Figure 1Inferotemporal (IT) theta: generation, task-modulation, phase-resetting, and comparison of spontaneous and event-related. Add average CSD map to show that initial sink is same as theta but different frequency. Move modulation of information specificity to show that maximum encoding happens during upper level sink
A. Location of the linear microelectrode array in anteroventral IT is indicated with a yellow arrowhead on a sagittal MRI taken with the electrodes in situ. The probe is in the lateral aspect of the fusiform g., medial bank of lateral occipito-temporal s. (coordinates 38 lateral, 22 posterior, 11 down) (). Patient 1.
B. Potential gradient (PG) recorded between contacts at 150μ centers, during 2 Word Recognition trials (WR). Spontaneous background activity between trials is dominated by 4-5 Hz oscillations (↓) in middle to upper cortical layers (channels 6 to 14, periods with white background). Presentation of new or old words (red and green bars at bottom) suppresses the theta activity (↑), especially from ~300-800ms after word onset (periods with violet background).
C. iERSP plotted as a z-score relative to the pre-stimulus baseline for each frequency and channel, allowing the task-related modulation of theta to be examined and compared to other frequencies. In the new and old plots, the values are averaged across trials and Red indicates increases in CSD power in that frequency, latency and channel from baseline; blue indicates a decrease. In the new-old plots, iERSP is expressed as a t-test between trials with new versus old words. Red indicates more power to new than old words in that frequency, latency and channel; blue indicates less. Data is presented from a Size Judgment (SJ) task to words referring to objects or animals. Similar results were obtained in WR (see ), and a Verb Conjugation task where the subject detects regular past tense morphology (not shown). The initial response is a strong wideband increase in spectral power at ~130-300ms (pink background) in upper and especially middle layers (↓). New and Old are initially similar, but significantly diverge starting ~230ms, when the spectral power increase to the Olds abruptly ends, whereas that to the News continues until ~300ms (↓↓) resulting in significant new/old differences (⇓). During the next phase, lasting until ~800ms post-stimulus onset (violet background), theta activity profoundly decreases compared to baseline (^). The decrease is deeper to new words (^^) resulting in significant new/old differences (Δ). In the third phase, until ~1400ms (yellow background), theta increases in deep layers to old words (↑), resulting in significant old>new differences (⇑). The overall pattern of response as well as the effects of word repetition were similar in all tasks, regardless of whether repetition defined explicit recognition targets, or was incidental to task requirements.
D. Middle layer CSD waveforms from individual trials aligned on stimulus onset and superimposed. In the pre-stimulus period, the CSD is large, and dominated by low frequencies with no discernable phase-relationship (^). This activity is abruptly terminated by a sharp current sink (downward deflections) peaking at ~180ms (▲), evolving into synchronous theta oscillations (⇑⇓) that are more prominent for old (green, 31 trials) than new words (red, 23 trials). The sharp peak is ~65ms duration, corresponding to ~16 Hz. Theta source peaks are visible at ~300, ~500 and ~700ms, after which the theta rapidly loses its phase-locking with the stimulus. Raw waveforms were high pass filtered at 1 Hz. Trials were selected for display that evoked a large amplitude initial sink from 170 to 200ms on ch 12.
E. Laminar profile of theta-generating transmembrane current flows. CSD amplitude is plotted against time to show average waveforms of theta cycles in different cortical layers. These demonstrate that theta is generated by sinks in middle cortical layers, (▲; purple box) accompanied by sources in more superficial (↓; orange box) and (weakly) in deep layers (green box). Over time, these alternate with sources in middle layers (⇓) and sinks (⇑) in superficial and (weakly) deep layers. Separate waveforms are shown for the second through fourth theta cycles after old (green) or new (red) word presentation (these occurred approximately in the range of 300-800ms after word onset), and for the second through fourth theta cycles prior to the stimulus (blue). Amplitude profiles of the three waveforms across cortical layers are indistinguishable, implying that event-related and spontaneous thetas are generated by similar or identical intracolumnar synaptic circuitry. Theta is generated through alternating sources and sinks in middle and superficial cortical layers with accompanying sources in superficial and deep layers.
F. CSD amplitude plotted against cortical depth at the trigger time in Figure 1E. The most prominent features of theta generation are a middle layer sink (▲) with superficial (⇓) and deep sources (↓). No difference can be observed between the amplitude profiles of theta generated after the stimulus to new (red) or old (green) words, versus spontaneous theta in the prestimulus period (blue).
G. Theta phase (from the Hilbert transform) averaged with respect to stimulus onset, demonstrating phase-locking with trial onset. Middle-layer phase shows positive peaks at ~140, 340 and 540ms (⇓) after stimulus onset, i.e., at 5Hz. Phase becomes random after ~800ms. Prestimulus phase is also random and thus averages to zero. Note that the Hilbert phase is a nonlinear measure because it abruptly transitions from –π to +π, and thus may appear slightly different from the ITC measure.
H. Event-related averaged CSD. As reported elsewhere (), the initial response is a sink in layer IV at ~180ms (△). Phase-resetting aligns the CSD waves so that traditional averaging methods reveal an evoked response that includes strong theta frequency oscillations (↓) even though theta power has decreased from the pre-stimulus period.
I. Spectral power, averaged on the peak of the sink in Layer III, increases in a wide band up to ~150Hz. Scale same as C except pixels with zscore less than 3 have been masked as green.