Visual evoked EFP latencies correlate with RT

8.3 Results

high for close and low or negative for intermediate to far electrode eccentricities (Fig. 22F).

The repetition of the analysis with all trials resulting in slower than or equal to median RT is summarized in Fig. 22B. The RF eccentricity groups 10° and 12° showed significant differences between attentional conditions (AI: –0.06 and 0.013 respectively, Fig. 22F).

The AI was previously shown to be influenced by the duration of a trial [Sharma et al. 2015].

Therefore, the trials were split by the duration between cue and target LCA dimming onset into short (< median trial length) and long trials (≥ median trial length) to test whether the trial duration influences the attentional modulation. Using only long trials, RF eccentricity groups of 1° to 3° showed significantly higher BGP for the attend-in condition (Fig. 22E, AI between 0.015 and 0.018). RF eccentricity groups of 11° and 12° showed significantly higher BGP for the attend-out condition (AI between –0.01 and –0.012). Utilizing only short trials for the analysis revealed significantly higher BGP activity for the 10° and 11° groups for the attend-in condition (AI –0.007 and –0.006, Fig. 22D and G).

Taken together, the BGP activity of electrodes with RFs near to (< 4°) and far from (> 9°) the attentional target was significantly influenced by covert attention. For fast RT and long trials, AI was positive for nearby and negative for far located electrodes. The attentional modulation disappears for groups with RF eccentricities near to the target when short or slow RT trials were used for the analysis, while a significant difference remained for electrodes with far distance to the target letter. Electrode groups with RFs of intermediate distance to the target letter showed no significant modulation between the attend-in and -out condition.

8.3 Results

positive deflection with a small negative bump (345 ms). After the onset of the cue, a positive peak with a plateau (from 74-110 ms post cue onset) was evoked, followed by a negative peak (142 ms). The onset of the target LCA dimming evoked a small negative (68 ms post LCA dimming onset) and a positive deflection (134 ms).

The amplitude and latency of the stimulus-induced VEP of the occipital cortex recorded with EEG is correlated with the subsequent RT [Donchin & Lindsley 1966; Eason et al. 1967;

Hartwell & Cowan 1994]. In order to test whether this holds true for EFPs from V1, a possible correlation between the EFP response peaks evoked by the LCA dimming of the target letter and the RT was investigated.

For each target letter, an electrode was selected that was highly modulated by the luminance change of the LCA (most stimulus-selective electrodes). For each electrode and target letter the mean over the trial-averaged EFP signal in between ± 25 ms from 68 ms post LCA dimming onset was calculated. The time window was chosen to cover the negative peak response that was evident in the ERP post LCA dimming. The electrode which had lowest mean EFP response for a target letter was chosen as the stimulus-selective electrode for that letter. Due to the fact that only the lower right quadrant of the visual field was covered by the RFs of the electrodes, some letters did cause no or only a modest modulation of the EFP.

Thus, target letters were rejected from further analysis whose selected electrode showed a negative peak amplitude in the trial-averaged EFP higher than –0.5 × 10^–5 V. Five out of 15 target letters were rejected. For the remaining most stimulus-selective electrodes, all available trials were sorted by the normalized RT (normalized by the condition- and session-wise RT

Fig. 23: Grand average ERP of the EFP over electrodes and normal trials. Single red lines indicate a change in the stimulation (1. stimuli display onset, 2. cue onset and 3. target LCA dimming onset). The space between the broken-dotted red lines represents a pseudo-random time interval which was cut out because of the trial-alignment.

8.3 Results

median, see Materials and Methods). An example of an electrode is shown in Fig. 24A right.

Additionally, the EFP activity averaged over the first, intermediate and last 50 trials (non overlapping, sorted by RT) was plotted (Fig. 24B right). The same was done for the time post stimulus display and post cue onset (see Fig. 24A left and middle, and Fig. 24B left and middle). The latency of the EFP activity evoked by the target dimming was shortest for fast RT and longest for slow RT, while the latency of the display onset was more or less constant between RT groups. The cue onset did not evoke a clear negative peak in this example.

In order to quantify the observed latency shift, the 50% fractal area latency of the negative response from 51 to 140 ms post LCA dimming onset with a threshold of 0 z-score was calculated for all single trials of the selected electrode (for details, see Chapter 7.2.7.1).

Single-trial RTs were divided into thirds to sort the latencies into three groups: fast, medium and slow RT trials. These groups of single-trial latencies were tested for statistical differences between them. Five out of ten of the selected electrodes showed significant differences between the grouped latencies (mean ω²: 0.024; p < 0.05). The post hoc test revealed a

Fig. 24: Analysis of the correlation between evoked EFP responses and RTs. Data of the best-selective electrode of the target letter R is used in this figure. A: Single trials were sorted by normalized RT from fastest to slowest and plotted for the different onsets. B: From the sorted trials the first, intermediate and last 50 trials (non-overlapping) were averaged for three different stimulation onsets. The onset latency evoked by the target dimming is shifted to the right for slower RTs.

8.3 Results

significant difference between slow and fast RT for all of the five electrodes. All other pair-wise comparisons were only significant for ≤ 1 electrodes.

The previous analysis was repeated, but this time, RT groups were defined by shuffling RT values and by dividing these into thirds to serve as control. Using shuffled RT groups did not result in significant differences between any groups.

The analysis of the attentional modulation of EFPs from V1 revealed that the attentional modulation was higher in long lasting trials in comparison to shorter trials. In order to investigate whether the trial duration has an influence onto the EFP response latency, the EFP latency analysis was repeated with trials sorted by the time between cue onset and target LCA dimming (groups: short, medium and long trials). This analysis revealed a significant latency difference between the medium and the short trials group for only two electrodes (p < 0.025) and between the fast and the short trials group for a single electrode (p = 0.04).