Figure 2.4 for the details on stimulation condition. The shape-tracking task is schematically shown at the bottom (see Figure 2.3 for a comparison). Solid blue line corresponds to all data (1346 trials), solid red line corresponds to data satisfying the criterion in Equation 2.23 (1114 trials for V4 and 1308 trials for V1), and dashed blue line corresponds to data not satisfying the criterion (232 trials for V4 and 38 trials for V1).
Epidural Intracortical
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2377 389
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Conditions: Trials: Conditions: Trials:
1346 1352342 362V1
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V4
Figure 3.9: Number of available trials for all recording sites after applying the data selection criterion. Overview of the stimulation conditions, as well as the number of all trials is presented under the corresponding arrays. Electrodes selected for further analysis are highlighted with the blue color.
LU STS
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Figure 3.10: Schematic illustration of the monkey’s brain with selected electrodes.
The epidural array is shown on the left hand side, and the intracortical arrays are shown on the right hand side. Selected electrodes are highlighted with the blue patches. The letters A-E mark the example recording sites (see Figures 3.11 and 3.12 for a comparison). The brain surface curvature is not accounted for in this picture for simplification purposes.